Study On Effects Of High Pressure Air Spray On Turbocharged Automotive Diesel Engine

With more attention to protecting the world environment, as one of the worst pollutant, it is urgent for automotives to reduce exhaust emissions and to improve the thermal efficiency. Therefore, it is the chief problem for a diesel engine to reduce pollutants emissions and to improve the thermal efficiency. The use of exhaust-gas turbocharger is one effective method to optimize the performance of the diesel engine because it can remarkably enhance the boost pressure, improve the quality of combustion process and reduce the emissions of the diesel engine. However, the exhaust-gas turbocharger can cause insufficient power at low speed and heavy smoke at transient operations due to its turbo-lag. Especially at low speed and high load operations, the problem of insufficient power is more obviously. To solve this problem, on the conditions of little change and cost, and as much as possible to optimize the performance of the diesel engine, the high pressure air spraying method to the turbine is adopted, which could increase the in-take air intake, thereby, the air-fuel ratio will increase too, then to improve the conditions of in-cylinder combustion process. Therefore, the exhaust emission and the thermal efficiency are improved.A repeatable transient condition control system has been realized by using a MCS51 single chip processor (SCP) as core control element and a step motor as real time actuator. Two kinds of typical transient operations at constant speed and constant torque can be realized by a conventional eddy current dynamometer combined with the developed transient condition control system. After developing the communication function of eddy current dynamometer, air flow-meter, fuel flow-meter and smoke opacity meter the measured data can be taken in analog signal for after treatment. After designing data acquisition program a high frequency A/D (Analog/Digital) converter board labeled as PCL 818HG can convert the analog signals mentioned above to digital signals and feed to computer. So, the performance of a diesel engine under transient operating conditions can be measured by this A/D convert board system. Which supply us a credible hardware system to study effects of high pressure air spray (HPAS) on a tubocharged automotive diesel engine.To research effects of HPAS on performances of a diesel engine well, some nozzles to dose high pressured air were designed. The responsing characterics of the diesel engine's performances parameters under different nozzles were studied. Under low speed and high load operations and transient operations, by analyzing those parameters response characteristics, conclusions could be summarized as follows: 1. At steady operations, there is a workable operation range in which the airinjection method is helpful for improving the performances of the diesel engine.The minmal workable load for decreasing the smoke emission will increase withthe speed increasing under the same nozzle, and will advance with the nozzlediameter increasing under the same speed. At transient operations, it's no benefitfor improving the performances of diesel engine to pre-spray some high pressure air before the diesel engine accelerating. The air injection time confirmed is greatly affecting the performances of diesel engine improved. Especially in the five seconds after the accelerator reaches the top, air injection method can greatly decrease the smoke emission.2. At low speed and high load operations, air injection is good for improving the power performance and smoke emission when the economy is not deteriorated. At the full load of lOOOrpm, the 3.0mm nozzle will increase the torque output about 46Nm, and will decrease the smoke emission about 14 units, that is, compared with the original engine, the torque output increases 7% and the smoke emission decreases 40%. At transient operations, the air injection method could greatly decrease the smoke emission, but it's no obviously effect to increase the power performance and could be neglected. It's same with the economy, and when the load is too low, the effective thermal efficiency decreases about 1% to 2% compared with the original engine.3. At low speed and high load operations, the extent that air injection improves the power output and smoke emission, will increase too when the load of engine increases. It's obviously to improve the power performance and smoke emisson, when the air injection flow is at the range from 27 kg/h to 42kg/h. When load is at the range from 80% to 100%, the top extend of the power and smoke emission improved is commonly in the speed from 900rpm to HOOrpm. At the full load of 900rpm, the 3.0mm nozzle will increase the torque output about 65Nm, or about 11% compared with the original engine, and will decrease the smoke emission about 36 units, or about 51% based on the original engine.4. At the transients in constant speed and increasing torque (CSIT) and the transients in constant torque and increasing speed (CSIT), the three nozzles which respective diameter is 2.5mm, 3.0mm and 3.6mm, and their flow is at the range from 27 kg/h to 42kg/h, are used in the experiments of the engine parameters' response course with the air injection course. From the results, smoke emission greatly decreased which resulted by fresh air intake increased and the exhaust gas diluted. The 3.0mm and 3.6mm nozzles are close in the smoke decreased extent. At the speed of 800rpm and the transient rate of 15s, the top extent of the smoke emission decreased is about 13 units, or about 38% compared with the original engine at the 3.6mm nozzle. At the transients in constant torque and increasing speed, with the speed increased, the rate of load decreases step by step, the original smoke emission is very low, so air injection is little effect on smoke emission decreased.5. At low speed and high load operations, the concentration of NO, and HC in the exhaust gas will much less using the air injection method than the original engine, for the exhaust gas is diluted by the flow of air injection. The extent that air injection decreases the concentration of NO* and HC in the exhaust gas will be decrease with the load increasing. The three nozzles, which diameter is 2.5mm,...