Higher Compression increases Thermal Effieciency... But Why?

[NorthWet]

Way back in the last millennium, I was taught that increasing the compression ratio (CR) yields an increase in Thermal Efficiency (TE). With the CR range used in most cars, roughly an equivalent increase, percentage-wise. (10% increase in CR yields 10% increase in TE.)

 

What I don't remember is.... why? What, exactly, is the mechanism, the factor, the condition that causes this?

 

Is it simply higher pressure? Or, is it higher temperature caused by the increased pressure?

 

Is it that the fuel and oxidizer are simply in closer proximity to one another? Or that the reactant molecules are not separated in space by as much non-reactant molecules?

 

Is it a combination of some of these factors? Or none of them?

 

I am curious, as most of the time car engines are run at a low load factor. Freeway cruising typically requires less than 15HP, well below peak TE for typical engine speeds. Variable-cylinder technology is used to disable some cylinders so that the others operate with a higher load factor and thus higher effective CR and TE. Is it possible/practical to give similar results by some other means, by playing with whatever factor in CR effects TE?

Edited June 15, 2009 by subeman90

[Jerry DeMoss]

Higher compression ratio is yeilds higher temps inside the combustion chamber by decreasing the "squish area" inside dome to piston interface area.

 

Therefor requiring the higher octane/high test stuff because it has a higher flash point. The lower octane like 85 or 87 would predetonate before the piston reached top dead center on a higher compression engine.

 

I hope that makes some sense.

[NorthWet]

...

Therefor requiring the higher octane/high test stuff because it has a higher flash point...

IIRC, this is not accurate. Detonation of the endgas has little to do with flash point per se, and more to do with the increasing pressure and temperature (over time) "cooking" the endgas and breaking down the more unstable molecules into molecules more susceptible to spontaneous ignition. Pure octane is less prone to being "cracked" than pentane, heptane or septane. You can have 2 engines with equivalent CRs, and one may have a lower octane requirement than the other because it limits the time that the endgas is exposed to dissociating heat and pressure. Such things as spark plug location (effects max distance the flame front has to travel), swirl and turbulence, quench (squish area), and engine speed all have an effect on the endgas.

 

I think temperature might be too simplistic, because that implies if you could raise the intake charge's temperature by 10% Kelvin (about 40C or 100F *edit - Oops, should be 30C or 70F. Did the math while tired. - end edit *) that you would also increase its power (or decrease its rate of fuel use) by about 10% (assuming detonation didn't wipe out the benefits). That could mean that a non-intercooled turbocharged engine might be the best way to go for part-throttle fuel efficiency. I think that at the very least that automakers would not have deleted hot-air risers on newer engines.

 

*edit - Forgot to mention that the dissociation in the endgas is greatest at peak temperature and pressure, not at those (normally) created prior to ignition. FWIIW... - end edit *

 

Please do not take my comments as criticism or saying that you are wrong. My arguments/comments are rather indirect and somewhat "anthropic" (something has to be a particular way otherwise we wouldn't be where we are), that if something simple existed then the automakers would be exploiting it. (PLEASE, no conspiracy theories. Ferrari/Porsche/racing teams wouldn't hobble themselves to appease OPEC/Military-Industrial-Complex/The Illuminati.) At the same time, my goal means I am mis-anthropic, because I believe that there is something that the automotive geniuses are not exploiting. (My mind is a confused place at times! )

 

A supposition: What would happen if you increased the amount of non-reactants into the intake charge when at part-throttle? This would increase the effective CR, and the charge temperature, at the time of ignition. (This does introduce other combustion-time issues, which might be interesting to explore...) Would this increase the TE?

Edited May 2, 2009 by NorthWet

[nipper]

because i feel lazy today

 

http://en.wikipedia.org/wiki/Compression_ratio

http://auto.howstuffworks.com/question90.htm

[NorthWet]

because i feel lazy today...

Lazy works for me... just have to wait until later to check them out.

 

First thought is that Wikipedia is good for general info, but no really technically useful, as it is user submitted and not critically reviewed. (Come to think of it, I recall chasing references through Wikipedia last year and got nothing truly useful.) I also feel that "How Things Work" can be oversimplified for the common person.

 

Check them out in a bit...

[nipper]

Lazy works for me... just have to wait until later to check them out.

 

First thought is that Wikipedia is good for general info, but no really technically useful, as it is user submitted and not critically reviewed. (Come to think of it, I recall chasing references through Wikipedia last year and got nothing truly useful.) I also feel that "How Things Work" can be oversimplified for the common person.

 

Check them out in a bit...

 

But i read Wiki and wont pass it on if it is wrong. i just didnt feel like typing all that out today.

 

 

Its only compression ratios. if you want a deep meaning thermodynamics explanation on a molecular basis you have to wait till i feel better.

[Log1call]

Thermal efficiency increases because there is a proportionaly smaller surface area in the combustion chamber to absorb the heat.

 

Horsepower is not guaranteed from an increase in compression ratio. The change in compression ratio tends to narrow the power band. The narrow power band means you can have either more torque or more horsepower at peak reading. The increased peak power(or torque), is over a narrower, and probably higher, rev range than before.

 

To utilise the increased compression ratio to it's best advantage, and get more horsepower, we need to retune other things like intake, cam, exhaust.

 

It is possible to increase the compression ratio and get neither more horsepower nor more torque.

[82bratavenger]

If there is no guarantee of more HP or torque, why is there such a discussion on replacing the stock EA81 pistons with either EA71 or EA82

pistons? What then would be the advantage or disadvantage to the piston swap and compression ratio change. Just fishin' for ideas here other than the ones floating around in my head.

[bheinen74]

If there is no guarantee of more HP or torque, why is there such a discussion on replacing the stock EA81 pistons with either EA71 or EA82

pistons? What then would be the advantage or disadvantage to the piston swap and compression ratio change. Just fishin' for ideas here other than the ones floating around in my head.

 

reason. people are looking for the extra 1, 1.5, or 2 hp -which is a big jump for a BRAT Gen1, or gen 2

[82bratavenger]

I agree being an owner of a gen 1 73hp EA81 brat. I have started a conversion using used parts as a test to see if there is a noticable difference. If I can get an extra 2 hp to turnmy 28" mudders I could use that.

[turin]

There are two issues with thermal efficiency (if we're talking about thermal efficiency the way that I understand - amount of work done by the engine, divided by the input heat).

 

One issue is the ideal-case issue. On a PV diagram, there is some closed curve that is traced by the gas. As the gas travels one complete cycle of this curve, the area inside the curve is the work, and the heat difference depends on the details of the curve. However, imagine that, simplistically, the same amount of heat is put into the gas (from the spark). Then, as the compression ratio increases, the area inside the gas curve increases, and so the thermal efficiency increases.

 

The other issue has to do with the fact that the gasoline engine is not ideal (not only because the working fluid enters and exits the engine). So, there is heat leaving the gas when it is not supposed to, and the heat initiated by the spark is not really a constant. To minimize the heat that leaves the cylinder, minimize the area of the cylinder when the heat is added. So, higher compression ratio makes this area smaller.

[vic/se]

because i feel lazy today

 

http://en.wikipedia.org/wiki/Compression_ratio

http://auto.howstuffworks.com/question90.htm

 

Don't have to be lazy lol. It's quite long to explain all.

I used to be a subscriber of hot rod mag. and they wrote a long but easy to understand article about the internal compression engine!