Modelling of Self-Ignition in Spark-Ignition Engine Using Reduced Chemical Kinetics for Gasoline Surrogates

Ahmed Faraz Khan,Philip John Roberts,Alexey A Burluka

doi:10.3390/fluids4030157

Ahmed Faraz Khan, Philip John Roberts + Show 1 more

Open Access

https://doi.org/10.3390/fluids4030157

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Abstract

A numerical and experimental investigation in to the role of gasoline surrogates and their reduced chemical kinetic mechanisms in spark ignition (SI) engine knocking has been carried out. In order to predict autoignition of gasoline in a spark ignition engine three reduced chemical kinetic mechanisms have been coupled with quasi-dimensional thermodynamic modelling approach. The modelling was supported by measurements of the knocking tendencies of three fuels of very different compositions yet an equivalent Research Octane Number (RON) of 90 (ULG90, PRF90 and 71.5% by volume toluene blended with n-heptane) as well as iso-octane. The experimental knock onsets provided a benchmark for the chemical kinetic predictions of autoignition and also highlighted the limitations of characterisation of the knock resistance of a gasoline in terms of the Research and Motoring octane numbers and the role of these parameters in surrogate formulation. Two approaches used to optimise the surrogate composition have been discussed and possible surrogates for ULG90 have been formulated and numerically studied. A discussion has also been made on the various surrogates from the literature which have been tested in shock tube and rapid compression machines for their autoignition times and are a source of chemical kinetic mechanism validation. The differences in the knock onsets of the tested fuels have been explained by modelling their reactivity using semi-detailed chemical kinetics. Through this work, the weaknesses and challenges of autoignition modelling in SI engines through gasoline surrogate chemical kinetics have been highlighted. Adequacy of a surrogate in simulating the autoignition behaviour of gasoline has also been investigated as it is more important for the surrogate to have the same reactivity as the gasoline at all engine relevant p − T conditions than having the same RON and Motored Octane Number (MON).

Highlights

The knock phenomenon limits the compression ratio and the thermodynamic efficiency of a spark ignition (SI) engine
A discussion has been made on the various surrogates from the literature which have been tested in shock tube and rapid compression machines for their autoignition times and are a source of chemical kinetic mechanism validation
Work of Mehl et al [12] demonstrated that the sensitivity of a surrogate can be correlated to the slope of the negative temperature coefficient (NTC) region, dlog(τign )/dT, while the values of the auto-ignition delay time τign in the NTC region depend on the anti knock index (AKI) of the surrogate

Summary

Introduction

The knock phenomenon limits the compression ratio and the thermodynamic efficiency of a spark ignition (SI) engine. Chemical kinetic mechanisms are routinely validated against shock tube and rapid compression machine measurements of the ignition delay times (τign ) of various gasoline surrogate fuels The conditions for these laboratory measurements are similar to those which prevail before ignition in homogeneous charge compression ignition (HCCI) or controlled autoignition (CAI) engine, and the recent kinetic mechanisms, e.g., [4,5,6], perform remarkably well in these regimes. Application of such gasoline surrogate mechanisms have been seen in various studies demonstrating autoignition predictions in HCCI and CAI engines, [4,7].

Practical Gasoline Surrogates

Chemical Kinetics Schemes for Gasoline Surrogates

Gasoline Surrogate Formulation

Supporting Experiments

Results and Discussion

Conclusions