mccan:

Otto cycle engines are gasoline engines -- all the fuel burns at once and the resultant hot gases expand to drive the piston down. Since they expand without any further input of heat, the temperature and pressure drop rapidly as the volume of the cylinder increases with piston movement. Much of the heat gets absorbed by the cylinder walls or goes out the exhaust -- heat and noise out the tailpipe.

Carnot cycle engines are theoretically adiabatic -- no temperature or pressure change as the piston moves down and the cylinder volume increases. This is achieved by continuously injecting fuel as the piston moves down, continuously increasing the amount of hot gas in the cylinder. The result is less heat loss due to free expansion and less heat and noise goes out the tailpipe.

This would be great if you could inject variable amounts of fuel over a fixed injection period, but what really happens is that the amount of fuel per millisecond is about the same while the duration of injection changes as more fuel in injected.

True adiabatic diesel engines have been built as research projects, but they have a single rpm and load combination at which they are truely adiabatic. Laboratory curiosities.

Hope this helps. It's rather complicated. Basic physics, though!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!