What mechanism dictates efficiency in a gas turbine and an internal combustion engine?

I seem to recall from my A level physics that the efficiency of an internal combustion engine was related to the Carnot cycle.

Or

Which is used in the ingenious Toyota hybrid powertrain.

Thermodynamics determines that the maximum efficiency of a heat engine is the input minus the output temperatures expressed as a percentage of the input temperature using an absolute temperature scale such as Kelvin.

Where does the 'maximum' come into that determination?

It means that no matter how hard you try there is a predictable upper limit to efficiency based on the input and output temperatures. John

As the page says, the Carnot cycle provides an upper bound on the possible efficency (asssuming various standard and uncontroversial thermodynamic assumptions).

#Paul

Like most things enfgineerinfg there is more than one way to make a mathematical model.,

Bernouilli/Newtonian.

Carnot/energy analysis.

It is pretty easy to see that energy extracted cannot exceed energy in combustion minus energy leaving the tail pipe.

You don't need to analyse that with Carnot,

Gas turbines would be efficient if the exhaust was not so damned hot, since combustion temperatures are in fact very high.

The combined cycle turbvine that uses those exhaust gasses to heat a steam boiler is the most efficient internal combustion derived heat engine we have.

Turbocharged diesel and petrol engines with very high combustion temperatures - generally achieved with high effective compression ratios and very weak fuel air mixes - are getting close though.

You can always waste energy in other ways than through the exhaust pipe