| Environment and energy are two major thesises of the world. Since the emission legislation is becoming more and more strictly, together with the crisis of resource, it is put forward by people that higher requirement for better engine performance.At the end of last century, mixture fuel supplying LPG mopeds, replaceing of gasoline ones, got a significant progress in energy conservation and limiting emission.Domestic small LPG engines were in the first phase of mixture fuel supply system which replaced gasoline fuel system, which got a significant progress in energy conservation and limiting emission. Currently, the total number of mopeds is considerable. StageⅢemission legislation(nearly equals to EuroⅢemission limits) will come into force in our contry in July 2008. The mopeds will also face the relevant regulations. So it is quite necessary to improve technology for stricter emission legislations.In this paper, research was done on a small air-cooled, single cylinder, LPG mixture engine, with the purpose of optimizing emission and controlling cost. Open-loop controlling of spray timing, spray width and ignition timing was realized by fixing throttle body, nozzle, throttle position sensor, engine temperature sensor and ECU on the engine. Finally, the emission performance was greatly improved, which supplied reference to improving small engine emissions.Utilizing the high-speed real-time data acquisition and controlling system, the effect of engine performance was mainly researched by adjusting fuel injection time, spray volume and ignition advance angle online. Systematically studied the laws of the effect of excess air coefficient and advanced ignition angle on LPG engine emission and power performance. Besides that, optimization strategies were devised depending on loads-mainly controlling CO at low and middle load; controlling NOx ignition strategies at part load and enhancing power as the main target at full load. Matching MAP of spray and ignition were made under steady operation conditions. The purpose of improving power performance and reducing emissions were obtained. Main experiments and conclusions in this paper are showed as follows:1. The extension-excess air coefficient (φα) had great influence on HC emission. Whenφαwas in the range of 0.85~1.1, HC volume emission was never higher than 500×10-6, but it increased significantly whenφαbecame higher or lower.2. NOx emission was increased whenφαwas growing gradually during a mixture concentration range (from 0.8 to 1.2). Whenφαwas about 1.05 the NOX emissions had the highest value and then decreased whenφαwas keeping on increasing. However, the peak value didn't differ at different speeds or loads.3. CO emission was decreased constantly whenφαwas increasing from 0.8 to 1.2.Speed and load did not have great influence on CO emission.4. Along with the increasing ofφα(between 0.8 and 1.2), engine torque increased at first and then reduced succeeding. Whenφαwas about 0.95, the engine got the maximum output torque. Whatever the air fuel mixture became richer or leaner, the performance of the engine would turn out to be worse.5. The ignition advance angle had considerable influence on the engine torque.When the angle was retarded from 35°CA BTDC(camshaft angle before top dead center) , torque increased slightly at first and then reduced gradually. Besides that, the corresponding ignition angle at maximum torque point was becoming bigger when speed became higher. When speed was greater than 6500r/min, the corresponding ignition angle at maximum torque point reached the limit of the engine.6. At all loads and speeds, HC and NOX emission value became higher followed by the increasing of ignition advance angle. The respond of NOX emission was more sensitive.7. The range of start spray width should be controlled between 2300μs and 2700μs.On the promise that the start combustion was reliable, the less the spray width was, the less the peak volume of HC was.8. When the ambient temperature became lower, HC emission turned to be worse while the necessary spray width for reliable start increased.9. The HC emission was greatly influenced by the start voltage. Slightly increase on the voltage would increase the reliability of start greatly in the start process.10. The CO emission was controlled lower than 3.5% after optimization. Compared with the original engine, the CO emission was decreased by 45% averagely. The HC emission was also obviously decreased after matching. The HC emission was decreased by 46% averagely. The dynamic performance of engine was improved at full load and small load. The NOX emissions were reduced by 30% at most and by 11% averagely after optimization.
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