| Braking system is one of most important systems in vehicle.Braking performance is related to the life safety of driver.With the improvement of society,the safety of vehicle is becoming more important in China,and a series of regulations have been promulgated to force the vehicle manufacturers to improve the active and passive safety of vehicles at the source.Especially in commercial vehicle,due to its relatively severe using environment,the transportation environment of heavy duty,in addition to the mountainous condition of southwest and QINGHAI-TIBET area and altitude,the brake system can’t meet its use requirements.In recent years,a series of auxiliary braking systems,such as hydraulic retarder braking,exhaust braking,engine in-cylinder braking and other technologies have been applied to commercial vehicles.In particular,the in-cylinder braking technology can meet the demand of heavy duty vehicles for braking performance due to its relatively higher braking power.Therefore,this paper embarks diesel engine air system,turbo system,in-cylinder brake,four stroke/two-stroke conversion path as technologies,studies the single factor and its coupling under the condition of altitude,the performance of the diesel engine braking mechanism,in order to push related technology synergy in the application of heavy-duty diesel engine in-cylinder brake.Based on an 8.4L single-stage turbocharged diesel engine as the research object,this paper conducts the universal characteristic test and emission characteristic test of the original engine,and analyzes the advantages and disadvantages of the original engine.Secondly,the one-dimensional thermodynamic simulation software is used to build and verify the thermodynamic model of the original engine based on its structure and test data.Based on the thermodynamic model of diesel engine,the mechanism of exhaust brake and the characteristics of altitude are analyzed.Secondly,DOE analysis is carried out on the in-cylinder brake profile,a group of in-cylinder brake profile is optimized and designed,and the working process and energy distribution in the cylinder are analyzed and studied in collaboration with the turbo system.Finally,an inlet and exhaust valve driving device is designed,which can working with four stroke working process(combustion condition),two stroke working process(motoring condition)and cylinder deactivation.The main research contents and conclusions are as follows:According to the different altitude working process of the exhaust braking coupling turbo system,the research shows that: at 2200 rpm,compared with the motoring working condition,the pump gas loss pressure PMEP increases by 395% with single-stage turbo exhaust brake condition,and the pump gas loss increases by 198.8% with the two-stage turbo exhaust brake condition.This is the main factor of exhaust brake to improve the braking torque.The brake torque of exhaust brake condition is significantly larger than the motoring torque.Compared with motoring condition,the increasing of single-stage turbo exhaust brake condition is that:1400rpm(134%),1800rpm(150.6%)and 2200rpm(143.7%).The increasing of two-stage turbo exhaust brake conditions is that: 1400rpm(122.5%),1800rpm(125.4%)and 2200rpm(99.8%).Under exhaust brake conditions,PMEP decreases significantly with increasing of altitude,so that the braking power of different turbo systems decreases with the increase of altitude,and exhaust braking still has certain limitations.The study on the altitude of the in-cylinder brake coupled with turbocharge system shows that: GIMEP increases from 3.2bar to 7.2bar and then decreases to 5.2bar as the nominal opening duration moves from before TDC to after TDC.There is a duration and opening time to maximize GIMEP,which is also the key to improve the brake power of diesel engine by in-cylinder brake.Based on DOE analysis,the optimal design of the in-cylinder brake profile makes the maximum brake power reach-167.5k W.Compared with the single-stage turbo system,the brake power at 1400rpm/1800rpm/2200 rpm,in-cylinder brake of the two-stage turbo is increased by 23.1%,17.8% and 26.6% respectively.At 1400rpm/1800rpm/2200 rpm,the matched regulated two-stage turbo system increases the brake power by 61.1%,74% and 77.3%,respectively.At 2200 rpm,the altitude increases from sea level to 4000 m,and the braking power decreases from 167 k W to 127 k W for single-stage turbo system;Two-stage turbo system reduced from 197 k W to 157 k W;The regulated two-stage turbo system is reduced from 278 k W to196 k W.Based on the characteristics of two-stroke in-cylinder brake and four-stroke in-cylinder brake and altitude,an inlet and exhaust valve driving mechanism is designed innovatively,and the model is constructed.the simulation results show that: the designed intake and exhaust valve driving device can work with four-stroke working process(combustion condition),two-stroke working process(motoring)and the cylinder deactivation.Based on the design of the inlet and exhaust valve drive device,the diesel engine inlet and exhaust brake CAM profile and the inlet and exhaust valve brake lift are designed.Compared with four-stroke diesel engine,the NIMEP(net mean indicating pressure,NIMEP)of two-stroke diesel engine is significantly higher than that of the four-stroke diesel engine when the two-stroke diesel engine does work twice within720 degrees: the NIMEP increasing of 14,00 rpm is 28.5%,38.7% and 41.4% respectively.Two-stroke diesel engine has the brake power of 460 k W at 2200 rpm.Especially,the braking torque power of two-stroke diesel engine is significantly higher than that of four-stroke diesel engine,which is 1800rpm(58.9%)and 2200rpm(65.3%).Compared with the four-stroke diesel engine,the NIMEP of the two-stroke diesel engine is relatively higher,and the increasing of the two-stroke diesel engine at 2000m: 1400rpm(43.4%),1800rpm(65.6%)and 2200rpm(79.5%).At 4000 m,the increasing are as follows: 1400rpm(46.5%),1800rpm(74.3%)and 2200rpm(89.5%).At 2200 rpm,as the altitude increases from sea level to 4000 m,the two-stroke braking power decreases from 460 k W to 354 k W,but is still higher than the four-stroke braking power at sea level.Finally,based on the optimized in-cylinder brake profile,the brake power of the two-stage turbo are greatly increased by 1000rpm(647%),1600rpm(460.9%)and 2200rpm(282.7%).At the same time,the regulated two-stage turbo system is matched,it can further improve the power of 1600rpm(669.9%)and 2200rpm(440.2%)at middle-high speed.The inlet and exhaust drive device and the inlet and exhaust brake CAM profile based on the design can further improve the brake power,especially the braking power at high speed of 2200rpm(792.8%). |
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