Air intake of auto engines
A 4 stroke water cooled engine reaches an operating temperature of about 95°C (just under boiling) or about a 60°C rise over ambient. Ambient is an average of 300° Kelvin, or about 20%. This means air entering the engine experiences about a 20% temperature increase. Its still "connected" to the rest of the atmosphere, so is the same pressure. Since its a higher temperature, the same pressure means about 80% density, relative to the air at ambient. So an engine works on 80% density air, and exhausts the same. This does not affect engine operation directly, because the amount of energy is determined by the fuel burned, which also determines the pressure increase inside the engine.
The maximum temperature rise is based on the oxygen. Once its used up, no more fuel will burn. It does not matter what the density is, the maximum rise only depends on oxygen.
The maximum pressure however, depends on the density times the temperature 80% density air will yield 80% of the pressure as 100% density air will.
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An exhaust powered turbo charger is a form of heat recycling
Typical exhaust manifold temperature may be anywhere from 120°C and up to several hundred. The density is the same as the intake density. The pressure will be increased however, because the cylinder was closed until exhaust, and heated. The pressure relative to the pressure envelope will be another 20% to as much as 100% at full power.
This extra heat is disbursed by dissipation and by some useless work accelerating air out the tail pipe. An exhaust driven turbo charger gathers that energy, and aids the engine in two ways.
First, it makes the intake pressure higher, about the same as the exhaust. The exhaust pressure previously robbed the engine of some exportable power, but equalizing exhaust and intake pressure either "adds" power to the intake stroke, or removes the back pressure from the exhaust stroke, depending on how you look at it. Either way its a significant energy recovery. This phenomena will increase fuel economy somewhat.
A secondary benefit, is that with higher intake pressure, the air density inside the cylinder can go up, even above 100%. This does not change the effective fuel efficiency, but it will increase the maximum power the engine is capable of. A given air load for a cylinder will have more oxygen, will burn more fuel, and that heat energy from the extra fuel burned will add to the maximum pressure/power of each stroke.
So, there is a successful man-made heat engine that does some recycling of heat. Proof of feasibility.
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