抄録
For spark ignition (SI) engine, lean operation can provide improvement of thermal efficiency relative to that of stoichiometric operation. However, the cycle-to-cycle variations of combustion increase with air dilution, and become unacceptable. To gain the benefits of lean operation, the ability to ensure stable, complete and fast combustion is required. As a method to realize stable lean operation, this study changes spark discharge pattern using a custom inductive ignition system to examine the influences of discharge energy and discharge duration on SI combustion at lean-stability limit. First, the spark discharge pattern was changed in air at atmospheric pressure. The results show that the discharge duration increases with increasing discharge offset, but the total discharge energy decreases. However, for excessive discharge offset, spark discharge from two spark coils occurs separately, which results in the increase of total discharge energy. Second, fired operation was conducted at excess-air ration (λ) of 1.0 and 1.67 while only changing spark discharge pattern at the same spark timing. For stoichiometric operation, the influences of spark discharge pattern on the combustion was not observed. On the other hand, for lean operation of λ=1.67, the combustion phasing was retarded with increasing discharge offset. However, there is no relation between the total discharge energy and the combustion phasing. Rather, high correlation between the combustion phasing and the integrated discharge energy varied after spark discharge was observed at specific crank angle. Finally, around lean-stability limit, spark discharge pattern was changed. Among spark discharge patterns, spark discharge pattern with discharge offset of 0.2ms can shorten the combustion duration with advanced spark timing and provide the largest improvement of thermal efficiency. On the other hand, spark discharge pattern with discharge offset of 0.1ms enables the leanest operation (λ=1.97) with COV of IMEPn less than 5% by shortening the duration from spark timing to CA5.
