Experimental Research Of The Impact On Low Temperature Combustion Using Oxygenated Biofuels With Different Molecular Structure

Using exhaust gas recovery(EGR)to realize low temperature combustion(LTC)is an important way to meet future emission regulations on diesel engine.EGR could largely reduce NO_x emission,but there is a soot-bump area at high EGR rates.Improving diesel fuel properties by blending suitable alternative biofuel is the simplest way to reduce soot while still maintaining low NO_x emission,which could meet the energy diversified development requirement and reduce the reliance on traditional oil resources.The current study was conducted on a modified single-cylinder diesel engine.In addition to pure diesel,the fuel used in the experiment involved three oxygenated hydrocarbons with different ring molecular structures,including cyclohexanone,cyclohexanol and 2,5-dimethylfuran(DMF),as well as another three oxygenated hydrocarbons with chain molecular structures,including 2-hexanone,n-hexanol,and isopropyl ether.These fuels were blended with diesel separately to form six blended fuels with the same oxygen content(4%wt.).The experiments were carried out at 60mg/cycle equivalent diesel at different EGR.The results indicate that the effects of oxygenated fuels on combustion and emissions are mainly affected by the cetane number and oxygen content,while the combustion is mainly affected by the cetane number.The ignition delay of blending fuels is longer and the peak of heat release rate is higher than neat diesel,which is more obvious at high EGR region.Oxygenated additives with ring structure present more significant influence on combustion than chain structureones,andtheignitiondelayranksinthisorder:DMF>cyclohexanol>cyclohexanone.Longer ignition delay is beneficial for the fuel-air mixing process,thus lower soot emission can be obtained.The effect of the cetane number and oxygen content can be isolated by adding a small percentage of cetane improver(2-ethylhexyl nitrate(EHN))to the blended fuels.The results show that the soot reduction capabilities of different oxygen functional groups increase in this order:alcohols<ketones<ethers.The oxygenated fuels with ring molecular structures tend to produce more soot compared to these ones with chain structures.Besides,oxygen functional groups located in the middle of the carbon chain is more conducive to reduce soot emission than the ones located at the end of the carbon chain.In general,it is observed that DMF shows the most advantage on achieving high efficient and clean low temperature combustion.In the following study,the blending ratio of DMF was increased to 40%and experiment was carried out at different loads,wherein the NO_x emission was fixed at0.4g/kW·h by adjusting EGR.The results indicate that the soot emission can be further reduced with higher blending ratio.However,the excessively high maximum pressure rise rate(MPRR)posed challenge to the operation and noise control of the engine.Therefore,injection strategy optimization and combustion mode comparisons were was conducted to improve the MPRR issue.The results show that retarding CA50 could effectively reduce MPRR and soot emission by introducing less EGR.Applying pilot-main injection strategy could only reduce MPRR with small pilot-main interval and the shorter ignition delay resulted in higher soot emission.Within DMF-Diesel dual-fuel RCCI combustion mode,MPRR also decreases largely along with higher soot emission due to the deteriorated mixing process with directly injected diesel at this condition.The THC and CO emissions also increase due to the high volatility of DMF.Nearly zero soot emission at wide range of CA50 can be obtained by increasing the DMF premixed ratio up to 80%,while the MPRR is still lower than that of DMF40 blended fuel.