이론공연비 압축착화 엔진에서 연소 특성 및 후처리 장치의 배출가스 저감에 관한 연구

Abstract

In present study, the main objectives are to analyze the optimal operating conditions and evaluate the conversion efficiency of after-treatment in stoichiometric compression ignition (SCI) engines with simulated-EGR. The reason is that the investigation of simulated-EGR is especially useful in the fundamental understanding in order to analyze the effect of constituents in recycled exhaust gas with real-EGR. In addition, the applying after-treatment devices technique is the key point of methodology to reduce the exhaust emission in stoichiometric compression ignition (SCI) engines. Therefore, the smoke reduction efficiency and conversion efficiency of exhaust emission is experimentally investigated by diesel particulate filter (DPF) and three-way catalyst (TWC) under stoichiometric condition, respectively. The experimental engine system mainly consists of the test engine, intake system, exhaust system, engine control system, fuel injection system, emission measurement instruments, and data acquisition system. The test engine is controlled by an ECU, which is based on reconfigurable embedded control system. In order to evaluate the performance of after-treatment, the exhaust system consists of exhaust gas surge tank, exhaust gas heating system, diesel particulate filter (DPF), three-way catalyst, and emission measurement instruments which consists of emission bench and smoke meter. In particular, the stoichiometric diesel combustion is important to improve the use of oxygen in combustion chamber because the oxygen is insufficient. Therefore, in order to analyze the influence by reducing the oxygen mole fraction of intake gas into the combustion phenomena inside the compression ignition (CI) engines under increasing the equivalence ratio up to stoichiometric condition, the O2 mole fraction is reduced from 21 to 11.16 % by adding only N2 to intake gas. As the results, the maximum IMEP slightly decreases when the equivalence ratio is richer. In particular, the stoichiometric condition has around 15% loss. This result shows that the stoichiometric combustion need not result in extreme deterioration of the engine performance. In addition, the combustion duration for conventional injection timing is remarkably reduced as equivalence ratio reaches stoichiometric. It is means that the stoichiometric combustion regime has a mainly premixed combustion phase except mixing-controlled combustion phase. Especially, the reduced oxygen in combustion chamber causes incomplete combustion in which the mixture is over-mixed by long ignition dwell and mixing time, leading that THC and CO emissions increases significantly. In present study, the stoichiometric diesel combustion is obtained by two methods: the one is used by reduced O2 mole fraction by adding only synthetic gas of N2 and the other is used by the simulated-EGR. Because the investigation of the effect of the intake mixture applying to simulated-EGR is important to evaluate the influence of main constituents in recycled exhaust gas with real-EGR, both methods have the same O2 mole fraction of 11.16 % but the simulated-EGR additionally has CO2 of 4.82 L/min. As the results, under conventional injection timing, the heat rejection efficiency is maintained around 60 %. In particular, the heat rejection efficiency with simulated-EGR is low compared to that with reduced O2 mole fraction by adding only N2. In addition, the trend of IS-CO with simulated-EGR is higher than that with reduced O2 mole fraction by adding only N2. The reason is the thermal effect by high specific heat capacity of CO2 suppresses the oxidation of CO emission due to reduction of combustion temperature. Therefore, the increased IS-CO with simulated-EGR is directly associated with the reduced heat rejection efficiency. Since the applying after-treatment devices technique is the key point of methodology to reduce the exhaust emission of stoichiometric compression ignition, the smoke reduction efficiency and conversion efficiency of exhaust emission is experimentally investigated by DPF and TWC under stoichiometric condition, respectively. As the results, the conversion efficiency of NOx is high and is provided around 75 % reduction for stoichiometric combustion. In particular, the NOx emission is reduced almost zero-level by TWC in stoichiometric diesel combustion. It can be guessed that TWC may reduce NOx emission although the stoichiometric diesel combustion under high-loads has relative high level of NOx emission. In addition, the conversion efficiency of CO and THC are also high and is provided around 90 and 60 % reduction, respectively. Furthermore, the smoke reduction efficiency of soot is high and is provided over 99 % reduction. The result means that the large amount of soot for stoichiometric diesel combustion is effectively reduced by DPF.