Improvement of fuel economy and reduction of hazardous exhaust emissions are critical issues in the development of internal combustion engines. In this situation, traditional empirical methods for developing new engines and certifying new fuels only have led to minor improvements. To achieve the required rate of innovation, completely new approaches are required which include new ignition system and combustion concept. Among many new technologies, plasma-assisted combustion has shown great promise for combustion enhancement. The plasma ignition can be accomplished by thermal and non-thermal plasma. Thermal plasma for combustion originated more than one hundred years ago with internal combustion engines and spark ignition systems. However, there were corrosion and erosion problems in the ignition system because of thermal equilibrium state of electron and molecules. Therefore, in thesedays many researchers pay attention in non-thermal plasma for ignition and combustion. It provides new possibilities for ignition and flame stabilization.
A number of researchers have examined the potential for microwave to influence combustion properties. At low frequency, momentum transfer due to ions was the prominent mechanism. Ionic wind and ion drift effects could dramatically alter low speed laminar flame structure and be used for soot production control. On the otherhand, predominant flame coupling was through the collision of free electrons with neutrals in the high frequency range. High frequency electric fields flame effects have been studied by a number of groups and measurements have shown global increases in flame speed, increases in flame temperatures, and corresponding increases in concentrations such as hydroxyl radical. However, most of the studies were conducted with conticuous microwave and flame. The researches under engine-like condition are rarely done.
In this study, microwave-assisted plasma ignition system was developed and applied in both constant-volume combustion chamber (CVCC) and single cylinder gasoline engine.The microwave delay was confirmed about $8 \mu s$ . The microwave was mixed with direct current voltage source and delivered into igniter. For the fundamental accessment of the igniter, light emission from spark event was analyzed with spectrometer under vacuum and atmospheric ambient pressure condition. The combustion experiments were performed under varous equivalent ratio and ambient aire pressure conditions in the CVCC. Finally, the single cylinder engine test was performed to investigate the effect of microwave ejection on engine performance and emission characteristics.
From the experiment, microwave assisted plasma ignition showed higher level of nitrogen and oxygen radicals than conventional sprak system. The microwave-assisted plasma ignition also provide plenty of advantageous radicals such as $O_2^+$ into the discharge zone. The results of combustion tests in CVCC showed that the initial flame kernel size was bigger with microwave ejection which showed 20% enhancement under lean condition. Based on the in-chamber pressure analysis, mivrowave-assisted plasma ignition had faster ignition process than conventional spark system. However, these advantages were diminished in the high pressure and equivalence ratio. To overcome this limitation, a high power magnetron (3kW) was utilized to assure combustion enhancement even in higher ambient aire pressure. The hydroxyl radicals in the flame front was captured by ICCD camera and 310 nm band pass filter. From the images, the intensity of the radical increased with microwave ejection which showed larger area because of increased flame speed. Finally, from engine experiments, the microwave ejection extended lean limit up to 1.55 with stable operating characteristics. On the other hand, the conventional spark ignition system showed lean limit at 1.3. Based on the extended lean limit, the fuel consumption was improved up to 6.5%. Meanwhile, the combustion phase was advanced and the peak of in-cylinder pressure and heat release rate were increased with microwave ejection because of higher flame propagating speed. In terms of emissions, the unburned hydrocarbon and carbon monoxide can be decreased significantly with microwave ejection due to more complete combustion. However, the nitrogen oxide emissions were increased.
현재 DISI엔진은 희박연소 (lean burn), 배기가스 재순환 (EGR; exhaust gas recirculation), 그리고 터보차져 등의 기술의 적용과 함께 발전하고 있다. 하지만 이러한 기술이 접목됨에 따라 연소 안정성은 떨어지게 된다. 따라서 문제점을 해결하기 위해 최근 기존의 스파크 플러그 보다 초기 화염핵의 크기가 크며 전파속도를 향상시킬 수 있는 새로운 플라즈마 점화시스템을 개발 및 적용하는 연구들이 수행되고 있다. 본 연구에서는 기존의 스파크 플러그 대비 초기 화염핵 생성이 크고 화염 확장 속도를 향상시킬 수 있는마이크로파 보조 점화기를 적용하여 가솔린 엔진의 성능 및 배기특성 개선에 대한 가능성을 알아보았다. 2.45 GHz의 마이그네트론을 이용하여 마이크로파 보조 플라즈마 점화기를 구축 하였으며 정적 연소실에서 기본적인 연소실험을 통해 연소개선에 대한 효과를 살펴보았다. 엔진실험은 0.5L급 단기통 직접분사식 가솔린엔진에서 수행하였으며 연소상과 배기배출물 (CO, HC, NOx)를 기존의 스파크 점화와 비교하였다.
