Effect of air dilution and effective compression ratio on the combustion characteristics of a HCCI (homogeneous charge compression ignition) engine fuelled with n-butanol

Biobutanol, i.e. n-butanol, is a promising biofuel of internal combustion engines. In this work, HCCI (homogeneous charge compression ignition) operation was achieved on a single cylinder four-stroke n-butanol engine through altering EVC (exhaust valve closing) timings and/or IVC (intake valve closing) timings when the amount of fuel injected in a cycle was fixed. To retard HCCI autoignition timing, the effect of air dilution (excess air) and ECR (effective compression ratio) on combustion event was investigated. The results show that air dilution and the decrease in ECR can retard the autoignition timing of n-butanol and effectively decrease the MPRR (maximum pressure rise rate) of the HCCI engine. However, the delayed autoignition timing caused by diluted air is restricted by combustion instability. Furthermore, in the case of air dilution, the indicated thermal efficiency is higher and the temperature at IVC timing is lower while the in-cylinder temperature to initiate autoignition is higher compared with that of decreased ECR. In addition, air dilution is more effective in reducing the MPRR, but leads to longer combustion duration as compared to decreased ECR.