High Density-Low Temperature Combustion in Diesel Engine Based on Technologies of Variable Boost Pressure and Intake Valve Timing

A concept of high density-low temperature combustion (HD-LTC) is put forward in this paper, showing potential of its high thermal efficiency and very low engine-out emissions by engine experimental and CFD modeling study. A single cylinder test engine has been built-up equipped with mechanisms of variable boost pressure and intake valve closing timing (IVCT). By delaying IVCT and raising boost pressure to certain values according to engine loads, the in-cylinder charge density is regulated much higher than in conventional engines. It is found that the high charge density can play the role of rising of heat capacity as exhaust gas recirculation (EGR) does. Thereby low temperature combustion is realized with less EGR (about 18~19% oxygen concentration) to achieve very low NOx and soot emissions, which is extremely important at high and full loads. In addition, high density of charge promotes fuel air mixing rate so as to promote burning rate, especially, in the later phase of engine combustion, which leads to higher efficiency. However, effects of charge density on NOx emissions are complex due to its two-fold effects on combustion process: one is increasing charge heat capacity and inhibiting temperature rising, another is promoting mixing rate and burning rate and then increasing temperature rate.