Combustion and the design of future engine fuels

Abstract

The fast diminution of readily extractable sources of fossil fuels, particularly oil, and concerns about global warming are leading to the creation of many new potential fuels. Their practicability must be assessed in terms of a wide range of physico-chemical properties, in relation to the operational aerodynamics in different engines. This article concentrates on those properties related to combustion and these are discussed in detail for some fuels with contrasting properties. Intrinsic fuel properties include volumetric energy, vapour pressure, heat of reaction, latent enthalpy of vaporization, and the relative volumes of energy that engines can breathe. Important combustion properties include the minimum ignition energy, laminar burning velocity, Markstein numbers for strain and curvature, flame extinction stretch rates for positive and negative stretch, stretch factor for flame instability, turbulent flame burning and quenching, autoignition delay time, excitation time for autoignition heat release, research and motor octane numbers and cetane number (CN). Such properties are manifest in a variety of aerodynamic contexts: for example, the nature of autoignition depends on spatial reactivity gradients and the acoustic speed. Particular problems can arise in characterizing engine knock when operational regimes are outside those in which the octane and CNs are determined. The approach adopted in this article does not assume any unique alignment between fuels, mode of combustion, and power unit, and different possibilities are discussed.