Theoretical Study Of Advanced Diesel Engine Air System And Its Application In Low Temperature Combustion Process

Today‘s theme of engine technology is energy conservation and emission reduction. Diesel engine low temperature combustion and high-density LTC technology had been proven to achieve ultra-low NOx and soot emissions with high thermal efficiency simultaneously. The development of advanced air system mainly includes layout scheme design, operation parameters calibration, and its control strategy development. Advanced air system not only ensures the engine has appropriate fuel air equivalence ratio, oxygen concentration and exhaust pressure, but also must be coordinated with combustion chamber and fuel system. Therefore, it is very important to understand and master the way to design an advanced air system. In this dissertation, for the purpose of achieving high thermal efficiency, low NOx and soot emissions, key technology of air system design, complicated physical and chemical processes involved in the new concept combustion are further studied by engine experiment, CFD numerical simulation and finally theoretically summerized.The match of two-stage turbocharging system is calculated to select initial turbocharger MAP. We used the GT-Power software to simulate the turbocharger working condition and confirm that the selected turbochargers can avoid the surge area, air-resistor area and make the whole conditions operate in high efficiency area.By the coordination of the turbocharger system, RIVCT and fuel control strategy, we apply the RIVCT under the whole operations. The exhaust gas driving power decreases due to using the RIVCT. However, the increased compressor efficiency would result in the reduction of required driving power at the same time. As it turns out, the intake pressure would increase slightly or remain the same. The fuel mixed more homogenously with the charge air because of a longer ignition delay caused by RIVCT which can lessen the effective compression ratio and temperature during the compression stroke.Experimental study was performed to compare the effects of high- and low-pressure exhaust gas recirculation loops on thermal efficiency and emissions. As high- and low-pressure EGR loop have different impact on the efficiency of air system, it is concluded that in low speed condition, because the exhaust energy is insufficient for the turbocharger to boost available air, the LP EGR loop has the advantgy obtaining high thermal efficiency and low emissions. But in high speed condition, the exhaust energy is usually relatively over high, using HP EGR loop will get high thermal efficiency and low emissions due to reduction of exhaust pressure.Under steady-state operations, variations in cycle-to-cycle and cylinder-to-cylinder combustion were found to have negligible effects on overall engine performance. However, during transient operations, we found significant improvements in performance by improving EGR mixing length. This dissertation studies the EGR mixing length and RIVCT on non-uniform EGR rate, equivalent ratio, heat release rate, transient response and smoke. It is found that the key to optimize the response performance and smoke is making equivalent ratio increase gradually and keep being under the critical equivalent ratio.At the same time, we put forward the theory named the six lowest principles. It is found that there exist the critical point which can be called minimum charge density when the overall performance of the engine changes with the charge density increasing. The minimum charge density coupling with fuel injection strategy would achieve the lowest fuel consumption and minimum exhaust energy, with the HD-LTC combustion theory applied. Reasonable control inter-cooled intensity, pressure ratio distribution and turbocharger efficiency make the minimum exhaust energy meet the lowest driving power.