High Compression Equal Better mpg?

[CavalierKing]

I got to thinking, and even though it doesn't fit my application does a high compression engine save on how much gas it takes to run in comparison to another block of the same size.

I.E.

2.2L 4Cylinder 9:1 V.S. 2.2L 4Cylinder 10:1 or 11:1 or even 12:1 (if possible)


I am not sure if my thinking is correct, but I believe one of two things.

1) The higher compression will take less fuel to cause an explosion to generate power of the lesser compressed engine.

2) The high compressed engine will take more fuel to make same amount of energy due to the higher compression.


I am unsure on which is correct, but I do know since their is less room for fuel, and air it would theoretically take less fuel/air in order to cause an explosion due to the compression in the cylinder.

I am unsure what is everyones opinion?

[virago250]

I would also like some concrete test on the engine compression question, but in reality to change compression is pretty expensive. I do know that higher compression means higher octane to keep spark knock away. and I would think it would take the same amount of fuel and air because the piston would still travelle the same distance and draw in the same amount of air but maybe the higher compression would push out the exahust valve faster. If you are really good with engines I would look into a differnt camshaft to get better mileage. Something that does not open the intake valve as long and has a small amount of scavengeing when the exauhst opens should use less fuel and air, but probly sacrifice some power.
Any data on cams I would also like to see

[rh77]

I did some experimentation with the TSX (premium fuel listed as "required"). The high compression engine lost 2-3 MPG in a single-blind test using 87 Octane under similar conditions as the 91 Octane fuel. At least 3 tanks of each were tested in an A-B-A manner.

However...

Higher compression generally means higher horsepower, less FE, etc...

RH77

[GasSavers_Ryland]

higher compression in theory is making more efficent use of the combustion, and helps to increase torque (your friend) that is part of the reason them being in high performance engines, it also takes a bit more quality controle, and persision in designing, and running the engine, and after you produce more power because of the more efficent design who really wants to use that for good?
on my moped they simply add thicker head gaskets for mopeds that are sold in states with a 25mph moped speed limit insted of a 30 or 35mph limit, the engine produces less power and goes slower, they are otherwise the same engine, remove the head gasket on one of those little two strokes, and you will gain torque and peek power and because it's turning at close to the same RPM, it's sucking in about the same amount of gas, but it alows you to gear it higher for higher speeds, droping your engine speed, or rasing your top speed, and increasing youre gas mileage.

[cfg83]

Hello -

Isn't this part of the fuel economy advantage of diesel engines? Isn't there a relationship between their higher-compression and their lean-burn capability?

CarloSW2

[Bill in Houston]

The compression ratio is a limiting factor in the engine's maximum possible efficiency.

If you can bump up the compression ratio without getting any other bad behaviors, it's free money.

[Bill in Houston]

Originally Posted by cfg83

Hello -

Isn't this part of the fuel economy advantage of diesel engines? Isn't there a relationship between their higher-compression and their lean-burn capability?

CarloSW2

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

[retrorocket]

While there are many problems with too high of a compression ratio there are many advantages to having as high of a ratio as the engine design and fuel permit.

On most engines the compression ratio is directly tied to the expansion ratio.
Just the other side of the stroke.

If we did not have to worry about abnormal combustion then we would see more power for the same amount of fuel as the compression ratio went up.

The energy comes form what we normally dump out the exhaust.

We could say as the compression ratio approaches infinite the exhaust temp approaches absolute zero.

As the compression ratio goes up the exhaust temp goes down and peak combustion temp goes up. (so Carnot efficiency goes up because there is a greater temp spread between the high and the low)


Really where else can it come from if more power is made from the same air fuel charge.

Compression really helps fe as long as you do not get into a situation where the mixture has to be fattened up to cool the combustion chamber or the ignition timing has to be retarded to avoid detonation or pre-ignition.

Small bore engines generally can handle more compression ratio than large bore engines and combustion chamber design has a great effect as well.

Engines with reverse cooling systems can also handle more compression ratio.
(this just means engines that cool the head first instead of last)

The exhaust valve is the main problem and it can be difficult to cool off enough to not cause problems in a high compression engine.

Even though E85 has less energy content if an engine has very high compression it can get about the same mpg on it as a regular engine can off gas.

The reason is that E85 is much much more resistant to abnormal combustion
than gas is.

So puting E85 in a gasoline engine is really a waste because a gas engine with
a standard gasoline compression ratio just can not utilize the fuels best quality.

All that being said it is actually very easy to run at a higher compression ratio.

Many people on this site do it all the time. Dynamic compression ratio is what really matters and you increase that when ever you give the engine more throttle.

If you are operating at 15 inches if vacuum then you are really only running half of the engines static compression ratio at best!

This is why a car gets better mpg in 5th gear rather than 2nd or 3rd.

As long as the air fuel ratio does not go really fat then you get the best mileage at wide open throttle in 6th or 7th or 8th gear!

Why? because your dynamic compression ratio is highest at wide open throttle.

Your engine is much more thermally efficient there.

[bugsih]

To save fuel in most of our cars and bike you need to have low rpms to burn less fuel and if your motor is high compression good luck lugging it, it will knock bad. I have a 2002 honda RC51 which is a 1000cc v-twin and its a high compression motor and it can not be lugged at all without some violent knocking and vibration so I don't see how you would save gas by having higher compression. Look at the metro or the Vx which is designed for fuel economy, I'm pretty sure you'll find they are not high Compression.....

[Drew1d]

Increased Compression generally means:
- Increased torque (at same rpm as low compression engine) Yay power!!!
- Increased knock. (you'll need higher octane gas)
- Decreased engine life (with more blowby gasses)

Why it won't save fuel:
The majority of the engine control systems on a modern fuel injected engine are to ensure Stoichiometric ratio. (Or air/fuel ratio if you speak english)

When an engine piston is at BDC, (Bottom Dead Center) the cylinder is filled with the air and gas at ideally the 14.7 to 1 ratio. (I'm not taking into account EGR or anything like that) I don't believe the compression(at Top Dead Center) will have any effect on fuel consumption. (at least at the same rpm)

Now, you could argue, you could up shift sooner if more torque is made in the low end of the RPM range. But, generally speaking it's expensive to rebuild engines for higher compression and usually done for performance, not fuel economy.

But I like your thinking.