Study On Smoothness Of Gasoline Engine Direct Start-Stop Process

In recent years, with the rapid development of Chinese automobile industry, automobile industry has become a pillar industry of national economy, but the rapid growth of domestic cars has brought a serious challenge of Chinese energy and environment. Consequently, energy saving and environmental conservation have great significance for promoting the continuable development of Chinese automobile industry and the national economy.As the most important power and machinery, internal combustion engine is widely used in all kinds of fields. In the future for a long time, oil and natural gas will be still the main transport energy .As the machinery with the highest thermal efficiency, internal combustion engine will be still the most important transport power. With the advance of energy conservation requirements and limiting harmful emissions, gasoline engine need not onlysave energy but also reduce emissions.Carbon dioxide mainly comes from engines. 12 percent of atmospheric carbon dioxide comes from vehicles emission, and the carbon dioxide in the exhaust emissions in direct proportion to fuel consumption. In order to reduce CO2 emissions, fuel consumption need to be further reduced. Consequently, applying the direct start-stop technology can effectively reduce CO2 emissions.Comfort of the car is further demanded by people. When vehicle works at some conditions such as start, idle, normal driving and parking, good smoothness and low noise is the quality of judging the quality of modern vehicle. Automobile engine have the main effect on the smoothness and noise quality. With the spread and application of the direct start-stop technology, engine can stop working when vehicle work at idle condition. Because engine need frequent start-stop, uncomfortable vibration can not be overlooked.In this paper, smoothness of direct start-stop process in gasoline engine was studied in Jetta gasoline engine. The impact factors of vibration in the direct start-stop process were found. The preparations of studying smoothness for start-stop technology in GDI enginewere done. The main work and conclusions in this paper were listed as follows:1. The theory of reversal was analyzed when engine stopped. A detection system for engine stopping phase was studied. A basis of first injection for start-stop technology was provided. Ensuring the premise of stable combustion, the first fuel injection made smoothness and emission become very good.2. The theory of running resistance was analyzed in the process of engine start. Mathematical model of resistance torque was built based on cylinder pressure Pgas and crankshaft angularω. Simulation calculation for engine's major running resistance and the total running resistance were done. The resistance torque cylinder pressure was calculated based on the measured cylinder pressure diagram.3. The excitation source of engine vibration was analyzed, and parameters of vibration measure and the evaluation criteria of smoothness were introduced. In this paper, the four-cylinder engine's vibration was analyzed. It was found that centrifugal inertial force, the centrifugal moment of inertia and first band reciprocating inertia force has struck a balance for four-cylinder engine. Only second bands reciprocating inertia force can not be balanced, which is the root causes of engine vertical vibration. The evaluation of engine start-stop smoothness includes both subjective and objective physical evaluation. Objective physical evaluation was studied in the page.4. When engine started, start vibration were produced by both the motor dragging and ignition time. By analyzing the factors of vibration during the start process, effects of different initial position on engine vibration were studied. Starting vibration that caused Different cooling water temperature, staring speed and the first injection pulse width were analyzed. The cause of stopping vibration was analyzed, and the impact of charge obstruct on stopping vibration was studied by experiment. Some specific conclusions are as follows:(1) Air pressure generated by motor's dragging is the force of starting vibration. Time of intake valve closure determines air pressure that generated by motor's dragging. Effect of different piston's position on starting vibration was studied. The experiment results showed that vibration will be reduced even if the piston stop after the intake valve closed. The vibration is the most smallest when the piston stops at 30°CA before top dead center. Heat engine's start is similar to cold engine.(2) Simulation calculation results shows that the engine cooling water temperature has little effect on the resistance moment. It has main effects of frictional resistance moment. As cold engine's start, its biggest resistance moment is 126.82N·m, and its average resistance moment is 19.09N·m. As heat engine's start, its biggest resistance moment is 103.50N·m, the average resistance moment is 6.65N·m. Therefore, the change of cold engine's friction resistance moment is bigger. Engine output torque and overturning moment are same, while their directions are opposite. Changes in frictional resistance moment can make overturning moment fluctuate, which lead to the vibration of the engine bearings. Consequently, when the initial position of piston is same, cold engine's starting vibration are bigger than heatengine's starting vibration.(3) Dragging speed have a certain effect on starting vibration. High-speed start's torque fluctuations are smaller. The speed fluctuation's decrease makes vertical inertial force reduce, which reduce starting vibration. In addition, high-speed start's speed increased rate is higher. Engine speed is up to idle speed (800r/min) in 0.3s, which can quickly neglect both the engine's inherent resonance frequency and DC motor's low-speed resonance zones. So starting vibration reduces clearly.(4) First injection pulse has large effect on starting vibration. Experiment showed that it can't be ignited when first injection pulse width is lower than 9ms. At this time, motor drag's vibration is just starting vibration. At the same speed, with the increase of fuel injection pulse, the amplitude of vibration also increases. At the same fuel injection quantity, with the increase of speed, the ignition delay period increases, which accumulate more combustible mixture. So the pressure rise rate and maximum combustion pressure increase, which make vibration increase.(5) The causes of engine stopping's vibration were analyzed. The effect of charge obstruct on engine stopping's vibration was studied. Charge obstruct can enlarge vibration damping, which reduce greatly the time of free vibration. Engine can neglect quickly resonance zones. Consequently, resonance can be retrained effectively.