Efficacy of Add-On Hydrous Ethanol Dual Fuel Systems to Reduce NOx Emissions From Diesel Engines

Abstract

Aftermarket dual-fuel injection systems using a variety of different fumigants have been proposed as alternatives to expensive after-treatment to control NOX emissions from legacy diesel engines. However, our previous work has shown that available add-on systems using hydrous ethanol as the fumigant achieve only minor benefits in emissions without recalibration of the diesel fuel injection strategy. This study experimentally re-evaluates a novel aftermarket dual-fuel port fuel injection (PFI) system used in our previous work, with the addition of higher flow injectors to increase the fumigant energy fraction (FEF), defined as the ratio of energy provided by the hydrous ethanol on a lower heating value (LHV) basis to overall fuel energy. Results of this study confirm our earlier findings that as FEF increases, NO emissions decrease, while NO2 and unburned ethanol emissions increase, leading to no change in overall NOX. Peak cylinder pressure and apparent rates of heat release are not strongly dependent on FEF, indicating that in-cylinder NO formation rates by the Zel’dovich mechanism remains the same. Through single zone modeling, we show the feasibility of in-cylinder NO conversion to NO2 aided by unburned ethanol. The modeling results indicate that NO to NO2 conversion occurs during the early expansion stroke where bulk gases have temperature in the range of 1150–1250 K. This work conclusively proves that aftermarket dual fuel systems for fixed calibration diesel engines cannot reduce NOX emissions without lowering peak temperature during diffusive combustion responsible for forming NO in the first place.

Hydrous Ethanol Steam Reforming and Thermochemical Recuperation to Improve Dual-Fuel Diesel Engine Emissions and Efficiency

“…Overall, soot emissions decreased drastically, reaching near 90% reduction from CDC levels at high FEF. This result is somewhat contrary to our earlier hydrous ethanol PFI work [1][2][3] which showed that soot emissions remained high even at significant FEF. With reformed ethanol, as FEF increased, engine combustion was more premixed with significantly less directly injected diesel.…”

“…However, the high load condition experienced only a minor increase before decreasing, indicative of the high CE seen during this condition, providing further evidence that ISR operation is most beneficial at high loads. Unlike our previous work with hydrous ethanol [1][2][3], no unburned ethanol was present at engine outlet conditions. Instead, THC emissions consisted mainly of methane, depicted in Figure 6 and smaller amounts of acetaldehyde and formaldehyde.…”

“…This shows that either the reformer achieved near full conversion of ethanol, or any ethanol leaving the reformer was oxidized during combustion. However, our previous work has shown that at even low replacement ratios, direct ethanol fumigation results in significant unburned ethanol emissions [1]. This means that it is more likely that the ISR reactor achieved near 100% conversion efficiency of hydrous ethanol for all cases.…”

“…Water removal in anhydrous ethanol refining is energy intensive. Allowing water fractions up to 25% has been shown to decrease energy requirements for corn-ethanol refining, thus improving its renewability [1]. However, water contained in hydrous ethanol has no heating value, requires additional energy to vaporize at the point of consumption and acts as a diluent, presenting challenges for its practical use.…”

“…Our previous work [1][2][3] explored the use of hydrous ethanol in diesel dual-fuel combustion modes using a stock engine fuel injection calibration to determine how effective such a system could be if implemented as an aftermarket solution. When introduced through port fuel injection (PFI), hydrous ethanol achieved stable combustion with negligible impact on engine performance and up to 60% fumigant energy fraction (FEF), defined as the ratio of energy provided by the ethanol over total fuel energy.…”