[citation][nom]The_Trutherizer[/nom]I've always wondered why they don't use sterling engines on the part of a car's exhaust where it connects to the engine. It get's red hot there and could probably power one or even two small sterling engines to provide more power to the electric engine if it is a hybrid.[/citation]
A regular engine already takes advantage of temperature differentials. The greater the temperature difference between the combustion chamber and the heat sink (the cylinder walls), the higher the engine's efficiency. Most of the heat energy you could capture with a stirling engine can already be captured with a regular engine simply by making the cooling system more effective.
If you replaced the cooling system a small stirling engine, yes it'll generate energy, but the less effective cooling means the engine loses efficiency. You're basically shifting energy generation from the ICE to the stirling engine, not generating additional energy from the heat differential. For there to be net additional power generation, you need a really large stirling engine which can handle the large amount of heat flow normally handled by the cooling system. And when it gets that large, the additional energy you burn carrying the extra weight around usually consumes more energy than it generates.
Stirling engines typically weigh a lot more per Watt (or hp) than an internal combustion engine. For transportation applications, that means you're almost always better off using a bigger ICE rather than tacking on a stirling engine. The story changes for fixed static operations. There, stirling engines can be useful if you can bear the initial investment and additional maintenance costs (again, typically a stirling engine produces fewer Watts per $, so the temptation is simply to use your money to buy a bigger ICE). Note that steam engines and the steam turbines used in nuclear power plants are essentially heat engines, like a stirling engine. That is, they derive all their mechanical energy from heat differentials, rather than from expansion of combustive gases.