Key Technologies Of Equivalent Combustion Natural Gas Engine And Its Application

The definition of equivalent combustion is the combustion of fuel and air in the engine cylinder in chemical equivalent.The system that equipped with a three-way catalytic converter could release extremely low pollutant emissions,but compared with lighted natural gas engine on commercial vehicles,it confronts a heavier heat load and poorer reliability.Relying on the National High-Tech Research and Development Plan（863 Plan）and Chongqing’s Key Industry Common Key Technology Innovation Special Key R&D Projects,a certain type of diesel engine model is applied to develop a gas engine that meets the ChinaⅥemission standards.The thesis is an in-depth study of the key technologies of gas equivalent combustion engines,in particular the thermal overload problem and emission performance.The topic is proved to be both meaningful in theoretical and applicable in engineering fields.Based on the initial prototype solution for the developed commercial vehicle natural gas engine,the Boost and Fire models of engine thermodynamics development and simulation are established.The models are used to predict the performance and emissions of natural gas engines;to analyze the impact of compression ratio and valve phase on engine torque,gas consumption rate,HC and NOx emissions;to consider the influence of the shape of combustion chamber on the power and heat release rate of the engine.On this basis,the engine cylinder head,piston,camshaft overlap angle,ignition system and air supply system are designed and optimized.Besides,the temperature field of the piston is simulated and analyzed,then the results are compared with the actual measurements to confirm the accuracy of the model.To solve the problem of high engine heat load for EGR-free equivalent combustion engine,the piston is optimized with an oil cooling cavity structure and the cylinder head is made with a double-layer water jacket structure,which improves cooling efficiency greatly.Software and hardware of the electronic control unit ECU are developed.In terms of hardware,the modular hardware design idea is adopted,the power module,microprocessor module,signal acquisition processing,power driver,communication interface and other modules are designed in detail,and they provide the basis for the implementation of the equivalent combustion control strategy.The stiffener heat dissipation design is applied in the housing,and sealing groove design is applied at the joint of the connector to solve the heat dissipation and waterproof problems of the ECU.In terms of software,a closed-loop electronic control system for natural gas engines is constructed,which can precisely control and adjust the air intake value,natural gas injection value,and ignition advance angle.The air volume is estimated based on the orifice flow equation,and the actual measured value of the pressure sensor of the intake manifold is used to conduct close-loop correction on the virtual manifold pressure of the inflation model.The partial pressure effect of CNG inert gas on manifold pressure is also considered,which guarantees the precise measurement of intake air volume.Finally,the closed loop design of the linear oxygen sensor ensures that the transient air-fuel ratio control window is kept within the optimal conversion efficiency window of catalyst,achieving the goal where warm-up emissions would be almost zero.A small sample simulation test was carried out on the special three-way catalytic converter matching natural gas engine.The light-off characteristics,high-efficiency conversion window and NH₃ generation pattern of the catalyst are discussed and analyzed.The results of matching test show that different airspeeds have a greater influence on the light-off temperature of CH₄ and NOx,and a smaller influence on that of CO.For the pre-stage catalyst,whether it is fresh or aged,the catalyst performance of the Pt/Pd/Rh scheme is better than that of the Pd/Rh scheme at the same precious metal content,including light-off performance andλwindow performance.During the light-off test,the temperature range of the NH₃ formation is between 150-300℃.After the catalyst is aged,the NH₃ formation peak shifts to high temperature range.The calibration and performance tests of natural gas engines are carried out respectively in the paper.The effects of ignition advance angle on the maximum explosion pressure of the engine and emissions are analyzed.The emission and reliability tests of natural gas engine are carried out.The results indicate that the engine can run reliably without knock under the EGR-free condition,with the structural innovation of piston and cylinder head.The emissions of CO,NMHC,CH₄,NOx,PM,PN and NH₃ of the engine are 16.13%,3.38%,10.38%,28.11%,12.5%,25.17%and 58.70%of these limits in the ChinaⅥemission standards,reaching the leading level in China.The engine bench matching test of three-way catalytic converter is carried out and the conversion efficiency of CO,CH₄ and NOx reaches 98.06%,97.05%and 98.84%respectively,with engine hot started.The test of NH₃ which a new emission from the ChinaⅥemission standards is conducted.The results show that there is no NH₃ in the original exhaust,and the NH₃ emission in cold start test is lower than that of hot start test in WHTC cycle test.The three-way catalytic converter will produce NH₃ and peak nearly400 ppm.The emission of NH₃ decreases with the dilution of air-fuel ratio of natural gas,the strategy using micro concentration target air-fuel ratio+ASC after treatment solution can meet the emission requirements of new added NH₃ emission limits in the ChinaⅥemission standards.