There are basically four ways to control Stirling power output. These are to change the temperature, or the pressure, or the phase angle, or to waste power with a bypass valve. Each has its good and bad points.

Temperature control is very fuel efficient, and simple to accomplish by just turning the burner up or down. The problem is that it is slow.

Pressure control can also be fuel efficient, and response time is quite rapid. This is the method used in road vehicles NASA and General Motors and Ford have developed. The problem here is complexity, as the several hundred PSI must be introduced into and removed from the engine quickly in accord with throttle position. This takes sophisticated compressors, reservoirs, servo loops, and so on. Too costly and trouble prone.

Phase angle control can be rapid, and fairly simple mechanically, but not very fuel efficient.

Wasting power sounds like the worst possible choice, and for cars and trucks it is, but it turns out to be OK for aircraft.

Here's why. The "mission profile" only requires that a small part of the flight needs rapid power control. During taxi, takeoff, and landing the pilot needs to be ready to instantly add power. Most of the trip, however, is made at a steady climb or cruise power setting, and it is during this time that fuel efficiency is of paramount importance.

So, the aircraft Stirling powerplant uses temperature control to set the power. Takeoff and landing is made at maximum temperature. When landing, the unneeded power is wasted with the bypass. During this time operation is fuel inefficient, but it gives the pilot the ability to apply power instantly if needed, and doesn't last long anyway.

Courtesy :

http://www.qrmc.com/towarticlesstirling.html