Investigation Of Novel Hydrous Ethanol Dual-Fuel Strategies For Reducing Emissions From Fixed-Calibration Diesel Engines

Abstract

Oxides of Nitrogen (NOX) from diesel engines are a major source of air pollution globally. Some dual-fuel combustion modes in diesel engines have demonstrated the potential to both reduce engine NOX and increase efficiency. However, new engines that allow complete control of engine parameters have the opportunity to enable reactivity-controlled compression ignition (RCCI) combustion which simultaneously limits the in-cylinder production of NOX and soot emissions. Given their longevity, legacy diesel engines will continue to be used at older emissions levels. This body of work investigates conventional dual-fuel strategies and novel fuel pretreatment architectures to allow mitigation of NOX emissions from fixed calibration legacy diesel engines. This thesis presents an investigation of conventional and novel dual-fuel combustion strategies, to reduce engine out NOX emissions while maintaining thermal efficiencies using hydrous ethanol and diesel as fuel sources. Direct use of hydrous ethanol has been shown to significantly reduce the energy input during the bio ethanol production process, improving the life cycle energy balance and economics compared to currently produced anhydrous ethanol. In this work, a thorough experimental investigation was conducted to characterize the performance and emissions of conventional dual-fuel strategies such as fumigation and port fuel injection (PFI), and novel thermochemical recuperation strategies such as reformed exhaust gas recirculation (REGR) and integrated steam reforming (ISR). The operability range of REGR and ISR was experimentally explored to determine optimal operating conditions. One of the primary findings of this work is that conventional dual-fuel strategies are shown to have little effect on NOX emissions until high fumigant energy fractions (FEF) are used where combustion instability leads to poor performance and high unburned hydrocarbon (UHC) emissions. Results indicate that UHCs facilitate the conversion of NO to NO2 during the expansion stroke of the engine causing lower NO emissions at the expense of higher NO2. This work is the first to numerically illustrate the effect of unburned ethanol on this NO to NO2 conversion. Therefore, retrofitting fixed calibration diesel engines with dual fuel strategies is not found to be an effective strategy for reducing NOX emissions, contrary to many literature claims. With the ability to control engine injection parameters, single fuel RCCI operation using an integrated REGR reactor partially reforming diesel fuel mixed with exhaust gases is demonstrated to significantly lower NOX emissions from a light duty diesel engine. This work is the first to experimentally demonstrate “single” fuel RCCI using a thermally integrated reforming reactor in a diesel engine. In these experiments, low reactivity reformate was introduced into the engine as part of the EGR system. Significantly advanced pilot and main injection were used to achieve RCCI operation with near zero levels of soot and NOX emissions. However, variability in local reformer equivalence ratios made REGR operation difficult to control where lean operation caused excessive reactor temperature and deactivating the catalyst, while running too rich led to poor conversion efficiency. To overcome the disadvantages of PFI injection of hydrous ethanol while incorporating the benefits from REGR, an ISR reactor was constructed and demonstrated to reduce emissions while increasing the efficiency of fixed calibration dual-fuel engines. The ISR reactor effectively produced hydrogen and methane syngas using hydrous ethanol as the secondary fuel source and used exhaust heat to promote the endothermic reactions. At high load and FEF, thermochemical recuperation (TCR) was achieved, yielding greater than 100% reforming efficiencies. When introduced to the engine, this generated syngas drastically reduced soot emissions, with minor benefits to NOX overall.