Experimental Study On Combustion Fundamental Theory And Control Strategy Of Diesel Low-temperature Combustion

Low Temperature Combustion (LTC) was considered as a new combustion process and widely investigated in the world. NOx and soot emissions can be simultaneously reduced to extremely low levels because of the lowered combustion temperature by the use of heavy exhaust gas recirculation (EGR). The fundamental theory and combustion control strategy was systematically investigated in the thesis, including the effects of combustion parameters, charge compositions, fuel properties, the coupling of injection strategy and EGR on combustion, performance and emission characteristics.Firstly, the effects of combustion parameters on combustion and emission characteristics were investigated in the diesel engine. The results show that EGR is the most effective method to reduce NOx emissions. As EGR rate increases, NOx emissions can meet emission regulations of different stages. The factors affecting soot is complex. As EGR rate increases, the soot increases slightly before Soot-Bump occurs, while soot decreases sharply after Soot-Bump peaking. With heavy EGR, low NOx and soot can be realized while CO and THC deteriorate significantly and fuel consumption rate increases up to 10-20%. With higher boost and charge density, the soot and NOx emissions can be reduced effectively and the Soot-Bump was observed at a higher EGR rate. Higher injection pressure has little effects on NOx emissions for light and medium EGR, but it can reduce soot emission effectively and improve fuel economy. Therefore the author considers that the medium EGR before Soot-Bump was observed is the future trend for diesel combustion control technology. Employing medium EGR and combustion optimum, the emissions can be reduced without deterioration of the combustion.Then, the emission results with various EGR rates show that the proportion of NO₂ in NOx has the similar trend like soot. NO₂ emission increases firstly as the EGR rates increase, then the NO₂ emission reach the maximum value and the proportion of NO₂ in NOx is over 40% corresponding to the maximum soot emission. Therefore, a further study on the NOx formation mechanism is necessary for LTC. As EGR rate increases, higher molecular weight hydrocarbons emissions reduce, while lower molecular weight hydrocarbons emissions increase especially for methane and formaldehyde at heavy EGR rate. The concentration of particulate matter decreases firstly then increases, and lastly increases again with the increase of EGR rate. The concentration is lowest at the inflection of Soot-Bump corresponding to the 15% oxygen concentration, and it is highest at 12% oxygen concentration. The accumulation diameter begins to increase when the oxygen concentration decreases to 15%. With the further decrease of oxygen concentration to 9.1%, the nucleation diameter begins to increase while accumulation diameter decreases, and the diameter of particulate matter is less than 50 nanometers. So the increase of small diameter particulate is another problem for LTC with a heavy EGR.The mechanism study of EGR on diesel LTC shows that the dilution effects of N2 and CO₂ in EGR are similar to the NOx reduction. That is, the NOx emissions mainly depend on oxygen concentration. N2 and CO₂ have different effects on soot. With the same oxygen concentration, the dilution effects of CO₂ are more effective than that of N2 for the reduction of soot. The main reason is that CO₂ has higher specific heat capacity which leading to a longer ignition delay. The numerical simulation shows that the combustion can not avoid NOx and soot forming regions with the oxygen concentration of 13% for N2. While with the oxygen concentration of 15% for CO₂, the combustion can avoid simultaneously NOx and soot forming regions. Furthermore, the chemical reaction between soot and CO₂ may be another explanation on the great reduction of soot with CO₂ dilution.The effects of fuel properties on combustion and emission characteristics show that diesel-n-butanol fuel and diesel-gasoline blending fuel can reduce soot emission greatly, while have little penalty on CO, THC and NOx emissions. Therefore, a lower cetane number, a higher oxygen conten, and or a better volatility can all effectively reduce soot emission for diesel LTC and even avoid the Soot-Bump. It also shows that the fuel properties should be one of main control parameters for the low temperature combustion.Lastly, the author investigates the coupling control of injection strategies and EGR rates. Based on fuel economy, emissions and mechanical load characteristics, the best CA50 in the research is 4deg.ATDC. With medium EGR rate and CA50 of 4deg.ATDC, proper pilot-injection mass and timing can realize lower NOx and soot emissions, higher thermal efficiency, lower combustion noise without the increase of CO and THC. Meanwhile, post-injection can further reduce soot emission. But the degree of reduction depends on the in-cylinder temperature and mixing rate between air and fuel after post-injection.