The Combustion Characteristics And Mechanism For Diesel/Ethanol Dual Fuels

The increasing depletion of fossil fuels and increasingly severe environmentalproblems is the great challenge for the internal combustion engine industry. Lookingfor a suitable alternative fuel has become a research hotspot. Ethanol as one of themost promising renewable bio-fuel has been used as alternative fuels worldwide sinceit could been blended with gasoline. Due to ethanol’s high octane number,diesel/ethanol dual fuel, rather than pure ethanol could be used in conventional dieselengine to maintain or even reduce emission and fuel consumption. However, thenature of such dual fuel compound combustion has not been clearly understood. Tofurther investigate such combustion characteristic, experiments on engine bench,constant volume bomb and low pressure laminar premixed flame were carried out,numerical simulations are also performed with detailed chemical kinetics mechanisms.A skeletal mechanism model is proposed and validated based on experimental results.The constant volume bomb is used to investigate into the ignition characteristicsof diesel under atmosphere of ethanol-air mixture. Introduction of ethanol extends theignition delays and flame left length. As the initial temperature increases, the dieselignition delay becomes shorter. Such ignition delay could also be found on the realengine used in this study.For emissions characteristics during the engine bench test show that ethanolsubstantially suppress the production of NOx and soot. However, emissions of HC,especially aldehyde, illustrate a considerable increase under dual fuel combustionmode.Mole fractions of combustion intermediates of the dual fuel flames are measuredby low pressure laminar premixed flame combined with synchrotron vacuumultraviolet photoionization mass spectrometry. Experimental results show that theintroduction of ethanol has no substantive effect on the dissociation and oxidation ofdiesel (n-heptane) whereas it does greatly increase the mole fractions of aldehydes,especially acetaldehyde. In addition, ethanol significantly reduce the mole fraction ofPAH which are generally regarded as soot precursor. Chemical kinetics simulation of the oxidation of n-heptane as for diesel referencefuel and n-heptane/ethanol blends is carried out based on CHEMKIN softwarerespectively. Calculation results show that both cool-flame reaction at lowtemperature and negative temperature coefficient of n-heptane region are extended,resulting in a delay of overall reaction process. Further analysis of reaction path andproduction rate indicate that the interaction of between the two fuel molecule proceedvia the free radical pool in which the OH·and HO2·is the most important free radicals.Addition of ethanol at low temperature, converses OH· into relatively inactive ofH2O2, thus inhibits the first step H+abstraction of n-heptane, resulting in an inhibitionof the entire fuel oxidation process.A skeletal mechanism model of the oxidation of n-heptane and ethanol isproposed on the basis of the analysis above. The model includes50reactions and41species. Predictions of the model are validated with detailed mechanism on ignitionand testing data from a HCCI engine. The results show that the skeletal model yieldshigh consistency compared with detailed mechanism.