High compression pistons

Luebben · #1 08-20-2009, 02:42 PM

Let me know if I am thinking wrong. If I fly at 8000 feet or higher most of the time, wouldn't higher compression pistons benefit me? The atmosphere is thinner, the engine produces less power because of that. Could I not regain some of the lost power by increasing my compression from 8.5 to 1 to 9.5 to 1? I have read the compression threads but I did not find information about high compression and high altitude flying. Thanks in advance!!!!!

rbibb · #2 08-20-2009, 03:00 PM

Well since higher compression pistons allow the engine to produce more HP at sea level they allow a proportionately more HP to be produced at altitude.

You start with more you end up with more.

Of course there is no free lunch - you will burn more fuel as well.

But, all in all, more HP is better than less.

Ah Physics - the laws that simply must be obeyed.

douglassmt · #3 08-20-2009, 05:26 PM

Indeed, physical laws. My favorite from the bathroom wall in the physics building in college:

186,000 miles/second (the speed of light)
It's not just a good idea, it's the law!

Toobuilder · #4 08-20-2009, 05:29 PM

Quote:

Originally Posted by rbibb

...Of course there is no free lunch - you will burn more fuel as well...

Actually, the opposite is true. High compression does not increase the displacement of the engine, it simply allows the air that you do have to produce more work. That's why modern cars are going to increasingly higher compression... It increases the efficiency of the engine.

airguy · #5 08-20-2009, 05:37 PM

Correct - the quantity of fuel burned is a function of the swept volume (displacement times RPM) and the mixture. It has nothing to do with compression ratios. Higher compression ratios in piston engines give a higher thermodynamic efficiency.

There is another thing to consider though - if you ever want to run mogas, those high compression pistons are going to cause you heartburn in the form of detonation. You are pretty locked in to 100LL if you run 9.5:1.

frankh · #6 08-20-2009, 05:38 PM

Engine suppliers recommend 8.5:1 max for autofuel. But I bet 9.2:1 would be ok as well..not that I intend to experiment.

Frank

asav8tor · #7 08-20-2009, 05:57 PM

I thought about this also and even considered 7:1 to burn regular gas. Then I though a max mpg combo (though reducing total performance) might be a high compression build but limit the MP. The enriching at full power argument is probably NA if you take off at significantly reduced power. I have taken off at 20". This would be a compromise build to get max miles at min $.

Ted Johns · #8 08-20-2009, 06:58 PM

Quote:

Correct - the quantity of fuel burned is a function of the swept volume (displacement times RPM) and the mixture. It has nothing to do with compression ratios. Higher compression ratios in piston engines give a higher thermodynamic efficiency.

Not exactly. The details matter. Increasing the compression raises both thermal and volumetric efficiency.

Increasing thermal efficiency improves the BSFC of the engine.
Increasing volumetric efficiency improves the ability of the engine to fill the combustion chamber with fuel - air mixture.

Both are responsible for the increase in HP due to an increase in CR.

The increase in VE allows an engine to burn more fuel.

Most hot rod mods to an engine focus on improving VE (porting, bigger valves, camshafts...). Increasing CR is one of the few that improves both VE and TE.

Toobuilder · #9 08-20-2009, 07:27 PM

Quote:

Originally Posted by Ted Johns

Not exactly. The details matter. Increasing the compression raises both thermal and volumetric efficiency...

Ummmm...

VE is an expression of the ability to fill a cylinder during a combustion event. In reality, one cylinder of a Lyc 360 is 90 cubic inches, but it is a truly rare event to have a naturally aspirated engine gain 100% VE. VE is simply tied to the ability to fill and expel the cylinder volume each combustion cycle, which is tied directly to valve timing, port configuration and induction/exhaust design (and some "shrouding" of the valve by the cylinder wall and piston). How does "smashing" a fixed volume of air more (i.e. high compression) improve the ability of the engine to ingest or expel the air?

I'll give you that the exhaust tuning will be altered because of the higher compression, but with an aircraft engine, the exhaust is rarely even close to tuned to begin with.

Ted Johns · #10 08-20-2009, 07:47 PM

Quote:

How does "smashing" a fixed volume of air more (i.e. high compression) improve the ability of the engine to ingest or expel the air?

It's not a large effect, but increasing the CR increases the intake signal, for several reasons.

Stronger vacuum signal, due to the smaller amount of residual gas in the combustion chamber.

Stronger vacuum signal in the valve overlap region due to the increase in exhaust port velocity, due to the higher BMEP. This effect is improved with tuned exhaust, but does not depend exclusively upon it.

Both the VE and the TE improvements are small for the CR changes being discussed here.