In this research, the effect of variable valve timing and lift was studied to improve thermal efficiency of a diesel engine, while maintaining low engine-out emissions. At high load conditions, early closing of one intake valve to the other or early intake valve opening realizes an enhancement of actual swirl motion without increased pumping losses, and retarded intake valve closing reduces the effective compression ratio, which both result in an increase of EGR ratio and an advanced fuel injection timing. Consequently low NO_x formation and an improved thermal efficiency can be achieved simultaneously. At low load conditions, heat release is small and fuel is dispersed with swirl because of the small fuel injection quantity, therefore increasing the effective compression ratio by early intake valve closing is effective to reduce HC. As mentioned above, variable valve system can meet the requirements from combustion at various engine operating conditions, and, as a result, achieved 40% engine-out NO_x reduction and 4% fuel consumption reduction in the NEDC. Furthermore, low-end torque could be increased by 40%, utilizing exhaust pressure pulsation by matching of exhaust valve opening timing, and overlap of intake and exhaust valve opening around TDC of intake stroke. To realize above effects, a new piston chamber with deep valve pockets was developed as well.
展开▼
