Study Of Diesel Engine Combustion And Emission Under Start Conditions Based On Cycle-by-Cycle Control

The diesel engine predominates in the heavy-duty vehicles because of its high power density, low fuel consumption, and high work reliability. The noise, vibration and emission of diesel are improving gradually with application of the advanced technology, such as turbocharging, high pressure injection and multi-valve configuration and so on and these result in that the diesel engine is more and more popularizing to the cars and the light vehicles. Wide attention has been paid to the diesel engine emissions as the amount of the diesel engine increases, corresponding emission control regulations have been enacted to control the emissions and the standards become more and more stringent.Emissions under the cold start conditions are significant for overall emissions. The combustion of the diesel engine is delayed by the deterioration of the combustion boundary conditions in the cold start that results in the incomplete combustion or a particular misfire cycle, leading to the high emissions. The PM emission rises in the cold start in addition to the expected HC and CO emission increase. Study on the combustion optimization and emission control in cold start is definitely important to realize extra-low emission from the diesel engine.A control and measure system was developed in this paper. The experimental studies of ignition, combustion and emissions in the cold start were carried out based on cycle and process control. The effects of the combustion boundary conditions on the combustion and emissions were determined and the corresponding optimization strategy was put forward.The main achievements are as follows.1. A transient measure and control system was developed based on cycle-by-cycle control and sample according to the features of the diesel engine cold start. The consistency of initial combustion boundary conditions and the synchronization of instantaneous parameter measurement were ensured and the cycle-by-cycle emissions measurement was realized.An electronic control system was developed with the modularized design of the control task, which ensures the accurate control and measurement of the fuel parameter, the auxiliary gas injection and the single-cycle sample. The optimized control on the idle speed stability was achieved under different speed conditions through fuzzy PID controller with online parameter adjustment, providing a requisite guarantee to investigate the process from start to idle.2. The optimization tests of the control strategy of the diesel engine idle speed stability showed that the Fuzzy PID control had obvious advantages against the classic PID control. It can be applied to the diesel engine idle stability control. With the Fuzzy control strategy, the speed could be smoothly adjusted to the target speed within 30 cycles no matter what the fuel amount was adopted to start diesel engine under low temperature, and the idle speed fluctuation range was no more than±5r/min.The fuzzy PID control rules were achieved through the diesel idle control tests as follows: when the speed is near the idle target, the value of KP and KI should be a little bit high, which ensures the engine responses promptly to the minimum speed variation and stabilize the speed to the vicinity of the target value quickly. When the speed overshoot is large, KP and KI take smaller values, leading to reduction of the input change rate, so that the engine speed can converge slowly to the target and doesn't oscillate.3. The analysis on the historiess of the combustion and emissions during in the start process of the diesel engine showed that there are four typical in-cylinder pressure trace patterns of diesel engine under start conditions. They represent different combustion states and have different effects on the instantaneous speed and their emissions have different typical behaviors. The child-mother peak in-cylinder pressure trace represents normal combustion and makes a great contribution to the instantaneous speed. Both the twin peak in-cylinder pressure trace and the child-pregnancy peak in-cylinder pressure trace represent incomplete combustion. The CO and NOx emissions are little in the operation with the twin peak in-cylinder pressure trace and it makes the instantaneous speed increase. There are little CO and no NOx in the emissions under operation with the child-pregnancy peak in-cylinder pressure trace and it makes no contribution to the speed building-up. The singleton peak in-cylinder pressure trace represents the misfired combustion. It is useless to the engine start. The emissions under the operation of the singleton peak in-cylinder pressure trace only include high HC.The whole start process of the diesel engine may be divided into three phases: the initial, the transitional, and the steady period. The transition period during the cold start of the diesel engine is long and may last more than 70 cycles, but it shortens during the warm start and the engine may stable within about 10 operation cycles, then transforms to the steady idle period. Compared with the results during the warm start, the number of the misfire and incomplete combustion cycles during the cold start is higher which results in the deterioration of HC and CO emissions obviously. The HC emission during the cold start reaches 5-7 times of HC emissions during the warm start, and the CO emission may reach one or two orders of magnitude higher. The results showed that the transitional period is the key to optimize the cold start control of the diesel engine.4. Effects of coolant temperature on combustion and emission of the first cycle and the transitional cycle during the cold start of the diesel engine were investigated and the results show that the ignition and combustion of the first cycle are still unsteady even with high fuel supply (80mg/cycle) when the coolant temperature as low as 10℃. When the coolant temperature exceedes 20℃and the ignition succeeds in the first fuel injection cycle and the combustion becomes stable.When the coolant temperature is 10℃, because of much fuel injected, though there is ignition, the HC emission is high. When the temperature exceedes 20℃the CO and HC emissions decrease greatly, but the HC volume fraction still exceedes 400×10^-6 and the CO exceedes 0.9%.The analysis of combustion and emission from 9th to 13th cycle within the transitional period were conducted when the medium fuel (35mg/cycle) was injected. The results showed that when the coolant temperature is 20℃, the ignition delay is long, the combustion is unstable, the HC and CO emissions are 3 times higher than the emissions at 40℃, and the NOx emission is low because of the low combustion temperature. When the coolant temperature exceedes 30℃, the combustion stability is improved greatly, the ignition delay shortens, the peak in-cylinder pressure increases, HC and CO emissions decrease and the NOx emission increases greatly. The HC volume fraction is lower tan 100×10^-6 and the CO is lower than 0.07%.Compared with the condition with high fuel injection, the combustion of the transitional period is improved and the HC and CO emissions from the firing cycle decrease greatly when the injected fuel decreases. The maximum HC drops by a factor more than 3 times and the CO drops is up to one order of magnitude when the coolant temperature exceedes 30℃.5. The effects of the fuel injection per cycle on the first cycle during start were investigated. The results showed that extra lean or extra thick mixture would result in the deterioration of the HC and CO emissions. There is an optimum fuel injection per cycle to realize the minimum HC and CO emissions and the air/fuel ratio 22 (50mg/cycle) was found in this paper.6. Optimization of the fuel injection control strategy under start conditions was performed.When the diesel engine is started under low temperature, the control strategy to let the engine start with a certain constant fuel injection, there may be misfire cycles during the initial and transitional periods. Therefore, an optimized control strategy was put forward, namely, the engine starts with high fuel injection and then decreases step by step the fuel injection during the transitional period until it operates at idle condition. This strategy was validated to decrease significantly the misfire cycles.The start condition is improved when the intake air and coolant are preheated. The start is fast and the emissions in the start period are low when the fuel injection is as much as 1.5².5 times of fuel injected in the stable idle operation. If the control strategy to let the engine start with high fuel injection and then enter into idle with fuel stepping down during the transitional period is adopted, the fuel injection at the final step before entering idle operation control should be set to the same scope.7. Studies of the combustion and emission behaviors during the whole process of the diesel engine start were carried out. The results showed that when the coolant temperature is higher than 20℃, there is a clearly falling trend of HC, CO and particulate emissions. The combustion stability is significantly improved. Therefore, when the coolant temperature is below 20℃, the auxiliary measures such as coolant and intake air heating should be considered to optimize the combustion and reduce the emissions during start of the diesel engine.There is an obvious peak of the HC and CO emissions in the engine start under different thermal parameters, and then it slowly declining vs. time. The HC and CO emissions are lowest under the conditions of intake preheat and the peak of HC emission scaled by volume fraction is less than 80×10^-6. At first 20s during start, CO emission at 10℃is 4 times of that at 40℃and 8 times of that with the intake air heating at 50℃.A peak of the NOx emission appears at different coolant temperatures, but it declines sharply then presents an upward trend. The peak NOx rises with the increase of the coolant temperature and the NOx drop amplitude after the peak decrease with the increase of the coolant temperature. Applying intake air heating, the peak of the NOx may reach 1.5 times of that the coolant temperature at 40℃. And it dose not drop immediately, but remaines roughly unchanged.The PM emission decreases greatly as the coolant temperature raises when the coolant temperature is 10℃, the total PM emission within the first 40s is twice as much as in 40℃and this is almost 4 times of the PM emission when the intake air is preheated.8. The experiments concerning the auxiliary gas injection illustrated that the auxiliary gas should have good volatility and ignition property to improve the diesel cold start.The proper control on the ether amount injected into intake port can eliminate the misfire and incomplete combustion cycle completely and improve the combustion stability in the diesel engine start. Too much ether injected results in the ignition occurring before the TDC and the large compress negative work because of too early combustion. Tests showed that the misfire is eliminated and the combustion is improved greatly when the injected ether volume fraction is 3000×10^-6. The drop of PM emission within the cold start exceeds a half at the ambient temperature is 12℃.Propane injection not only appears no positive effect on the ignition during the start, but also deteriorates the combustion stability. The HC emission increases proportional with the amount of propane injected.The main innovational points in this paper are listed as follows.1. In this study, a control and measure system was developed based on cycle-by-cycle control and sample. The system mainly includes instantaneous speed and in-cylinder pressure measurement system, the single-cycle exhaust sampling system, electronic fuel injection system, combustion boundary active control system. The consistency of the initial combustion boundary conditions is ensured to realize the start operation control and the synchronization of the real-time high-speed multi-parameter data acquisition is realized. The developted single-cycle sample system resolves the transient and single-cycle emission measurement during the start of the diesel engine. With the control and measure system the investigation can be performed to study the histories of the combustion and emissions in the start process of the diesel engine and it overcomes the obstacle in the way of measuring the instantaneous parameters.2. Two methods based on the cycle-by-cycle control and on the whole process were presented to study the performance of the diesel engine under start conditions. The combustion and emission behaviors of single or several cycles during different start phases could be investigated through the cycle-by-cycle study method with the control and measure system. The mechanism of the variations of combustion and emission during the start could be revealed. There are four typical in-cylinder pressure traces when the whole process study method was adopted. The diesel engine start process was divided into three phases to analyze the combustion and emissions within the process. The cycle-by-cycle study method emphasizes to reveal the mechanism and optimize the control strategy and the process study method emphasizes the macroscopical trend and the overall effect.3. The optimized fuel injection control strategy under start conditions were put forward.When the diesel engine is started under low temperature, the control strategy to let the engine start with a certain constant fuel injection, there may be misfire cycles during the initial and transitional periods. Therefore, an optimized control strategy was put forward, namely, the engine starts with high fuel injection and then decreases step by step the fuel injection during the transitional period until it operates at idle condition. This strategy was validated to decrease significantly the misfire cycles.The start condition is improved when the intake air and coolant are preheated. The start is fast and the emissions in the start period are low when the fuel injection is as much as 1.5².5 times of fuel injected in the stable idle operation. If the control strategy to let the engine start with high fuel injection and then enter into idle with fuel stepping down during the transitional period is adopted, the fuel injection at the final step before entering idle operation control should be set to the same scope.