Investigation On Combustion Characteristics Of A Homogeneous Charge Compression Ignition Engine Fuelled With N-Butanol/Ethanol-Gasoline Blends At High Exhaust Gas Dilution

n-Butanol is a very promising bio-alternative fuel of internal combustion engines in the future.Homogeneous charge compression ignition（HCCI）combustion can effectively improve the thermal efficiency of conventional gasoline engines.The combination of n-butanol and HCCI engine can reduce the dependence of vehicle on fossil energy.By the use of negative valve overlap strategy,the combustion and emission characteristics of HCCI engine fuelled with different n-butanol/ethanol-gasoline blends were investigated on a single cylinder gasoline engine at indicated mean effective pressure（IMEP）≤0.3 MPa and equivalent air-fuel ratio.The results can be drawn as follows:The engine fueled with n-butanol/ethanol-gasoline can operate stably in the HCCI combustion,and autoignition timing is related to alcohol types and its content in the blends.With the increase of n-butanol/ethanol volume fraction in the blends,ignition is advanced,combustion duration is shortened,the maximum pressure rise rate is increased,and the indicated thermal efficiency and IMEP is decreased at the same speed,valve timing and lift,and equivalent air-fuel ratio.At the same alcohol volume fraction,autoignition timing of HCCI engine for the blends with n-butanol is earlier and combustion duration is shorter than ethanol-gasoline blends,while IMEP and indicated thermal efficiency are lower for n-butanol-gasoline blends relative to those of ethanol-gasoline blends.In the operating conditions with exhaust gas recirculation（IEGR）more than 38%by mass in the mixture,with the IEGR increases,autoignition timing for gasoline is postponed,but autoignition timing for n-butanol and ethanol is advanced.Alcohol-gasoline fuel types and blend ratios also affect the emission characteristics of HCCI engines.With the increase of n-butanol blend ratio,oxides of nitrogen emissions from HCCI engines decrease.At 1500 rpm,as the n-butanol/ethanol blending ratio increases,formaldehyde emissions,acetaldehyde emissions,ethylene emissions,and methane emissions of the HCCI engine increase gradually,but aromatic hydrocarbon emissions will drastically decrease.The propylene emissions decrease for the ethanol-gasoline,but increase for the n-butanol-gasoline.At the same alcohol volume fraction,the acetaldehyde emissions and methane emissions of the n-butanol-gasoline were lower than those of the ethanol-gasoline,respectively,but the formaldehyde emissions,ethylene emissions,and aromatic hydrocarbon emissions were higher than those of the ethanol-gasoline.Both decreased effective compression ratio（ECR）and air dilution methods can retard ignition timing of n-butanol HCCI engine,reduce the maximum pressure rise rate and improve the indicated thermal efficiency.At 1500rpm and 11.3mg of fuel injected in a cycle,the ignition time is postponed from-6.8^oCA ATDC to 3.3^oCA ATDC though two different methods,in case of air dilution,the combustion duration is longer,the maximum pressure rise rate is reduced more rapidly and indicated thermal efficiency is higher compared with that of decreased ECR.The coefficient of variation of HCCI engine increases in case of air dilution.The instability of combustion limits the use of air dilution methods to delay the ignition time.IEGR,molecular structure of fuel and in-cylinder temperature at IVC timing(T_IVC)all affect low-temperature combustion.The results show that the ignition timing of n-butanol is earlier than that of iso-octane,as IEGR increases from 0%to50%,the ignition time of iso-octane is postponed more crank angle than that of n-butanol.At the low-temperature stage,the mainα-hydroxybutyl produces n-butyraldehyde and HO₂·through the addition of oxygen.This reaction inhibits OH·low-temperature chain branching reaction,but it promotes the production of HO₂·and the accumulation of H₂O₂.The primary carbon atoms and branched chain structures in the iso-octane molecule increase the activation energy of oxygenation and isomerization reaction,which not only inhibits the OH·low-temperature chain branching reaction,but also is not conducive to the production of HO₂·and H₂O₂.The H₂O₂ peak concentration of n-butanol is 6.24 times that of isooctane.When the temperature exceeds 1000K,the chain reaction of OH·generated by H₂O₂ is accelerated,leading to earlier ignition timing of n-butanol.The H₂O in the residual gas can promote the decomposition of H₂O₂ to OH·.n-Butanol is prone to produce HO₂·and accumulates H₂O₂ during low-temperature oxidation,which is beneficial for n-butanol to produce more OH·under high exhaust gas dilution conditions.When T_IVCVC are 555K,575K and 595K,the inhibition of residual exhaust gas on low-temperature combustion process of n-butanol and iso-octane has a similar tendency.