에탄올연료 SI 엔진에서 공연비 및 에탄올-가솔린연료 혼합비율에 따른 희박연소 및 배기특성에 관한 연구

Abstract

본 논문은 바이오에탄올 연료 및 에탄올-가솔린 혼합연료를 SI엔진에 적용하여 다양한 엔진운전조건하에서 공연비 및 에탄올-가솔린 혼합비율에 따른 엔진성능특성, 엔진연소특성, 희박연소특성 및 배기 배출물 특성을 실험적으로 규명하여 각 성능 특성별 최적의 공연비 및 에탄올 혼합비율을 도출하는 것을 목적으로 한다. 본 연구에서는 배기량 1591cc, 압축비 10.5인 4기통 SI엔진을 사용하였으며, 실험엔진의 성능특성을 제어하기 위해 75kW 와전류 전기동력계를 사용하였다. 실험엔진의 점화시기, 연료분사시기 및 분사기간 등을 정확하게 제어하고, 흡입공기온도, 냉각수온도 및 배기가스온도를 제어 및 모니터링 하면서 기관의 연소압력, 열발생율, 최대압력 변동계수 및 도시평균유효압력 변동계수 등을 도출하였다. 또한 연소과정에서 발생하는 CO, HC, NOX 및 CO2 등의 배기 배출물을 분석하여 배출가스 특성을 분석하였다. 실험연료로는 순수 가솔린 연료와 순수 에탄올 연료를 기본으로 하여 가솔린 연료에 에탄올 연료를 부피기준으로 20%혼합한 E20, 40%혼합한 E40, 60% 혼합한 E60 및 80% 혼합한 E80의 6개 연료를 사용하였으며, 실험결과는 가솔린 연료와 바이오에탄올 연료 및 에탄올-가솔린 혼합연료를 비교, 분석하였다. 실험결과, 공연비 변화에 대한 제동토크, 제동평균유효압력, 제동출력특성은 에탄올 연료가 가솔린 연료에 비하여 우수한 성능을 나타내었으며, 에탄올-가솔린 혼합연료의 경우 에탄올 혼합비율이 높을수록 성능특성이 가솔린에 비하여 우수한 것으로 확인되었다. 공연비 변화에 대한 연소실 압력 및 열발생률은 에탄올 연료가 가솔린 연료에 비하여 높았으며, 에탄올-가솔린 혼합연료의 경우 에탄올 혼합비율이 높을수록 연소압력 및 열발생률이 상승하는 경향을 나타내었다. 또한 최대연소압력 변동계수 및 에탄올-가솔린 혼합연료의 도시평균유효압력 변동계수를 분석한 결과 에탄올 연료의 우수한 엔진운전안정성을 확인할 수 있었으며, 특히 에탄올 혼합비율이 높은 E80 및 E100의 경우 희박연소한계 (LBL, lean burn limit)가 λ=1.4 영역까지 확장되는 것을 확인할 수 있었다. CO, HC, NOX 및 CO2 등의 배기 배출물 특성을 분석한 결과 에탄올 연료의 배기가스 배출량이 가솔린 연료의 배출량에 비해 전반적으로 저감되었으며, 혼합연료의 경우 에탄올 혼합비율이 높은 연료에서 배기가스 저감 효과가 큰 것을 확인할 수 있었다.; A study of lean combustion and exhaust emission characteristics in a SI engine fueled with ethanol according to air-fuel ratio and ethanol-gasoline fuel blending ratio

The purpose of this paper is to investigate the effect of air-fuel ratio and ethanol-gasoline blending ratio on engine performance, engine combustion, lean combustion and exhaust emission characteristics in a ethanol fueled SI engine using bio-ethanol fuel, which was recommended as promising alternative fuel for gasoline fuel, and ethanol-gasoline blended fuel under various engine operating conditions. The results of engine performance, combustion and exhaust emission of bio-ethanol and ethanol-gasoline blended fuel were compared with those of pure gasoline fuel. Based on those results, the optimized air-fuel ratio region for bio-ethanol fuel and the most effective blending ratio of ethanol-gasoline fuel will be investigated and the environmentally friendly characteristics of bio-ethanol fuel will be also clarified.

In order to achieve this purpose, a 1591cc and 10.5 of compression ratio with four cylinders SI engine was tested on 75kW eddy-current dynamometer to control engine speed and engine load. In addition, engine control parameters, such as ignition timing, fuel injection timing, duration of fuel injection, engine operating conditions, intake air temperature and coolant temperature, were controlled by universal ECU (Engine control unit) by electronic engine control system. The exhaust emissions such as carbon oxides (CO), unburned hydrocarbon (HC), nitric oxides (NOX) and carbon dioxides (CO2), which was produced from combustion process, were tested by emission analyzer. The test fuels were pure gasoline and pure bio-ethanol fuel, the purity ratio of anhydrous ethanol was 99.9%, and ethanol fuel was blended to gasoline fuel by 20% (E20), 40% (E40), 60% (E60), 80% (E80) based on fuel volume, respectively and all of theses six fuels were tested in SI engine.

The results showed that engine performance characteristics, such as brake torque, BMEP (brake mean effective pressure) and brake power according to air-fuel ratio and ethanol-gasoline blending ratio were increased in bio-ethanol fuel and higher ethanol blended fuel than those of gasoline fuel. BSFC (brake specific fuel consumption) was increased in bio-ethanol fuel and higher ethanol blended fuel but BSEC (brake specific energy consumption), considering the lower heating value of each test fuel, was decreased in bio-ethanol fuel and higher ethanol blended fuel than those of gasoline fuel. The combustion characteristics and lean combustion characteristics such as, cylinder pressure and ROHR (rate of heat release) were slightly higher in bio-ethanol fuel and higher ethanol blended fuel than those of gasoline fuel. The COV (coefficient of variation) of maximum cylinder pressure according to air-fuel ratio was decreased in bio-ethanol fuel than that of gasoline fuel. In addition, the COV of IMEP (indicated mean effective pressure) of bio-ethanol fuel and higher ethanol blended fuel was lower than that of gasoline fuel and the LBL (lean burn limit) was expanded to λ=1.4 in higher ethanol blended fuel, E80 and E100. The overall quantity of exhaust emissions, such as CO, HC, NOX and CO2 were decreased in bio-ethanol fuel and higher ethanol blended fuel.

From these results, the excellent environmentally friendly characteristics of bio-ethanol fuel were clarified and the optimized air-fuel ratio region for bio-ethanol fuel and the most effective blending ratio of ethanol-gasoline fuel was also investigated by experimentally.