There are several reasons for the superior fuel economy. An internal combustion engine takes in new air and fuel for each stroke, saving nothing from the previous one. But the Stirling re-uses the same heat energy on successive strokes, the fuel is only needed to make up the losses. The second reason is that the fuel is always burned full lean, at the best air/fuel ratio, while normal aircraft engines actually use gasoline as a coolant. Expensive coolant. The Stirling also uses the exhaust from the burner to preheat the incoming combustion air. Since the Stirling exhaust is cool, it is obvious that less energy is being thrown away.

Compare this with today's powerplants. Suction (use crankshaft energy to pull new air and new fuel in), Compression (use crankshaft energy to compress the mixture), Expansion (burn the fuel, raising the pressure to push the piston down and impart energy to the crank), Exaust (use crankshaft energy to force the burned mixture out). Note that three cycles out of four require energy, only one provides it. No wonder today's powerplants vibrate so much. And what does the engine do next? It begins again with brand new air and brand new fuel, saving nothing from the cycles that have gone before.

In our engines today, combusting the mixture of fuel and oxygen creates heat, which mostly heats the nitrogen which makes up 78% of air. Increased nitrogen pressure is main thing that pushes the piston down in a conventional I.C. engine. Stirling engines prefer a lower-molecular-weight gas, because it takes less heat energy (i.e., less fuel) to produce a given pressure change in a small molecule. Air has a molecular weight of about 28.9, while Helium is only 4. So air takes about seven times more energy to produce a given force. This is somewhat comparing apples and oranges, and I don't want to get mired down in the thermodynamics here. The Stirling is certainly more fuel efficient than the powerplants we're flying now, but not seven times as good. Stirlings aren't even twice as good. (But again, the learning curve is ahead.) It's very doubtful that SFC will ever be twice as good as today's quoted numbers, but I think it's possible that real world fuel consumption could improve by a factor of two.