Analysis On Performance And Power Improvement Methods Of Diesel Engine Under High Exhaust Backpressure

Diesel engines are widely used in power plant of various ships due to its advantages of high output power and high thermal efficiency.This kind of diesel engine will work under water for a long time and face the environment of high exhaust backpressure brought by water depth and fluctuating exhaust backpressure brought by sea waves.At this time,various performance parameters such as exhaust temperature and output power will change to different degrees,which will not only threaten the reliability of diesel engine working for a long time,but also make it difficult to meet the power demand of ships.In order to solve this problem,the performance and improvement methods of diesel engine under high exhaust backpressure is studied in this paper.A performance prediction model of turbocharged diesel engine in fluctuating high backpressure was established and calibrated with test data.The variations of engine performances and key parameters under different period of backpressure fluctuation and cycle-average backpressure were investigated.The results show that high backpressure significantly deteriorates engine output,which can be reduced by up to 69% at 1.65 bar,and exhaust temperature rapidly reaching the limitation.Under fluctuating backpressure condition,the performance and operating parameters of the internal combustion engine show hysteresis loops around the quasi-steady performance,and the unsteady hysteresis loops expand when the fluctuating period and cycle-average backpressure decreases.The design of the engine gas system and the difference of sensitivity of turbine pressure ratio to backpressure are the reasons of engine performance hysteresis phenomenon under fluctuating backpressure condition.A variety of power improvement methods for high exhaust backpressure diesel engines are designed based on electrically-assisted turbocharger technology,and the restrictions and power improvement effects of each method are compared and calculated based on the performance prediction model.The results show that the topological structure selection and optimization results of electrically-assisted turbocharger are mainly restricted by the operating range of the main turbocharger.The parallel scheme has the best effect at 1.35 bar backpressure,and the diesel engine power can be restored to 93.1%.When the back pressure increases to 1.65 bar,the series scheme at low-pressure stage can provide enough air intake and the diesel power can be restored to88.1%.For WP7 turbocharged diesel engine,the intake and exhaust pipes and valves were redesigned,a new electronic control system was established and calibration.A high backpressure diesel engine test-bed with advantages of low cost,high safety and easy to be reformed is established.Based on this test-bed,the variations of engine performances and key parameters under different diesel engine speed and the power improvement effect of electrically-assisted turbocharger series scheme at low-pressure stage was tested and studied.The changes of exhaust temperature,air intake,output power,fuel consumption rate and in-cylinder pressure curve are analyzed.The results show that the sensitivity of performance parameters to exhaust backpressure is not affected by diesel engine speed.Exhaust temperature exceeding is the most important factor to restrict the engine output power under high exhaust backpressure.The electrically-assisted turbocharger can greatly increase the air intake of high backpressure diesel engine and reduce the exhaust temperature,so as to realize the diesel engine power recovery under high backpressure.