Global Efficiency Optimization Of The Controllable Mechanical Turbo-compounding System Of Vehicle Heavy Duty Diesel Engines

The utilization of pressure energy in the exhaust of internal combustion engine is now becoming an important technology in fuel economy enhancement and emission reduction. Mechanical turbo-compounding system was used in this paper. The power turbine is arranged downstream the charging turbine to further recover the waste pressure energy in the exhaust out of charging turbine. The waste energy is recovered as mechanical power coupled directly to the crankshaft. Considering the low efficiency of power turbine under off-design conditions and the deteriorate influence of power turbine on engine under low load, a controllable mechanical turbo-compounding system was put forward. Power turbine bypass valve and continuously variable transmission were adopted to adjust the operating conditions of charging turbine and power turbine according to operating condition of the engine.GT-SUITE simulation models of engine with controllable mechanical turbo-compounding system and a heavy duty truck were established. The calibration of the model shows that model error of exhaust flow rate, exhaust temperature and engine power is below 4%, 6% and 5% individually. The results indicate that the model is accurate enough to support the investigation work.Firstly, the influence of key design parameters such as flow area of both power turbine and charging turbine, transmission ratio on the global efficiency was studied. Matching strategy of charging turbine, power turbine and transmission ratio was put forward based on the investigation. The highway driving cycle was selected here as the typical driving cycle. The distribution of engine operating condition was firstly analyzed to choose the matching point of power turbine, charging turbine and transmission ratio. The results indicate that the global efficiency increased by 5.4% at rated operating condition and fuel consumption of 100 km decreased by 1.33%.Secondly, continuously variable transmission was used to adjust the rotating speed of power turbine in order to optimize the isentropic efficiency at the off-design point. The effect of bypass valve on the operating points of charging turbine and power turbine was also investigated. The relationship between recovered power and decrement of engine power was studied aiming at uncovering the strategy of bypass valve. The results indicate that the bypass valve should be open under engine low load condition and additional pumping loss can be avoided. On the contrary, the bypass valve should be closed under middle and high load condition.Thirdly, the turbo-lag is an important factor that influences the fuel consumption of engine under driving cycle. The bypass valve of power turbine was used to improve the transient response of intake pressure due to its influence on the outlet pressure of charging turbine. The influence of bypass valve on the intake pressure was firstly investigated and a control strategy of auxiliary boosting using power turbine bypass was put forward to get rid of turbo-lag phenomenon. The results indicate that the fuel consumption of 100 km was reduced by 2% under highway driving cycle and 3.4% under bus driving cycle.