Isolated Low Temperature Heat Release in Spark Ignition Engines

Low temperature heat release (LTHR) has been of interest to researchers for its potential to mitigate knock in spark ignition (SI) engines and control auto-ignition in advanced compression ignition (ACI) engines. Previous studies have identified and investigated LTHR in both ACI and SI engines before the main high temperature heat release (HTHR) event by appropriately curating the in-cylinder thermal state during compression, or in the case of SI engines, timing the spark discharge late to reveal LTHR (sometimes referred to as pre-spark heat release). In this work, LTHR is demonstrated in isolation from HTHR events. Tests were run on motored single-cylinder engines and inlet air temperatures and pressures were adjusted to realise LTHR from n-heptane and iso-octane (2,2,4-trimethylpentane) without entering the HTHR regime. LTHR was observed for a lean n-heptane-air mixture at inlet temperatures ranging from 60°C to 100°C and inlet pressures of 0.9 bar (absolute). For temperatures below 60°C LTHR was not detected and for temperatures above 100°C measurements could not be taken due to the presence of HTHR. No LTHR was detected for iso-octane at 0.9 bar inlet pressures for the same conditions. Following predictions from chemical kinetics modelling in CHEMKIN (and previous studies), intake pressures were increased to 1.1 bar and 1.5 bar, which successfully led to the realisation of LTHR from iso-octane. The effect of temperature, pressure, and engine speed on the presence, intensity and phasing of LTHR are presented alongside pressure-temperature trajectories of the in-cylinder gases to explain the trends.