Performance Simulation Of Diesel Engine With Miller Cycle Based On Two-Stage Turbocharging

Under the pressures of energy crisis and environmental pollution, the energy saving and emission reduction technology of diesel engine is widely discussed. A Miller cycle changes the intake valve closing angle to reduce the effective compression ratio, resulting in the expansion ratio higher than the compression ratio. It is a high thermal efficiency cycle. With the progress of the turbocharging and Variable Valve Actuation technologies, the development of the Miller cycle gets lots of technical support. This paper has mainly studied the effect of the complementary gas path technology patterns with the Miller cycle and the turbocharging technology on the economy and emission performance of the diesel engine.In this paper, simulation model of a four-cylinder diesel engine was established by AVL Boost software, and performance of the engine under the rated speed with different loads in a Miller cycle was analyzed. With the Miller cycle, the engine with single turbocharger cannot meet the air input requirement under high load condition. Therefore, a two-stage turbocharging system was calculated, the high and low pressure turbochargers were selected, and the two stage turbocharging model was established based on the original diesel engine according to the parameters of these turbochargers. The performance of the engine with Miller cycle by means of early intake valve closing on high, medium and low speed was calculated, the effects on gas exchange, combustion, economy, emission performance with different Miller cycle cases were analyzed, and the optimum intake valve-closing angle of Miller cycle was selected according to the standard of lowest brake specific fuel consumption.The results show that, (1) In external characteristic condition, the diesel engine with two-stage turbocharger gets a better power performance at low speed, however, the power performance at high speed is decreased. In addition, with the two-stage turbocharger system, soot emission reduces at each speed while NOx emission keeps the same. (2) Except condition of the full load speed characteristics in high speed, the diesel engine with two stage turbocharger system in Miller cycle can reduce fuel consumption to some extent. With the advance of the intake valve closing angle, brake specific fuel consumption of the engine decrease firstly and increase later. The fuel consumption decrease more obviously in medium and small load than in high load at each speed. (3) When the early intake valve closing angle is small, NOx emission reduces with the advance of intake valve closing angle, while the soot emission increase a little in this two-stage turbocharging engine. Under all conditions, NOx emission of the optimal case with Miller cycle selected according to the standard of lowest brake specific fuel consumption is lower than the original engine. Soot emission of the optimal case is lower than the original diesel engine under the high speed condition.