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.

This paper introduces, and presents experimental validation for, an on-engine applicable control framework for fuel-flexible combustion of diesel–biodiesel blends. The approach is based on changing two of the closed-loop targeted control variables used by the engine control module (ECM): (1) replacing exhaust gas recirculation (EGR) fraction with combustible oxygen mass fraction (COMF); (2) replacing total injected fuel mass with total injected fuel energy, including replacing start of main injection timing with end of main injection timing. It is shown that the stock ECM control structure with pure biodiesel (B100) produces 38 per cent more brake-specific nitrogen oxides (NO x ) compared to pure conventional diesel (B0). However, new results presented here with the proposed control framework show that B100 can be made to produce lower brake-specific NO x , 2 per cent higher brake thermal efficiency, 50 per cent lower brake-specific particulate matter, and 1.2 dB lower combustion noise than B0. Benefits of, and novel contributions related to, this strategy are that it is generalizable to other engine systems, is physically based, does not require modified or additional engine calibration, and maintains the stock B0 performance. In essence, the approach presented defines the biodiesel blend control problem as closed-loop targeting of COMF and injected fuel energy, paving the way for future work in controller design to achieve these targets in real-time, on-engine situations.