Ignition by Shielded Glow Plug in Natural Gas Fueled Direct Injection Engines

Injected natural gas requires some form of ignition assist in order to ignite in the time available in a diesel engine combustion chamber. A glow plug — a heated surface — is one form of ignition assist. Ignition by glow plug results in a single site of ignition from which the flame must propagate to other jets in the ignition pattern. The goal of this work was to determine what factors affect how the flame propagates from this initial ignition site to the remaining unburned mixture. The combustion of natural gas jets under diesel engine conditions was studied over a range of temperatures with a glow plug shield using a CFR engine as a rapid compression device. The results showed that of all the factors considered it is the inter-related geometries of the injection pattern, combustion chamber, and glow plug shield that are most dominant in controlling combustion rates and fuel utilization, because those factors determine the distribution of fuel in the combustion chamber. Ignition of adjacent gas jets requires a flammable path between jets, which is achieved: 1) through mixing between the entrainment regions of adjacent jets and 2) through mixing along the cylinder wall of adjacent jets that are spreading along the wall. Ignition by either of both of these pathways can provide high fuel utilization and combustion rates and low combustion variability. Autoignition of an adjacent jet due to heat release from ignition of the first jet was not observed in these experiments with two jets.