Reactivity controlled compression ignition combustion in a light duty diesel engine using alternative fuels

Abstract

Reactivity controlled compression ignition (RCCI) combustion has been introduced to implement controllable, clean, and high thermal efficiency without undermining the advantages of premixed combustion. However, simultaneous autoignition introduced by RCCI combustion affects the combustion under higher load operations. A series of experiments was conducted on a light duty diesel engine operated in the RCCI mode to study the effect of oxygenated fuel blends, fuel inhomogeneity, combustion timing, rate of heat release, exhaust gas recirculation and load extension. Gasoline, ethanol and methanol as port base fuels and diesel as base direct fuel were used for the experiments. The engine was tested at steady state conditions. A mixture of alcohol fuels and diethyl ether (DEE) as high reactive fuel presented inhomogeneity in an actual engine combustion and resulted in high temperature heat release (HTHR) at two different stages. The addition of diethyl ether in the ethanol blend resulted in dual phase heat release combustion and advanced ignition phasing of the prevailing heat release and suppressed the peak pressure rising rate and knocking tendency. It governed the end-gas heat release pattern and improved the indicated mean effective pressure (IMEP). Resultant dual phase heat release and dominant premixed combustion enhanced the fuel oxidation and reduction of soot precursors. Alternatively, diethyl ether addition increased the in-cylinder maximum pressure. With regard to the RCCI hypothesis, higher reactivity of DEE will enhance oxidation of hydrocarbons, thus resulted in lower HC emissions. Changing gasoline /diesel RCCI composition with gasoline /diesel-n-butanol has slight effects on engine IMEP reduction and combustion parameters. By raising the presence of n-butanol in the fuel mixture (in this case from 0% to 40%), the premixed ratio of optimum IMEP value decreased by as much as 15% (from rp=85% to rp=70%). It was found that high reactive fuel can be used as blending portion of port fuel to effectively control combustion timing and extend the high load region. In the present study, the three dimensional model with detailed chemical kinetics was also employed to investigate the second law analyses of reactivity controlled combustion with iso-octane / nheptane.