Abstract. Based on engine bench test system, the HC emission behavior of a lean burn LPG engine during constant speed and changing (increasing an d de creasing) torque transient conditions was investigated. Results show that HC is significantly deteriorated under the experimental operating conditions. HC concentration value is higher than the steady-state value by 2.3% an d 14.2%, respectively. T he response of the actual torque lags behind the desired torque by 2.047s and 2.109s, respectively, and leads to the actual speed deviates from the desired value by 4.5% and 3.7%. T he deviation of the actual speed causes the fluctuations of IAA, which is the key reason for the deterioration of HC under increasing torque operation. The control of fuel valve cannot meet the demand of the actual LPG mass durin g the transient operating conditions, leads to the actual LPG mass value deviate from the target value by 4.7% and 8.8%, respectively. And this is also the main responsibility for the actual AFR deviate from the target value by 5.1% and 8.2%, respectively. During the decreasing torque operation, caused by the deviation of the actual AFR, the mixture is too rich, which results in the deterioration of HC.

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