Study On The Air-Fuel Ratio Controller For LPG Engine

This paper presents an air-fuel ratio (A/F) controller in LPG engine. The method of controlling the A/F combines the open and closed loop systems and gradually improves the accuracy of the LPG injection duty ratio by rough regulation, smooth regulation and micro-regulation. This A/F controller could quickly response and regulate the LPG injection time to meet different engine running conditions and make excessive air coefficient (a) in normal operation range of the three-way ca alyst system (TWC).The task in the open loop system is to get LPG injection duty ratio in pre-set two dimension MAP according to practical engine operation condition, while the task in the closed-loop system is to calculate LPG injection duty ratio by the Approaching Increment method or the Proportion and Integral (PI) Controller according to the signal of the oxygen sensor. To eliminate the oscillation phenomena of the signal of the oxygen sensor in low-speed condition, this paper develops the Stable Increment method. On the other hand, the function of the rough regulation is the same as the open loop system. The A/F controller adopts the Approaching Increment method in the smooth regulation and the PI controller for non-oscillation or stable increment method for low-speed oscillation in the micro-regulation.Experiments with the A/F controller in HONDA EC2500C engine demonstrated that the response time is between 3 and 5 seconds when the oxygen signal lies in non-oscillation waveform state, while the response time is between 8 and 10 seconds when the oxygen signal lies in oscillation waveform state. Compared with the controller system in the WT615 Turbo-Charged CNG Engine, the transient respond performance which normalizes a in the desired range in this A/F controller is better and the steady performance which keeps a in the desired range is almost as same as the WT615 engine. Compared with the controller system in the British Leyangd CFI Engine, though the transient respond performance is clearly poorer ,the steady performance in this A/F controller is significiantly better.