| Summary: | The potential of using low temperature heat release (LTHR) to improve engine efficiency is of particular interest because of its effect on autoignition chemistry in spark ignition (SI) and advanced compression ignition engines (ACI). A recently developed method has been shown to isolate LTHR in SI engine conditions, paving the way for more detailed and sophisticated analyses of LTHR characteristics and chemistry under realistic engine conditions. Previous LTHR modelling work has suggested that residuals and residual chemistry have a significant impact on LTHR characteristics. Until now, studies on isolated LTHR have focused on single-component surrogates.
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In this work, the effect of primary reference fuels (PRFs), binary blends of n-heptane and iso-octane (2,2,4-trimethylpentane), alongside blends with ethanol on LTHR characteristics was studied. Tests were conducted on a single-cylinder engine with SI geometry, motored (with the ignition system disabled) but with fuels injected. PRFs containing 50 and 75% isooctane by volume (i.e. PRF50 and PRF75) were tested, as well as pure n-heptane and iso-octane. In addition, blends of n-heptane and ethanol containing 10, 20 and 50% ethanol by volume were also tested. The blends were tested with inlet temperatures of 40°C to 140°C at an inlet pressure of 0.9 bar (absolute).
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To simulate the effect of high temperature combustion residuals, a skip spark duty cycle was employed for ignition of a PRF80 blend. A Fourier-Transform Infra-Red (FTIR) exhaust gas measurement system was employed to analyse the change in mixture composition as a result of experiencing LTHR. Ignition delay and homogeneous charge compression ignition (HCCI) simulations were performed with a gasoline surrogate chemical kinetic mechanism on CHEMKIN to assist with analysing the experimental data. At the conditions tested, the presence of iso-octane in the blend strongly reduced the intensity of LTHR whilst neat iso-octane exhibited no signs of LTHR due to its relatively lower reactivity at the conditions tested. Ignition delay contours were used to explain the results observed at the conditions tested and the HCCI modelling showed that as well as the n-heptane portion of the blends being almost entirely consumed during LTHR, significant fractions of iso-octane and ethanol were also consumed alongside it — as opposed to pro-rated consumption or single component consumption of n-heptane. Substituting ethanol into the blend reduced the LTHR intensity more dramatically because of ethanol’s interaction with n-heptane’s low temperature oxidation reactions. Skip firing was shown to have a small impact on the LTHR characteristics in cycles immediately after high temperature SI combustion cycles.
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