Glow Plug Assisted Ignition and Combustion of Methanol in an Optical DI Diesel Engine

An experimental study of the glow-plug-assisted ignition and combustion of pure methanol (M100) was conducted using a modern-technology, 4-stroke, heavy-duty DI diesel engine that has been modified to provide extensive optical access into the combustion chamber. For comparison purposes, results also are presented for a two-component paraffinic diesel reference fuel with a cetane number of 45 (CN45). A 1200-rpm, moderate-load operating condition was studied. Images of direct luminosity from the combustion chamber are used along with thermodynamic analyses of cylinder pressure data to identify differences between the ignition and combustion characteristics of the two fuels. The M100 data show significant departures from the traditional diesel combustion features exhibited by CN45. Whereas CN45 readily autoignites at the conditions studied, M100 does not. The glow-plug-assisted ignition of M100 was found to be strongly dependent on glow plug (GP) temperature and proximity to a fuel jet. The direct luminosity images show that M100 ignition occurs at the GP, followed by combustion propagation first to the two jets straddling the GP, then to the adjacent two jets, and finally to the last two jets emanating from the 6-hole, centrally-located injector nozzle. Three spikes in the apparent heat release rate are observed to correspond to these pair-wise jet ignition events. Spatially integrating the direct luminosity image data suggests that in-cylinder soot concentrations during M100 combustion are at least 2 orders of magnitude lower than those during CN45 combustion. Peak heat release rates during M100 combustion are only half of those achieved during the premixed burn phase of CN45 combustion, resulting in noticeably quieter engine operation.