IGNITION ENERGY TESTING OF THE QUARTER WAVE COAXIAL CAVITY RESONATOR WITH AIR- LIQUEFIED-PETROLEUM-GAS MIXTURES

Significant environmental and economic benefit could be obtained from engines that utilize lean mixtures ignited with high energy sources. To help reach this goal, the quarter wave coaxial cavity resonator (QWCCR) igniter was examined as an ignition source. Evaluation consisted of comparative ignition tests with liquefied-petroleum-gas (LPG) air mixtures of varying composition. Combustion of these mixtures was contained in a closed steel vessel with a pre-combustion pressure near one atmosphere. The resonator igniter was fired in this vessel with a nominal 150 W microwave pulse of varying duration to determine ignition energy limits for various mixtures. Mixture compositions were determined by partial pressure measurement and the ideal gas law. Successful ignition was observed through a view port in a combustion chamber. Microwave pulse and reflected power was captured in real time with a high-speed digital storage oscilloscope. Ignition energies and power levels were calculated from these measurements. As a comparison, ignition experiments were also carried out with a standard non-resistive spark plug where gap voltage and current were recorded for energy calculations. Results show that ignitable mixtures around stoichiometric and slightly rich compositions are combustible with the QWCCR using similar levels of energy as the conventional spark plug, in the low milli-Joule range. Combustion energy for very lean mixtures could not be determined reliably for the QWCCR for this prototype test, but are expected to be lower than that for a conventional spark. Given the capability of high power, high energy delivery, and opportunity for optimization, the QWCCR has the potential to deliver more energy per unit time than a conventional spark plug and thus could be considered be as a lean ignition source.

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