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Old 10-11-2013, 10:02 PM   #3 (permalink)
Occasionally6
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The A/C in a car is a heat pump.

The car heater is using otherwise wasted heat so there's nothing to gain by doing it in any other way. You can either pump the coolant through the radiator or through the heater core. It's pretty much the same pump effort required either way.

Exhaust (and coolant) heat might be used to evaporate a low boiling point fluid and useful work extracted from that in a turbine. (Exactly as occurs in a steam turbine, just lower pressure and temperature.) BMW were prototyping that using ethanol as the working fluid a few years ago.

Yes, a gas turbine in the exhaust can be use to extract work. If not connected to an electric generator it can geared to the crankshaft. See turbocompounding.

Exhaust gas from an ICE is at fairly low temperature and pressure so the efficiency is pretty low. That wouldn't matter where the gas is free and otherwise wasted but cost kills it in practice.

In a spark ignited engine, where the gas flow is throttled for much of the engine operating range, there is a lot of variation in gas flow rate so it's not going to be possible to optimise a turbine in terms of efficiency.

Faster warm would be possible with a heat pump but you have to put energy in to get the heat back. Even allowing for the multiplication effect of a heat pump, it's going to cost much more than you can save. Again, engine coolant heat is otherwise wasted anyway.
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