The characterization of two-stage ignition effects on late injection low temperature combustion using biodiesel and biodiesel blends

The first stage of ignition in saturated hydrocarbon fuels (in diesel combustion) is characterized as low temperature heat release (LTHR) or cool flame combustion. LTHR takes place as a series of isomerization reactions at temperatures from 600K to 900K, and is often detectable in HCCI, rapid compression machines, and early injection low temperature combustion (LTC). The experimental investigation presented attempts to determine the existence of LTHR behavior in late injection low temperature combustion in a medium duty diesel engine with both petroleum diesel and biodiesel fuels and to determine the influence of such behavior on LTC torque and emissions. Three experiments were performed to meet these objectives: the first studies two operating modes (conventional combustion with -8° after top dead center injection timing and 0% EGR and low temperature combustion with 0° after top dead center injection timing and nominally 42% EGR level) with standard petroleum diesel, palm biodiesel, and soy biodiesel; the second studies a sweep of EGR level from 0% to nominally 45% with petroleum diesel and palm biodiesel with a constant injection timing of 0° after top dead center. The third and final experiment utilized petroleum diesel, soy biodiesel, and blends from the two fuels (20 and 50% soy biodiesel) to see the influence of viscosity and density on LTHR. LTHR is apparent in all fuels' rates of heat release profiles at the LTC operating conditions. Diesel fuel LTC displays a longer and more intense LTHR phase. Lower amounts of LTHR in the palm biodiesel causes less sensitivity to EGR, less instability, and produces better torque and emission characteristics. Density and viscosity only change the shape of the LTHR duration, while cetane number or ignition quality affects the length of the LTHR duration.