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.

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A substantial opportunity exists to reduce carbon dioxide (CO2) emissions, as well as dependence on foreign oil, by developing strategies to cleanly and efficiently use biodiesel, a renewable domestically available alternative diesel fuel. However, biodiesel utilization presents several challenges, including decreased fuel energy density and increased emissions of smog-generating nitrogen oxides (NOx). These negative aspects can likely be mitigated via closed-loop combustion control provided the properties of the fuel blend can be estimated accurately, on-vehicle, in real-time. To this end, this paper presents a method to practically estimate the biodiesel content of fuel being used in a diesel engine during steady-state operation. The simple generalizable physically motivated estimation strategy presented utilizes information from a wideband oxygen sensor in the engine’s exhaust stream, coupled with knowledge of the air-fuel ratio, to estimate the biodiesel content of the fuel. Experimental validation was performed on a 2007 Cummins 6.7 l ISB series engine. Four fuel blends (0%, 20%, 50%, and 100% biodiesel) were tested at a wide variety of torque-speed conditions. The estimation strategy correctly estimated the biodiesel content of the four fuel blends to within 4.2% of the true biodiesel content. Blends of 0%, 20%, 50%, and 100% were estimated to be 2.5%, 17.1%, 54.2%, and 96.8%, respectively. The results indicate that the estimation strategy presented is capable of accurately estimating the biodiesel content in a diesel engine during steady-state engine operation. This method offers a practical alternative to in-the-fuel type sensors because wideband oxygen sensors are already in widespread production and are in place on some modern diesel vehicles today.

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.