A Real-Time Chemical Equilibrium Mechanism for Control-Oriented Combustion Models 2021-26-0327

To enable closed loop combustion control, in-cylinder pressure needs to be sensed and analyzed in real time on engine control units (ECU). Therefore, computationally light weight models are necessary. One way to reduce computational effort is to adapt the way how the composition of dissociating flue gas is calculated. The widely used chemical equilibrium mechanism developed by Olikara and Borman [10] describes the gas phase products of burnt hydrocarbon fuels and is used as a reference throughout this work. The reaction scheme considers eleven species and seven elementary reactions. Four different approaches to reduce computational effort while maintaining enough accuracy have been explored. As first approach, complete combustion is used to derive mass fractions of CO₂, H₂O, O₂ and N₂. Secondly, mass fractions at various in-cylinder conditions are calculated using the reference mechanism and are stored in tabulated form. Subsequently, linear interpolation is used to estimate mass fractions at any input condition. Thirdly, the mechanism is reduced to six species and two reactions. Mass fractions are then calculated on the fly during runtime. To achieve minimum deviation in total enthalpy, the equilibrium constants for each reaction are adjusted. A wide range of in-cylinder conditions such as pressures up to 200 bar, rich to lean fuel air mixtures and a temperature range of 1000-3000 K is considered during the constant’s optimization. In the fourth approach mass fraction ratios of CO/CO₂ and H₂/H₂O at various in-cylinder conditions are calculated using the reduced mechanism and stored in tabulated form. Again, linear interpolation is used to estimate mass fractions at any input condition. This approach shows the largest reduction in computational time and requires only little storage space for the tables. Further, there is only slight maximum variation in enthalpy of mixture of 2.30 % compared to the reference mechanism. Thus, this approach is chosen for further validation. Measured in-cylinder pressure traces are analyzed using a standard two zone analysis model. Heat release rates, zone temperatures and thermal properties such as du/dp, dR/dp, du/dT, dR/dT are compared with the results that have been derived using the reference mechanism. There are only minimal differences especially regarding all variables describing combustion phasing such as MFB50 or start and end of combustion. Further, the new mechanism is 22 times faster. All together this new approach can help enable more detailed real time combustion control strategies.