λDSF: Dynamic Skip Fire with Homogeneous Lean Burn for Improved Fuel Consumption, Emissions and Drivability 2018-01-0891

Dynamic skip fire (DSF) has shown significant fuel economy improvement potential via reduction of pumping losses that generally affect throttled spark-ignition (SI) engines. In DSF operation, individual cylinders are fired on-demand near peak efficiency to satisfy driver torque demand. For vehicles with a downsized-boosted 4-cylinder engine, DSF can reduce fuel consumption by 8% in the WLTC (Class 3) drive cycle. The relatively low cost of cylinder deactivation hardware further improves the production value of DSF.

Lean burn strategies in gasoline engines have also demonstrated significant fuel efficiency gains resulting from reduced pumping losses and improved thermodynamic characteristics, such as higher specific heat ratio and lower heat losses. Fuel-air mixture stratification is generally required to achieve stable combustion at low loads. However, stratified operation suffers from higher particulate emissions and the required quiescent combustion chamber compromises the efficiency of homogeneous lean and stoichiometric operation at higher loads. Alternatively, a homogeneous lean burn engine would switch to throttled, stoichiometric operation at low loads, which would incur a significant fuel consumption penalty.

This article introduces the concept of λDSF (pronounced lean-DSF), which integrates dynamic skip fire with homogeneous lean burn. Through individual cylinder deactivation, λDSF can extend high efficiency and stable homogeneous lean combustion to low loads. λDSF would also allow for an optimized combustion chamber design, minimize emissions through homogeneous operation and higher exhaust gas temperatures, and improve drivability by reducing discrete mode switches between stoichiometric and lean operation. Vehicle simulations with λDSF for a C-segment vehicle with a 2.0 l, 4-cylinder, Miller cycle engine project a reduction in CO2 emissions by 4.6% to 9.7% from the baseline homogeneous lean burn configuration depending on the drive cycle (CAFE, WLTP, NEDC, JC08). Results also showed benefits for lean exhaust aftertreatment and virtually eliminated stoichiometric-lean transitions once DSF is enabled.