Ultra-Low Emissions Aimed Two-stage Combustion Feasibility Study

Currently at home and abroad the research of efficient and cleaner diesel new-type combustion mode mainly bases on microcosmic concentration-temperature (φ-T) schematic diagram of the cylinder combustion reaction district. Optimize the burning path by the control of mixing process (organize different combustion stage mixture) and chemical process (control combustion reaction district atmosphere), avoid the region of NOx and PM emissions and realize ultra-low emission. Two-stage Combustion mode with low temperature combustion and high temperature combustion is proposed in this study, which has ultra-low emission potential. By adjusting the first stage fuel injection parameters (injection quantity, injection pressure, injection timing,etc), form quasi homogeneous mixture with equivalence ratio below two in compression stroke, realize LTC combustion with cold-flame exothermic stage. Spay the second phase fuel (Control injection parameters) to mixture atmosphere of Appropriate temperature and concentration in cylinder, to extend the second phase fuel ignition delay and form Combustible mixture area of Low discrete degree around fuel beam for guaranteeing the second phase fast and complete combustion. Higher combustion temperature and shorter heat transfer time are generated against traditional combustion reaction, thus NOx and PM emissions are decreased sharply and the second phase burning with high-temperature combustion characteristics is realized.The research of simulation and experiment about Two-stage Combustion mode was conducted on a CA6DL2-35E3 electronic high pressure common rail turbocharged inter-cooled heavy diesel engine. Main work contents and conclusion is as follows:1. Measurement and control test bench was set up. Monitor the engine running parameters and record the measurement results by using high speed sensor and millisecond timescale A/D data acquisition system. Cylinder pressure curve is measured with the magnetoelectric type encoder and DS -9100 combustion analyzer.Use MEXA7200E car exhaust analyzer to collect and analyse engine exhaust concentrations, use AVL439 FSN meter to measure smoke opacity value of engine exhaust. Fuel injection parameters are regulated independently by using development ECU and INCA calibration system, such as injection mode, injection pressure, injection timing.etc.2. Build engine simulation platform. Aimed at this test engine, first use Pro/E to plot engine combustion chamber graphics, second use the self-created CMC -o-grid template which bases on Match piston method form STAR-CD to divide grid, thus calculation model is established for combustion process simulation of combustion engine.3. By adjusting injection parameters the simulated experimental research shows that: According to the intake flow of the engine, appropriately adjust the first stage fuel injection quantity, injection pressure and timing, LTC combustion with cold-flame exothermic stage occurs in compression stroke before BDC.Spraying the second phase fuel into the appropriate temperature, concentration atmosphere area, high temperature burning stage can be realized, which combusts quickly and completely (compared with the traditional combustion way). Select 1650r/min,25% load, two phases injection quantity 1:1,43℃A interval Angle between main injection timing and pilot injection timing, rail pressure and main injection timing are consistent with single phase mode, thus two-stage combustion is realized, NOx and soot emissions are reduced up to 47.8% and 24.6% respectively according to single phase combustion. Reducing NOx and PM emissions potential is testified by using this new burning mode.4. Engine test bench tests were conducted according to simulation conclusion. Use the INCA calibration system and development ECU to adjust injection parameters, explore this topic of new burning mode proposed. Quite a lot of research shows that:choose two-stage injection mode i, when two phases injection interval angle is more than 40°CA, adjust the proportion of injection quantity between two sections, the performance curve of combustion heat release rate has three-peak burning of two stages with the cold-flame exothermic chemical process. In two-stage combustion mode, keep the first stage injection parameters constant, only change main injection timing. CO and HC emission concentration is not influenced by the main injection timing, but NOx emission concentration declines with the main injection timing delay, with smoke opacity maximum value found. So it fails to reduce the NOx and PM emission level simultaneously.In double-stage exothermic mode, change the proportion of injection quantity between two sections, Other fuel injection parameters remain unchanged. NOx and HC emission concentration level is affected slightly, and CO emission concentration and smoke opacity value change dramatically in two-stage combustion model, so adjusting injection quantity scale can be used to optimize CO emission concentration and smoke opacity value.5. The post injection influence on two-stage combustion model has been researched in this paper. Post injection quantity witch comes form main injection quantity is added into two-stage combustion model.The highest average temperature and the maximum breakout pressure in cylinder decrease, but the average temperature of other strokes except combustion stroke in cylinder increases obviously. The NOx and HC emission concentration basically is not influenced by the change of post injection timing. But CO emission concentration increases at first, then decreases,on the contrary smoke opacity value first reduces,then increases. So adjusting post injection timing is a means witch is used to adjust CO and PM emissions.