Another side to high comp.

[jim_geo]

I've just recieved my latest issue of Light Plane Maintanance in this issue their is an interesting artical titled Aircraft Engine Basics. It appears to be the first of a five part series and what I've find interesting is the paragraph that follows.

"Volumetric efficiencies tend to be better at higher compression ratios, and exahust temperatures tend to be lower (since the cumbustion gases expand more before being let out). Which is another way of saying that more useful work is extracted from the combustion process."

Perhaps the statement about exaust gas temperature is obvious to others but for me it's a bit of an eye opener. Perhaps their are some other thoughts on this.

[N200PF]

I wish I knew enough to comment...

- Peter

[LooseNut]

Hmm, very interesting ... that makes good sense, theoretically.

However, I wonder if there are other factors that come into play and overwhelm this "gas expansion cooling" effect.

Now, how does one apply this interesting bit of information? For example a high compresion engine is also producing more horses than a low compresion engine of equal displasement ... so is it fair to compare a high copmpression engine of, say 360 cubes (higher hp) with a low compression engine of 360 cubes (lower hp) ... or is it fair to compare a high compression 180 hp engine (smaller displacement) with a low compression 180 hp engine (larger displacement)?

The question is, what happens in real applications? Do high compression engines really show lower exhaust temps?

... still, very interesting.

[jim_geo]

LooseNut, I guess I should say it right here, I don't buy the statement. Even if there is 'extra' cooling on the decompression side of the stroke there's going to be some 'extra' heating on the compression side. That's just the nature of the beast. My thoughts on this are that if exhaust gas temp is going down and it's being caused by higher compression then something else is at play. Almost surely it would have to be followed by more heat transfer to the cylinder heads (where else would heat be going?). Which would be, I think, an indication of better spark timing and, or perhaps just better all around fuel burn. I don't know, that's why I posted this thread. Your question made me ponder a moment. Shouldn't engines be sized by a standard compression this way an O-360 with say an 8.5:1 ratio would be called an O-360 and one with a 10:1 might be called a O-380. The same volume of air squeezed into a smaller space I'm thinking mimics a larger displacement. How is it fair to compare them? I think it's not.