In this paper, a gas engine system capable of stable operation at any mix ratio of city gas 13A and biogas was developed. The gas engine system consists of a spark-ignition gas engine, an additional electric throttle valve for fuel and our own control algorithm. The engine is a 3-cylinder 1.6-l engine that was originally used for co-generation, and the fuel throttle valve was added to respond to different fuel compositions. The control algorithm was also designed to adjust the fuel and air ratio to attain a higher generation efficiency and lower NOx emission with different mix ratios of city gas 13A, biogas and load. Before developing the controller, the effect of the mix ratio on generation efficiency and NOx emission was investigated under various load conditions. The following summarizes the experimental results: a control algorithm using the Wobbe index for mixed fuels was formulated; this index determines the target fuel-to-air ratio. Next, operation tests were performed under varying fuel mix ratios and loads by applying the control algorithm to the gas engine. The target engine rotational speed and exhaust O2 concentration was realized in 5s when the biogas fraction varied from 20% to 40% and from 70% to 40%. When the load was also varied from 9.4kW to 0.5kW and from 0.5kW to 9.4kW at a constant rate, the rotational speed and exhaust O2 concentration achieved the target values in 20s. Under both transient operation conditions, the engine system met the NOx emission requirement, and the results indicate that the simple hardware modification to a conventional gas engine and our original control algorithm are capable of achieving stable engine operation at any mix ratio of city gas 13A and biogas.
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