A simple thermodynamic model of combustion in Otto cycle engines has been published and shown to exhibit chaotic cycle to cycle variations in certain regimes. However, these chaotic regimes are far from any physical limit. When the model is modified to predict observables within their physical ranges, the chaotic behaviour disappears. It is not possible from these results to claim that the fuel thermo-chemistry alone can induce chaotic behaviour. By elimination, this would support the view that cycle-to-cycle variations are caused by the effects of turbulence on fluid flow and combustion in engines, and not by chaotic phenomena arising from the non-linearity of fuel thermo-chemistry.