Thermal effiency

[Jared_80]

How much does the surface to volume ratio affect the thermal efficency of an internal combustion engine? I was thinking that if they increased the size of the cylinders that they could increase power and efficency along with it. I have heard that the avarage engine looses 63% of it's possable power in the form of heat. Just as a weird example I compared the surface area of the cylinder walls of a 4 inch cylinder to that of a 7 inch cylinder. The 4 inch had a nearly even 1:1 ratio (for every 1 cu inch of volume it had 1 square inch of cylinder wall) The 7 inch piston would have a 1.7:1 ratio 70% less surface area per cu inch. I know that a 7 inch piston in a street car would be crazy (though the torque would be amazing) but is my idea correct would it raise thermal efficency? Or would it loose effecency due to the long distances that the flame front would have to travel to combust all of the fuel? aka unburnt gas. And how much weight could you save by making fewer larger cylinders? Yes I know that the redline would still be in the grunting range, and it would take about 30 seconds for it to rev up.

[454Casull]

Yes. The higher the volume:surface area ratio, the less power lost to escaping heat, relative friction, etc.

That's why the huge diesels in marine applications can reach 50% total efficiency...

[Jared_80]

WOW normal engines only make like 18-23% right? How large of a cylinder do those engines use? Is there a formula to find the aprox max RPM for an engine? Since larger cylinders have less surface area their spacific weight (weight to displacement ratio) would be less right?

[454Casull]

Very large.

And good luck with finding a formula to calculate max RPM...