| Short-chain alcohols are considered as the most potential substitutes for internal combustion engines.In recent years,they have become one of the highlights in the research of internal combustion engines.As alternative fuels,alcohols are generally mixed with gasoline or diesel or in the form of dual fuels.However,in the current studies,the mechanism of the effects of alcohols on the combustion and emissions of internal combustion engines is not yet clear.The differences in the effects of between different alcohols on engines are also less involved.In this paper,the effects of alcohols on diesel’s ignition delays in a rapid compression machine(RCM)and engine combustion and emission performance were studied based on the carbon chain length of linear alcohols(ethanol,n-propanol,n-butanol,n-pentanol),the oxygen content of fuel blends and the molecular structures of butanol isomers(n-butanol,iso-butanol,tert-butanol,sec-butanol).First of all,rapid compression machine experiments of alcohol/diesel blends were carried out.The experimental pressure and equivalence ratio are 10 bar and 1 respectively,with a temperature range of 670~850 K.Alcohol can increase the ignition delay of diesel.In the same blend ratio,the effects of alcohols on the increase of ignition delay are ethanol ≈ npropanol > n-butanol > n-pentanol,iso-butanol ≈ sec-butanol > n-butanol > tert-butanol.The effect of alcohols on the promoting the ignition delay of diesel is the dilution effect and the competation for OH with diesel in the low temperature reactions.However,t-butanol has a relatively pronounced low-temperature reaction and has minimal effect on the ignition delay of diesel fuel.The engine load characteristics of different fuels were measured on a single-cylinder diesel engine.It was found that various kinds of alcohols can obviously delay the combustion phase of diesel engine and prolong the ignition delay.Their effects is similar with that in the rapid compression machine.The longer ignition delay will result the shorter combustion duration and the higher maximum temperature and pressure in the cylinder.In the terms of main gas emissions,the blending of alcohols will cause the CO emissions to increase obviously under small loads and slightly decrease slightly under heavy loads.NOx emissions are slightly higher than diesel,while there is not obvious difference between different fuels.HC emissions slightly increase compared with that of diesel fuels.PM emissions are higher than that of diesel fuels in total number,but the geometry mean diameters decrease obviously.The characteristics of three kinds of unregulated emissions were studied.Through the reaction path analysis,it was found that the formaldehyde emission of iso-butanol/diesel is the highest because the formaldehyde producing path of iso-butanol is the richest and iso-butanol can be directly oxidized to formaldehyde after Hatom abstraction reactions.Comparing different alcohols,the efficiencies and emissions of n-pentanol and tert-butanol are more advantageous than other alcohols.Finally,the combustion and emission performances of butanol isomers/diesel in two kinds of low temperature combustion mode,EGR and PCCI were studied.It was found that with the increase of EGR rate,the combustion phase difference of various fuels is more significant.When the EGR rate is more than 40%,the combustion in cylinder deteriorates obviously and the emission increases sharply.In the PCCI mode,the CO emissions were significantly higher than that of the original engine,with a little decrease of NOx emissions and a little increase of HC emissions.The reduction in PM emissions is remarkable.Unregulated emissions such as formaldehyde,acetaldehyde,ethylene and methane have increased.In PCCI mode,the engine injection and EGR strategy need to be further optimized to achieve the best engine performance. |
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