Studies on a correlation based Wiebe function for semi-predictive combustion simulation in SI engines fuelled with hydrogen-methane mixtures

The present work investigates on possible semi-predictive capabilities of a Wiebe function in simulating the combustion development in a normally aspirated spark ignition engine, to be used as a first approach tool for engine design purposes. Based on a wide experimental database, including cycle per cycle pressure measurement in cylinder and inlet/exhaust runners, through a commerce simulation software a turbulent combustion model was set up and assumed as reference. Based on the values of the characteristic Wiebe function parameters estimated at each operating condition of the experimental database, and using statistics tools, correlations were set up as non-linear polynomial relationships where Wiebe parameters are expressed as function of the main engine working variables and of the laminar flame speed estimated at combustion start, providing the possibility of referring to different fuels. Simulations were carried out, with both Wiebe and turbulent combustion model, on a 1D model of the tested engine, in the range of 1000-4500 revolution per minute and 0.4-1.0 bar of manifold absolute pressure, with spark advance set for Maximum Brake Torque Timing, and referring to three fuelling options, that is gasoline, methane and hydrogen + methane mixture, the latter in two different percent compositions. MBTT was set by optimization algorithms meeting the constraint of avoiding knock by means of a Douaud-Eyzat approach, based on induction time estimate. For hydrogen+methane mixtures, literature models were selected and implemented to estimate induction times, to avoid knocking conditions, and laminar burning flame accounting for dilution due to exhaust gas residuals. Comparison of the results of the Wiebe combustion model were made with the results of the reference turbulent combustion model, considering Brake Mean Effective Pressure and Brake Specific Fuel Consumption, in terms of percent differences. The comparison shows that a Wiebe approach in combustion modelling may provide acceptable results in some limited operating conditions. Assuming the engine fuelled with gasoline, it was found that the Wiebe model approach gives results relatively similar to the turbulent model more or less in the medium-low range of MAP and RPM. Assuming the engine fuelled with methane and with a hydrogen-methane mixture with 0.02 and 0.1 hydrogen mass fraction respectively, the range where differences between the two combustion simulation approaches are less noticeable is a little more wider.