Development of Inductive Quarter Wave Radio Frequency Coaxial Resonant Cavity for a Compressed Natural Gas Ignition

The function of the ignition systems is to inject the required energy into the combustion chamber in order to ignite the air-fuel mixture. The amount of the injected energy depends on many factors such as the kind of combustion gas used, air-fuel ratio, and the combustion pressure. To meet the future fuel economy demands the vehicle engine manufacturers try to design engines that run on lean or ultra lean air-fuel ratios. These lean mixtures require more efficient energy injection mechanism to be ignited. The conventional ignition systems use spark plugs with a narrow spark gap to ignite the air-fuel mixture. The spark plug ionizes the mixture in the gap volume between the electrodes by means of DC high voltage source which is established from the energy stored in a magnetic coil. The drawback of such systems to meet the lean mixture demands is the need to increase the DC voltage between the gap electrodes in order to ignite the lean and ultra lean mixtures. The spark is localized around the spark plug volume and the size of the plasma channel is small. Moreover, going to higher DC voltage levels cause shorter life time for the spark plugs. The use of high energy plasma to ignite the mixture has recently gained interest. Many researches have been carried out on studying the Quarter Wave Radio Frequency Coaxial Cavity (QWRFCC) as a microwave plasma generator. This plasma source has many advantages: the mixture has lower breakdown voltage when using microwave frequencies, the device is working as electric field amplifier, and there is no electrode degradation. In this work a coaxial cavity is developed to be used as an ignition source. The cavity is intended to work at household microwave frequency range (2.45GHz). The dimensions of the cavity have been calculated along with lumped parameters to be used in the RLC electrical model to analyze the behavior of the cavity under the load variation. From simulation and experimental results, It is noted that the radio frequency cavity is able to initiate plasma kernel larger than the conventional spark plug with less input energy requirements. This cavity is planned to be used as an ignition source in a compressed natural gas engine (CNG engine) where the air-fuel ratio (φ) is around 25:1.

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