Lean-burn gas engines are operating worldwide because of having an advantage of lower NOx emission and higher thermal efficiency than those of stoichiometric gas engines. The modern lean-burn gas engines, especially medium and large size, have the pre-combustion chamber technology. On the contrary, there are some problems that originate in the spark plug. Particularly near the ignition plug located in the center, the fuel gas density is lean, affected by the lean-gas mixture coming from the main combustion chamber during the compression stroke and the fuel gas density near the wall is rich. The lifetime of ignition plug is likely to be shorter than those used in the conventional theoretical mixture gas combustion engine, because the required voltage for the plug is high, which reaches 20–25 kV or more.

The authors and their colleagues have studied a combustion method of using micro-pilot fuel oil instead of spark plug as an ignition source in recent four years to provide a solution for the above mentioned technical problems. The energy of micro-pilot fuel oil is equivalent to 1% of the total thermal input, but the energy of the pilot fuel oil is several thousands times of the spark ignition. According to the author’s study, NOx emission level is defined by the amount of pilot fuel oil. But only about 1% fuel can meet the NOx target. NOx emission level meets TA-Luft of 500 mg/m3N @ 5% O2. Even the regulation of 200 ppm @ 0% O2 in the Japanese large cities can be achieved, this level is almost corresponding to the half TA-Luft.

This paper describes the performance being desired for gas engines through the service-experience in co-generation fields and also describes the newly developed gas engine corresponding to a 1000 kW class, which has micro-pilot fuel oil ignition method. This engine has the same performance of a diesel engine, BMEP of 2.3 MPa and brake thermal efficiency of 43%.

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