The reported discrepancy between theory and experiment for external combustion Stirling engines\nis explained by the addition of thermal resistance of the combustion gasses to the standard\nCarnot model. In these cases, the Stirling engine ideal efficiency is not as is normally reported\nequal to the Carnot cycle efficiency but is significantly lower. A new equation for ideal Stirling engine\nefficiency when the heat is obtained through external combustion without pre-heating the air,\nis presented and results for various fuels tabulated. The results show that petrol and diesel, internal\ncombustion engines (Otto cycle) have a higher ideal efficiency than the Stirling engine.\nWhen comparing thermoacoustic engines heated by wood, efficiency should not be quoted as a\npercentage of the Carnot efficiency, but against a figure 48% lower than Carnot. The effect is not\nseen with electrically heated rigs, solar or nuclear fission heated engines.
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