Effect of Intake Temperature and Engine Speed on the Auto-Ignition Reactivity of the Fuels for HCCI Fuel Rating

Paper in proceeding, 2021

The current research octane number (RON) and motor octane number (MON) gasoline tests are inadequate for describing the auto-ignition reactivity of fuels in homogeneous charge compression ignition (HCCI) combustion. Intake temperature and engine speed are two important parameters when trying to understand the fuel auto-ignition reactivity in HCCI combustion. The objective of this study was to understand the effect of high intake temperature (between 100 and 200 °C) and engine speed (600 and 900 rpm) on the auto-ignition HCCI reactivity ratings of fuels using an instrumented Cooperative Fuel Research (CFR) engine. The fuels used for this study included blends of iso-octane/n-heptane, toluene/n-heptane, ethanol/n-heptane, and gasolines with varying chemical compositions and octane levels. The CFR engine was operated at 600 and 900 rpm with an intake pressure of 1.0 bar and an excess air ratio (lambda) of 3. It was found that the relative HCCI reactivity ranking of the gasolines with a RON of 98 was constant at both engine speeds and high intake temperatures (150 °C and 200 °C). This implied that for HCCI fuel ratings, there exists a threshold intake temperature beyond which further increases in temperature do not change the relative rankings of the fuel's HCCI reactivity. At these high intake air temperatures, changes in engine speed between 600 and 900 rpm did not affect the HCCI ratings of the RON98 gasolines either. However, the effects of intake temperature (between 150 and 200 °C) and engine speed (600 and 900 rpm) did become apparent when additional gasoline octane levels and chemical compositions were investigated.