Transient Behavior of Glow Plugs in Direct-Injection Natural Gas Engines

Abstract

Glow plugs are a possible ignition source for direct injected natural gas engines. This ignition assistance application is much different than the cold start assist function for which most glow plugs have been designed. In the cold start application, the glow plug is simply heating the air in the cylinder. In the cycle-by-cycle ignition assist application, the glow plug needs to achieve high surface temperatures at specific times in the engine cycle to provide a localized source of ignition. Whereas a simple lumped heat capacitance model is a satisfactory representation of the glow plug for the air heating situation, a much more complex situation exists for hot surface ignition. Simple measurements and theoretical analysis show that the thickness of the heat penetration layer is small within the time scale of the ignition preparation period (1–2 ms). The experiments and analysis were used to develop a discretized representation of the glow plug domain. A simplified heat transfer model, incorporating both convection and radiation losses, was developed for the discretized representation to compute heat transfer to and from the surrounding gas. A scheme for coupling the glow plug model to the surrounding gas computational domain in the KIVA-3V engine simulation code was also developed. The glow plug model successfully simulates the natural gas ignition process for a direct-injection natural gas engine. As well, it can provide detailed information on the local glow plug surface temperature distribution, which can aid in the design of more reliable glow plugs.

Numerical studies of the ignition characteristics of a high-pressure gas jet in compression-ignition engines with glow plug ignition assist: Part 1—Operating condition study

“…Typically, natural gas is ignited within 2 ms after fuel injection, which at 230 r/min corresponds to less than 3° of crankshaft rotation, and the swirl changes very little during that time period. Since the purpose of this computation is focused on natural gas ignition delay, simulation of fuel injection and ignition can be started at the time of injection, using the swirl intensity as a boundary condition and using the simplified computational model shown in Figure 2 to represent only the constant-volume combustion chamber 13,14 where the fuel injection and ignition occur. The conditions inside the combustion chamber at the start of injection (SOI), summarized in Table 2, hence were employed as the initial inputs in the modeling, unless otherwise specified.…”

“…To accurately simulate the transient heat transfer behavior of the GP on the time scale of an engine cycle, a GP discretized model was developed and implemented to simulate the gas ignition process by a hot surface. 13,14 In this model, the solid GP surface was discretized into small cells that match each sub-layer gas cell surrounding the GP, and then heat transfer equations were employed to calculate not only the radiative and convective heat loss from the GP surface but also the conductive heat transfer inside the GP. This GP model is used to predict the temperature change on the GP surface in the radial and axial directions during the natural gas injection and combustion process.…”

“…Agarwal and Assanis 11 and Papageorgakis et al 12 used the KIVA-3 engine computational fluid dynamics (CFD) code to study the natural gas injection, ignition and emissions in a GP-assisted direct-injection engine. Cheng and Wallace 13 and Cheng 14 developed computational models for gas injection and GP ignition, integrated them with KIVA-3V and then employed the modified code to explore fuel injection and ignition inside a modeled version of the modified CFR engine combustion chamber used for the experiments. Nevertheless, uncertainties were introduced into the simulated results, not only because several key engine geometries, such as the fuel injector shape, were different between the experimental setup and the KIVA model but also because the simple chemical kinetic model used in the simulation does not adequately reflect the natural gas ignition process over a wide range of temperatures.…”

Numerical studies of the ignition characteristics of a high-pressure gas jet in compression ignition engines with glow plug ignition assist: Part 2-Effects of multi-opening glow plug shields

“…Based on this engine configuration, a KIVA computational model was developed to represent only the geometry and structure of the CFR engine’s external combustion chamber, 38,44–47 where the fuel injection and ignition occurred, as shown in Figure 2(b). The engine model has been validated by experimental data 48,49 that indicates the computational model can well simulate the jet penetration, fuel mixing, and hence ignition and flame propagation.…”

“…38 Here, this engine model will be briefly introduced (the mesh setup is provided in Appendix 2), and more details about this model can be found in previous papers. 38,44–47…”