Abstract
This study developed an innovative microwave plasma combustion system to improve fuel economy and achieve higher efficiency for spark ignition (SI) engines. Microwave plasma technology was combined with an active ignition system, which generates plasma by applying a high-frequency voltage to the standard spark discharge. A standard ignition coil was used. The high-frequency voltage was applied at frequencies ranging from kHz to MHz; the voltage exceeded the inductive discharge voltage and was less than the capacitive discharge voltage. The high-frequency voltage was applied just after the SI timing of the standard spark system, and the pulse width was changeable. A magnetron supplied microwaves to the cylinder through a waveguide, co axial cable, mixer unit, and center electrode of a non-resister spark plug. The mixer unit functioned as an isolator to prevent adverse current due to the high frequency and microwave oscillation. Engine modifications were not required for this ignition system. During the active ignition and microwave operating cycles, the heat release was stable and the indicated mean effective pressure (IMEP) fluctuation was relatively low. Without microwave oscillation, the initial combustion period became unstable and there were some partial burn cycles. The initial combustion period and IMEP were strongly correlated. Therefore, the stable, short initial combustion period led to a high IMEP and less combustion fluctuation. To optimize the microwave oscillation timing, the effect of the microwave timing on the torque was investigated. This showed that the torque curve had two peaks when plotted against the timing of the microwave oscillation: the first peak was related to the microwave plasma generation, which utilized the high-performance ignition as a source of the plasma, and the second peak was outside the range of the high-performance ignition timing. This means that the microwaves improved the initial flame development, not the ignition, suggesting the possibility of further fuel economy improvement by enhancing flame propagation under ultra-lean or high emission gas recirculation (EGR) conditions.
