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Extending the lean limit or/and exhaust-gas-recirculation (EGR) limit/s are necessary for improving fuel economy in spark ignition engines. One of the major problems preventing the engine to operate at lean conditions is stable and successful initial ignition kernel formation. A repeatable, stabilized ignition and early flame development are quite important for the subsequent part of the combustion cycle to run smooth without partial burn or cycle misfire. This study aims to develop an innovative plasma ignition system for reciprocating combustion engines with an aim to extend lean limit and for high pressure applications. This ignition system utilizes microwaves to generate plasma as an ignition source. This microwave plasma igniter is much simplified device compared to conventional spark plug. The microwave plasma ignition system consists of microwave oscillator, co-axial cable and microwave discharge igniter (MDI).

An in-spark-plug flame sensor was developed to measure local chemiluminescence near the spark gap in a practical spark-ignition (SI) engine in order to study the development of the initial flame kernel, flame front structure, transient phenomena, and the correlation between the initial flame kernel structure and cyclic variation in the flame front structure, which influences engine performance directly. The sensor consists of a commercial instrumented spark plug with small Cassegrain optics and an optical fiber. The small Cassegrain optics were developed to measure the local chemiluminescence intensity profile and temporal history of OH*, CH*, and C2* at the flame front formed in a turbulent premixed flame in an SI engine. A highresolution monochromator with an intensified chargecoupled device (ICCD) and spectroscopy using optical filters and photomultiplier tubes (PMTs) were used to measure the time-series of the three radicals, as well as the in-cylinder pressure.

A small Cassegrain optics sensor was developed to measure local chemiluminescence spectra and the local chemiluminescence intensities of OH*, CH*, and C2* in a four-stroke spark-ignition (SI) engine in order to investigate the propagation characteristics of the turbulent premixed flame. The small Cassegrain optics sensor was an M5 type that could be installed in place of a pressure transducer. The measurements could be used to estimate the flame propagation speed, burning zone thickness, and local air/fuel (A/F) ratio for each cycle. The specifications of the small Cassegrain optics sensor were the same as those used for previous engine measurements. In this paper, measurements were made of several A/F ratios using gasoline to fuel the model engine. The performances of two Cassegrain optics sensors were compared to demonstrate the advantages of the new small sensor by measuring the local chemiluminescence intensities of a turbulent premixed flame in the model engine.

This study aims to develop innovative plasma combustion system to improve fuel economy and achieve higher efficiency without any modification of current engine configuration. A new plasma generation technique, that used a combination of spark discharge and microwave, was proposed. This technique was applied to gasoline engine as an ignition source, which was intensive and stable even in lean condition. In this technique, firstly, small plasma source was generated by spark discharge. Secondly, microwave was radiated to the plasma source to expand the plasma. The microwave power was absorbed by the plasma source and large non-thermal plasma was formed. In non-thermal plasma, the electron temperature was high and the gas temperature was low. Then many OH radicals were generated in the plasma. The frequency of the microwave was 2.45 GHz because we used a magnetron for microwave oven. Magnetrons for microwave oven were high efficiency and reasonable.

The objective of this study was to develop an innovative microwave-induced plasma ignition system to improve the fuel economy of a current engine and achieve a higher efficiency without any configuration modifications. A new plasma generation technique was proposed for a stable and intense ignition source. A microwave plasma combustion system was developed consisting of a spark plug, microwave transfer system, and control system. A magnetron, like that found in a microwave oven, was used as a microwave oscillator. The spark plug had a microwave antenna inside that generated plasma in the engine cylinders. The microwave transfer system transmitted microwave power from the oscillator to the antenna. Combustion experiments were performed using a single-cylinder research engine. The microwave plasma expanded the range of lean operating conditions. The single-cylinder engine had an indicated mean effective pressure (IMEP) of 275 kPa at an engine speed of 2000 rpm.