Comparison of Simulated and Experimental Combustion of Biodiesel Blends in a Single Cylinder Diesel HCCI Engine 2007-01-4010

The effect of biodiesel content on homogeneous charge compression ignition (HCCI) engine performance has been investigated both experimentally and by computer simulation. Combustion experiments were performed in a single cylinder HCCI engine using blends of soy biodiesel in ultra low sulfur diesel, with concentrations ranging from 0 to 50 vol% and equivalence ratios (Φ) from 0.38 to 0.48. Data from the engine tests included combustion analysis and exhaust composition analysis with standard gaseous emissions equipment. The engine utilized a custom port fuel injection strategy to provide highly premixed charges of fuel and air, making it possible to compare the results with single zone chemical kinetics simulations that were performed using CHEMKIN III, with a reaction set including 670 species and over 3000 reactions. The reaction mechanism incorporated equations for the combustion of a paraffinic fuel, n-heptane, and an oxygenated component, methyl butanoate, as well as reactions for the formation of NO_x. The zero-dimensional model did a reasonably good job of predicting the HCCI combustion event, correctly predicting intake temperature effects on the phasing of both low temperature heat release (LTHR) and the main combustion event. It also did a good job of predicting the magnitude of LTHR. Differences between the simulation and experimental data included the dependence on biodiesel concentration and the duration of both LTHR and the main combustion event. The probable reasons for these differences are the changing derived cetane number (DCN) of the model fuel blend with biodiesel concentration, and the inability of the model to account for stratification of temperature and Φ. The simulation also showed that concentrations of intermediate species produced during LTHR are dependent on the magnitude of LTHR, but otherwise the addition of biodiesel has no discernable effect.