No abstract

Alternative fuels are gaining importance as a means of reducing petroleum dependence and green house gas emissions. Biodiesel is an attractive renewable fuel; however, it typically results in increased emissions of nitrogen oxides (NO x ) relative to petroleum diesel. In order to develop hypotheses for the cause of increased NO x emissions during diffusion-dominated combustion in a modern diesel engine, an effort incorporating both experimental and modeling tasks was conducted. Experiments using a 2007 Cummins diesel engine showed NO x and fuel consumption increases of up to 38% and 13%, respectively, and torque decreases up to 12% for soy-biodiesel. Fuel properties and ignition delay characteristics were implemented in a previously validated engine model to reflect soy-biodiesel. Model predictions are within 3.5%, 7%, and 9.5%, respectively, of experimental engine gas exchange (airflow, charge flow, and exhaust gas recirculation (EGR) fraction), performance (work output, torque, and fuel consumption), and NO x emission measurements. The experimental and model results for the diffusion combustion-dominated operating conditions considered here suggest that higher biodiesel distillation temperatures and fuel-bound oxygen lead to near stoichiometric equivalence ratios in the rich, premixed portion of the flame as well as higher combustible oxygen mass fractions in the diffusion flame front which together result in increased biodiesel combustion temperatures and NO x formation rates.

Stock engine design and decision making target optimal performance with conventional diesel fuel, leading to suboptimal results for biodiesel. The main result of this study is the determination of the appropriate engine decision making for the air/fuel ratio (AFR), exhaust gas recirculation (EGR) fraction, injection (rail) pressure, and start of main fuel injection (SOI) in a modern common rail diesel engine using variablegeometry turbo charging and operating with 5% (B5) and 20% (B20) soy-based biodiesel fuel mixtures to minimize brake-specific fuel consumption (BSFC) while adhering to strict combustion noise, NO x and PM emission constraints. In so doing, this effort determined to what extent the optimal AFR, EGR fraction, rail pressure, and SOI settings can (1) overcome the well-known "biodiesel-NO x effect" and (2) mitigate the impact of lower biodiesel energy density on BSFC for B5 and B20 biodiesel blends. Study findings indicate that lower AFR, higher EGR fraction, and earlier start of main injection can completely eliminate biodiesel NO x increases with blends of soy-based biodiesel in a modern diesel engine. While the BSNO x reductions were achieved with acceptable BSPM and noise, it was not possible to reduce the BSFC for B5 and B20 to conventional diesel levels.

No abstract

Alternative fuel vehicles are gaining importance as a means of reducing petroleum dependence. One attractive option is biodiesel, a renewable diesel fuel produced from plant or animal fats, since it significantly reduces carbon monoxide, unburned hydrocarbon, and particulate matter emissions as well as carbon dioxide when considered on a full life cycle basis. However, biodiesel combustion also typically results in increased fuel consumption and nitrogen oxide (NOx) emissions relative to petroleum diesel. In order to determine the cause of and develop mitigation strategies for increased biodiesel fuel consumption and NOx emissions, an accurate simulation model was developed and validated. Key fuel properties as well as ignition delay characteristics were implemented in a previously validated whole engine model to reflect soy-biodiesel fuel. The model predictions were within 5% of experimental results for most values at the three operating points. Using this biodiesel model, the “biodiesel NOx effect” was linked to the near stoichiometric equivalence ratios for biodiesel.