Simulation of HCCI-addressing compression ratio and turbo charging

This paper focuses on the performance and efficiency of an HCCI (Homogenous Charge Compression Ignition) engine system running on natural gas or landfill gas for stationary applications. Zero dimensional modeling and simulation of the engine, turbo, inlet and exhaust manifolds and inlet air conditioner (intercooler/heater) are used to study the effect of compression ratio and exhaust turbine size on maximum mean effective pressure and efficiency. The extended Zeldovich mechanism is used to estimate NO-formation in order to determine operation limits. Detailed chemical kinetics is used to predict ignition timing. Simulation of the in-cylinder process gives a minimum λ-value of 2.4 for natural gas, regardless of compression ratio. This is restricted by the NO formation for richer mixtures. Lower compression ratios allow higher inlet pressure and hence higher load, but it also reduces indicated efficiency. Given indicated mean effective pressure, IMEP and a fixed friction, FMEP the best brake efficiency was attained at compression ratios of 15:1 to 17:1, according to the simulations. Full system simulation using three different turbines, showed that the required inlet pressure could not be reached. At these low loads a high compression ratio enables lower inlet temperature. This provides higher mass flow and hence power output. The higher compression ratio also increases the indicated- and brake efficiency. Very small turbines or advanced turbo charging technologies seems necessary in order to give acceptable specific power and brake efficiency.