The Simulation Analysis Of The Two-Stage Combustion Mode On Diesel Engine

To adapt to the more and more stringent emission regulations, all over the world is inthe study of new combustion concepts on diesel engine, there are several new combustionconcepts which are being focused on at present: Low temperature lean combustion such asHCCI and PCCI; Low temperature combustion with the equivalence ratio near Φ=1such asLTC; High temperature lean combustion such as UNIBUS. And all these new combustionconcepts are using two methods to change traditional combustion mode:1.control thecombustion temperature by EGR;2. solve the problem of diesel hard to evaporate byreinforcing the mixture.Therefore, the main methods to change the traditional combustionmode is to adjust EGR and Injection parameters which are very important to combustion，and this method is very affective to reduce the NO_x and SOOT emission at the same time.But all these new combustion concepts are just suitable to small-medium load, so thisresearch proposes such a combustion strategy: first of all, achieve the first-stage lowtemperature combustion by controlling the combustion atmosphere and the mixtureformation strength, this method can made the combustion happened near the lowtemperature area, so it can pass the “two peninsula” area to achieve ultra-low emissioncombustion; then, in order to expand the load range,it will be coupled with a second-stagecombustion after the low temperature combustion, and this second-stage combustion willhappen out of the SOOT peninsula due to the very rich mixture, so this two-stagecombustion mode will expand the load range on the basis of ultra-low emission.To explore and analysis this two-stage combustion mode, a lot of numericalsimulations were carried out on CA6DL2-E3diesel engine in this paper. First of all,simulate the low-temperature stage and the second stage combustion respectively, thenadjust the coupling strategy according to the variation of the boundary parameters, finallyachieve the coupling of the two-stage combustion.The main work and conclusions were summarized as below: 1. Simulation platform is set up. Commercial code STAR-CD is used to simulate in thispaper, and all the boundary parameters setting and the confirm of the models and algorithmsare according to CA6DL2-35E3diesel engine.2. Choose the operation point of1650r/min、120Mpa and40mg/cys as a basis simulationpoint according to the test data. Change the combustion path by adjust the EGR rate(i.e.10%、20%、30%、40%and50%) and injection timing（i.e.6°CA BTDC、15°CA BTDC、20°CA BTDC、30°CA BTDC and40°CA BTDC）, finally take the point of EGR rate40%、injection timing30°CA BTDC as the first stage combustion simulation point. The NO_x andSOOT emission can be reduced to78%and64%respectively.3. Simulate the atmosphere after the first stage combustion and make it as the initialconditions of the second stage combustion. The results show that there are almost no NOxin the second stage combustion and the SOOT emission is just as much as40%of thetraditional combustion. So it can sure that the second stage combustion is feasible toexpand the load range while keep the ultra-low emission characteristics.4. Keeping the total amount of fuel constant and changing the fuel amount of each stage to40-24、44-20、48-16and52-12, couple the two stage combustion by adjusting the injectionlaw. It turns out that the CO and HC emissions in the two stage combustion mode willincrease greatly; almost all the NO_x emissions are produced in the first stage and the SOOTemission mainly due to second stage; it turns out that the D4(52-12) point can expand theload range effectively while reducing the NO_x and SOOT to58%and65%at the sametime.5. Analyze the traditional combustion mode and the two-stage combustion mode by usingthe Ф-T principle diagram, and the process from fuel evaporating to combustion end can beseen very well in this picture. Sum up the area of the two-stage combustion mode in theФ-T principle diagram by analyzing the combustion path.