Autoignition prediction capability of the Livengood–Wu correlation applied to fuels of commercial interest

The integral method proposed by Livengood and Wu has been traditionally used to predict the occurrence of knock on spark-ignition engines. Due to its simplicity and low computational demand, this is a method of great interest for the prediction of another autoignition phenomenon, such as the onset of combustion in compression ignition or homogeneous charged compression ignition engines. However, the simplicity of the method is a consequence of the restrictive assumptions considered during its development, which may limit the applicability of the equation. In this study, the validity of the correlation proposed by Livengood and Wu has been evaluated at different initial operative conditions under pure homogeneous charged compression ignition combustion mode for fuels with practical interest (hydrogen, methane, ethanol and n-heptane). The integral method has shown very good prediction capability for the fuels, which do not present two-stage heat release (hydrogen, methane and ethanol) except in those cases when the onset of combustion is very delayed. When cool flames appear (as in the case of n-heptane), the integral method overpredicts the autoignition times since it does not consider the first stage of heat release. In these cases, the prediction of the integral method may be improved if the whole combustion process is considered as two individual processes. This approach shows fairly good prediction capacity although it is unpractical since the simulation of the second-stage combustion requires the previous calculation of the composition of the mixture and the temperature increase at the end of the first stage. Finally, two alternatives to the original integral method are tested which keep its simplicity and universality while taking into account both first and second heat release, one of them showing better results than the original Livengood and Wu equation.