An investigation of multiple spark discharge using multi-coil ignition system for improving thermal efficiency of lean SI engine operation
Abstract Lean spark ignition (SI) engine operation can provide improvement of the thermal efficiency relative to that of stoichiometric SI operation. However, the cycle-to-cycle variations of SI combustion increase with increasing 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 enable stable lean operation, this study investigates the effects of multiple spark discharge on lean SI operation using a multi-coil ignition system that features ten spark coils for a single spark plug. First, the effects of multiple spark discharge on the combustion phasing and the combustion duration are examined for lean operation (excess-air ratio ( λ ) = 1.67) under a constant spark timing. The results show that both the total discharge duration and the total discharge energy increase with the extension of time interval between spark discharges ( Δt i ) for multiple spark discharge. For the multiple spark discharge of substantially extended Δt i , a number of restrikes occur after the spark blowouts due to the weak discharge energy release rate. However, both the total discharge duration and the total discharge energy do not contribute directly to the change of initial combustion phase. Instead, the effective spark discharge energy exists within the total discharge energy, which actually contributes to the change of initial combustion phase. When the effective spark discharge energy is high, it leads to the advancement of the initial combustion phase, resulting in the number of knocking cycles increases with the increase of knocking intensity. Second, additional experiments were conducted under ultra-lean conditions in the λ = 1.82&n 1.82–1.97 range since the effects of multiple spark discharge on SI combustion are much more pronounced for the leaner operation. The results show that the net indicated thermal efficiency increases quite linearly with increasing λ for all multiple spark discharges considered. Especially, for the multiple spark discharge of Δt i = 0.2 ms, a proper spark discharge energy is generated from the spark timing to a certain timing after the first start of spark discharge, which advances both the spark timing and the initial combustion phase. A combination of the advanced spark timing and the advanced initial combustion phase leads to the advancement of main combustion phase, and eventually shorten the combustion duration. Because of shortened combustion duration, stable ultra-lean operation at around λ = 1.94 c 1.94 can be achieved with the highest net indicated thermal efficiency of 47.0%. Highlights Effective spark discharge energy exists, which influences on the combustion phasing. Too strong spark discharge energy leads to knocking combustion. Different spark discharge characteristics are observed for lean SI operation. Initial and main combustion phasing can be advanced by the multiple spark discharge. Thermal efficiency of SI engine is mainly improved by shortening combustion duration.
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