Consider reactants (fuel/oxidizer) contained in a piston cylinder arrangement (constant pressure) or a closed container (constant volume). They will react at each and every location within the gas volume at the same rate. This means that there are no temperature or composition gradients within the mixture, there is a single temperature and a set of species concentration suffice to describe the evolution of the system. By applying conservation laws and solving differential equations with the initial conditions defined, we get temporal evolution of temperature and species concentrations. There will be a sudden rise in temperature/pressure/change in some species concentration, after some period of time. This time is called ‘Homogeneous Ignition delay time’.