The influence of blending n-butanol at 20% by volume on the ignition delay times for a reference gasoline was studied in a rapid compression machine (RCM) for stoichiometric fuel/air mixtures at 20 bar and 678 K-858 K. Delay times for the blend lay between those of stoichiometric gasoline and stoichiometric n-butanol across the temperature range studied. At lower temperatures, delays for the blend were however, much closer to those of n-butanol than gasoline despite n-butanol being only 20% of the mixture. Under these conditions n-butanol acted as an octane enhancer over and above what might be expected from a simple linear blending law. The ability of a gasoline surrogate, based on a toluene reference fuel (TRF), to capture the main trends of the gasoline/ n-butanol blending behaviour was also tested within the RCM. The 3-component TRF based on a mixture of toluene, n-heptane and iso-octane was able to capture the trends well across the temperature range studied. Simulations of ignition delay times were also performed using a detailed blended nbutanol/TRF mechanism based on the adiabatic core assumption and volume histories from the experimental data. Overall, the model captured the main features of the blending behaviour, although at the lowest temperatures, predicted ignition delays for stoichiometric n-butanol were longer than those observed. A bruteforce local sensitivity analysis was performed to evaluate the main chemical processes driving the ignition behaviour of the TRF, n-butanol and blended fuels. The reactions of fuel + OH dominated the sensitivities at lower temperatures, with H abstraction from nn-butanol and the blend. At higher temperatures the decomposition of H 2 O 2 and reactions of HO 2 and that of formaldehyde with OH became critical, in common with the ignition behaviour of other fuels. Remaining uncertainties in the rates of these key reactions are discussed.

The impact of n-butanol blending on the combustion, autoignition and knock properties of gasoline has been investigated under supercharged spark ignition engine conditions for stoichiometric fuel/air mixtures at intake temperature and pressure conditions of 320 K and 1.6 bar, respectively, for a range of spark timings. A toluene reference fuel (TRF) surrogate for gasoline containing toluene, n-heptane and iso-octane has been tested experimentally in the Leeds University Ported Optical Engine (LUPOE) alongside a reference gasoline and their blends (20 % n-butanol and 80 % gasoline/TRF by volume). Although the gasoline/n-butanol blend displayed the highest burning rate, and consequently the highest peak pressures compared to gasoline, TRF and the TRF/n-butanol blend, it exhibited the least propensity to knock, indicating that addition of n-butanol provides an opportunity for enhancing the knock resistance of gasoline as well as improving engine efficiency via the use of higher compression ratios. The anti-knock enhancing quality of n-butanol on gasoline was however observed to weaken at later spark timings. Hence, whilst n-butanol has shown some promise based on the current study, its application as an octane enhancer for gasoline under real engine conditions may be somewhat limited at the studied blending ratio. As expected based on previous ignition delay studies, the TRF showed an earlier knocking boundary than the rest of the fuels, which may possibly be attributed to the absence of an oxygenate (ethanol or n-butanol) as present in the other fuels and a lower octane index. Overall, the TRF mixture gave a reasonable representation of the reference gasoline in terms of the produced knock onsets at the later spark timings for the pure fuels. However, on blending, the TRF did not reproduce the trend for the gasoline at later spark timings which can be linked to difficulties in capturing the temperature trends in ignition delays around the negative temperature coefficient region observed in previous work in a rapid compression machine (

A global uncertainty analysis is performed for three current mechanisms describing the low temperature oxidation of dimethyl ether (Aramco Mech 1.3, Zheng et al. 2005, Liu et al. 2013) with application to simulations of species concentrations (CH 2 , H 2 O 2 , CH 3 OCHO) corresponding to existing data from an atmospheric pressure flow reactor, and high pressure ignition delays. When incorporating uncertainties in reaction rates within a global sampling approach, the distributions of predicted targets can span several orders of magnitude. The experimental profiles however, fall within the predictive uncertainty limits. A variance based sensitivity analysis is then undertaken using high dimensional model representations. The main contributions to predictive uncertainties come from the CH 3 OCH 2 +O 2 system, with isomerisation, propagation, chain-branching, secondary OH formation and peroxy-peroxy reactions all playing a role. The response surface describing the relationship between sampled reaction rates and predicted outputs is complex in all cases. Higher-order interactions between parameters contribute significantly to output variance, and no single reaction channel dominates for any of the conditions studied. Sensitivity scatter plots illustrate that many different parameter combinations could lead to good agreement with specific sets of experimental data. The Aramco scheme is then updated based on data from a recent study by Eskola et al. which presents quite different temperature and pressure dependencies for the rates of CH 3 OCH 2 O 2 CH 2 OCH 2 O 2 H and CH 2 OCH 2 O 2 H OH+2CH 2 O compared with currently used values, and includes well skipping channels. The updates from Eskola worsen the agreement with experiments when used in isolation. However, if the rate of the CH 2 OCH 2 O 2 H+O 2 channel is subsequently reduced, very good agreement can be achieved. Due to the complex nature of the response surface, the tuning of this channel remains speculative. Further detailed studies of the temperature and pressure dependence of the CH 3 OCH 2 O 2 +O 2 , CH 2 OCH 2 O 2 H+O 2 system are recommended in order to reduce uncertainties within current DME mechanisms for low temperature conditions.

The ability of a mechanism describing the oxidation kinetics of toluene reference fuel (TRF)/nbutanol mixtures to predict the impact of n-butanol blending at 20% by volume on the autoignition and knock properties of gasoline has been investigated under conditions of a strongly supercharged spark ignition (SI) engine. Simulations were performed using the LOGEengine code for stoichiometric fuel/air mixtures at intake temperature and pressure conditions of 320 K and 1.6 bar, respectively, for a range of spark timings. At the later spark timing of 6 °CA bTDC, the predicted knock onsets for a gasoline surrogate (toluene reference fuel, TRF) and the TRF/n-butanol blend are higher compared to the measurements, which is consistent with an earlier study of ignition delay times predicted in a rapid compression machine (RCM, Agbro et al., Fuel, 2017, 187:211-219). The discrepancy between the predicted and measured knock onsets is however quite small at higher pressure and temperature conditions (spark timing of 8 °CA bTDC) and can be improved by updating a key reaction related to the toluene chemistry. The ability of the scheme to predict the influence of n-butanol blending on knock onsets requires improvement at later spark timings. The simulations highlighted that the low-intermediate temperature chemistry within the SI engine end gas, represented by the presence of a cool flame and negative temperature coefficient (NTC) phase, plays an important role in influencing the high temperature heat release and consequently the overall knock onset. This is due to its sensitisation effect (increasing of temperature and pressure) on the end gas and reduction of the time required for the high temperature heat release to occur. Therefore, accurate representation of the low-intermediate temperature chemistry is crucial for predicting knock. The engine simulations provide 2 temperature, heat release and species profiles that link conditions in practical devices and ignition delay times predicted in an RCM. This facilitates a better understanding of the chemical processes affecting knock onsets predicted within the engine and the main reactions governing them.

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