| Small non-road spark igniton engines are large capacity, wide use and low price,the global production is more than 50 million units per year.China’s annual production is about 22 million units and more than 80% for export. The advanced technologies are difficult to be used because of the constrains by the structural characteristics, the use and the low price of the small non-road spark ignition engines, that more than 95% are still using the carburetor. In this paper,the domestic large production,four-stroke air-cooled 168F is the research prototype.The researching work includes optimization of the inlet and exhausting port,model and calculation of the two-pahse flow in the carburetor,low emission fuel delivery characteristics and study of the combustion process.In the case of not using the catalyst,the improved 168F is expected to meet the EPA’s phase 3,the future EU’s "Stave V" emissions regulations and the requirement of the deterioration by the coordination of the "quantiy" and "time".The steady flow testing bench was designed.The intake and exhausting port were tested,the maximum flow coefficient of the intake and exhausting port is 0.488 and 0.508 respectively .The 3-D computation software FIRE was the tool for researching the effects of the valve lift,the shape of the port,the deepth and the inclination angle of the port on the flow properties.Based on the simulation results,the shape,deepth and draft angle were optimized.The influence of different casting technology on the design of the port was analyzed and the evaluation value of the port of the smll non-road SI engine was given.The optimized cylinder head was tested,the results showed that the flow coefficent of the intake increased by 18.27% and the flow coefficent of the exhausting increased by 10.69% at the maximum valve lift.The rated power increased by 6.22%and the specific fuel consumption decreased by 3.08%.The CO and HC specific emissions decreased by 10.78% and 14.37% respectively,NOx increased by 9.37%,HC+NOx decreased by 5.77%.Experimental study combined with theoretical analysis,the low emission fuel delivery characteristics to controlling the emissions were defined that the rich mixture was needed under the heavy load to inhibiting the formation of NOx,the lean mixture was needed under the medium and light loads to decreasing the emissions of HC,the rich mixture was needed under idle for maintaining a stable operation to reducing the formation of HC.To studying the factors for the two-phase flow and fuel delivery characteristics of the carburetor,the computation model of a circular tube was established.Based on the model of a circular tube,the computation models of main and idle fuel delivery system of the carburetor were extablished and the calculation program was wrote by the mathematical software Matlab.The calculation results showed that the fuel injection period continued to the expansion stroke because of the inertia of the fuel,the structural parameters of the main fule delivey system had the great influence on the concentration of the mixture under the medium and full loads and very small influence on the the concentration of the mixture under the light loads and idle mode,the structural parameters of the idle fule delivey system had no influence on the concentration of the mixture under the medium and full loads,the air-fuel two-pahse flow was very important to promoting the broken and atomization of the fuel due to the air from the air bleed tube.The structural parameters of the carburetor were optimized for reducing the emissions.The test results showed that the optimized carburetor had a good consistent with the low emission fuel delivery characteristics.The emissions results showed that the NOx emissions decreased by 32.07%,CO decreased by 11.64% and HC decreased by 15.63%,the specific emissions of the engine could meet the EPA pahse 3 emissions regulation.But there is not enough margin of deterioration for the HC+NOx which need to be further reduced.The pressures inside the cylider were measured,then the heat relase rate and temperature were calculated with adjusting the ignition advanced angle under the rated load.The experimental results showed that the compression stroke would consume more power when the ignition angle was advanced too much due to the higher pressure before the top dead center,the expansion stroke would loss more power when the ignition angle was retarded too much due to the late combustion.The emissions,brake power and fuel consumption were measured too.Combining analysis of the working process and the performances,the best ignition advanced angle was-22℃A and 3℃was retarded compared to the origin ignition advanced angle.The CO specific emission was 249.8 g/kW·h ,HC+NOx was 8.54 g/kW·h with -22℃A ignition advanced angle.The emissions from the optimized 168F could meet the EPA phase 3 and EU "Stage V"implemented in 2019 emissions regulations and have enough margin for deterioration. The researching works have achieved the expected results. |
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