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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 combustion process in diesel engines deposits soot on the in-cylinder surfaces. Previous works have suggested that these soot deposits eventually break off during cylinder blow-down and the exhaust stroke and contribute significantly to exhaust soot emissions. In order to better understand this potential pathway to soot emissions, the authors recently investigated combusting fuel-jet/wall interactions in a diesel engine. This work, published as a companion paper, showed how soot escaped from the combusting fuel jet and was brought in close proximity to the wall so that it could become a deposit. The current study extends this earlier work with laser-extinction measurements of the soot-deposition rate in the same single-cylinder, heavy-duty DI diesel engine. Measurements were made by passing the beam of a CW-diode laser through a window in the piston bowl rim that was in-line with one of the fuel jets.

Significant reductions of particulate matter (PM) and nitrogen oxides (NOx) emissions from diesel engines have been realized through fueling with water-fuel emulsions. However, the physical and chemical in-cylinder mechanisms that affect these pollutant reductions are not well understood. To address this issue, laser-based and chemiluminescence imaging experiments were performed in an optically-accessible, heavy-duty diesel engine using both a standard diesel fuel (D2) and an emulsion of 20% water, by mass (W20). A laser-based Mie-scatter diagnostic was used to measure the liquid-phase fuel penetration and showed 40-70% greater maximum liquid lengths with W20 at the operating conditions tested. At some conditions with low charge temperature or density, the liquid phase fuel may impinge directly on in-cylinder surfaces, leading to increased PM, HC, and CO emissions because of poor mixing.

To better understand the factors affecting soot formation in diesel engines, in-cylinder soot and diffusion flame lift-off were measured in a heavy-duty, direct-injection diesel engine. Measurements were obtained at two operating conditions using two commercial diesel fuels and a range of oxygenated paraffinic fuel blends. A line-of-sight laser extinction diagnostic was improved and employed to measure the relative soot concentration within the jet (“jet-soot”) and the rates of soot-wall deposition on the piston bowl-rim. An OH chemiluminescence imaging technique was developed to determine the location of the diffusion flame and to measure the lift-off lengths of the diffusion flame to estimate the amount of oxygen entrainment in the diesel jets. Both the jet-soot and the rate of soot-wall deposition were found to decrease with increasing fuel oxygen-to-carbon ratio (O/C) over a wide range of O/C.

Over the past decade, laser diagnostics have improved our understanding of many aspects of diesel combustion. However, interactions between the combusting fuel jet and the piston-bowl wall are not well understood. In heavy-duty diesel engines, with typical fuels, these interactions occur with the combusting vapor-phase region of the jet, which consists of a central region containing soot and other products of rich-premixed combustion, surrounded by a diffusion flame. Since previous work has shown that the OH radical is a good marker of the diffusion flame, planar laser-induced fluorescence (PLIF) imaging of OH was applied to an investigation of the diffusion flame during wall interaction. In addition, simultaneous OH PLIF and planar laser-induced incandescence (PLII) soot imaging was applied to investigate the likelihood for soot deposition on the bowl wall.