posted on June 12, 2012 00:00
Engine Downsizing and Turbos: More MPG and More Power?!
By Khiem Dinh
Khiem Dinh is an engineer for Honeywell Turbo Technologies at the time of this writing. All statements and opinions expressed by Khiem Dinh are solely those of Khiem Dinh and not reflective of Honeywell Turbo Technologies.
In an extremely rare instance, government regulations have actually made two seemingly conflicting goals both better. "To have one's cake and eat it too" as they would say. With rising fuel costs and environmental concerns, regulatory bodies across the world are demanding reduced fuel consumption and lower emissions. Previously, this would imply smaller engines with reduced horsepower; two things counter to the wants of performance enthusiasts. Enter the modern turbocharged engine. Through turbocharging, engines can now deliver a wider torque curve, more power, and substantially improved fuel economy. The fuel economy gains are through engine downsizing and downspeeding. We're going to show you how turbos achieve all of these goals.
First, we need to look at naturally aspirated engines. The torque curve of a naturally aspirated engine is greatly influenced by the volumetric efficiency (VE) of the engine. There are many factors affecting VE: intake and exhaust manifold lengths, cam profiles, cam timing, intake and exhaust port lengths and geometries, number of valves, bore vs. stroke, rod ratio, etc. Generally, all of these variables can be optimized to provide the best VE, and therefore torque, in a narrow rpm range. This also means that VE and torque are sacrificed in other portions of the rpm range. Old school V8 hot-rodders are well aware of this fact when they swap in a big cam; they end up with the lopey idle and weak bottom-end torque to gain the top-end torque and horsepower. In effect, they reduced the VE in the lower rpm range for better VE in the upper rpm range. Modern engines have continually employed new technologies to minimize these trade-offs in the pursuit of better VE and torque across the rpm range.
The technologies probably having the greatest impact in improved VE and torque in the greatest range are related to the valves and cams of the engine. The factors related to valves affecting VE include lift, duration, overlap, and timing. In order to have optimal VE across the rpm range, the factors have to be adjustable. One of the first mass produced systems to address this was Honda's VTEC system which essentially had two cams in one; one set of cam lobes were optimized for lower rpm operation while the other set of cam lobes were designed to maximize VE in the upper rpm range. Since that time, many new systems are now in production enabling variable valve lift, duration, and timing allowing engineers to extract the best torque from of an engine across all rpm ranges. A few of these systems are the BMW VANOS (Variable NOckenwellenSteuerung which is German of course), Nissan VVEL (Variable Valve Event and Lift), and Mitsubishi MIVEC (Mitsubishi Innovative Valve timing Electronic Control). The newest kid on the block that I am aware of is the Fiat Multi-Air system which has so much adjustability that it can actually open and close the intake valve twice during the intake stroke.
|The Evo X uses Mitsubishi's MIVEC adjustable cam timing system to achieve a fat powerband.
|Nissan's VQ37 features the VVEL system to get the most out of the naturally aspirated engine.
The holy grail of all torque curves is the flat torque curve and all of these valve systems aim to achieve the table top shape. However, as much as they try, they are still limited by the physics of cams, valves, lifters, rockers, and every other part in the valvetrain which prevents them from getting the same VE across the entire engine speed range. Therefore, the torque curve is not flat.
|Here is a chart showing the VE curve for an Acura NSX and also how VTEC affects the curve. Firstly, the curves are not even close to flat. The second observation shows how having two optimized cams profiles (the two cam profiles used in the VTEC system) improved the VE across the entire engine speed range due to having to make fewer compromises on cam profile.
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Tuesday, June 12, 2012 7:25 AM
Great article. I've been wondering for a while, though, does anyone know what long term effect running higher boost has on engine life? There's a reason BMW/Mercedes chose the boost level they did, and I'm guessing it has to do with the fatigue life of critical components such as con rods. Getting it to not detonate and put down mega numbers is nice and all, but how long can you do it for?
Tuesday, June 12, 2012 8:35 AM
Isn't the heat produced essentially creating a heat treatment on certain parts? Extra power/heat is torture for electronics, so radiating more in the engine compartment should require more insulation.
So more heat, more insulation, more weight, poorer chassis balance, means there is no free lunch.
Still cool to be on puff as the needle spins up!
Tuesday, June 12, 2012 9:37 AM
The problem with smaller turbocharged engines is that when they are under load, they are also under boost, which dramatically increases fuel consumption. Case in point: the Ford 5.0L N/A V8 vs the 3.7 turbocharged V6 in the new F-150s.
While under minimal load (highway cruising and around town), the turbo V6 gets significantly better fuel economy than the V8. However, when towing, the turbo V6 gets much, MUCH worse than the V8, because the V6 is under boost and sucking down much more fuel. The point is that engine downsizing and turbocharging is only an effective means of increased fuel economy if the bulk of the engine's use is not under heavy load (such as when towing).
Additionally, real-world test data is even more confusing. The Chevrolet Cruze, using a small turbocharged 4 cylinder, does not seem to get as good fuel economy as the larger-displacement naturally-aspirated Ecotec it replaced.
Tuesday, June 12, 2012 10:00 AM
All sorts of weird factors come into play, which make me both want to agree and pick apart things like this.
Regarding engine load... for most applications, it's a reasonable assumption that relatively little time is going to be spent at high load, so stuff isn't designed for it. For a relevant example, I'm willing to bet money that more or less any performance type gasoline turbo engine can't reject enough heat from the pistons at efficient air/fuel ratios to have them live long-term. So you richen things up because hey, WOT isn't going to be on a fuel economy test cycle, whereas pistons breaking will certainly come around to bite you.
Lowered compression ratio in turbocharged downsized engines works against you too; it hurts thermal efficiency. You can compensate for that in mid-load situations by having a turbo that's making slight boost, but that's then creating exhaust backpressure. Direct injection helps a lot here and is going to be one of the keys in low load efficiency of modern turbo engines, but it's expensive and the aftermarket stuff for DI isn't there yet to my knowledge.
I'd really like to see internal details on one of the old R8 engines; I'm willing to bet there's some interesting stuff going on with piston cooling.
Tuesday, June 12, 2012 12:30 PM
Everyone has brought up great points. However, turbo motors are just plain tougher than N/A motors. Turning up the boost a little will not affect engine life to a high degree. There are safety margins built into everything OEM. It's just been that recently N/A motors have been pushed to the limits of "safe" tuning at the OEM level. Turbo motors do not need to be pushed nearly as hard to get the results they are looking for.
Side note: Turbo motors can get 100% VE (or higher I believe) where N/A motors cannot reach more than about 75%.
Yes, boost burns fuel. But how often are you under high load during the life of your grocery-getter's life? REALLY small turbos kind of suck due to building boost at low engine speeds under light load. The Caliber SRT4 is a good example of this.
My love for turbo motors runs deep and I do not see any downsides to running a turbo motor other than cost to build. They are built tougher and have more parts which equals more money out of my pocket. OEMs have been building turbo diesels forever. No one thought to complain about those. Why complain now when it's gasoline?
Tuesday, June 12, 2012 1:24 PM
Ech... various points. Yes, I'm bored at work. ;)
If turbo motors are tougher than N/A ones, it's solely because they're built that way so the OEMs don't have to worry about warranty work, and even then not necessarily. One easy case in point is the USDM WRX engine; cast pistons, and crank, rods and rod bolts not significantly different than the NA engines. I remember mention that the GT-R is using cast oil gallery pistons as well; more provision for cooling certainly but cast. I don't pretend to know the internals for every boosted engine out there, but I know that in the GM build manual for the Ecotec they specify changing a lot of the internals at power levels not that far over stock. Put simply, just as modern N/A engines are being designed so that there's not much reserve strength, so are quite a few modern boosted engines.
Volumetric efficiency doesn't change much under boost (ignoring things like backpressure from the turbine) - the engine isn't taking in any more volume of air, it's just that the air is more dense. It may seem like a silly distinction, but it's important for some purposes. Also, most modern N/A engines are well above 75% VE at peak torque; some of the well optimized race engines are above 120% VE.
Turbo diesels and gasoline diesels have a lot of differences, both by nature and by design. Turbodiesels (excepting things made by Audi and Peugeot for LeMans) are designed to operate at pretty much constant RPM and load for most of their lives, and that load to be a large portion of their rated output... so everything is substantially overbuilt compared to production gasoline engines. By nature they're operating lean of stoichiometric... because they're diesels and not gasoline engines. They don't need to worry about detonation (because they're diesels) or high piston crown temperatures (because they're designed for it) so they operate more efficiently under boost. Gasoline engines are richening up significantly under boost.
Don't get me wrong, I'm actually for the idea of downsized turbocharged engines. It's just harder than it might seem to get actual efficiency benefits out of them.
Tuesday, June 12, 2012 2:19 PM
In my experience, one downside to the downsizing with turbos is throttle modulation.
I owned a 09 chevy cobalt ss turbo. it had GMs tune on it which created around 21-23 psi of boost on the stock turbo depending on the weather conditions. It would just spin the tires in a straight line in 2nd gear. there was no way to modulate the throttle without dropping boost/power and then having to build it. I could short shift to 3rd, but my clutch didn't like that. The problem is worse in the corners. the problem only got worse when i had it tuned.
I now own a 5.0 mustang and love the fact that I can modulate the gas to get the right amount of wheel spin on a launch. its also nice to be able to go 8/10ths in a corner right to the limit of adhesion. I couldnt do that in my downsized 2.0turbo. Drive by wire is still fucking stupid though, along with the electric steering. i'm begging for some feedback in the wheel.
Tuesday, June 12, 2012 3:18 PM
Comparing turbo-diesels and gasoline diesels is like comparing spark plugs and fuel injectors. Although they're both internal combustion engines, they operate on rather different principles.
Tuesday, June 12, 2012 4:10 PM
apples & oranges
@matt --> most "tuners" don't offer warranty like how "manufacturers" must do..
BTW WTF is a "gasoline diesel" ??
you talkin S-I? C-I? HCCI? HVCE?
Tuesday, June 12, 2012 4:25 PM
I meant "turbocharged gasoline engines."
Tuesday, June 12, 2012 5:29 PM
@ Dusty: Why is the turbo engine sucking down more fuel just because it's under boost?
I think the EcoBoost is sucking down more fuel because it's revving much lower and tows more effortlessly. This causes the driver to be more comfortable maintaining 70mph up that grade, where in a gas engine, the annoying throttle load and engine revs of the NA motor would cause him to back off.
Driver confidence in the engine (and its perceived longevity) definitely come into play. Not exactly apples to apples, but I know that if I tow with my Cummins Dodge like I did with my F-150 (keeping it around 55-60mph), it gets much better mileage (around 19-20mpg towing Project G20). However, since the truck is so understressed, I often find myself just setting the cruise at 70-75mph, and wafting along on CTD torque. The result? 16-18mpg.
Tuesday, June 12, 2012 7:53 PM
Well, if the object is to have close to 14.7 Air/Fuel ratio, putting more air into the cylinders (as in under boost) requires more fuel to be injected in, as well...
Remember, a 2.5L engine under 14.7 psi of boost is sucking in as much air as a 5.0L naturally-aspirated engine.
Tuesday, June 12, 2012 8:01 PM
@Jason, generally, more weight is saved by going to the smaller and adding turbos. Yes, insulation is important to protect other components in the engine bay.
@Dusty, it depends on the duty cycle and where the engine is going to be operating. I rarely see a truck towing and I bet the vast majority of F150s don’t even have a tow hitch attached. The Ecoboost F150 serves a good number of customers who only use the bed of the truck to make Home Depot runs. For those 5x a year they might need to tow, the Ecoboost F150 CAN do it. For the other 350+ days a year, it’ll save a lot of fuel compared to the naturally aspirated gasoline V8. If anyone needs to do serious towing, they should opt for the Powerstroke diesel. We have a Chevy Cruze as one of our company cars. Using 87 octane in moderately warm temps (80+F) vs. 91 octane will cut the fuel economy 20%-25%; it should really be a premium -required vehicle. Engines can be sized too small. The first gen Prius only used a 1.5L and Toyota switched to a 1.8L to improve fuel efficiency. Some of my own real-world experience, I got the same gas mileage in my old Nissan 200sx SE-R when it was NA and turbo (SR20DE). Of course, when it was turbo, it made double the power. A friend had the GA16DE in his 200sx which would get 40mpg hwy vs. my 30 mpg. So it’s fair to say a turbo GA16DE would get significantly better gas mileage than the NA SR20DE while still have a lot more power.
To address durability, companies design parts to meet a specific fatigue life target at a loading and duty cycle. If you keep the loading the same but increase the duty cycle, the life will be shorter. If you increase the loading and keep the duty cycle the same, the life will be shorter. Increase load and duty cycle, and life really goes down. That’s why an engine can last 100k miles no problem in a street car, but only be good for 5k racing miles.
@Kenku, the old R8 info should be out there. I have a tech document Audi published on the R10 diesel somewhere. Internal combustion engine technology is still evolving. The 1.6L gasoline, direct-injection, turbo engine being used in the DeltaWing is running a pretty high compression ratio (I can’t remember exactly, but want to say it’s 11.0-12.0) and going to run stoichiometric A/F using 100octane (RON I think) fuel. Its BSFC target is in the 225-230 g/kWh range, so a major improvement over the current typical 270 g/kWh for gasoline turbo engines.
@b Drecksage, I think your experience is more with the throttle mapping on the Cobalt than the turbo. I’ve auto-x’d/road course tracked my old turbo Nissan SE-R (also FWD) and Evo (along with a few friends WRXs/STIs) and Mini Cooper S with no throttle modulation issues. Keep in mind that pretty much all the cars in World Touring Car Championship (WTCC) are 1.6L gasoline turbo; I think many being part of the Global Race Engine strategy, so the engines are also used in WRC. The one car I did have serious throttle modulation issues on? An Infiniti G35 sedan with the NA VQ35 and the automatic transmission (rental car for a Nissan convention). The throttle mapping was horrible and it had super aggressive throttle tip-in. So as soon as you tipped into the throttle, it would give a lot of throttle (to make it feel faster off the line I assume). I’m very smooth with my throttle inputs and no matter how light I was on tip-in (I do mean just barely touching it), the rear of the car would get kicked out sideways when trying to accelerate out of the corners because the electronic throttle would give it a lot of throttle vs. the tiny pedal angle. The stability control would then kick in the rear brakes to bring the back-end back inline. The rear pads were smoking pretty good after the first session. Ironically enough, turning off the VDC (stability control) cured the throttle mapping issue; so turning off the stability control made the car safer.
Tuesday, June 12, 2012 9:06 PM
@spdracerut Let me try to explain a little better.
these are just generalizations, not exact obviously.
wot in cobalt ss=300hp
9/10ths throttle in cobalt=150hp since the throttle isnt all the way open, the bpv is open to stop compressor surge.
1/2 throttle in cobalt =150hp cause lack of boost
get what i'm trying to articulate? I know its not exact.
i wouldnt expect to have this problem with awd cars cause you dont have to worry about wheel spin until high hp/wet/snowy conditions. Also, I wouldnt expect it to happen with race cars cause of the added traction.
i also never had the problem with the 06 supercharged cobalt ss I had because at 9/10ths it didnt bleed a lot of the boost.
Tuesday, June 12, 2012 9:07 PM
@spdracerut also with the g35....that is the STUPID drive by wire!!!!! :)
Tuesday, June 12, 2012 9:26 PM
@ drecksage: I experienced the same with my T25 setup on my SE-R way back when, as anything above 1/2 throttle was max boost, and that is precisely why we went with an internal wastegate setup with no boost controller on Project G20. With the stock wastegate and no controller, it is much easier to manage.
@ Dusty: yes, a turbo engine at 14.7 psi of boost will burn more fuel, but not for long before the truck and trailer are bouncing off the speed governor. My point is that both trucks represent the same aero load, so it should take the same amount of energy to motivate them, regardless of engine aspiration. Since a given air mass entering the NA engine is given a fairly similar amount of fuel as the turbo engine, why would it burn so much more fuel? The only reason I can think of is the Ecoboost driver feels his engine is less stressed, and maintains a higher speed.
Tuesday, June 12, 2012 10:09 PM
Willing to be a given air mass entering an OEM turbo engine in a pickup truck won't be given all that similar of an amount of fuel as to the NA engine though. Under load they're going to have to be fairly conservative.
Wednesday, June 13, 2012 12:01 AM
@b drecksage, that's why I mentioned tracking my old turbo FWD Nissan SE-R :) Also a t25 setup with JWT S3 cams. On a road course, I ran straight wastegate pressure. I only used the boost controller for more straightline speed when required. And also the Mini Cooper S.
A lot of it has to do with turbo sizing and calibration (fuel, timing, boost mapping, cam phasing, throttle mapping). I didn't like the stock boost mapping on the Evo as it had a lot of error correction. The error correction caused the boost to spike and overshoot the boost target when you rolled into the throttle, so maybe that's what was happening on the Cobalt. I took out the almost all error correction (from ~20% to 2% IIRC) which basically made it very similar to running a straight signal from the boost source to the wastegate.
Wednesday, June 13, 2012 8:35 AM
Turbocharged engines generate more heat; perhaps this is the reason. All I know is that every test I've seen comparing the 5.0L F-150 to the turbo F-150 shows the turbo motor has far worse gas mileage when towing and hauling under the same conditions.
However, when the truck is not stressed and simply taking Sandra Soccer Mom to the grocery store, the turbo V6 does get better fuel economy.
The only point I'm trying to make is that using a smaller-displacement turbocharged motor to achieve better fuel economy does not work in all circumstances. A vehicle that is used in a high-stress environment would benefit from the higher-displacement motor.
Wednesday, June 13, 2012 10:13 AM
@Dusty, you are correct. So that brings into play the whole analysis of intended use and duty cycle. I did the same analysis when I had my Evo and switching to the S2000. How often MUST I use the back seat and larger trunk capacity of the Evo vs. the S2000? The answer was basically never, so I ditched the Evo for the S2000 for: superior gas mileage, cheaper running costs, and a few other things.
Of course, if everyone did this analysis, the majority of SUVs and CUVs wouldn't exist. In Europe, they have to do this analysis because fuel costs are 2-2.5x here so they have to get good MPG. So it's no surprise the most common car sizes are of the Ford Fiesta and Focus.
But like I said, technology is always marching along. I'm looking at an article on the DeltaWing and it has a 12.0 compression ratio, will run 100 RON (~94-95 oct here) gasoline, direct injection, and run stoichometric A/F at WOT giving it a targeted BSFC of 220-225 g/kWh. That's much closer to a diesel than a current gasoline engine. At WOT, a typical current turbo gasoline may have a best BSFC of 270, but at full load and full power, it could be in the neighborhood of 330 g/kWh. So this engine used in the DeltaWing is a massive improvement in fuel efficiency.
Wednesday, June 13, 2012 10:57 AM
A quote I'm seeing about the RML engine in the DeltaWing has it at 230g/kWh on the dyno, but that's nitpicking. Interesting factoid is that the Honda RA-168E turbo F1 engine was doing 264g/kWh back in the late 80s in its efficiency setting, at a *lot* more RPMs than the DW engine and without direct injection or the like. Interesting comparison in a way actually; they're both similar displacement, both running on 100RON "gas", and the Honda motor was only running 1.5bar over atmospheric boost; I wonder what the DW is running.
One paper I did see on the R8 (race) engine had it that the piston crowns were a lot more conventional looking than production direct injection gas engines because part load, stratified, combustion wasn't a design consideration. Wonder what the DW's looks like; pictures I've seen have them hiding it.
Wednesday, June 13, 2012 11:48 AM
The DW is quoted to be running 2bar. It doesn't state if that's gage or absolute, though I suppose it's gage.
Interesting on the RA-168E, it did 314g/kWh in 'best power' mode.
Wednesday, June 13, 2012 3:05 PM
@ Kenku: a turbocharged engine usually runs about 1 point lower A:F than a NA engine under WOT. Less than 10% difference, so pretty similar.
@ Dusty: Don't know what to say. Both should consume about the same amount of fuel steady state traveling. I still think the seat:steering wheel interface is the culprit.
Wednesday, June 13, 2012 3:32 PM
Doing a bit of a rethink about what I said... okay. Given the same torque output in both cases... 10ish% richer per airflow, sure, and I'll grant that's not *that* much.
The problem is the air consumption. To get the same torque output, you need a bit more airflow due to lower compression ratio hindering thermal efficiency, and a bit more airflow to compensate for not being able to run best torque spark timing due to knock issues... and that before someone puts regular unleaded in it instead of premium.
I don't have data on the Ecoboost to say how much that would cost in BSFC. It's going to be something though.
Wednesday, June 13, 2012 3:35 PM
And yeah I'm changing my reasoning, before anyone calls me on it. ;) I'm not an expert, I'm thinking this stuff through on the fly.
Wednesday, June 13, 2012 5:00 PM
A larger-displacement engine needs to work at a lower percentage of its total engine output to create a specific amount of torque. So a 5.0L V8 would have to work less hard to output 300 lb-ft of torque than a 3.7L V6. In fact, in order for that V6 to produce 300 lb-ft of torque, it must be under boost, and thus consuming more fuel. In other words, the turbo V6 must operate at a higher "duty cycle" than the V8.
Think of the two engines as fuel injectors, with the turbo V6 being a "smaller" injector.
Wednesday, June 13, 2012 5:59 PM
@ Kenku: The lower compression should be offset by higher cylinder pressure under boost.
@ Dusty: yep, except we're not talking about AC motors, we're talking about gasoline engines. Both engines are making the same power at the crank to motivate the vehicle. In addition, the NA engine working at a "lower duty cycle" is less efficient, as pumping losses are higher. This is why a smaller engine, assuming RPM can be kept constant, gets better mileage. Being at a lower throttle opening for the same power output is less efficient, not to mention the added friction of moving more pistons (that are also larger and going farther), valves, etc, for the same crank speed.
A smaller engine under boost should not burn a lot more fuel than a larger one under vacuum if they're under the same demand. Boost does not cause increases in fuel consumption, increased power demand does.
Wednesday, June 13, 2012 6:42 PM
Dusty and Rockwood, you guys are missing 3 important pieces of information: the BSFC maps for each engine and where the towing operating points would be. I did some digging around and a NA gasoline engine has a typical best BSFC of around 240 g/kWh and the turbo gasoline around 270 g/kWh. The engine in the Prius can get 220 g/kWh, but uses atkinson cycle to do it. Anyway, under high load towing, a bigger NA engine probably gets better mileage than a smaller turbo engine just based on BSFC.
Don't forget that a lot of the fuel economy improvement from the smaller engine comes in city driving cycle too where you sit and stop. When just idling, it takes less fuel to keep the smaller displacement engine spinning than the bigger engine.
Wednesday, June 13, 2012 8:55 PM
Khiem: I had mentioned that the turbo engine will be about 10% less efficient earlier. :)
Monday, June 18, 2012 10:54 AM
If the goal is good power and high mileage in a gasoline engine, the best solution to me seems to be few cylinders, relatively large displacement and few components.
For example, if you need 115 horsepower for your Fiat 500 the current solution would be something like a 1.5L DOHC, 16 valve 4 cylinder. This engine is going to need 5 crank bearings, at least 6 cam bearings, a big complicated timing system and an altogether pretty high level of stress.
Why not build a SOHC, 8 valve, 2 liter twin instead? Make it a flat or V twin and you'd need only two crank bearings. Make it pushrod and you need only two cam bearings. And since you only need 58 hp/liter, you're going to be running some pretty mild cam timing. Granted, you'd also need a counter-balance shaft, but still, sounds to me like you'd be experiencing a lot less oil shear, a lot less frictional loss in an engine like this. You could also run it at much lower engine speeds because it's only got to make 58 hp/liter.
Monday, June 18, 2012 11:01 AM
Probably more energy wasted in pumping losses and cylinder friction than in valvetrain/bearing friction, not to mention packaing constraints with a larger engine without increased friction from whacky rod-ratios, etc.
Monday, June 18, 2012 11:22 AM
A two liter twin is going to package considerably better than an inline four, even with taller deck heights. I would also think it's easier to control cylinder friction in two slightly larger cylinders (volume will increase with piston size much faster than ring circumference) than four smaller cylinders.
A lot of the most efficient engines out there seem to follow this recipe. LSx, Porsche 944, Fiat Twin Air, 3.8L Buick, etc. On the other hand, it's almost impossible to be more inefficient than small engines with lots of cylinders and high revs. Think of all those Italian 3 liter V12s that get 8 mpg.
Monday, June 18, 2012 12:53 PM
How would a V motor package better in a FF car, especially one displacing 2.0L? What about tooling costs?
And yes, rings circumference goes down, but what about the whacky rod-ratios you would get from anything but a severly oversquare engine? 4 cylinders also helps spread the load out from power production, so I question if they'll be able to make the ringlands thin enough to meet moder emissions standards, and I wonder how smooth it would be (I know they're internally balanced, but bangs half as often and twice as powerful probably wouldn't go unnnoticed).
I also think a 2.0L 2-banger would be a slug.
In any case, the primary motivation for downsizing is reduced pumping losses. A 2.0L engine will still attempt to pump 2.0L of air every time it turns, even if it doesn't need 2.0L of power.
Turbos and 6spd transmissions take care of the need for high revs, and I wouldn't call a 4-banger "a lot of cylinders" by any means.
Those Italian V12s got 8mpg for a lot of reasons. I don't think excessive cylinder friction was chief among them.
Monday, June 18, 2012 2:04 PM
I think you are considerably overestimating the size of a two liter twin. You can carry a 140 cube Harley V twin in your arms, ditto with a big BMW bike flat twin. The Fiat twin air is likewise tiny. Hell, I can' think of any twin that isn't externally tiny.
If it's an inline twin, mount it transversely and you have an engine that's half an inch taller than an I4 and half the width. Actually, it would probably still be shorter than the I4 because we're using a SOHC or pushrod configuration that will be shorter than a DOHC setup.
If it's a V or flat twin, mount it north south and take advantage of the fact that it's only 1 cylinder thick.
And let's think about the ring lands. We are talking the same displacement per cylinder as the biggest modern Vortec Chevys. And 58 hp per cylinder isn't much either.
If pumping losses are such a big deal, why don't we see a direct correlation between engine size and mpg? Why don't we see better mileage out of, for example, a 6 liter Ferrari V12 than a 7 liter Corvette V8?
I would explain this with lower engine speeds (my reason for larger displacements), fewer cams, bearings, valves, rods and pistons dragging through the oil on every revolution (my reason for smaller piston counts and two valve heads), and a more efficient combustion chamber with more quench (my reasoning behind relatively large bores and two valve heads).
And I don't think I agree with your position that a 2 liter twin would draw more air than a 1.5 liter four and therefore have more pumping losses. With a lower revving, larger displacement engine you could also run taller gears. This means it will be easier to run my hypothetical twin at low rpms than the typical four banger. A 2.0L twin running at an average speed of 2,000 rpm is going to take in the same amount of air, all else equal, that a 1.5L four running at 3000 rpm would.
I'm not against downsizing, I would just do it by cutting the cylinder count. Big twins or triples for econo cars, 2.5-3.0 liter fours for the family sedans, 5 liter sixes for pickup trucks.
Monday, June 18, 2012 2:08 PM
Whoops, screwed that up. A 2.0 at 2,500 rpm would be the same air as a 1.5 at 3,300, rpm.
Monday, June 18, 2012 4:23 PM
Very true, but a water-cooled production passenger engine with a 100k warranty and PZEV requirements, and an air-cooled production motorcycle where 30k miles before a rebuild is doing good and practically no emissions requirements are two completely seperate things. My guess is a passenger engine V-twin would be considerably larger than what you'd find in a Harley.
Length (snout to tail, crankwise) is a greater concern than width in a transverse FF car. A longitudinal FF car would put the engine ahead of the axle, which isn't so good for packaging or handling dynamics, not to mention tooling costs.
8.1L Vortecs consume enough oil to almost be considered a diesel engine. Google "8.1 Vortec oil consumption" and you'll see plenty of this. My in-laws' have one. 1 quart per 1k miles is about what it consumes, and we're talking about 28% more power out of the same design. Since we're talking about similar valvetrains to the 8.1L Vortec (2V per, pushrod or SOHC), the only remaining way to get more power density is to rev higher, which would be murder for such a motor unless you're talking paint-can sized cylinders on a seriously oversquare engine.
Apples to apples good sir! Ferrari V12 vs. Corvette is not apples to apples. One is a nearly bespoke racing engine cum production car engine, the other is a mass-produced truck powerplant cum sports car powerplant. A good apples to apples would be the Corvette itself: Z06 vs. GS. Same suspension, tires, 3.42:1 axle ratio, chassis, etc. 24mpg for the Z06, 26mpg for the GS. The difference? 0.8L of displacement.
More apples to apples: My father-in-law's construction company has a fleet of service bed trucks. The 6.8L V10 powered F350s get about the same mileage as the 5.4L V8 F350s (10-12mpg, depending on driver and location). More cylinders, similar mileage. The V10s actually spin faster as well (most have 4.10:1 axle ratios vice 3.73:1 for most of the V8s).
Why wouldn't a 2.0L draw more air? A 1.5L 4-banger is more than capable of motivating a 2800lb econobox at 2000rpm, and the now de-facto 6 speed transmission (or CVT) is more than capable of a good enough gear spread to allow for this.
Downsizing just cylinder count, but not displacement doesn't make a huge amount of sense. A NA 3.0L engine will need to rev to about 6000rpm to make the 240hp required to compete in the family sedan class, and a 3.0 4-banger is going to be a buzzy bastard at 6000rpm. If you use a turbo, you don't need 3.0L of displacement to get there. On top of that, a 3.0L engine is still going to be making lots of vacuum at 70mph on the freeway, and vacuum is bad for fuel economy.
In the end, it's far easier to re-tool a 2.0L four into a 1.6L four than it is to switch from a 2.0L four to a 2.0L twin, and I'd be willing to bet the reduced friction of cylinder paring (with no decrease in displacement) results in less efficiency increase than dropping displacement.
Monday, June 18, 2012 11:31 PM
I think you overestimate the buzziness of a 3.0L four banger making 240 hp.
And remember, this refinement issue is a problem they solved very nicely in the mid 80s. We'd almost have to be better at it now.
That said, the more I think of it, the more I agree that 1,000cc per cylinder is probably too much for the harshness, vibration and oiling reasons you mention. 750cc, though, would let you have a triple with over 2 liters of displacement, and that's LS2 cylinder volumes - which are certainly doable.
I still think you are mistaken about the air thing. If we have a 16 valve, 1.5 liter four making 35 hp (about what you'd need to keep that econobox doing 60 mpg), it will be using the exact same amount of air as a 2 liter twin making 35 hp (assuming equal frictional losses, which would probably favor the twin, and equal VE losses, which would likely favor the four). The difference, it seems, would be that the twin would likely be spinning much slower than the four while making 35 hp. You can get rid of the vacuum problem by simply using taller gears.
As for packaging, here's what I'm trying to illustrate.
Here's a Fiat 500 with the 1.4L inline four. http://i68.photobucket.com/albums/i12/xcentox/DSC_0004.jpg
Here's the same Fiat 500 with the TwinAir two cylinder.
Now, the comparisons we've brought up. I'm only half joking when I suggest the poor but equal fuel measures you've observed in the 5.4 and 6.8 Tritons are a matter of God, in his wrath, striking down vengefully on Ford for making an abomination so evil as the "mod" motor.
Seriously though, I don't have enough experience with those big trucks to know first hand. I've heard that the Triton V10 is one of the least fuel efficient engines of the last 20 years, but I don't know.
As for the Vettes, I have two points. First, I would point out that the LS7 is not only bigger than the LS2/3, but that it also revs higher and that's bad for efficiency. Also, it's interesting to note that the LS7 and LS2/3 are by far the most fuel efficient big-league sportscar engines out there. The only ones I can think of that get close are lower on cylinder count - Porsche flat sixes - or similar on revs and similar on cylinder count - the turbo BMW V8s. The really zingy V8s, regardless of displacement, are horrible. The V10s and V12s, regardless of displacement, are likewise horrible. Zingy V12s mean you will never get out of single digits!
I won't disagree that more displacement, so long as there's enough power to keep the revs down in the smaller engine, does eat away at efficiency. However, I would argue that the displacement disadvantage is dwarfed by rev and cylinder count, camshaft/other friction source issues.
In other words, I think that power density by displacement is a relatively minor concern. Power density by external dimensions and by unit of consumption makes more sense.
Finally, tooling. The big three have been cutting cylinders off their engines for decades. Can't imagine they chose to do so because it's hard and expensive.
Tuesday, June 19, 2012 1:21 PM
I think the level of refinement needed in a car engine now is much greater than it used to be, and oversized cylinders (bore x stroke standpoint) make things more difficult. This is the #1 gripe about the current VQ37HR. What was acceptable in the 80s is not at all acceptable these days. When I drive an SR20-powered car now, even being used to diesel clatter, I'm amazed that I put up with the buzziness, but as recently as 10 years ago, it seemed perfectly fine.
I am not mistaken about the air thing. Neither engine will be operating at 100% load, so the engine will be fighting against a partially-closed throttle to maintain speed. That partially-closed throttle is where your pumping losses are increased. So, that larger engine will need more air to maintain 35hp than the smaller one because some of that combustion energy is wasted on breathing through a smaller hole. This is the core reason behind downsizing, and turbos have made it possible to downsize without the power deficiency usually seen by it.
The V10-powered Super Duties are gas hogs, but it has more to do with brick-like aerodynamics, huge curb weights, sloppy automatic transmissions, huge rolling resistance tires, etc.
For this discussion, the LS7's higher redline is moot. The LS3-powered GS and the LS7-powered Z06 have the same 3.42:1 axle ratio. They are both at the the same RPM on the freeway, yet the Z06 gets worse mileage. The LS7 even has a higher compression ratio, which should help with fuel efficiency.
I can think of not a single V12-powered car where fuel efficiency was even a remote concern.
If just removing cylinders were the key to fuel efficiency, don't you think the OEMs would've done so? Why spend money on expensive turbochargers when they can just make 3.0L 4-bangers?
My last example on why pumping losses are more important is the diesel engine. A diesel engine offers very little engine braking, despite stratospheric compression ratios, 4-valve/cylinder pushrod valvetrains, huge strokes, etc. Why? No throttle body. How do to the Big Three address this in their large diesel trucks? Exhaust brake. What is an exhaust brake? It's basically a throttle body in the exhaust stream that stops up the exhaust much like a throttle body stops up a gas engine's intake manifold. The result? Enough engine braking to keep 23,000lbs of truck and trailer from accelerating beyond just about any preset speed on a 7% grade. All three big diesels, both 6 and 8 cylinder, are all equally worthless at engine braking without this.
An exhaust brake is not necessary on their gasoline counterparts, as the gas engines create plenty of engine braking on their own through the throttle body.
Cutting cylinders off of an existing design is one thing, switching cylinder arrangement is another. There's a reason why Toyota went to Subaru for its flat-four.
Tuesday, June 19, 2012 2:12 PM
If you have taller gears with the larger engine, which you would, the larger engine would be working at higher load. Higher load equals less pumping loss. Lower rpm equals less everything loss. Run a VANOS type system and this becomes a total non-issue, for the diesel reasons you mention.
A two liter engine taking in air from 2 liter gulps, 2500 times a minute, will consume the exact same amount of air as a 1.5 doing it 3,333 times.
When I said the LS7 was higher revving, I was trying to say that where the LS2/3 redlines at 6500 rpm, the LS7 is designed to rev to 7,000 rpm. It's higher strung in addition to being larger.
Your point about V12s is valid, but do you know any modern Porsche flat sixes that were designed for fuel economy? We are talking similar level of performance, similar design goals and very different economy outcomes.
As for why OEMs don't make large displacement engines with few cylinders, tax and insurance brackets in Europe and Asia are a good place to start. Another reason is that most of the engines with fewer than four cylinders aren't available in the US, probably for marketing reasons. I would also submit that a lot of engines, past and present, fit the sort of design brief I'm describing. LSx, 3.8L Buick, Iron Duke (crappy but fuel efficient), Toyota 3rz-fe, 2tr-fe, 1ar, 2ar, Ford 3.8L V6, Porsche 2.5/2.7/3.0 inline fours, all of the V6s you get base in the pony cars, etc.
Tuesday, June 19, 2012 3:09 PM
VANOS reduces pumping losses slightly, but all gasoline engines will have part throttle pumping losses because they are throttling airflow to limit power. Diesels limit power by fuel injection. Air restriction, even when it happens at the valves, is still pumping loss. Until they start limiting power by fuel instead of air, this will always be a problem with gasoline engines.
Why can't you run taller gears with a smaller engine? Are you talking about running a large enough engine to maintain 750rpm at 75mph or something?
Yes, your math is correct. Why can't the 1.5L engine turn 2500rpm?
What does the LS7's redline have to do with fuel consumption? We're not talking about fuel consumption at maximum engine speed, we're talking about fuel consumption at 65mph (or any other speed you choose). Both have the same gears, so both will be turning the same RPM. In any case, the larger displacement LS7 gets worse mileage.
In any case, yes, reducing cylinder count does help, as does displacement. Increasing engine displacement and reducing cylinder count for the sole purpose of reducing RPM and friction is not a reasonable way of increasing fuel efficiency, IMO.
Monday, June 25, 2012 6:54 PM
You can't run taller gears with smaller engines because smaller engines don't make as much horsepower at low rpms. Because of this, you have to turn higher rpm to achieve any desired speed. Thus, the 110 hp 2.0 engine will make more power at 2500 rpm than the 110 hp 1.5 engine and can, call it 45 hp for the 2.0. The 1.5 will need probably closer to 3000 rpm to make 45 hp.
The LS7s redline indicates cam timing, heads, intake and exhaust more tuned for higher rpm. Big cams and huge ports aren't good for low speed efficiency.
Tuesday, June 26, 2012 11:27 AM
How much power do you need to motivate a sub-3000lb econocar at freeway speeds? I doubt that either engine would be operating at WOT to maintain 70mph. In fact, the old-school 1.5L SOHC engine in my buddy's Civic CX had no problem with 80mph (2600rpm) with 4 dudes in it (~750lbs), 4 snowboards in a rack on top (read: additional areo load), and gear for the weekend in the trunk (150lbs?). Car cruised along at 80mph at less than 1/2 throttle, and would pull most hills without downshifting.
You'd be surprised what CAD has done to cylinder heads these days and keeping low speed efficiency up. I got almost exactly the same fuel mileage in my P10 switching to a VVL motor, which has MUCH larger ports than a regular SR20DE, has a shorter runner intake manifold, much stiffer valve springs, the small cam profile on the SR16 cams is about the same size/slightly larger than the stock SR20DE cams, and compression is marginally higher (0.8 points).
Tuesday, June 26, 2012 5:52 PM
If a 1.5 can make the necessary horsepower at 2500 rpm, the 2.0 could do it at a lower rpm. More displacement = more torque at the same throttle opening. This means you can run the engine at a lower rpm to get the same, required hp. Let's say you desire 45 hp at half throttle. You are going to be turning higher rpm in the smaller engine to achieve that, all else equal.
This is purely anecdotal, but I just did a motor swap from a DOHC 1.5L I4 (4.10 gears) to a 2.0L DOHC I4 in my project car. In order to keep up with traffic with the 1.5, I was shifting at 3000 rpm plus and getting fuel mileage between 18 and 25 mpg. With the 2.0, I'm shifting at 1500, 1800 rpm. I'm getting 22 to 25 mpg now, with the same gears. My highway mileage is about the same, largely because the 1.5L required frequent trips down to 4th gear on the highway and the 2.0 never does, my city mileage is considerably better because my average rpm is probably half what it was before.
If I'd gone with the 3.54 gears in the 2.0's stock transmission, I wouldn't need to do 3500 rpm on the highway anymore and the advantage would be even greater.
With your engine, btw, I'd guess the higher compression and similar cams are cancelling out your bigger ports.
Tuesday, June 26, 2012 6:16 PM
That said, a turbo, provided you could run it close to stoichiometric under boost, would let you go crazy running super high NA loads for highway cruising and then shifting to boost when you need the extra power.
Tuesday, June 26, 2012 9:40 PM
Modern gasoline direct injection turbo cars can run stoichiometric under boost in the lower half of the rpm range, so where you do a lot of your city driving.
Wednesday, June 27, 2012 11:26 AM
@ Daewoo: Understood, but there's a point of diminishing returns on reduced RPM from an NVH and driver satisfaction standpoint. A car that gets 75mpg but is absolutely tortuous to drive is worthless to the manufacturer if it doesn't sell. For example, all the engine downsizing/optimization in the world is nothing compared to making a car a 3-wheeled 20' long teardrop for highway MPG. No one would drive/buy it though.
Wednesday, June 27, 2012 11:02 PM
True. If I were the king of a car company with either a bad reputation or no reputation (think Mitsubishi, Suzuki, Daewoo, Daihatsu), and I was going to revamp my engine lineup, I'd do it like this.
1.5L SOHC flat twin with around 100 hp for city cars and entry level Civic-sized cars.
3.0L SOHC flat four with around 180 hp for sporty Civic-sized car, entry level Camry-sized car.
3.0L DOHC flat four with around 230 hp by means of direct injection and better heads. Make this the sporty engine for the Camry-sized car and the base engine for the big cars.
3.0L turbo DOHC flat four with around 300 hp. Make this the hotrod engine for the smaller cars and, perhaps with a touch more power, the level 2 engine for your big car.
4.5L SOHC flat six with around 300 hp. Trucks, vans and big cars.
4.5L turbo DOHC flat six with about 400 hp. Here's your premium sedan, premium engine.
The entire thing would be super modular as well. Same pistons, same rods, same deck height. Only two sets of valves. Only three different crank castings. Only two types of valvetrain setup. Only three types of fuel injection.
Redline everything at 6,000 rpm and you've got good efficiency, low stress, low COG and class leading packaging because you've got the fewest cylinders for any comparable competitor.
Monday, July 02, 2012 9:32 AM
I don't think you'll be able to get a 3.0L engine to rev to 6000rpm without reliability/NVH concerns.
I'll finish this too-long-for-even-me conversation with this:
Increasing displacement not only increases pumping losses, it increases friction in relation to increases in displacement, maybe even negating most of the gains received from reduced RPM. Remember that while the crank and cam are only turning 1500rpm (or whatever value you choose), the increased displacement means that the larger piston is moving farther with each stroke. The larger engine's pistons will not only be travelling at similar speeds to the smaller engine, they'll also experience higher rates of acceleration near TDC. I think that with modern roller valvetrains and microfinishing, friction losses from the crank and valvetrain aren't nearly as high as from the actual piston travelling through its stroke.
Monday, July 02, 2012 1:17 PM
750cc per cylinder is commonplace right now, is it not?
Monday, July 02, 2012 4:55 PM
In a 4-cylinder? Nope. With the long stroke you'll need and OHC valvetrain, that motor is going to be really wide (if you do F4), or have a really shitty rod:stroke ratio. LS motors (what you're using for the 750cc model, I assume) are V motors and pushrod, which will allow for taller decks.
If you do I4, you'll REALLY have some NVH issues.
I suppose you could do a V4, but that would have all the weight/balance issues of a V6 (though slightly smaller) with none of the benefits (since engine length is not really a concern with a V6).
There's a reason manufacturers haven't gone this route. If just reducing cylinder count, and not displacement were it, they'd love it as they won't have to add another supplier and cost to their engine production (turbochargers).
Monday, July 02, 2012 5:04 PM
I had a longer response but it got messed up, damnit.
Anyways, expect to see the high volume to be in 4-cylinder, 1.4L-2.0L, 350cc-500cc (GM, Ford, Subaru, Nissan, Audi/VW, BMW, Mazda, EVERYONE). There will be little volume in any 4-cylinders above 2.0L. 1.0L 3-cylinders are coming too.
Next next high volume will be V6s. Currently, the bigger ones of those are 3.5L-3.7L (Nissan, Honda, Toyota, etc) for 580cc-620cc per cylinder. But expect all those to start going downsized to 3.0L like BMW and Audi have already done. I want to say that Honda and GM are working on 2.8-3.0L 6-cylinders right now.
V8s like Audi and BMW are 4.0-4.4L for 500-550cc per cylinder. V8s are going to be low-volume and pretty much the domain of sports cars and trucks. I think really only GM uses a displacement/cylinder of 750cc, so very small peanuts when looking at the total volume of vehicles sold.
Tuesday, July 03, 2012 10:58 AM
Khiem: Ford has their 6.2L V8 (775cc/cylinder), but there isn't a single fuel-economy-minded vehicle with that kind of cylinder displacement.
Tuesday, July 03, 2012 2:48 PM
Manufacturers have already solved the issues you bring up. Toyota currently makes hundreds of thousands of 700cc/cylinder four cylinder engines (across three engine families). Porsche's 3.0L inline 4 cylinder was a very good engine 20 years ago, and we're better at NVH now then we were then. There are multitudes of 4 plus liter six cylinder engines. The small block Chevy is, by far and away, the best selling engine of all time. This makes it the opposite of small peanuts. It is also the most versatile engine in its class and one of the most fuel efficient as well. In fact the only engines that can compete with it are large displacement Ford V8s (trucks, muscle cars) and large displacement Porsche F6s (sports cars). This means that my proposal is neither new nor radical.
Look at the best fuel economy engines in a given class and you will find that, almost invariably, they have low cylinder counts and average or larger than average displacements. This is because you can run them at lower engine speeds with the same load (pumping losses) as their smaller or higher cylinder-count competitors.
Tuesday, July 03, 2012 3:00 PM
Think of it this way, a two generations old 6.0L pushrod V8 is smooth enough for Cadillac, a class leader in fuel economy and makes 400 hp. Why on earth wouldn't we see 200 hp, class leading fuel economy and smooth enough-for a Cadillac with a 3.0L I4 with a counter balance shaft? Or even more than that, a naturally balanced 3.0L flat four?
And I also think you're misunderstanding a big part of this. I'm not proposing a 3.0L 4 as a competitor to a 1.5L 4, I'm proposing it as a competitor to a 2.5L V6. I would compete with that 1.5L 4 with a 2.0L triple. This, it seems to me, is completely uncontroversial. Big Toyota/Porsche/GM 4s absolutely cream small V6s on fuel economy. Big V6s cream small V8s, big V8s cream V10s and V12s.
Wednesday, July 04, 2012 5:20 PM
What 3L 4 cylinder does Toyota have in the US market?
What 700cc/L 6 is currently sold other than Cummins?
Which class leading fuel economy car uses a larger engine to accomplish this? Mazda went smaller for Skyactive, Toyota went smaller for the Prius, the 1.8L/2.0L Sentras get better FE than their 2.5L platformmates, despite shorter gearing and similar revs, Chevy went smaller for the Ecotec.
Hell, this whole article is about engine downsize for FE. Why would manufacturers do that?
A V8 is naturally balanced. An I4 is not. The larger the 4, the larger the vibrations. This is not an obscure fact. A large F4 would be insanely wide. Using pushrods to accommodate a larger engine to save on friction is oxymoronic.
A 2.5L V6 will likely make more power, pollute less, have less pumping losses, and similar/less friction than a 3.0 4. Did you not read my post about your larger engine moving larger pistons farther and incurring more friction loss? Did you not read my post about the complete lack of engine braking in a diesel?
A large 3 cylinder will be an NVH nightmare.
Wednesday, July 04, 2012 5:29 PM
BTW, the larger, lower revving and taller geared Mazda3 gets worse mileage than its smaller platformmate. Why? Pumping losses and friction from a bigger engine.
Thursday, July 05, 2012 1:24 AM
Did you not read my several posts listing the engines that comply to my paradigm? Please do so before posting yet again about how my entirely uncontroversial proposal has NO PRECEDENT IN THE HISTORY OF THE UNIVERSE!!!!
Did you ever figure out what I'm actually proposing? I agree that a 1.8L i4 will get better gas mileage than a 2.5L I4. Not controversial. However, this difference is usually very small. What is not small, as I will prove below, is the difference between a small V6 and a large I4.
Therefore, it follows that large per cylinder displacement (up to about 750 cc/cylinder) is well worth the costs if you can get rid of extra cylinders. Ie, a 2 liter triple beats a 2.0L I4. A 3.0L four beats a 3.0L six.
Larger throttle openings make fewer pumping losses. If you want fewer pumping losses, run in a higher gear. Very simple. More cylinders make more friction surface area than bigger piston rings. Pie(r)^2 and all.
The NVH thing is demonstrably false. We have millions of engines like I describe and they're fine. You have ignored these engines several times, but that really doesn't change the facts.
Now, as to my point regarding large engines with smaller cylinder counts. This is actually harder than I thought because nobody makes small V6s anymore (because they are largely pointless) and everybody makes big 4 bangers, but we'll do the best we can.
First, let's consider small SUVs and trucks, since they are the most common vehicles where consumers can choose between small V6s and big I4s within the same class.
2012 Toyota Highlander 2wd 2.7L I4, 22 mpg combined. 2012 Tacoma 2.7L I4 2wd, 22mpg combined. Toyota Venza 2.7L I4 23mpg combined.
Compare this with the direct competitors from Ford, ie the 3.0L V6 (smallest in the range) Ford Escape FWD, 20 mpg combined. The much smaller and more aerodynamic Fusion 3.0L V6 just matches the giant, brick-like Venza SUV-thingy at 23 mpg combined.
The Chevy Captiva 3.0 V6 (smallest V6 in the range) gets a combined 19 mpg. Contrast this with the 2.9L I4 in the similarly sized and less aerodynamic Chevy Colorado, which gets 21 combined mpg. The Equinox, also the with the smallest in range 3.0 V6 manages 19 mpg combined.
I couldn't find any major manufacturers who built a sub-3L V6 in their 2012 model year cars or trucks. This is surely evidence of the amazing superiority of small, multi-cylinder engines. In a related note, I couldn't find a single large manufacturer who didn't offer a 4 cylinder engine of at least 2.4L. This is surely evidence large 4 bangers suck balls.
But maybe this is all an apparition. Maybe we will see the economy amazingness of small, multicylinder designs over the crappy, large displacement/cylinder designs in big league sports cars producing ~400-500 hp. I chose them because it's the only other segment I can think of where you will have comparable cars with engines that encompass both my preference and your preference.
In my corner, we have the Corvettes, the Porsche 911s, Mustangs, Nissan GTRs, and the Mercedes SLS. In your corner, we have all the Ferraris, all the Maseratis, all the Lamborghinis, the BMW M3/M5/M6 and all the big Astons.
So, my side gets the following figures:
Corvette 6.2L V8, 430 hp 19 mpg combined. Corvette 7.0L V8, 505 hp, 18 mpg combined.
Porsche 911 Carrera S 3.8L F6, 400 hp, 22 mpg combined. Porsche GT3 RS 4.0L F6, 500 hp, 17 mpg combined.
Mustang 5.0L V8, 420 hp 20 mpg combined. Mustang 5.4L V8, 662 hp, 17 mpg combined.
Nissan GTR 3.8L V6, 530 hp, 19mpg combined.
Mercedes SLS 6.2L V8, 563 hp, 16 mpg combined. (I'm actually cutting you slack here. The new 5.5L V8 is 19 mpg combined).
Your side, which has generally smaller displacement and many more cylinders, with all those wonderful, small, efficient combustion chambers and trivial frictional losses, goes like this.
Ferrari Italia 4.5L V8, 562 hp, 14mpg combined.
Ferrari California 4.3L V8, 460 hp 15 mpg combined.
Ferrari FF 6.0L V12, 660hp, 13 mpg combined.
Maserati Granturismo/Quattroporte 4.7L V8, 435 hp, 15 mpg combined.
Lamborghini Aventador 6.5L V12, 691 hp, 13 mpg combined.
Lamborghini Gallardo 5.2L V10, 562 hp, 16 mpg combined.
BMW M5/M6 5.0L V10, 507hp, 13 mpg combined.
BMW M3 4.0L V8, 414 hp, 16mpg combined.
Perhaps you are going to argue that a GT500 engine is secretly designed for fuel economy?
Thursday, July 05, 2012 1:25 AM
My source for the mpg stuff.
Thursday, July 05, 2012 11:29 AM
@Daewoo, you're leaving out a copule very important factors: sales volumes and gearing (which is part of intended purposes).
Compare sales volumes for 4-cylinder engines 2.0L and below compared to larger than 2.0L. You'll find the volume on 2.0L and below is much much much greater.
As for NVH, OEMs are extremely strict which is why you're not going to see a 3-cyl 2.0L. They are only considering a 1.0L as the largest displacement for a 3-cyl.
You're also leaving out more considerations in your enigne/mpg comparison. The Vette, Mustang, and Viper all have very tall overdrives to bump up their fuel economy numbers to help with CAFE. I think they all reach top speed in 5th gear even and not 6th speed. Not to mention the Vette has that stupid 1st gear to 4th gear forced shift for fuel economy. Furthmore, the Vette, Porsche, Mustang, etc are all sports cars which have different NVH requirements compared to more typical passenger cars. Also, as the ARE sports cars, fuel economy is less of a concern. Ferrari, Porsche, and Lamborghini could give a rats ass about CAFE numbers.
For the exotics, it's about noise, feel, RPMs, and power. Do you think anyone wants to drive a Ferrari that only goes to 6.5k rpms? No, they want scream in the 458 of the flat-plane crank V8 reving to 8500rpms. Same deal with the Porshce. Again, sports cars have completely different NVH compared to normal passenger cars. A GT3RS is pretty damn uncompromised as a track vehicle that you can buy in a showroom, and even pretty hard core car guys would have a hard time dealing with the rough ride and noises of that thing in a daily driver role.
Of course, there is the matter of horsepower and horsepower sells. Given equal displacements, a higher cylinder count enigne will make more power than a lower one. Some examples in motorcycles are the middleweights and superbike. The Ducati 848 twin races with the Triumph 675 triple which races with the Japanese 600cc bikes. The Ducati 1198/1199 twins race with the 1000cc 4-bangers.
And hey, you left out my BMW example with the new 4.4L V8 making more power and getting better mileage than the old 5.0L V10 :P
Reconduct your analysis using normal pedestrain vehicles instead of sports cars (which have a COMPLETELY different set of requirements). Last I checked, Nissan sold a shit ton (yes, that's an official unit of measurement) more Versas and Sentras than it did GTRs.
So the car segments of interest are the sub-compact, compact, and midsize. So Fit/Civic/Accord, Mazda 2/3/6, Nissan Versa/Sentra/Altima, Toyota Yaris/Corolla/Camry, Ford Fiesta/Focus/Fusion, GM Aveo/Cruze/Malibu, Hyundai/Kia whatever they sell. And really, should look at the European makes because they sell a ton of small vehicles, so Audi/VW A3/golf/Jetta, BMW 1/3/5 series, Citreon, Peugoet, Seat, Skoda, Fiat, etc. Report back on engine sizes, sales volumes, MPGs, etc.
Thursday, July 05, 2012 1:50 PM
Khiem pretty much summed everything up. I will, however, add one more point:
Since you purport that friction, not pumping losses, is a major contributot to fuel consumption, how come the V8 diesels in C&D's most recent Big Three Diesel Shootout got significantly better fuel mileage than the 6 cylinder Cummins did?
15mpg for the V8s, 12mpg for the CTD. This removes part-throttle pumping losses from the equation (diesels don't have throttle bodies), all three engines are within .1L of each other, displacement-wise, all have the same valvetrain type, fuel injection type/method, turbo chargers, make power at similar RPMs, weigh similar amounts, and have similar frontal areas. The Cummins is turning about 15% higher RPM, and gets about 20% worse mileage. The numbers seem to add up that cylinder count had little to do with the MPG results.
Fact is, displacement has more to do with fuel economy than cylinder count. I'm not saying that reducing cylinder count will do nothing for MPG, but I am saying that reducing just cylinder count (but not displacement) will not give significant enough gains to overcome the negative aspects of an engine with overly large cylinders. Trust me, this is coming from a guy who regularly drives something that has almost 1L per cylinder. The V8 diesels from Chevy and Ford are considerably quieter and smoother.
Thursday, July 05, 2012 2:46 PM
I would love to compare small cars and compacts but there's a problem. Nobody makes small 6s anymore. Nobody. So, I would be comparing I4s to I4s, or if I wanted to, I could throw in the VW I5s. In other words, what information would I possibly find of relevance? I could compare very small I4s to European twins and triples if you prefer, but the lower displacement 4s are gonna get creamed.
The reason I chose trucks and sports cars is that they are the only two categories where you can compare engines with similar outputs and sizes but different cylinder counts.
You also claim that the multicylinder engines in the small displacement sportscar crowd are not designed for fuel economy. I agree. Are any of the engines making 400-690 hp I mentioned designed for fuel economy? Since we have similar design briefs for each of the engines in those sports cars, ie, make power and be exciting, it is entirely fair to compare their fuel economy when we decide to compare the effects of displacement vs. cylinder count. We find, as I show above, that large displacement/cylinder engines with fewer cylinders do better by a big margin. So if Porsche and Lamborghini both don't give a rat's ass about CAFE, why does the Porsche get 22 combined mpg while the Lambo gets 16 mpg? Why do we find the same thing with the Ferraris, Maseratis etc? We find the same thing with truck/small suv engines. Same brief - make torque, be tough, save fuel, same results - the 4s beat the 6s.
As for excluding your 4.4 BMW, I got my info on 2012 model year cars and the page didn't show the new M5/M6. I'm sure they do very well, but they're also turbo engines with a much lower rpm powerband and fewer cylinders than the engines they replace. Thus, I'm not sure they're instructional for this discussion.
Diminishing returns in the cylinder count/displacement per cylinder game + 4.88 inch stroke (!!!) + a very large engine turning 15% higher RPM = the V8s winning on economy.
There's a reason I don't propose a 5 liter single cylinder engine. At a certain point, all those problems you mention get big enough you're better off adding more cylinders. With modern direct injection and NVH requirements, I'd put that at about 600 cc/cylinder in a twin (about where we stop seeing comfortable twins in bikes and from Fiat), 700 cc/cylinder in a triple (which is where you stop seeing comfortable triples in the bike world), 750cc/cylinder in a four (which is about where you see OEMs dropping 4s in favor of 6s), 800cc/cylinder for sixes/eights/tens/twelves/whatever (which is about the most you'll see in a modern gasoline engine in a car).
I also have experience with large displacement/cylinder engines and they vary. A 300 cube Ford I6 is pretty harsh but you will be hard pressed to find a smoother, quieter engine than a 500 cube Caddy V8. On the other hand, you will be hard pressed to find a thrashier engine than a 3.0L Ford Vulcan V6. While a 4.5L Toyota I6 is a very smooth engine, the 4.6L mod motor of the same era is pure suck. The big Yota I4s are also smooth motors.
Thursday, July 05, 2012 3:13 PM
The Powerstroke's V8 has a stroke of 4.25", or only a 13% increase in stroke.
In any case, how does it not apply to this discussion? It's the EXACT same thing you proposed: decrease cylinder count, leave displacement alone (or even increase it). How are you going to get the displacement/cylinder increase you proposed? Dropping 2 cylinders results in a displacement/cylinder increase for a 6.7L engine of 32%. Were you just going to increase bore by 32% and leave stroke alone?
And yes, it was turning 15% faster. It also got 20% worse mileage. Shouldn't it have gotten similar mileage, or at least a penalty smaller than the RPM increase? Keep in mind the Ford has the frontal area of a small building, and the Ram is actually the most aerodynamic full-size pickup sold in the US.
Thursday, July 05, 2012 7:01 PM
“I would love to compare small cars and compacts but there's a problem. Nobody makes small 6s anymore. Nobody. So, I would be comparing I4s to I4s, or if I wanted to, I could throw in the VW I5s. In other words, what information would I possibly find of relevance? I could compare very small I4s to European twins and triples if you prefer, but the lower displacement 4s are gonna get creamed. “
Alright, let’s look at some of the main parameters engine designers have to deal with in deciding base engine design: cost, weight, packaging, power, torque curve, fuel economy, NVH. All of the auto companies have a whole lot of really smart people designing engines for them. Mazda decided on a 4-cylinder 2.0L for its new SkyActiv. Ford is doing 4-cylinder 1.6L-2.0L Ecoboost engines and a 3-cylinder 1.0L Ecoboost. GM and Fiat decided on 4-cylinder 1.4L engines. Why is that? All of these companies, Audi/VW, BMW, Ford, Fiat, GM, Honda, Toyota, Hyundai, etc can design engines however the heck they want. So why not make a 3-cylinder 2.0L? Ask yourself that.
“The reason I chose trucks and sports cars is that they are the only two categories where you can compare engines with similar outputs and sizes but different cylinder counts.”
And trucks and sports cars have completely different NVH requirements compared to more commonplace passenger vehicles.
“So if Porsche and Lamborghini both don't give a rat's ass about CAFE, why does the Porsche get 22 combined mpg while the Lambo gets 16 mpg?”
Because the Porsche is a 3.8L@400hp and the Lambo is 5.2L@562hp? I can make a smaller engine with less rated horsepower get better fuel economy too.
“Are any of the engines making 400-690 hp I mentioned designed for fuel economy?”
Yeah, actually the ZR1, Z06 and Mustang GT, GT500 are geared with a big overdrive 6th for highway fuel economy. @60mph, the ZR1 spins 1725rpms (20mpg hwy), Z06 spins 1300rpm (24mpg), Mustang GT spins 1900rpm (26mpg), Camaro SS spins 1400rpm (24mpg), Ferrari 458 spins 2600rpms (17mpg), Lambo Gallardo Superlegga spins 2500rpms (and gets 20mpg, surprisingly good) (data from Road and Track). The American cars are designed with the top gear to be a big overdrive to maximize fuel economy. The Italian supercars are designed to use the most out of their gearing for maximum performance at the sacrifice of fuel economy; they will hit top speed in their top gears, the American cars will not (they have to drop down to a lower/shorter gear).
Thursday, July 05, 2012 10:36 PM
Dimishing returns is why it's not a good comparison. That Cummins is over 1100 cc/cylinder, which is a great deal more than I suggested. You also wouldn't increase the bore 32%. Pie R squared and all. Giant engines also get less efficient as speeds increase quite quickly.
I might as well tell you to defend 2.0L V8s.
Why don't they make 2.0L triples? I suspect marketing reasons. More cylinders are perceived as "high tech." They have done this in motorcycles, successfully.
If you don't like my comparison of trucks/small SUVs and sports cars, please suggest another valid means of comparison. I don't know any. I'm also interested to hear how small SUVs/CUVs don't have to be smoother than econo boxes. My experience is pretty much the exact opposite.
I was careful to include comparable engine sizes and power outputs. The Porsche 3.8 is comparable to the M3 V8 in power and size. The only difference is the Porsche murders it for efficiency. Astons and Corvettes have similar sizes and power outputs. The only difference is that the Corvettes murder the Astons for efficiency. The GTR and 4.0RS are similar in size and power to the Lamborghini and Ferrari but beat them badly for efficiency. The Big Lambo is likewise comparable to the GT500 and ZR1, and likewise, the engine with more cylinders gets beaten up pretty badly for efficiency.
As for the "efficiency designs" of the engines with fewer cylinders, I sort of agree. With big displacements you can pull tall gears. With your multi-cylinder, small displacement engines, you simply cannot. It would be pretty grating and frustrating, I imagine, to do 60 mph at 1300 rpm in a 458 Italia. The 6s I mention seem to overcome this handicap quite nicely. Small displacement/cylinder engines, well, not so much.
Thursday, July 05, 2012 11:40 PM
How is it not a perfect comparison? Same displacement, one has fewer cylinders.
But, fine, let's see you wriggle out of this one:
Same truck. Same Trans. One is a 5.0 V8, one a 4.9 I6. EXACT same mpg.
Btw, 911 vs M3 is fail: 911 is considerably lighter, geared longer, small frontal area teardrop shaped, and 15% smaller engine.
Aston vs Vette fail: Aston is like 1000lbs heavier, is shorter geared, and has a humongoid frontal area.
Friday, July 06, 2012 12:45 AM
They probably don't make a 2.0L triple because the NVH sucks. I've ridden my buddies Triumph speed triple (1L triple), a Ducati Diavel (1.2L twin), and an old Buell XB12 (1.2L twin), and a Yamaha R1 (1L 4-cylinder). At idle, the Buell just about wanted to shake itself apart with the handlebars shaking everywhere. The Ducati was better, but still pretty good NVH through the seat. Speed Triple better yet but the smoothest, and highest HP was the R1 with the 4-cylinder.
The BMW M3 4.0L V8 spins 2600rpms at 60 (23mpg hwy), and the 2012 911 with the 3.8L flat-6 does 1600rpm@60mph (26mpg). Again, you're ignoring gearing. As I demonstrated in the article with my S2000, downspeeding has a significant effect on MPG.
Your argument of needing a big displacement to pull tall gears is not really valid. Example, the SR20 used in the SE-R vs. the GA16 used in the base Sentra. The base Sentra had the smaller engine with taller gearing giving it superior mileage.
If you want to check on the NVH of a ~2.0L triple, take the ~4L 6-cylinder and disable every other cylinder in the firing order so that it becomes a 3-cylinder. Not perfect, but rough approximation.
Friday, July 06, 2012 1:28 PM
As I said before, it's a case of diminishing returns. That's why I'm not advocating 1100 plus cc/cylinder engines. That's why I'm not asking you to advocate a 2.0L v8.
As for "wriggling," perhaps you should compare the "our user's average mpg" for the I6 and V8. I think you'll find the 4.9 does 17mpg and the 5.0 does 13.9. Your own source and all. How do you "wriggle" out of all those 3.0L v6s getting beaten by the big 4s?
My 911 thing is a fail? Ok. That 22 combined mpg came from a cabriolet. We look at a cabriolet S4 and get 21 combined mpg, this from an AWD car of about the same weight, 21 vs 16 for the M3 coupe. And you're math is way off. 200cc is 5% of 4,000cc. Oh, and the 500 hp 4.0L GT3 RS gets better mileage as well, so that theory goes down the shitter.
Aston vs Vette is a fail? This is getting funny, Rockwood. If it makes you feel better, compare the ZO6 to the 4.7L Vantage. Vette still wins. And yes, tall gears are a benefit of displacement, a benefit of a lower revving engine.
How about GTR vs Ferrari 458? Is this a fail as well? Surely the GTR only wins (and wins big) because of its smaller body, lower weight and efficient 2wd layout. Oh wait ...
And how about the enormous Gt500 vs the much less powerful M5? Or the Ferrari vs the SLS? Or the ZR1/GT500 vs the Aventador? Or any of the cars from the large displacement/cylinder category vs any of the cars from the small displacement/cylinder category? Because every single car I mentioned in my category will beat or match every single car in your category.
Friday, July 06, 2012 2:12 PM
The 2.0l triple thing was based off this:
They actually criticize it for being too smooth and civilized, for lacking the character you'd want in a big bike.
I completely agree with you about gears. You want to pull the tallest gears possible for fuel economy.
This brings me to the matter of high power density engines. Unless we're talking about forced induction, the only real ways to increase power density are bad for efficiency. Bigger ports hurt mpg. Bigger cams hurt mpg. Massively oversquare engines hurt efficiency. We need these things to make 100 hp/liter because we need to rev the engine to 8000 rpm. This engine will also really suck at 1500 rpm cruising.
Compare this to an engine making 75 hp/liter. For it, we can use small, efficient ports, mild cams and an undersquare design. We can also ditch the heavy valve springs, the friction inducing quad cams and ultra heavy duty everything. All of this is good for efficiency, all of it perfectly suited to doing 1500 rpm on the highway.
Furthermore, power density really doesn't matter when you think about it. Who cares if you can make 150 hp/liter if your 1.0l engine is heavier, externally larger, less efficient and less reliable than a 2.0L engine making 75 hp/liter? As such, I would prefer to look at power density from the standpoints of hp/cubic inch of external dimension, or hp/lb of engine weight.
With all this in mind, it seems to me that we want relatively lazy, efficiency optimized engines that don't make 100 hp/liter so that we can do 1500 rpm on the highway, we want fewer cylinders so we can package it better and we want a moderately larger displacement so we can match our 100 hp/liter competitor on power, if not power density.
IE, an LS7, which is cheaper, more efficient, just as powerful, smaller and lighter vs a smaller displacement Ferrari 575 V12. Ie a 3.0L, 30 mpg I4 for your 944 instead of a 12 mpg, 2.5L V8 in your Alfa Montreal.
That said, I agree you want to smallest displacement that will do the job, so long as it doesn't mean adding cylinders.
Friday, July 06, 2012 4:21 PM
How is retaining the same displacement, but removing 2 cylinders diminishing returns? I am not suggesting you run 1100cc per cylinder, I am just showing you a direct comparison of cylinder deletion. Both are running relatively huge strokes, mainly because they're diesels, and there is not much in the way of difference between them.
As for the fueleconomy.org site, I really believe you are grasping at straws now in atttempt to wriggle out of this. If I understand your stance correctly, you're suggesting we throw out scientifically repeatable MPG tests, with an precicely defined and strictly controlled environment, in favor of "user data"? I highly suggest you click the "details" link: both had only 2 vehicles each polled. No indication of operating environment, vehicle features, etc. I don't think extremely anecdotal evidence like that should throw out the EPA test, do you?
As for the rest, yes, your comparisons are not apples to apples. There are too many variables in all of your examples. Aerodynamics varies greatly in your comparisons, as does gearing, intended market, price (which effects whether or not the manufacturer cares about getting hit with a guzzler tax), weight, etc. The best example so far is the F-150 I posted.
Lastly, please stop including Ferraris and Lambos in MPG comparisons. It's like bringing up Michael Jordon or Tiger Woods during a best Quarterback debate. :)
Friday, July 06, 2012 7:29 PM
You are clearly not understanding what I mean when I say diminishing returns.
Cutting two cylinders off a 1.0L V8 and making a 1.0L V6 is going to have lots of benefits and very few drawbacks because a 180cc/cylinder engine is still tiny and you're going to get all sorts of friction benefits from running two fewer cylinders. On the other hand, a 6.7L six cylinder has massive 1100 plus cc/cylinders. At this size, the rod angles, inefficient combustion chambers and frictional losses are no longer worth it and you should add more cylinders. In other words, the benefits of fewer cylinders diminish at really large displacements. This is why I keep suggesting the sweet spot is in the 700-850cc per cylinder area.
Grasping at straws? It was your damned source. But anyway, using EPA only figures and many different model lines, the 4.9 usually beats the 5.0 and never loses.
Speaking of wriggling, why do the big 4s consistently beat up on the small 6s?
As for the sports cars, ALL large displacement/cylinder engines beat up on ALL small displacement/cylinder engines in ALL the US market sport scars offered in 2012. Seems pretty indicative to me.
And anyway, the Lambo was the star of MPG in multicylinder engines. :)
Friday, July 06, 2012 7:38 PM
Check out this comparo, btw. Exact same chassis, only 200cc displacement difference, almost 30% advantage for the engine with fewer cylinders.
Friday, July 06, 2012 11:27 PM
Mitsubishi Gallant likewise instructional.
Jeep Cherokee 2.5 I4 vs 2.8 V6
Sonoma/S10 2.5 I4 vs 2.8 V6
Saturday, July 07, 2012 12:57 AM
Most of the 4.9L vs. 5.0L clear winners had dissimilar gearing/transmissions (the V8 was "heavy duty" and was generally paired with the C6 3 speed auto). In most cases where it was actually the same transmission/gearing, it was VERY close (1mpg or less, highway).
The MX-3 had shorter gearing with the "performance" V6 vs. the fuel sipper 4.
The Galant V6 was a 4spd auto vs. 5 spd manual for the I4.
The Jeep 4 banger was TBI, the V6 had a 2bbl carb, and the 4s had smaller transmissions/transfer cases/differentials.
The S10/Sonoma used the same V6 as the Cherokee, and the "Iron Duke" 4 was TBI. I think the V6 was TBI as well, but that was by no means an efficient engine.
Either way, I'm done. Good luck with your 3-cylinder car company! :)
Saturday, July 07, 2012 3:21 PM
It's already here. Everybody makes big fours, nobody makes small 6s.
Saturday, July 07, 2012 6:10 PM
Sunday, July 08, 2012 8:01 PM
I stand corrected.
One company makes one very low volume, small displacement V6. Everybody makes a least one big four.
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