A study of the ignition delay characteristics of combustion in a compression ignition engine operating on blended mixtures of diesel and gasoline

• Main Author: Thoo, Wei Jet
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2016

The interest to study diesel-gasoline fuel mix for CI engine combustion had been motivated by the higher thermal efficiency of CI engine compared to SI engine which gasoline normally runs in and the report of having lower NOx and PM emissions for gasoline combustion in CI engine. The experimental CI engine was unable to run on 100% gasoline but able to run on gasoline blend as high as G80 with default SOI timing setting. 100% gasoline would not run despite it contains only 20% more gasoline than G80 due to its extremely longer ignition delay caused by the exponential increase of gasoline blend’s ID. Engine brake thermal efficiencies of all gasoline blends tested up to G80 were comparable and averaged at 24.2%, 33.8% and 39.8% for engine speed-load conditions of 2000rev/min 2.5bar BMEP, 2000rev/min 5bar BMEP and 2000rev/min 8.5bar BMEP, accordingly. This finding confirmed that gasoline blend could be a new alternative fuel that offers comparable performance to the liquid fuel market for CI engine. In Europe, diesel blended with a small percentage of biodiesel or ethanol has been common to liquid fuel market. The study focused on ID that was closely correlated to NOx and soot formations in engine cylinder instead of NOx and PM emissions at tailpipe. The longer ID of 100% gasoline in relative to diesel could go up to 14CAD resulted in increased proportion of premixed combustion to mixing-controlled combustion at the rate of 40 Joule per CAD increase in ID. This incremental premixed combustion proportion was ideal for low NOx and soot formations in CI engine. ID was able to be discriminated into physical delay, a period dictated by engine speed-load conditions and controlled fuel breakup, fuel vaporisation and fuel-air mixing; and chemical delay, a period dictated by fuel chemical kinetic mechanism and controlled the amount of heat released. This finding gave valuable insight to the fact that proportion of premixed combustion and mixing-controlled combustion were controlled by chemical delay. Zero-dimensional theoretical combustion study with chemical kinetic mechanism confirmed that the exponential increased ID trend of gasoline blends was attributed to chemical delay. Hence a gasoline blend close to 100% gasoline would have very lean premixed combustion and small mixing-combustion which correlated to very low NOx and soot formations in cylinder. In order to understand the NOx and soot formations in cylinder in detail, a 73species reduced chemical kinetic mechanism that could represent gasoline blend combustion in CFD was developed. This reduced chemical kinetic mechanism could be used for future CFD work to understand effect of interactions between physical processes (fuel breakup, fuel vaporisation and fuel-air mixing) and chemical processes (activation of fuel combustion chemistry) on NOx and soot formations in cylinder. This work founded an effective semi-automatic reduction methodology with MATLAB algorithms for developing the 73species CFD-compatible reduced chemical kinetic mechanism of gasoline blends. This platform made building a surrogate fuel’s reduced chemical kinetic mechanism from multiple detailed chemical kinetic mechanisms of single component fuels fast, accessible and friendly to users of all background. DRG reduction technique had been enhanced by the multiple-stage ROP and multiple-step DRG approaches. The multiple-stage ROP and multiple-step approaches increased the species size reduction of chemical kinetic mechanisms by additional 8% and 13.5%, accordingly. The additional species size reduction capability of both approaches would be beneficial for the reduction of chemical kinetic mechanism for CFD use which is practically limited to size of 100species for feasible computational errors and speed. Apart from the limitation for the percentage of gasoline blend that could be used in the experimental CI engine, the lower compressibility of gasoline blends in relative to diesel had caused the SOI timing to be retarded up to 3CAD in this pump-triggered type of injection system. This shift of combustion phase had no significant effect on the ID and heat-release characteristics. The combustion phase shift can be easily compensated by advancing the SOI accordingly.