A study of plasma-assisted ignition in a small internal combustion engine

A plasma-assisted spark plug is tested in a single cylinder, 34 cubic centimeter, 4-stroke internal combustion engine of the type used in small scale unmanned aerial systems. The spark plug, developed by Imagineering, Inc. of Japan, couples 2.45 GHz microwave frequency energy from a commercial magnetron into the electrodes of a standard geometry non-resistor spark plug. Tests are run at two different engine speeds, with and without the addition of 750 mJ of microwave pulses, and the intake charge is varied over a wide range of equivalence ratios. The initial results show that with the microwave-assisted ignition the lean burn limit was extended by 20-30% according to the coefficient of variation of the indicated mean effective pressure (COV _imep) staying within the stable operating limits. The results also demonstrate an improvement in the indicated mean effective pressure of 6-10% at strong operating conditions away from the lean limit. The minimum specific fuel consumption occurs at lean operating conditions of A/F = 17-20, where there can be significant limitations in the COV _imep for the standard spark plug, yet the microwave-enhanced spark plug can achieve stable operation. It is thus demonstrated that using the microwave-assisted spark plug allows for a wider range of stable operating conditions, which encompasses the region of greatest fuel conversion efficiency not achievable with the standard spark plug.