An Experimental Investigation of Combustion Chamber Design Parameters for Hot Surface Ignition

Abstract

Natural gas requires some form of ignition assist in order to autoignite in the time available in a compression ignition engine. Ignition assist using a glow plug — a heated surface — was investigated using an apparatus that consists of an optically accessible constant volume combustion bomb coupled to a single cylinder CFR engine through the spark plug port. Previous studies have shown the dominant effect of fuel injection pattern and glow plug shield geometry on ignition delay, combustion rate, and fuel utilization with 1–3 fuel jets. New work has been carried out to evaluate the ability of a shielded glow plug to ignite a full nine jet symmetrical fuel injection pattern. The sensitivity of ignition delay and fuel utilization to fuel injector angular alignment relative to the glow plug, glow plug shield opening angle, and glow plug power was analyzed using in-cylinder pressure data and exhaust hydrocarbon emissions concentrations. Two glow plugs, one conventional metallic and one ceramic, and two fuel injector nozzle orifice sizes were evaluated for their effect on ignition delay. The ignition and flame propagation process was observed using high speed images. Glow plug power was shown to have a dominant effect on ignition delay and fuel utilization, with a secondary effect from fuel injector angle and glow plug opening angle. The ceramic glow plug was shown to provide superior ignition assist while consuming less power than the metallic glow plug. The larger fuel injector nozzle size increased ignition delay times, likely due to increased convective cooling of the glow plug surface from the larger gas jet. Acquired images show that the smaller fuel injector orifice size created a flammable path in two distinct areas; along the periphery of the fuel jets and between the fuel jets. The higher mass flow rate and subsequent increased mixing of the larger fuel jets created flammable paths throughout the entirety of the combustion chamber.