Preliminary Testing of n-Butanol HCCI on High Compression Ratio Diesel Engines 2019-01-0577

The control of combustion phasing in homogeneous charge compression ignition (HCCI) combustion is investigated with neat n-butanol in this work. HCCI is a commonly researched combustion mode, owing to its improved thermal efficiency over conventional gasoline combustion, as well as its lower nitrogen oxide (NOx) and particulate matter emissions compared to those of diesel combustion. Despite these advantages, HCCI lacks successful widespread implementation with conventional fuels, primarily due to the lack of effective combustion phasing control. In this preliminary study, chemical kinetic simulations are conducted to study the auto-ignition characteristics of n-butanol under varied background pressures, temperatures, and dilution levels using established mechanisms in CHEMKIN software. Increasing the pressure or temperature lead to a shorter ignition delay, whereas increasing the dilution by the application of exhaust gas recirculation (EGR) leads to a longer ignition delay. These ignition delay simulation results are used as a guide for the experimental study of n-butanol HCCI combustion on engine tests. Experiments are conducted near engine mid-load (~7 bar IMEP) on two engines with high compression ratios (i.e. 16.2:1 and 18.2:1). Selected control variables including intake boost pressure, intake air temperature, and EGR are studied to examine their effects on achieving the desired combustion phasing for optimal thermal efficiency, which is identified to lie in a narrow crank angle window of 2 to 8 °CA after top dead center for the test engines under the tested conditions. The combustion characteristics and exhaust emissions in these tests are also reported. At a compression ratio of 16.2:1, a combination of intake boost and intake heating is required to achieve stable combustion within the crank angle window of high efficiency. At a higher compression ratio of 18.2:1, boosting the intake pressure alone is sufficient to reach target combustion phasing, however the pressure rise rate reaches limiting conditions. The application of EGR is therefore necessary at the high compression ratio to achieve the target combustion phasing without exceeding the operability limits. A combination of boost pressure regulation and EGR application is effective to achieve stable operation at the 18.2:1 compression ratio.