| Low temperature combustion(LTC) technology has been widely concerned for its advantages of high efficiency,ultra low NOx and smoke emissions. However, the operation range of LTC is mainly limited in moderate and low load, due to its particulate combustion mode. It is difficult to meet the demand of engine on load range.In this research, a diesel LTC was realized in a single diesel engine by the integrated application of negative valve overlap injection technology, variable valve timing( VVT) and supercharging control stragies. Also, effects of variable valve timing, variable valve lift, intake pressure, and etc on LTC combustion characteristics and emissions were studied. The research on load extension is further conducted by combing internal EGR, boosting and miller cycle technologies. Finally, the study on two combustion mode was conducted so that LTC engine could operate at all load conditions. The fuel injection compensation strategy and EGR control strategy was applied to improve the smoothness of IMEP and CA50 during mode switching process. The main conclusions are as below:Effects of the intake parameters and valve timing on LTC combustion characteristics were studied by one-dimensional simulation. The results show that EGR and effective compression ratio(controlled by the intake valve closing angle IVC) are critical for controlling combustion phase of LTC. Proper exhaust valve closing angle(EVC) is effective for promoting the evaporation of fuel and improving the thermal efficiency. The intake pressure Pin could be used as another key parameter to control the combustion phase of LTC.A fully variable valve mechanism was designed and validated by the experiment in a single diesel engine. Effects of variable valve timing and variable valve lift on LTC combustion characteristics and emissions were investigated by experiment and simulation. The results show that EVC and exhaust valve lift both have great impacts on internal EGR rate. High internal EGR rate is beneficial for enhancing the combustion efficiency and reducing smoke emissions. However, it also causes the advance of combustion phase and increase of NOx emissions. By controlling IVC, effective compression ratio could be reduced, and so the combustion phase was delayed. The reduction of the amount of the intake charge also leads to higher NOx emission, and so boosting should be applied to increase the amout of the fresh charge.Experimental research on load extension by combining internal EGR and boosting means is conducted. The results show that moderate internal EGR is effective in expanding the range of upper and lower limit. Increasing the intake pressure results in decreasing in the maximum pressure rate, so the increase of intake pressure is effective in extending the upper load limit of LTC. Also, the stability of combustion was improved due to the reduction in the trendency of missing fire.At low intake pressure, NOx emission first reached the limit condition during the process of increasing the load. When the excess air ratio exceeds 2.7, NOx emission could be limited less than 100 ppm. At high intake pressure, the maximum pressure rise rate was the major limiting condition. At engine speed 1500r/min, negative valve overlap period 30 oCA, and boost pressure 0.18 MPa, the upper load limit of LTC was increased by 68%.With the introduction of Miller cycle, the maximum IMEPg could be increased to 1.029 MPa by optimizing IVC and intake pressure, increased by 140%.The difference of combustion characteristic for CI and LTC results in large variation of IMEP during CI-LTC direct mode switching. The experiment shows that fuel injection compensation is useful for improving somoothness of IMEP during mode switching process. According to the optimization results, three transition fuel injection cycle and the concave type 2 fuel compensation rate curves was the best for reducing the variation of IMEP. Combustion characteristics between LTC and CI also lead to the large variation in combustion phasing(CA50) during mode switching. It is necessary to introduce EGR strategy to keep the stability of CA50 during the process of switching. EGR response is slower than mode switching response, so the EGR valve action should be ahead of the combustion mode switching. |
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