15

Compression Ratio - A Look From The Thermo Side

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.

You want more power and probably better fuel economy due to ever increasing fuel costs.  The government wants lower emissions.  So how do you achieve all those?  Increasing the compression ratio of the engine is one method.  My old school naturally aspirated SR20DE had a compression ratio of 9.5 back in the 1990s.  The S2000 came onto the scene around 2000 with a compression ratio of 11.0.  Now modern performance cars like the Ferrari 458 and Porsche 911 have a compression ratio of 12.5.  The upcoming Mazda Sky-G gasoline engine has an incredible 14.0 compression ratio achieved with some new technologies.  Even turbo cars are upping the compression ratio all in the name of more power and fuel economy.  The SR20DET had a compression ratio of 8.5 while the new Nissan Juke and Hyundai Sonata are 9.5.  The 911 turbo is 9.8 which is higher than my old naturally aspirated SR20DE!  So how does increasing the compression ratio do these things?  We’re going to use that thing called thermodynamics to show you.

We're going to look specifically at the 4-stroke gasoline engine.  If you need a quick refresher, read up on Suck, Squish, Bang, Blow.  Compression ratio deals specifically with the squish and bang; more squish (compression) makes a bigger bang (get your heads out of the gutter).  To see the effects of higher compression, we'll model the combustion process with the Otto cycle.

The Otto cycle makes a lot of assumptions like isentropic compression and constant volume heat transfer to simplify things, so stuff that is basically impossible in the real world.  While you can't get accurate values, you can use the Otto cycle to make comparisons.

 
 
Here's the Otto cycle plotted on a P-V (pressure-volume) and T-S (temperature-entropy) diagram along with two of the corresponding strokes in the 4-stroke cycle.  1 to 2 is isentropic compression (entropy(s) is a constant).  2-3 is heat (Q) addition, or fuel being burned.  3-4 is the power stroke (isentropic expansion).  4-1 is heat being lost, basically out the exhaust and the water jacket of the engine block.  The two strokes not shown are the intake and exhaust strokes.

 

This diagram is more representative of the actual curves in the 4-stroke process.  As you can see on the exhaust stroke, the pressure in the cylinder is a bit above atmospheric.  The pressure on the intake stroke is a little under atmospheric for a naturally aspirated engine.  These are both generalizations and represent some of the pumping losses in a gasoline engine.

 

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Comments
8695Beaters
8695Beaterslink
Sunday, May 15, 2011 7:21 PM
Perfect timing, I've been arguing with a few friends about how much boost a stock K24 can take. Now I can finally prove my point.
538
538link
Sunday, May 15, 2011 8:37 PM
So what has mazda done to be able to run such high compression on their SkyG engines?
VP
VPlink
Monday, May 16, 2011 1:12 AM
Some data regarding the SkyG:
http://www.mazda.com/mazdaspirit/skyactiv/engine/skyactiv-g.html
SkullWorks
SkullWorkslink
Monday, May 16, 2011 9:49 AM
stratified combustion,

detonation or preignition can only occur if there is a combustible in the chamber to combust. if you wait until you want the combustion to occur to inject fuel then it doesn't matter much what happened before that,

^the short version

Ockham
Ockhamlink
Monday, May 16, 2011 2:00 PM
In addition to stratified charge, Mazda is exploiting an old technique (Tri-Y headers). We horsepower freaks usually look at Tri-Ys as intake-improving devices, but they get spent charge out as well as they get fresh charge in. Less exhaust in the chamber equals less heat in the chamber, equals less tendency to knock, which lets you get away with a (for stock) nutty compression ratio.

I'd love to have a look at those headers. 60cm is crazy long for a production engine, and they'd be a welcome change from cast-iron logs.
spdracerut
spdracerutlink
Monday, May 16, 2011 4:23 PM
Direct injection is also a main enabler to the higher compression ratios of modern engines. The fuel is sprayed directly into the cylinder at very high pressure resulting in very small droplets. These droplets absorb the heat of compression by evaporation, dropping the temperature in the cylinder and reducing the chance of knock.
538
538link
Tuesday, May 17, 2011 4:42 PM
Just learned more tidbits thanks guys!
destrux
destruxlink
Thursday, May 19, 2011 5:20 PM
Wow, and I was shocked by Ford going with a 12:1 CR on their new 4 cylinder. Mazda always impresses me.
Wilmar
Wilmarlink
Saturday, February 25, 2012 3:06 PM
Perhaps this is basic, but is this the reason diesels are so much more efficient than spark ignition engines? Other than the fuel's energy density delta which is about 15% greater for diesel, the compression ratio is a lot higher in a diesel which would account for the remaining efficiency which I have seen generally stated as ~ 30% higher than spark ignition gas engine. Never thought about it before I saw this article.
spdracerut
spdracerutlink
Monday, February 27, 2012 4:01 PM
@Wilmar, that's part of it. Diesels are often geared to spin lower RPMs as they have an abundance of torque. Lower RPMs equals lower frictional and pumping losses. Diesels also do not have throttle plates reducing pumping losses as compared to gasoline cars. I think diesels also have lower combustion temps which means less heat loss to the coolant jacket and therefore higher efficiency; not positive on the lower combustion temps though the exhaust gas temps are certainly lower.
Zero 1
Zero 1link
Monday, December 31, 2012 5:48 PM
Wow, thanks for providing the link VP, very interesting read.
silhouette
silhouettelink
Monday, March 18, 2013 1:15 PM
If turbocharged engines consume more fuel due to the increase in air density and have a lower compression ratio as a result, then why does the current trend of downsizing engines with turbocharging yield better fuel economy numbers?
spdracerut
spdracerutlink
Monday, March 18, 2013 6:02 PM
@Chris, it has to do with engine downsizing and downspeeding. Read this:
http://www.motoiq.com/magazine_articles/id/2561/engine-downsizing-and-turbos-more-mpg-and-more-power.aspx
Lessendz
Lessendzlink
Monday, June 10, 2013 9:20 PM
higher compression leads to higher combustion temps.
on gasoline or E10 blends..
NOx emissions will increase acordingly
which is a major problem (IE for diesels) as it is a "Regulated Emission" in America

better "emissions" starts with the chemicals in the Fuel..
Not the mechanicals trying to capture pressure..
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