Study On Effect Of Hydrogen On Cyclic Variations In A Lean-burn Gasoline Engine

With the energy crisis and environmental problem becoming more remarkable, energy saving and emission reduction is the goal for global auto industry, which can be achieved by lean-burn technology. When the gasoline engine uses this technology, the thermal efficiency will be improved, fuel economy performance will be better and exhaust emissions will decrease. But the cyclic variations in spark-ignition engine restrict the level of lean burn, which would destroy the economy and emissions and go against our goal. In this case, the key to taking advantage of lean-burn technology is to control the level of cyclic variations in spark-ignition engine.Based on the Project from National Natural Science Foundation ―Study on Energy-saving Mechanism of HEV Equipped with Lean-burn Gasoline Engine with Hydrogen Direct Injection System and with Local Hydrogenous Mixture‖, this paper studies the effect of hydrogen on the cyclic variations in a lean-burn gasoline engine.The content and main conclusions of this paper are as follows:(1)There are three ways to set the ignition timing in the study of cyclic variations, namely the fixed ignition timing, the minimal coefficient of variation ignition timing and the minimum spark advance for best torque ignition timing(MBT), which have been compared in two fuel models. At last, we choose the MBT ignition timing for both models considering all aspects of this study.(2)The cyclic variations in a lean-burn gasoline engine can be lowered by hydrogen addition without negative influence on power performance and exhaust emission, which proves that this way works well.(3) In this study, the combustion duration is defined by the mass of fuel burned. The corresponding relationship between cyclic variations and combustion duration is established by comparing them under the same operating condition. When the combustion duration gets longer, the cyclic variations get more obvious. In this experiment, we find that the 0-10% mass of fuel burned combustion duration have a great influence on the cyclic variations. If only the 0-10% mass of fuel burned combustion duration is under 33°CA, the engine will run steadily.(4) As for the effect of hydrogen on the rules of cyclic variations varying with the ignition timing, we find that the rules of 0-10% MFB combustion duration and 10-90% MFB combustion duration varying with the ignition timing have not changed after hydrogen addition, but the combustion durations are shortened. The rules of cyclic variations varying with the ignition timing remain the same as before, but the ignition timing for the minimal coefficient of variation is delayed after hydrogen addition.(5)As for the effect of hydrogen on the rules of cyclic variations varying with the excess air ratio,we find that no matter if the hydrogen is added, the 0-10% MFB combustion duration and 10-90% MFB combustion duration get longer when the excess air ratio gets bigger,and the cyclic variations get more obvious when the excess air ratio gets bigger. But the coefficient of variation of the indicated mean effective pressure(Co VIMEP) decreases significantly when the hydrogen is added, especially meanwhile the excess air ratio is big, which proves that the limit of lean-burn gasoline engine can be broadened by hydrogen addition.(6) As for the effect of hydrogen on the rules of cyclic variations varying with the throttle percentage, we find that no matter if the hydrogen is added, the 0-10% MFB combustion duration and 10-90% MFB combustion duration get shorter when the throttle percentage gets bigger, and the 0-10% MFB combustion duration gets shorter than 33°CA after hydrogen addition. As a result, the rules of cyclic variations varying with the throttle percentage have been changed. Instead of dropping greatly with the throttle percentage grows, the Co VIMEP remains at a lower level and changes in a small range. When the excess air ratio is bigger, the rule reappears.(7) As for the effect of hydrogen on the rules of cyclic variations varying with the engine speed, we find that the rules of 0-10% MFB combustion duration and 10-90% MFB combustion duration varying with the engine speed have not changed after hydrogen addition. When the throttle percentage is bigger and the excess air ratio is smaller, the rules of cyclic variations varying with the engine speed remain the same after hydrogen addition. When the throttle percentage is smaller or the excess air ratio is bigger, the rules of cyclic variations varying with the engine speed have been changed by hydrogen addition, which enhances the adaptability of cyclic variations to the engine speed, so the Co VIMEP remains the same when the engine speed changes.In a word, hydrogen addition changes the rules of cyclic variations varying with the engine operating parameters. The cyclic variations of lean-burn gasoline engine will be improved by hydrogen addition while the other performances are as well as before, which makes it possible that gasoline engine can take full advantage of lean-burn technology to realize the goal.