Simulation Research On Diesel Engine Braking Performance Based On Fully Hydraulic Variable Valve System

With the improvement of the load capacity of commercial vehicles and the promulgation of regulations on auxiliary brake,the installation of an auxiliary braking system independent of the service brake has become a standard configuration for heavy commercial vehicles.Engine in-cylinder braking technology has stable braking effect and high braking efficiency,which has gradually become a research hotspot in the field of auxiliary brake.The paper uses the Fully Hydraulic Variable Valve System(FHVVS)independently designed by the group to realize Hydraulic Variable Valve Compression Release(HVVCR)brake and Hydraulic Variable Valve Two Stroke(HVVTS)brake,and explored their braking performance through simulation.The main work are as follows:(1)The construction of a six-cylinder diesel engine simulation model with GTPower software and the verification of model by experimental data.According to the structural parameters and technical parameters of the test prototype,the simulation model of the diesel engine is built and the parameter setting in the model is completed.The deviation between the test data and the simulation data is less than 5%,which prove that the simulation model is accurate and suitable for research of braking performance.(2)The overall structure and working principle of FHVVS are introduced,and the test bench of valve motion law is built.Because the test prototype is a single overhead camshaft engine,and the intake cams and exhaust cams are on the same camshaft and located on one side of the cylinder head,the intake valve and exhaust valve can be controlled at the same time.By the test bench,the function of FHVVS can be verified,and it is convenient to measure the law of valve movement under braking conditions.(3)The performance of HVVCR brake was explored.There is optimal opening time and maximum lift of decompression exhaust valve at each speed to maximize the braking power.The opening time and maximum lift of the decompression exhaust valve should be increased appropriately at high speed.Improving the geometric compression ratio and intake pressure of the diesel engine can increase the braking power.The HVVCR brake adopts two exhaust valve brake and the maximum cylinder pressure during braking process is only about 50%of single exhaust valve brake,which has the advantages of stability and reliability.FHVVS only needs an cycle to complete the transition from normal conditions to braking conditions under the speed of 1900r/min,which has the advantage of fast response.(4)The performance of HVVTS brake was explored.By adding two circular orifices with the diameter of 1.8mm on the exhaust valve sleeve,the HVVTS brake is realized.The intake valve lift and exhaust valve lift of the braking condition was measured by the test bench.The reopening of the intake valve can increase the braking power of the HVVTS brake and significantly reduce the instantaneous maximum temperature in the cylinder during the braking process.The braking power is increased by 18.3%and the instantaneous maximum temperature in the cylinder is reduced by 16.6%at the speed of 2100r/min.The HVVTS brake has the characteristics of high braking power at low speed and low braking pressure at high speed.The braking power under the speed of 1100r/min is 55%higher than the original engine brake and the maximum pressure at the speed of 2100r/min is reduced by 47.2%compared to the original engine brake.