Study of Reformer Gas Effects on n-Heptane HCCI Combustion Using a Chemical Kinetic Mechanism Optimized by Genetic Algorithm 2008-01-0039

Because of the potential for low NO_x emissions with high efficiency, HCCI engines could develop a significant niche in the engine world. However, HCCI engines suffer from a narrow operating range between knock and misfire boundaries because the ignition timing is only controlled by mixture chemistry and compression conditions. Varying combinations of operating parameters are required to obtain good combustion under different conditions and chemical kinetic models are widely used as an engine research tool. The performance of such models depends critically on the accuracy of the chemical mechanisms which are still under development and require some optimization, particularly for larger hydrocarbon molecules.

This study starts with a Chalmers University mechanism [1] which is well-proven for pure n-heptane but works less well for mixtures blended with significant amounts of reformer gas containing high fractions of H₂ and CO [2]. A Genetic Algorithm (GA) approach has been used to significantly enhance the base mechanism as tested against actual engine and shock tube data values. Data came from an HCCI engine fueled with heptane blended with 0% to 25% reformer gas. Engine operating conditions varied with equivalence ratio between ϕ = 0.4 to 0.8, intake pressure between 1 and 1.5 bar, speed of 700 to 800 RPM and EGR of 0% to 40%. A good agreement was also found on shock tube ignition delay with different initial conditions (P = 6 to 42 bar and ϕ = 0.5 to 3). The study showed that the genetic algorithm could significantly improve start-of-main-combustion timing prediction compared with the base mechanism by adjusting reaction parameters for key influential reactions.

The enhanced chemical kinetic mechanism was used to perform a detailed study of the thermal and chemical effects by which reformed fuel blending modifies HCCI engine combustion with a very low-octane base fuel, (ie. n-heptane). The study examined the contributions of key reactions to both heat and species production. Results show that base fuel replacement with reformer gas delays ignition timing and slows combustion, primarily due to reduced H₂O production, (the main source of heat release during cool flame reactions), and consequently a lower temperature rise during 1^st stage combustion. This diminishes the pool of available radicals from the cool flame ignition stage and thus delays the main ignition.