[thedoc (OP)]

Bruce Rodgers asked the Naked Scientists:
   Can a sterling engine recover a combustion engine's wasted heat?
What do you think?

[evan_au]

A Stirling engine can recover some of the waste heat, if it has a cold sink available to it.

But not all the waste energy - that would produce a perpetual motion machine.

• The policy of the Patent Office is "We will reject any application for a patent on a perpetual motion machine, unless it is accompanied by a working prototype."

There is a limit to the amount of energy that can be extracted by a heat engine. The theory was developed back in the days of steam engines, but it applies to jet engines and nuclear reactors too.
https://en.wikipedia.org/wiki/Carnot%27s_theorem_(thermodynamics)

Talk about evading obsolescence!

[syhprum]

If you are going to use it in a vehicle the heat is generally free but the cold is costly to obtain and to the best of my knowledge they have never been used in a road vehicle, a ship is an other matter where they may be usfull

[Carmela]

So if we want to recover low and very low temperature waste heat we can go for STIRLING ENGINE, which can recover any kind of low grade waste heat because it is external combustion engine and also its efficiency is very good.

[Bored chemist]

I suspect that the extra energy recovered is not worth the extra weight  of a 2nd engine and a more complex cooling system.

[alancalverd]

There was a lot of interest in Stirling engines as prime movers for motor vehicles in the 1960s. Like gas turbines, they can run on anything that burns and allow total combustion at all speeds, but are more economical than turbines at low speed. The general idea was to use a Stirling to feed an electric powertrain, to overcome the slow response of the Stirling (it generally has more inertia than an internal combustion machine) but the proposal predated modern batteries, inverters and brushless motors.

Worth a revisit. The future conversion of Stirling engines from organic fuel (LPG, road diesel, kerosene, biodiesel, ethanol or even chip fat) to pure hydrogen is a lot cheaper than building a fuel cell car from scratch.