A New Ignitior for Large-Bore Natural Gas Engines - Railplug Design Improvement and Optimization

It is a very challenging problem to reliably ignite extremely lean mixtures, especially for the low speed, high load conditions of large-bore natural gas engines. If these engines are to be use for the distributed power generation market, it will require operation with higher boost pressures and even leaner mixtures. Both place greater demands on the ignition system. The railplug is a very promising ignition system for lean burn natural gas engines with its high-energy deposition and high velocity plasma arc. It requires care to properly design railplugs for this new application, however. For these engines, in-cylinder pressure and mixture temperature are very high at the time of ignition due to the high boost pressure. Hot spots may exist on the electrodes of the ignitor, causing pre-ignition problems. A heat transfer model is proposed in this paper to aid the railplug design. The electrode temperature was measured in an operating natural gas engine. The engine tests showed that the lean-stability-limit (LSL) was extended from a fuel/air equivalence ratio of φ = 0.63 (0.02 J) or φ=0.59 (0.7 J) for spark plugs to φ = 0.535 using a magnet enhanced railplug with a discharge energy of 0.7J. The LSL was extended to φ = 0.54 for railplugs with delivered energy 0.15 J, a typical discharge energy for commercial capacitor discharge ignition systems.