A US Air Force Research Laboratory team hopes to develop a self-starting system for a hybrid piston-pulsed detonation engine (PDE), after demonstrating that both significant thrust and shaft power can be extracted from a test engine at the same time.

Although most PDE research efforts are aimed at overcoming technological hurdles to the use of pulse detonation as a means of producing thrust for high-speed vehicles, the Propulsion Directorate team says work on power extraction is just as crucial. Such a system, it says, could enable the PDE to work in much wider applications and allow jet-like operations.

In a PDE the operating cycle begins when fuel is mixed with air. Detonation is then initiated inside the simple tube which forms the engine. The detonation wave moves through the fuel-air mix and high-pressure gases fill the detonation chamber. The detonation wave then blasts out of the engine, reducing pressure at the inlet and allowing the cycle to start again.

The proof-of-concept hybrid engine consisted of a four-cylinder Yamaha Katana 1,100cc motorcycle engine modified with a spacer block to provide room for the four detonation tubes (one for each cylinder) of the PDE. These were placed perpendicular to the direction of the piston travel, and the spark timing adjusted so that the pressure from the detonation cycle occurred while the piston was travelling downward at its maximum velocity.

The engine was started with an electric motor, connected via the timing chain to the crankshaft. Air and fuel rates were adjusted to match detonation tube volume and engine frequency. A spark from the standard ignition system was then used to ignite the fuel air mixture and, once the engine completed several cycles, the clutch was remotely disengaged, separating the electric motor from the hybrid engine.

The team discovered that a critical starting-rotation speed has to be achieved to produce enough power to allow the engine to self-actuate and continue firing. The small proof-of-concept engine needed a critical detonation frequency of 5Hz. The maximum power produced was approximately 15hp (11kW) while still producing 50lb (0.22kN) of thrust.

Source: Flight International