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This study summarizes the peer-reviewed literature regarding the use of raw pyrolysis liquids (PLs) created from woody biomass as fuels for compression-ignition (CI) engines. First, a brief overview is presented of fast pyrolysis and the potential advantages of PLs as fuels for CI engines. Second, a discussion of the general composition and properties of PLs relative to conventional, petroleum-derived diesel fuels is provided, with emphasis on the differences that are most likely to affect PL performance in CI-engine applications. Next, a synopsis is given of the peer-reviewed literature describing experimental studies of CI engines operated using neat PLs and PLs combined in various ways with other fuels. This literature conclusively indicates that raw PLs and PL blends cannot be used as “drop-in replacements” for diesel fuel in CI engines, which is reflected in part by none of the cited studies reporting successful operation on PL fuels for more than twelve consecutive hours.

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 maximum extent of liquid-phase fuel penetration into in-cylinder gases is an important parameter in compression-ignition (CI) engine design. Penetration of the fuel is needed to promote fuel-air mixing, but over-penetration of the liquid phase and impingement on the bowl wall can lead to higher emissions. This maximum liquid-phase fuel penetration, or “liquid length,” is a function of fuel properties, in-cylinder conditions, and injection characteristics. The goal of this study was to measure and correlate the liquid lengths of fuels with wide physical property variations. The fuels were injected into a large range of in-cylinder temperature (700 to 1300 K) and density (3.6 to 59.0 kg/m3) conditions, at an injection pressure (140 MPa) that is characteristic of those provided by current high-pressure injection equipment.