i. An engine that has a centrifugal compressor fitted downstream of an axial compressor. The former assists the axial compressor when running outside its relatively narrow operating range, especially during starting. This adds to the flexibility of the engine.
ii. A compounded piston engine utilizes more than one method of delivering propulsive effort. Turbine wheels, which resemble windmills on a single axle, are placed in the exhaust of the piston engine. The exhaust gases hit the turbine wheel blades, causing the wheel to revolve. This, in turn, helps to rotate the crankshaft through a gear system. This extra energy boosts propeller power.