| A largely-produced and widely-used non-road small spark-ignition engine is investigated in this paper. The paper makes a study of the influence of squish combustion chamber on the performance and exhaust emissions of the engine.Firstly, different squish combustion chamber schemes are contrived based on the prototype,then CFD software is introduced to simulate the airflow movement in compression stroke of the prototype and new combustion chambers. By means of analyzing the simulation results, the scheme with improved turbulence intensity or improved turbulence velocity is selected for machining.In case of maintaining the compression ratio, air fuel ratio and intake charge efficiency, engine tests show that the engine with squish combustion chamber has the following performance: power output is decreased, HC emission is increased, NOx emission is decreased and the temperature of the cylinder head is higher than the prototype engine. It is considered that squish airflow quickens the combustion velocity, but unburned fuel inside of the squish clearance increases HC emission and decreases power output.Further experiments show that along with the increase of squish clearance, HC exhaust increases firstly and then decreases. It is proved that HC emission is effected by squish clearance. When the thichness of the squish clearance increases, more unburned fuel hides inside and makes HC emission increase. But when the thickness of the clearance increases continuously, the flame diffuses more sufficiently in squish clearance, HC emission is then decreased.Correlation between squish clearance and NO_x emission also has been proved by test results but with opposite trend.The performance experiments show that while losing some power and economy performance, HC+NO_x emission of the sample engine with squish combustion chamber is about 8.4% times lower than the prototype engine.
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