American Institute of Aeronautics and Astronautics 1 Improving the Performance of a Valveless Pulse Combustor using Unsteady Fuel Injection Tom Offord, Robert J Miller 1 , James R Dawson 1 , Jonathan J H Heffer 1 Whittle Laboratory, Department of Engineering, University of Cambridge, United Kingdom Sam Mason 1 , and Mark Taylor Rolls Royce plc, Derby, United Kingdom This paper describes an experimental investigation into the effect of unsteady fuel injection on the performance of a valveless pulse combustor. Two fuel systems were used. The first delivered a steady flow of ethylene through choked nozzles, and the second delivered ethylene in discrete pulses using high-frequency fuel injectors. Both fuel systems injected directly into the combustion chamber. The high-frequency fuel injectors were phase locked to the unsteady pressure measured on the inlet pipe. The phase and opening pulse width of the injectors and the time-averaged fuel mass flow rate through the injectors were independently varied. For a given fuel mass flow rate, it is shown that the maximum pressure amplitude occurs when fuel is injected during flow reversal in the inlet pipe, i.e. flow direction is out of the combustor. The optimal fuel injection pulse width is shown to be approximately 2/9 th of the cycle. It should, however, be noted that this is the shortest time in which the injectors can reliably be fully opened and closed. It is shown that by using unsteady fuel injection the mass flow rate of fuel needed to achieve a given amplitude of unsteady pressure can be reduced by up to 65% when compared with the steady fuel injection case. At low fuel mass flow rates unsteady fuel injection is shown to raise the efficiency of the combustor by a factor of 7 decreasing to a factor of 2 at high fuel mass flow rates. Nomenclature p = pressure  = phase angle  = pulse width  = density  = fuel conversion efficiency I. Introduction ulse combustors are large amplitude resonant thermo-acoustic devices in which the heat released by combustion couples with the acoustic field. The behaviour of thermo-acoustic devices was studied by Lord Rayleigh [1], who observed that when unsteady heat release occurs in phase with unsteady pressure oscillations, the oscillations are amplified. When heat release leads the pressure maximum the operating frequency is increased, and when heat release lags the pressure maximum the operating frequency is reduced. The ideal thermo-acoustic device, therefore, would be one in which all the heat release occurred impulsively at peak pressure. The efficiency of converting heat into the mechanical energy within the wave is known as Rayleigh efficiency. In most pulse combustors fuel is injected at a constant mass flow rate throughout the cycle. This is achieved by having choked fuel injection nozzles. The fluid dynamics within the combustor are then relied upon to ensure that the heat-release varies in phase with the pressure to sustain resonance. Consequently, alterations to the geometry of the combustor 1 AIAA member. P