Exhaust Gas Recirculation (EGR) and lean burn utilize the diluents into the engine cylinder to control combustion leading to enhanced fuel consumption and emissions. However, with high diluent rates, the occurrence of excessive cyclic variation brings about undesirable combustion instability within the engine cylinder resulting in the deterioration of both engine performance and emissions.
In this study, the effects of EGR was primarily investigated on the performance and emissions with a single cylinder liquid-phase LPG(Liquefied petroleum gas) injection engine, in a wide range of EGR rates, engine operating conditions and LPG proportions. After that, an analytic approach was also investigated on the three different effects of EGR which are dilution, thermal, and chemical effect. A fuel burn rate based on the single-step reaction rate was also developed and incorporated in a thermodynamic cycle calculation to account for the EGR.
As EGR rate was increased, NOx emission was reduced while HC emission was increased. Pumping loss reduction by EGR improved brake specific fuel consumption (bsfc) and EGR lowered exhaust gas temperature. Moreover, LPG proportions made a difference on the performance and emission characteristics.
Proper stratification of mixture and diluents could improve the combustion stability under high dilution environment. To examine EGR stratification within the cylinder, fast-response solenoid valve was used in the midst of EGR line and its timing and duty were controlled. Supplying EGR in both homogeneous mode and stratified mode, in-cylinder pressure and emissions were measured. With a 30% EGR rate, the peak heat release was 10% higher, accompanied by shorter combustion duration in the stratified EGR (EGR#1) case than in the homogeneous EGR (EGR#4) case. Addition of EGR into a lean mixture of λ=1.2 led to a 3% further reduction in fuel consumption, which was attainable only with stratified EGR but not with homogeneous EGR. With more than a 20% EGR rate, NOx emission further reduced with less increase in hydrocarbon emission with stratified EGR than with homogeneous EGR.
The effects of the diluents on the flame propagation characteristics were investigated in an optically accessible single cylinder engine. A series of bottom-view flame images were taken from direct visualization using a UV (ultra violet) intensified high-speed CCD (Charge coupled device) camera. Flame propagation patterns showed that the flame speed was less decreased by the stratified EGR than homogeneous EGR.
Planar laser induced fluorescence (PLIF) technique was applied to investigate the stratification of the in-cylinder charge. Acetone as a tracer was seeded into EGR to get the EGR distribution within the engine cylinder. It was found that EGR occupied the low part of cylinder through the stratification process and this could help reducing burn duration.
EGR effects on combustion in spark ignition (SI) engines were quite different from those in diesel engines. Firstly, dilution effect is negligible in SI engines because there is no decrease in oxygen concentration in the mixture by the admission of EGR as an additional charge into the cylinder. Secondly, thermal effect is the most significant effect among the three effects of EGR. The thermal capacity of the mixture increases as much as the EGR content and this made flame temperature low and burn duration long. Thirdly, chemical effect has a trade-off between the increase of $CO_2$ or $H_2O$ concentration and the decrease of flame temperature with EGR rate. Resultantly, the slow burn with EGR is attributed to the decrease in the flame temperature which results mainly from the increased thermal capacity (thermal effect) with relatively small effect of the dissociation of EGR (chemical effect).
배기가스재순환(EGR; Exhaust gas recirculation 이하 EGR)은 희박연소와 마찬가지로 연료소모율과 배출가스 성능을 향상시키기 위하여 희석제를 이용하여 연소를 제어하는 것이다. 그러나 희석률이 너무 크면 연소불안정성이 야기되어 과도한 사이클변동이 발생하게 되고 따라서 엔진출력과 배출가스 성능이 모두 악화된다. 본 연구에서는 다양한 EGR율과 엔진운전조건 및 LPG 조성에 대하여 단기통 액상분사식 액화석유가스(LPG; Liquefied petroleum gas 이하 LPG) 엔진의 출력과 배출가스 성능에 미치는 EGR 효과를 주로 연구하였다. 이러한 결과로부터 단단계 반응율에 기초한 연료연소율을 사용하여 EGR의 세 가지 효과인 희석효과, 열 및 화학적 효과에 대한 열역학적 사이클 해석을 시도하였다. 기존의 EGR 연소의 특성을 살펴보면, 엔진에 EGR율을 증가시키면 질소산화물은 배출이 감소하지만 탄화수소 배출은 증가한다. 한편, EGR 유입에 의한 펌핑손실의 감소는 연료소모율을 낮추고 배출가스 온도도 떨어뜨린다. 이러한 현상은 LPG 연료의 조성에 따라 영향을 받는 것으로 나타났다. 고농도의 희석조건에서 혼합기와 희석제의 적절한 성층화는 연소안정성 향상을 가져온다. 엔진 실린더 내에서 EGR 성층화를 연구하기 위하여 EGR 유로에 고응답 솔레노이드 밸브를 장착하고 밸브 작동시기와 기간을 제어하였다. 기존의 균일한 방식(EGR#4)과 성층화 방식(EGR#1)을 모두 사용하여 엔진에 EGR을 공급하면서 연소압력과 배출가스를 측정하였다. EGR 30%를 공급하였을 때, 성층화된 EGR을 적용한 경우는 균일한 경우에 비하여 최고 열발생율이 10% 더 큰 값을 보였다. 그리고 희박한 혼합기에 EGR을 사용한 경우 성층화된 경우만이 연료소모율이 향상되었다. EGR율에 따라 질소산화물이 감소하였으나, 성층화 EGR의 경우 탄화수소 배출량이 덜 증가하였다. 화염 가시화를 통하여 희석제가 화염전파에 미치는 영향을 조사하였다. 고속디지털카메라를 사용하여 단기통 가시화 엔진에서 피스톤창과 광증폭기를 거쳐 연속 화염화상을 취득하였다. 화염전파 형상을 관찰한 결과 성층화된 EGR의 경우 균일한 것에 비하여 화염속도가 덜 감소하였다. 평면레이저유도형광법을 사용하여 실린더 혼합기의 성층화를 조사하였다. 실린더 내 EGR 분포를 얻기 위하여 아세톤을 EGR에 첨가하였다. 엔진에 공급된 EGR이 성층화과정을 통하여 실린더 하층부에 분포하였고, 이것이 연소기간을 줄이는 원인이 된다.
