Affiliation:
1. University of Oxford, Engineering Science, UK
2. Southwest Research Institute, USA
Abstract
<div>Low-temperature heat release (LTHR) is of interest for its potential to help
control autoignition in advanced compression ignition (ACI) engines and mitigate
knock in spark ignition (SI) engines. Previous studies have identified and
investigated LTHR in both ACI and SI engines before the main high-temperature
heat release (HTHR) event and, more recently, LTHR in isolation has been
demonstrated in SI engines by appropriately curating the in-cylinder thermal
state during compression and disabling the spark discharge. Ethanol is an
increasingly common component of market fuel blends, owing to its renewable
sources. In this work, the effect of adding ethanol to iso-octane
(2,2,4-trimethylpentane) blends on their LTHR behavior is demonstrated. Tests
were run on a motored single-cylinder engine elevated inlet air temperatures and
pressures were adjusted to realize LTHR from blends of iso-octane and ethanol
without entering the HTHR regime. The blends were tested with inlet temperatures
of 40°C–140°C at equivalence ratios of 0.5, 0.67, and 1.0 with boosted (1.5
barA) conditions. The measured LTHR decreased with increasing ethanol content
for all conditions tested; iso-octane–ethanol blends with above 20% ethanol
content (by volume) showed minimal LTHR under engine conditions. These net
effects resulted from the combination of thermal effects (charge cooling) and
chemical effects (reactivity changes at low temperatures). The effect of
temperature, pressure, fuel composition, and equivalence ratio on ignition delay
times calculated from chemical kinetic modeling are presented alongside
pressure–temperature trajectories of the in-cylinder gases to explain the
trends. The underlying cause of the trends is explained by using a sensitivity
analysis to determine the contribution of each reaction within the chemical
kinetic mechanism to first-stage ignition, revealing the effect of introducing
ethanol on the OH radical pool and resulting LTHR intensity.</div>