From: David Roberson
I am a bit confused about your statement that the gasoline engine has a Carnot efficiency of 65%. If that calculation is just based upon the maximum temperature of the hot gas within the cylinder as compared to the cooler exhaust gas, then I would have to seek a reason for the extra losses encountered. Yes, the Carnot number for the ICE is based on the high temp of combustion near TDC - which does not change much in any type of gasoline engine - and low temp being the exhaust temperature, which is significantly over ambient. Higher compression helps on the high end, but there are limits. If you could squeeze more energy from the exhaust on the first pass, it would really help the efficiency - but there are a variety of interlocking reasons why an ICE cannot be "leaned-out" enough to lower the exhaust temperature. The best you can do is add a turbocharger. In a typical ICE of 30% thermal efficiency, roughly 35% of the losses are out the tailpipe and 35% are out the radiator. The engine must be kept cool so radiator losses are almost unavoidable without going to ceramics. The best TEG to capture exhaust heat operates at 5% thermal efficiency, so using one can only add (.05 x .35 = .018) which will raise the 30% original efficiency to less than 32% which is not enough to matter much. That is why few cars have them. The solution that works well --on paper-- to lower both of the major loss categories - out the tailpipe and out the radiator - is the six-stroke cycle, such as the design of Bill Crower and others - which has not yet gained traction, so to speak. Wiki has a basic article: http://en.wikipedia.org/wiki/Six-stroke_engine
