Surface to volume ratio and its affect on thermal effecency

[kachok25]

I know that reducing the surface to volume ratio in an internal combustion engine means that less heat is lost through the cylinder walls, but how much would it affect it, say everything else (CR, ignition timing, air to fuel ratio) being the same how much would my thermal effecency increase if I could reduce the surface area by say 15%? Is there any good formula for this?

[kachok25]

If 33% of the heat is lost through the cylinder walls and we reduce that surface area by 15% wouldent the energy savings be around 5%? since it is 1/3 by 15%. I don't know just some crazy guess, but it does make some sence, anybody have the phisicics background to back that up, or shoot it down.

[curtis73]

Yes, you could theoretically save 5% of the energy. A small percentage of that will show up as dynamic compression, (and a resulting retarding of the spark and/or richening the fuel curve) but most of it will go out the tailpipe. The other issue is that if you have 33% of the heat going through the cylinder walls, and you reduce the surface area by 15%, a bit will be conserved in combustion, but the cylinder walls will now absorb more of that heat. Keeping 5% more heat in the combustion from your hypothesis is only plausible if the iron and surrounding coolant don't absorb any more of the heat they're being given.

I think the small differences it would make don't offset other factors in what consumers want, like smoothness, quiet operation, small external size, and a flat torque curve. The small percentage of efficiency you could gain with a 15% drop in surface area would mean the public might have to go back to 3 cylinder engines As a physics and engineering example, you might be able to duplicate this by experimenting with equal displacement V8s and V6s, like a chevy. There exist both 4.3L V6s and 4.3L V8s with identical architecture. The V6 is just the V8 with two cylinders lopped off. The V8 would be hampered by a lot more surface area.

Another nice side effect to your proposal is that the smaller surface means less swept area and therefore less friction.

[kachok25]

Yea there would certainly be alot of other factors to consider, and I am not even saying it would be a good projoct, just an intresting case study. You could also build it as a lightweight high dispalcement setup, but the big downside would be tae very low redline prob around 4,500-5000 RPM even if it was balenced. How much differnece would redicing the swept area/friction make?

[curtis73]

Well, all other factors the same, reducing the stroke by 10% not only would reduce the friction surface by 10%, you also slow the piston speed which further reduces friction by the square root of delta speed.

[kachok25]

Originally Posted by curtis73

Well, all other factors the same, reducing the stroke by 10% not only would reduce the friction surface by 10%, you also slow the piston speed which further reduces friction by the square root of delta speed.

So are you saying a short stroked engien would have a higher efficency than say a perfect square engine? A longer stroke engine creats more torque so it can run at a lower RPM wouldent those factors balence out?