Study On Control Strategy Of Diesel Engine Under Start Conditions

Diesel engine is widely applied because of its high efficiency, economy, long life and other advantages. The proportion of diesel cars become lager and larger. With the wide use of diesel engine, its pollution became a major problem. The emissions of diesel engine, just like HC and CO, are lesser than gasoline engine, but NOX and PM emissions are high. There is special relationship between NOX and PM, when one decreases the other increases. Because of its features, the start performance of the diesel engine is poor with notable increase in PM and HC caused by misfire. The emissions from start to 40 seconds should be test in EuropeⅣregulations. How to control the emissions in start phase of diesel engine has become a serious problem. In this research, Optimization of the fuel injection control strategy was performed and the effects of auxiliary measures were studied under start conditions.The start process of diesel engine is a complex and transientional process. In this study, a high-speed data acquisition and real-time monitoring system was used to deeply study the combustion mechanism during start phase. The consistency of initial combustion boundary conditions and the synchronization of instantaneous parameter measurement could be created and the fuel injection could be controled exactly with this system under start conditions. Speed synchronous signals, cylinder pressure signals and analysis of the emission samples could be acquired.PID control was adopted to achieve stable idle speed. The experiments of PID parameters choosing showed: If the deviation E and deviation change EC are large, KP and KI shoud take small value to achieve smooth transition. If E and EC are small, the value of KP and KI should be a little bit high, which make sure that the engine responses promptly to the minimum speed variation and stabilizes the speed to the vicinity of the target value quickly. Otherwise, the speed may be unstable or fluctuate. The choice of KP, KI is affected by external conditions such as ambiement temperature and injection fuel. Small KP, KI should be adopted to avoid the speed fluctuating under low temperature and much fuel injection conditions.The fixed value of KP, KI leads to the defects of classic PID control and make the control can not realize online parameter adjustment. It is difficult to achieve the optimized result. So the Fuzzy-PID control, the combination of classic PID and intelligent control, was used. The method of Fuzzy-PID control can make KP, KI realize online parameter adjustment and can greatly enhance its adaptability to external conditions. By using this method the idle speed became very stable and responsed immediately with no fluctuations. In the test, the speed curve was very smooth. Fluctuations in the idle speed were controlled within±5r/min.During the test on different start fuel injection, it was founded that: The engine was hard to ignite in the first cycle by using small fuel injection, but the transition period was very smooth with no misfire cycles; Starting with large fuel injection, the engine could successful ignite in the first cycle, but there were a lot of misfire and incomplete combustion cycles in the transition period; The result was that the emissions was relatively low by using middle fuel injection. However, these control strategies were not optimized and the emissions were high.During the test, the start process should be divided into three stages: beginning period, transition period and stable idle speed period. In order to optimize the performance of emissions, these three satges should be optimized separately.Through the test, the author also found the reasons why the emissions were high by using large fuel delivery. When large fuel injection was used, the maximum speed was very high. When the engine changed into Fuzzy-PID control, its fuel injection changed rapidly. Because of the high-speed, the preparing time for ignition was short. Moreover, due to the fast increasing engine speed, the temperature of the body was very low. This also led to misfire in the transition period. When started with small fuel injection, the maximum speed of the engine was little higher than the idle, the decrease of fuel injection was relatively small and the speed of the engine was very low, so the preparing time for ignition was relatively long. Moreover, due to the slowly increasing engine speed, the temperature of body had increased significouldtly before the engine changing to fuzzy-PID control; this was more conducive to a smooth transition.Different idle speed performance was compared in order to optimize the start process. It was turn out that the emissions were the lowest when the idle speed was 900r/min. Large fuel injection was used to make sure that the engine ignites in the first cycle. How to optimize the transitional period was the key problem. The reasons why the engine misfired in the transitional period were analyzed. Two different control strategies were used to optimize this period: Smooth-dropped fuel injection and ladder-dropped fuel injection. By using smooth-dropped fuel injection strategy, the combustion of start process was improved, but not significant. The reason was that the fuel injection was reduced very slowly and the speed increased fast. It didn't fundamentally remove the shortcomings of the control strategy of large fuel injection directly changing into idle speed fuel injection. The combustion of start process was improved very much by using ladder-dropped fuel injection. It was mainly because that it reduced its fuel injection fast in the beginning of the start process and avoid the engine speed rising fast. Its maximum speed was also very low. The temperature of the body rose to a relatively high level before changing into fuzzy-PID control. It had a long burning time and a good state of combustion. So its emissions were greatlyreduced.Intake air heating and cooling water heating were used to reduce the emissions. By using these two auxiliary measures, the combustion of the engine had been improved dramatically when the engine started; the gas and particulate emissions were low. Intake air heating could raise the temperature of compression end and the fuel was easy to atomize. The improvement to the combustion was direct and rapid with intake air heating. Cooling water heating could raise the temperature of body and lubricants. So it reduced the friction. It could also heat the intake air in the pipe so it could make sure that the engine ignites at the first cycle and reduced the time of warming. In order to reduce the emissions, these two auxiliary measures should be considered, when the temperature is below 20°C.By using supplementary measures such as intake air heating and cooling water heating, the fuel injection of each cycle should be provided about 1~2.5 times than that of idle speed. This can make the engine start quickly with low emissions. If large fuel injection is used to make sure that the first cycle ignites, the turning point of the ladder-drop fuel injection should use this fuel injection too. From this fuel injection, the engine changes into fuzzy-PID control.High idle speed will increase the total amount of particulate emissions per unit time. In this study, when idle speed was below 900r/min, particulate emissions per unit time were low.