A Comparison of Combustion and Emissions of Diesel Fuels and Oxygenated Fuels in a Modern DI Diesel Engine

Two oxygenated fuels were evaluated on a single-cylinder diesel engine and compared to three hydrocarbon diesel fuels. The oxygenated fuels included canola biodiesel (canola methyl esters, CME) and CME blended with dibutyl succinate (DBS), both of which are or have the potential to be bio-derived. DBS was added to improve the cold flow properties, but also reduced the cetane number and net heating value of the resulting blend. A 60-40 blend of the two (60% vol CME and 40% vol DBS) provided desirable cold flow benefits while staying above the U.S. minimum cetane number requirement. Contrary to prior vehicle test results and numerous literature reports, single-cylinder engine testing of both CME and the 60-40 blend showed no statistically discernable change in NOx emissions relative to diesel fuel, but only when constant intake oxygen was maintained. The increased NOx emissions typically reported for oxygenated fuels are believed to be largely due to two factors: 1) the method used to control Exhaust Gas Recirculation (EGR), which is typically based on air mass or EGR rate rather than intake oxygen concentration, and 2) the shift in calibration set points (e.g., EGR, boost pressure, etc.) that result from the increased pedal demand needed to achieve the same torque with oxygenated fuels, due to their lower energy content. When compared at constant intake oxygen, results for NOx emissions, combustion noise and thermal efficiency were similar between the diesel fuels and the oxygenated fuels tested. A substantial reduction in particulate matter (PM) emissions was observed with both oxygenated fuels (91% reduction with CME and 97% reduction with 60-40 blend on average over various operating conditions), which was attributed to the respective oxygen content of those two fuels and its effect on the average oxygen equivalence ratio at the lift-off length. Elevated hydrocarbon emissions were initially observed with the 60-40 blend and were attributed to poor pilot burn caused by the combination of that fuel's low cetane number and low energy content. When the injection quantities of both the pilot and main injections were adjusted to compensate for energy content, the hydrocarbon emissions were reduced to a level similar to that of the other fuels tested. Under Low Temperature Combustion (LTC), fuel-related effects appeared to track with the cetane number and were relatively insensitive to fuel oxygen content.