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

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 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. Different approaches to reduce computational effort while maintaining enough accuracy have been explored. As first approach, complete combustion approach is used. Secondly, mass fractions at various in-cylinder conditions are calculated using the reference mechanism and 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. To achieve minimum deviation in total enthalpy, the equilibrium constants for each reaction are optimized. 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. 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. There is only slight variation in total enthalpy of 2.23%. Thus, this approach is chosen for further validation. Measured in-cylinder pressure traces are analyzed using a homogeneous two zone model. Heat release rates, zone temperatures, adiabatic burn temperature, heat losses and thermal properties such as du/dP, dR/dP, du/dT, dR/dT are compared with results that have been derived using the reference mechanism. There are only minimal differences especially regarding all variables describing combustion phasing. Further, the new mechanism is 20 times faster. All together this new approach can help to enable more detailed real time combustion control strategies.