| Application of spark-ignition to compression-ignition(SICI)mode to gasoline engine operating in narrow range for hybrid vehicles enables knock-free combustion under ultra-high compression ratio(CR).Meanwhile,gasoline blended with ethanol has low-carbon trend of energy and can further increase flame speed,enhance chemical reactivity at intermediate-to-high temperature and optimize the combustion phasing.In this thesis,the characteristics of auto-ignition and flame propagation for fuels containing ethanol were investigated based on experiments conducted on a rapid compression machine(RCM)and a thermal engine combined with numerical simulation.Through fuel-engine co-optima research,a dedicated combustion system with SICI applied has been developed.This new combustion system could result in high thermal efficiency under middle-to-high engine loads.The autoignition characteristic of 11 ethanol-gasoline surrogate fuels was investigated using RCM by decoupling octane number and sensitivity of each fuels.This investigation was focused on the reactivity variation,critical condition before autoignition,knock characteristic and dilution tolerance under lean-burn and high-CR conditions.Results show that auto-ignition is the necessary but not sufficient condition for pressure oscillation.The synergistic effect on auto-ignition between ethanol and toluene exists,which shortens ignition delay time.By adjusting the percentage of ethanol,intermediate octane number and sensitivity of the blended fuel could meet both requirements for high flame speed and low auto-ignition intensity and be adaptable for the strong dependency of ethanol on high-temperature in SICI mode.Regarding flame characteristics,results show that ethanol has a stronger dependence of pressure on flame speed than that of iso-octane and toluene in homogeneous mixtures.For iso-octane with great low-temperature heat release,H radical and critical reactions dominate pressure dependence on flame speed in negative temperature coefficient(NTC)and non-NTC region respectively.For toluene without NTC behavior,except for aforementioned two factors,three-body chain termination reactions on flame speed were also significant Furthermore,it is proved by simulation that flame speed had a positive correlation with stratification degree under enginerelevant conditions.Increasing temperature and pressure of mixture can extend lean limit in stratified mixtures.To verify ethanol's effect on SICI combustion,two methods of splash blending and octane number controlled blending were applied to forming the fuels with ethanol blends.Results of engine experiments also show the synergistic effect of species on auto-ignition between ethanol and aromatics.The constant volume ratio and maximum thermal efficiency are increased with increasing ethanol content,while particle number concentration is decreased.However,ethanol concentration exceeding the upper limit increases the risk of deterioration in nucleation particles.Ethanol under the thermodynamic state at top dead center shows the chemical inertness,which is the obstacle to further improve the characteristic of fuel economy and pollutant emission.Based on the above research for understanding the auto-ignition,flame propagation and engine performance with blended fuels,a dedicated combustion system for SICI was developed using 3-D simulation to study the in-cylinder flow,spray and achieve fuelengine co-optimization.Intake port and combustion chamber were redesigned.Based on the optimal combustion system,the three stage including “initial flame propagation-rapid flame-moderate autoignition” and indicated thermal efficiency over 45% were achieved with excess air ratio over 1.6 under medium-to-high engine loads.Furthermore,the method for avoiding accidental pre-ignition and identifying autoignition phasing was proposed under no in-cylinder pressure condition.It was validated under different operating conditions for reducing the application cost of SICI mode. |
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