Reducing the cycle-cycle variability of a natural gas engine using controlled ignition current

Running an internal combustion engine with diluted methane/air mixtures has a potential of reducing emissions and increasing efficiency. However, diluted mixtures need high ignition energy in a sufficiently large volume, which is difficult to accomplish. Increasing the spark duration has shown to be a promising way of delivering more energy into the diluted charge, but this requires a more sophisticated ignition system. This work focuses on evaluating the effects regarding enhancing early flame development, reducing cyclic variations and extending the lean limit using a new capacitive ignition system as compared to a conventional inductive ignition system. The new system offers the opportunity to customise the spark by altering the electric pulse train characteristics choosing the number of pulses, the length of the individual pulses as well as the time delay between them. Several different spark configurations were evaluated for the new capacitive system, including varying spark duration between approximately 0.5-1.2 ms and maximum current levels of 0.2-0.4 A. Results show that longer spark duration and faster initial energy deposition has a positive effect on the lean limit and the combustion stability, while the current level shows little or no effect on these variables. The more advanced capacitive system is capable of delivering more energy and longer spark duration, resulting in reduced cyclic variations and enhanced early flame development. With the longer spark durations of the capacitive system the lean limit was extended from = 1.65 to approximately = 1.75.