A Comparative Study of Directly Injected, Spark Ignition Engine Combustion and Energy Transfer with Natural Gas, Gasoline, and Charge Dilution

This article presents an investigation of energy transfer, flame propagation, and emissions formation mechanisms in a four-cylinder, downsized and boosted, spark ignition engine fuelled by either directly injected compressed natural gas (DI CNG) or gasoline (GDI). Three different charge preparation strategies are examined for both fuels: stoichiometric engine operation without external dilution, stoichiometric operation with external exhaust gas recirculation (EGR), and lean burn.

In this work, experiments and engine modelling are first used to analyze the energy transfer throughout the engine system. This analysis shows that an early start of fuel injection (SOI) improves fuel efficiency through lower unburned fuel energy at low loads with stoichiometric DI CNG operation. The charge dilution study then reveals that the optimization of fuel efficiency with EGR or lean burn requires a balance between the positive impact of lower in-cylinder heat losses and less pumping work and the negative impact of higher energy losses to the exhaust, via either sensible enthalpy or unburned fuel. Between these two dilution strategies, lean burn is more efficient than stoichiometric EGR operation at equivalent dilution levels. Flame propagation is then examined using the results of premixed, turbulent combustion simulations. This reveals that increasing EGR and lean burn for both DI CNG and GDI decreases the flame speed, and this is shown to have a more pronounced effect on CNG combustion. Using the premixed, turbulent flame theory of Bradley, it is demonstrated that changing the fuel from gasoline to CNG, increasing charge dilution, or increasing engine speed all promote the likelihood of bulk flame quenching, which correlates with the measured engine-out unburned hydrocarbon (UHC) emissions and combustion variability. This provides insight into the fundamental processes by which charge dilution impacts flame propagation and, thus, engine performance.