Development and Validation of a Quasi-Dimensional Model for (M)Ethanol-Fuelled SI Engines

Abstract

-Methanol and ethanol are interesting spark-ignition engine fuels, both from a production and an end-use point of view. Despite promising experimental results, the full potential of these fuels remain to be explored. In this respect, quasi-dimensional engine simulation codes are especially useful as they allow cheap and fast optimization of engines. The aim of the current work was to develop and validate such a model for methanol-fuelled engines. Several laminar burning velocity correlations and turbulent burning velocity models were implemented in a QD code and their predictive performance was assessed for a wide range of engine operating conditions.The effects of compression ratio and ignition timing on the in-cylinder combustion were well reproduced irrespective of the employed correlation or model. However, to predict the effect of changes in mixture composition, the correlation and model selection proved crucial. Compared to existing correlations, a new correlation developed by the current authors led to better reproduction of the effects of equivalence ratio and residual gas content and the combustion duration.For the turbulent burning velocity, the models of Damköhler and Peters consistently underestimated the influence of equivalence ratio and residual gas content on the combustion duration, while the Gülder, Leeds, Zimont and Fractals model corresponded well with the experiments. The combination of one of these models with the new correlation can be used with confidence to simulate the performance and efficiency of methanol-fuelled engines.INTRODUCTION -The use of sustainable liquid alcohols in spark-ignition engines offers the potential of decarbonizing transport and securing domestic energy supply while increasing engine performance and efficiency compared to fossil fuels thanks to a number of interesting properties [1,2]. The most significant interesting properties of light alcohols include:-High heat of vaporization, which causes considerable charge cooling as the injected fuel evaporates -Elevated knock resistance, which allows to apply higher compression ratios (CR), optimal spark timing and aggressive downsizing. -High flame speeds, enabling qualitative load control using mixture richness or varying amounts of exhaust gas recirculation (EGR). The potential of neat light alcohol fuels (methanol and ethanol) has been demonstrated experimentally in both dedicated and flex-fuel alcohol engines [1]. In dedicated alcohol engines high compression ratios enable peak brake thermal efficiencies up to 42%, while throttleless load control using EGR allows to spread the high efficiencies to the part load regions [1,3]. In flex-fuel engines the CR is limited due to knock constraints associated with gasoline operation, but still relative power and efficiency benefits of about 10% and NO x reductions of 5-10 g/kWh can be obtained over the entire load range thanks to more isochoric combustion, optimal ignition timing and reduced flow and dissociation losses [1].

Modeling End-Gas Autoignition of Ethanol/Gasoline Surrogate Blends in the Cooperative Fuel Research Engine

“…Residual Gas Submodel. A full-flow GT-Power 30 model of the standard CFR engine was used to calculate the residual gas fraction, taking into account the discharge coefficients of the intake and exhaust valves 31 and their measured lift profiles. Dependent upon the fuel (and hence the CCR), the calculated residual gas fraction ranges from 5 to 7% (see Table 3) under standard RON conditions.…”