Development of Innovative Microwave Plasma Ignition System with Compact Microwave Discharge Igniter

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Abstract
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). A semi-conductor device was used as microwave oscillator, a flexible co-axial cable transmitted the microwaves from microwave oscillator to MDI, and MDI acts as an ignition source. This microwave ignition system generates non-thermal plasma and allows the control of plasma lifetime, plasma intensity with changing microwave oscillation pulse pattern. The MDI can be applied to multi-point ignition to achieve higher thermal efficiency by improving not only ignition performance, but also faster combustion and knock reduction.In this study, plasma generation and ignition performance of MDI were investigated in constant volume chamber at high ambient pressure of 6 MPa. The results indicated that MDI can generate plasma under much high ambient pressures up to 6 MPa in both air and CO2 as ambient gases. The combustion performance of MDI was compared with commercial spark plug in constant volume combustion chamber by Schlieren technique, and in single cylinder optical engine. Preliminary results suggested that MDI delivered similar performance with spark plug.

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