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A predictive, quasi-dimensional simulation of combustion in a spark ignition engine has been coupled with a chemical kinetic model for the low temperature, pre-flame reactions of hydrocarbon fuel and air mixtures. The simulation is capable of predicting the onset of autoignition without prior knowledge of the cylinder pressure history. Near-wall temperature gradients were computed within the framework of the engine cycle simulation by dividing the region into a number of thin mass slices which were assumed to remain adjacent to the combustion chamber surfaces in both the burned and unburned gas. The influence of the near-wall turbulence on the temperature field was accounted for by means of a boundary layer turbulence model developed by the authors. Fluid motion in the bulk gases has been considered by the inclusion of a turbulence model based on k - ε theory while the flame propagation rate was predicted using a fractal flame model.

A new type of model has been developed to predict near wall temperature gradients and local instantaneous heat fluxes in a “motored” engine. The unburnt charge in an existing “phenomenological” model is divided into a number of discrete masses which are assumed to be “stacked” adjacent to the cylinder surfaces. A sub-model based on the one-dimensional Enthalpy Equation is applied to the system of discrete masses in order to predict the near wall temperature distribution. Predicted temperature profiles are compared with those measured by other researchers and show good agreement under both low and high swirl conditions. Local instantaneous heat fluxes are calculated from the near wall temperature gradients, and these also show good agreement with measured results. Near wall velocity and turbulence data have been used in modelling turbulent eddy transport processes rather than using conventional boundary layer theories.

An existing “phenomenological” computer model of the spark ignition engine combustion process has been used to reveal further information on flame development in such engines. A detailed flame map and pressure-time diagram ( from Ref ( 6 ) ) has been analysed to determine more precisely the 3-dimensional nature of the flame development across the CFR engine cylindrical disc combustion chamber. The conventional spherical flame assumption (centred at the spark plug) is found to be inaccurate with the extent of the deviation from sphericity varying with flame radius and compression ratio in both the vertical and horizontal (plan view) planes. The flame map provides input data in the form of enflamed volumes, heat transfer surface areas etc for the subsequent evaluation of turbulent burning velocities, mass burn rates, pressure-time diagrams etc.

A literature survey of the laminar burning velocity behaviors of iso-octane-air mixtures has been conducted with a view to recommending a suitable correlation for use in spark ignition engine combustion work. This comprised an initial study of some burning velocity measurement techniques and then considered laminar burning velocity as a function of unburnt gas temperature, dilution, equivalence ratio and pressure. To provide more definitive guidelines, especially with regard to the pressure effect, other hydrocarbon fuel burning velocity data has been reviewed. The recommended correlation utilises the Heimel and Weast (3) atmospheric pressure values with dilution catered for by the Metghalchi and Keck (44) modification. The pressure attenuation was found to vary with equivalence ratio in such a way that the negative pressure exponents increased as mixtures became weaker and richer. This is significant in terms of the trend toward combustion at leaner mixtures.

An extensive literature survey has been made of the formation mechanisms involved in NO generation within the flame front (“prompt NO”) and in the post-combustion gases. Computer models having a uniform temperature and composition in the burnt charge are showm to be inaccurate in predicting NO. Improvements can be achieved by the incorporation of a one-dimensional temperature gradient and partial mixing in the burnt gases. However, the development of models in which heat transfer is confined to a multi-element “thermal boundary layer” is required for the most accurate predictions. Limited results from one such model are presented.

A computer model has been developed to simulate the combustion process in a Renault IFP spark ignition research engine. The model enables the gradients in temperature and composition in the burnt gases behind the flame front to be predicted in a more accurate manner than has henceforth been possible. This is due to the incorporation into the model of heat exchanges due to mixing between adjacent zones in the, burnt gases. The temperature gradients in the burnt gases are reduced quite markedly as the turbulent eddy diffusivity, ɛ, of the flow in the combustion chamber is increased. The NO gradients are not so significantly affected. The ability of the model to predict accurate values of burnt gas temperature behind the flame front is verified from the correlations noted between predicted and spectroscopically measured burnt gas temperatures at certain locations in the combustion chamber. Good agreement is also obtained between the predicted and measured NO levels in the engine exhaust.

The development of a hot wire anemometry system to study the turbulent flow conditions in spark ignition engine combustion chambers is described. Measurements of a ‘mean’ flow velocity and a fluctuating flow velocity within certain frequency bandpass ranges are reported under ‘motoring’ engine conditions during the ‘combustion period’ of the engine cycle. Two types of combustion chamber design have been investigated - a ‘squish’ design and a cylindrical disc design.

The mathematical model of the compression, combustion, and expansion phases of the Renault IFP variable compression ratio research engine reported here is an attempt to combine as many as possible of the basic characteristics of engine combustion. Finite rates of flame propagation and heat release are computed on the basis of Semenov's theory. To allow for the effects of turbulence, Semenov's estimate of laminar burning velocity is multiplied by a term derived from flame speed measurements in the engine. Dissociation of the burned gases is compensated by chemical equilibrium and heat transfer data due to Annand. The computer model makes possible a parametric study of the effects of variables such as mixture composition, spark timing, compression ratio, engine speed, exhaust residuals and injected water as a means of controlling certain obnoxious emissions resulting from use of propane, isooctane, and benzene fuels. Experimental data corroborate the accuracy of the model.