Investigation of the Relationship Between DI Diesel Combustion Processes and Engine-Out Soot Using an Oxygenated Fuel

The relationship between combustion processes and engine-out soot was investigated in an optically accessible DI diesel engine using diethylene glycol diethyl ether (DGE) fuel, a viable diesel oxygenate. The high oxygen content of DGE enables operation without soot emissions at higher loads than with a hydrocarbon fuel. The high cetane number of DGE enables operation at charge-gas temperatures below those required for current diesel fuels, which may be advantageous for reducing NO_x emissions. In-cylinder optical measurements of flame lift-off length and natural luminosity were obtained simultaneously with engine-out soot measurements while varying charge-gas density and temperature. The local mixture stoichiometry at the lift-off length was characterized by a parameter called the oxygen ratio that was estimated from the measured flame lift-off length using an entrainment correlation for non-reacting sprays.

The results demonstrate that near-zero engine-out soot emissions can be achieved with the oxygenated fuel DGE at high engine loads, even under conditions that yield significant in-cylinder soot formation. In all test cases, significant soot formation during the fuel-injection period was indicated by natural luminosity measurements and was consistent with predictions based on the estimated oxygen ratio at the flame lift-off length, yet near-zero engine-out soot was observed in some cases. The absence of engine-out soot despite significant in-cylinder soot formation is attributed to oxidation of soot before the exhaust valves open. Consistent with this hypothesis, in-cylinder measurements show that most of the natural luminosity signal in high engine-out soot cases occurs after the end of injection and persists until the exhaust valves open. Most compellingly, engine-out soot is found to be proportional to the natural luminosity signal at exhaust valve opening as the quantity of fuel injected, charge-gas density, and temperature are varied.