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spdracerut posted on April 04, 2011 17:57
Compressor Efficiency and More
by Khiem Dinh
Compressor efficiency is a term thrown around whenever people mention forced induction, but what does it really mean? How does it affect an engine's performance? And what role does air temperature play in all of this? The cool thing about thermodynamics is that we can explain effects with equations. Using basic compressor equations, we'll put some numbers to the affects of air temperature and compressor efficiency on compressor power requirements and air temperature increase.
Some fundamental compressor equations are below.
I like working in SI units because doing calculations with English units sucks! So, mass flow rate is in kg/sec, the average constant specific heat value for air I used is 1.007 kJ/kg*k, and k for air is 1.4. The specific heat of air actually varies with temperature, but the change is basically nothing within the range of temperatures we're using, so I'm assuming a constant value. After calculating all the values in SI units, it is a simple conversion to English units. The table and chart I generated assumes: air inlet temperature of 298K/24.85C/76.7F, and 100% compressor efficiency (isentropic compression).
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Looking at the chart and table above, it becomes very obvious that increases in mass flow rate and pressure ratio require more power. Also, the graph shows lines of constant power. For a given compressor power, you can get a lot of flow and little pressure ratio, a lot of pressure ratio and little flow, or somewhere in the middle. To get a feel for what the numbers mean in the real world, we'll use 2.0L 4-cylinder engine as an example.
Automotive engines of this displacement and cylinder count will make roughly 500hp with a mass flow rate of 50lbs/min and a pressure ratio of 2.75, or about 25psi of boost. Looking at the table, a 100% efficient compressor would require 51.1hp! Looking at the compressor map for a GT3076, it shows a compressor efficiency of ~72% at this point. So the actual power requirement becomes 71hp. A GTX3582 has a compressor efficiency of 77% resulting in a power requirement of 66.4hp. That extra hp required over a 100% efficient compressor ends up as extra heat in the air. More efficient is better!
The other variable in the equation that's very important, but many people seem to neglect in the practice of building turbo cars, is the temperature of the air going into the compressor. Many people have the misconception that performance is unaffected because the intercooler will cool the air enough regardless of the air temp going into the turbo. What they are neglecting is the fact that compressor performance improves with cooler air. Said another way, it gets worse with taking in hotter air. Using values of 50lbs/min and a PR of 2.75, the table below shows compressor power required, change in temperature of the air (Delta T), and the exit temperature of the air based on the inlet temperature of the air.
Tuesday, April 05, 2011 1:04 AM
I've heard so many times people arguing that a "cold air intake" is worthless on a turbo car. It made me cry inside every time, because people are just plain uneducated. Ha ha
Tuesday, April 05, 2011 7:33 AM
I've always gotten more out of reading compressor maps than turbine 'maps', if you can call them that. I'd really like to see someone expound on turbine efficiency, and how to pick a turbine for a certain engine.
Tuesday, April 05, 2011 7:36 AM
Does the efficiency of the turbo become affected by the efficiency of the intercooler?
Tuesday, April 05, 2011 7:37 AM
Also.... this article makes me wonder, would there be any value in putting a small pre-compressor charge cooler, such as a compact and efficient water-to-air unit? Or would the additional restriction mitigate the benefit of the colder air?
Tuesday, April 05, 2011 7:37 AM
@2_Liter, yeah, there's a reason why every turbo car from the factory sucks air from a colder source at the front of the car/truck. You'd think just from that, people would get it. But they don't.... what can ya do?
Tuesday, April 05, 2011 8:12 AM
Now that's signal. Thank you for the article.
Tuesday, April 05, 2011 9:00 AM
Thermal Applications is coming back to me.
Tuesday, April 05, 2011 11:10 AM
XD All the gangsta phrases totally deviated me from the focus of the article! Too funny!!! LOL!!!
Tuesday, April 05, 2011 12:01 PM
@sethulrich, I think it can benefit the compressor IF it does not cause the compressor to go into surge. By introducing a flow restriction in front of a compressor, it reduces the pressure at the inlet and will increase the pressure ratio needed to achieve the same gauge pressure at the compressor outlet. If you can use some ram air, it'll help overcome the pressure drop of any heat exchanger placed ahead of the compressor. To be benefical, the air temp needs to be dropped a significant amount from ambient. You have to weigh in the cost:benefit ratio of adding such a system due to extra plumbing, weight, cost, etc.
Tuesday, April 05, 2011 12:52 PM
Hrrm. My Duster just has an air filter stuck on the turbo, which is near the back of the motor. I think a cool air intake is now in order...
Tuesday, April 05, 2011 1:57 PM
Seth, some people put a water/meth injection jet pre-turbo for this purpose. I suppose nawwwls would work too.
Tuesday, April 05, 2011 6:09 PM
Dusty, check out the engine bays of the Cobalt SS, VW Golf GTI, Evo X, WRX, etc. All those cars have their turbos on the back side of the engine next to the firewall and all of them have intakes getting air from a cold source. There is a reason to the madness :) Champ/Indy cars used to spray methanol in front of the compressor wheel back when they used the 2.65L turbo Cosworth engines. This helps as the methanol absorbs heat during compression which keeps the charge cooler. It's basically like intercooler during the compression process which improves efficiency. So like Jamal mentioned, people have injected water/meth pre-compressor to achieve the same purpose. There is a risk of causing compressor blade damage though; they aren't exactly designed to handle liquid.
Tuesday, April 05, 2011 6:19 PM
@Jason, about the intercooler, sorta.... An intercooler with a high pressure drop means the compressor needs to operate at a higher pressure ratio. Less pressure drop means the compressor doesn't have to work as hard.
Tuesday, April 05, 2011 7:36 PM
I'm pretty sure the 1.8t Gti has the turbo firewall side and the new 2.0t is located front side. I personally like the front facing exhaust port, like with the Hinds B-series due to easier access.
Tuesday, April 05, 2011 11:05 PM
@Der, I think the days of the exhaust manifold on the front side of the car are gone. It's all about cat light-off time on cold start for reduced emissons, so the shortest distance possible to the cat is required. So I'd expect every new transverse inline 4 made to have the exhaust mani up against the firewall. No more easy exhaust work.
Wednesday, April 06, 2011 11:33 PM
spdracerut - Bingo, and I think that putting the weight of the turbo closer to the cab helps balance too. Ultimately, the days of turbo manifold are numbered as well. We will all have to get very creative when upgrading turbos.
Thursday, April 07, 2011 7:44 AM
Personally, I think the integrated turbine housing/manifolds are great for OEM use. They are efficient, and provide excellent boost response. The big downside, which I think Peter was alluding to, is the difficulty in upgrading.
Thursday, April 07, 2011 1:22 PM
Actually, I think exhaust fabrication with the manifold/turbo on the firewall side of the engine is easier since you don't have to make an effective U-turn, then clear the crossmember, oilpan, and other bits as you go under the engine. With it mounted on the back of the engine, you just need to make it to the tunnel. The manifold would be more difficult to make from scratch, unfortunately. And yeah, one-piece manifold/turbine housings are great for OEM use. They also provide one less gasket to have to warranty, and the manifold to turbine gasket is usually the most failure-prone gasket in the exhaust.
Monday, April 11, 2011 8:16 AM
Great read! Q: How much load does a turbo shaft have to handle? Lets say a GTX35 at 25, 30, 35 psi for example. Q: Is it measured in HP or TQ? Q: Is max load found at lift throttle? Q: How much does a blow-off valve help or is it even needed? Q: What's the best way to maintain your turbo?
Tuesday, April 12, 2011 8:16 PM
@mxpop, so there's thrust loading going along the axis of the shaft and then I guess what could be called radial loading. So radial loading would be taken up by the journal bearings and ball bearing. Those affect what is called shaft motion. If you look at either compressor or turbine wheel head on, shaft motion is the shaft moving away from centerline. Thrust loading is taken up by the thrust bearing and also ball bearing; the design of the ball bearing cartridge is such that it handles both radial and thrust loading. In a journal bearing system, the journals handle radial and the thrust bearing the thrust. I guess radial load depends on the mass flow rate of and velocity of the exhaust going into the turbine wheel and air leaving the compressor wheel. Thrust load can be approximated by turbine pressure acting on the area of the diameter of the turbine wheel, and boost pressure working on the area of the diameter of the compressor wheel. If you have the same diameter turbine and compressor wheels, 40psi of exhaust pressure and 20psi of boost pressure, then you have a pressure imbalance pushing everything towards the compressor side. So a 51mm diameter wheel is about 2", so that's an area of 3.14 in^2, times 20psi. So a thrust load of about 63 lbs. Doing the math in my head, so a little rough. While WRC cars don't use BOV, it's hell on the bearings. Just imagine ou have a pump feeding a fire hose pumping 100 gallons of water a minute. You leave the pump running but suddening close the valve at the end of the hose. Bad things happen right? Same thing with having a turbo flow 60 lbs/min and then shutting the throttle plate. The turbo still has momentum keeping it spinning, but now it's trying to push a ton of air with the air having no where to go. So the purpose of the BOV is to vent the air. If not, it shocks the turbo really hard which often leads to bearing failures and wheels breaking. If the car has a no-lift shifting system where the throttle plate stays open, then not an issue. Best way to maintain any turbo is to make sure the oil is clean, not worn out (not much viscosity breakdown), and of proper temperature. Dirty oil can damage the bearings, whether they are journal or ball bearing. Low viscoscity due to wear or excessive heat means accelerated wear for either journal or ball bearing. A proper cool down and water cooling prevents oil coking that also destroys bearings.
 
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