Study On Pre-ignition Heat Release And Antiknock Performance Of Fuels With Different Octane Sensitivities

Gasoline compression ignition(GCI)is a promising combustion concept owing to its high thermal efficiency and low pollutant emissions.Interactions between fuel and engine are basic issues for GCI engines.In order to suppress knocking,engines often use high octane number fuel(RON> 90).But some studies show that low octane number fuel with RON = 60～80 is more suitable for GCI engines.In addition,because GCI combustion mode is a low temperature combustion,there will be a significant preignition heat release,which has an important impact on the combustion process.Therefore,in this paper,four kinds of low-octane fuels with different sensitivities were used to investigate the pre-ignition heat release phenomena.Meanwhile,the anti-knock performance of fuels with different sensitivities was also analyzed under high load.Firstly,the CHEMKIN software with well-validated surrogate and kinetic models was used to explore the pre-ignition heat release phenomenon of four toluene primary reference fuels(TPRF)with the same research octane number(RON)but different sensitivity(S).With addressing ignition characteristics and pre-ignition heat release,the role of fuel sensitivity,inlet conditions,and exhaust gas recirculation(EGR)on fuel reactivity was clearly demonstrated.The results demonstrate that in high-temperature low-pressure regime,the pre-ignition heat release only manifests as ITHR,and its magnitude increases with the increase of fuel sensitivity.Therefore,there is a positive correlation between fuel reactivity and fuel sensitivity in this regime.However,the preignition heat release is transformed into LTHR in low-temperature regime,and its magnitude increase with the decrease of fuel sensitivity,resulting in a negative correlation between the fuel reactivity and fuel sensitivity.In intermediate temperature and pressure regime,fuels of different sensitivities have similar pre-ignition heat release,and thus different sensitivities have similar reactivity.Finally,the effects of cooling effect and chemical effect of EGR on pre-ignition heat release are studied.The results show that the chemical effect of EGR has a significant effect on the pre-ignition heat release.As the EGR rate increases,the magnitude of pre-ignition heat release decreases.However,the cooling effect of EGR has no effect on pre-ignition heat release.Then,an experimental study was conducted on a single-cylinder gasoline engine to compare the indicated mean effective pressure(IMEP),combustion stability limits,and engine emissions at different injection timings(SOI),and then the best SOI is obtained.Under the optimal SOI,the role of fuel sensitivity in the combustion process was studied.Through the analysis of LTHR before auto-ignition,the effects of intake conditions and fuel sensitivity on the combustion phasing are summarized.The results show that LTHR only occurs when the SOI is sufficiently advanced,and earlier injection timing can achieve stable combustion,higher IMEP and lower NOx emissions.A strong LTHR can promote the advancement of the combustion phasing,and the magnitude of the LTHR is negatively correlated with the intake temperature and positively correlated with the intake pressure.Finally,the effect of octane sensitivity on LTHR was also studied,which shows that the impact of LTHR between different sensitivity fuels is mainly reflected in combustion phasing,and the onset of LTHR is more advanced for low sensitivity fuels.Finally,the suppression of knocking phenomena of different sensitivity fuels at different ignition timings is explored.The results show that as the spark timing advances,the peak in-cylinder pressure gradually increases,and when the spark timing is earlier,a severe pressure oscillation can be detected in the cylinder.For fuels with the same octane number but different sensitivities,for fuels with higher sensitivity,the ability to suppress knocking are stronger.