According to the Oxford Science Dictionary, petrol engines and diesel engines have 25 % and 35 % efficiency respectively, but they push at the ground directly. So, what is the efficiency of an aircraft engine which pushes against air which lets the plane push forward from the ground? I expect a lower answer. :-P
Oddly enough the opposite is true with gas turbines having manufacturers efficiency quoted at around 55% but this is efficiency based at full power or what is quoted as ‘baseload power’. Hence gas turbine engines used in aircraft are ‘tuned’ for maximum efficiency at operating cruise conditions.
Weird, considering what I would expect from science.
This might help also, its rather long winded but in summary, because essentially they are performing different jobs (Car engines have slightly differing mechanism to that of a piston craft or a turbofan) and thus have different conditions in mind when in design phase, it is difficult to compare combustion efficiency (presuming this is the efficiency you are referring to here) between car engines and aircraft engines.
But if you really wanted an answer, then the above answer is pretty much it (He’s a real pilot so might as well believe him 😊 ).
P.S Could there be by chance that you think that by the aircraft engine pushing against air, there is a decrease in efficiency? But the aircraft engines (certainly turbofans slightly different with props) are meant to push against the air and most of the thrust does exactly do that whilst in cars the heat generated from the combustion is essentially “waste energy” decreasing the combustion efficiency whilst in planes (again, in turbofans) even heat generated is used for thrust
Diesel and petrol engines are reciprocating engines. Hence the pistons change direction during the combustion cycle. This reciprocation plus the complexity of the power transmission chain (valves, valve drive chains, crankshafts, con rods and all the supporting bearings etc.) added to thermal and acoustic energy generation reduce the power efficiency.
Gas turbine engines work on through flow pressure profile to control combustion. Combustion itself takes part in an annular combustion chamber with no moving parts. The pressure flow generated by the combustion of fuel is used by power turbines to transfer power to the compressor and the auxiliary drive chain thus drawing power and reducing efficiency. Heat and noise further reduce efficiency and the thrust generated at the back of the engine.
However the relatively simplistic flow and power generation are far more efficient than a reciprocating engine. The inefficiencies are derived from the fact that the aerodynamic efficiencies of both the compressor and the power turbine structure can only be properly tuned for one speed.
Various systems help to adjust the efficiencies at the front end of the compressor (Variable Inlet Guide Vanes) but the core of the compressor is generally fixed.
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