In the ongoing quest to reduce fuel consumption, some engine manufacturers are incorporating devices that deactivate the valve train in some cylinders in an effort to reduce pumping losses—one of the primary energy losses in throttled spark ignition engines. Current deactivation strategies designate fixed sets of cylinders to be deactivated, resulting in two or three operating modes. Tula’s DSF, however, decides whether or not to fire a cylinder on a cycle-by-cycle basis.

DSF also deactivates cylinders in a manner that achieves the load demanded while avoiding objectionable noise and vibration. Throughout this process, the air mass is increased in each firing cylinder to produce more torque.

One of the benefits of DSF is the wide range of firing pattern selections to minimize the resonance modes. This capability of DSF provides us the opportunity to choose firing patterns which produce surprisingly lower NVH than V8 at equivalent power. Besides NVH considerations, our FTP cycle data for L94 engine (employed in 2010 GMC Denali) show fuel economies of 19.92 mpg at DSF versus 17.34 mpg at V8 mode. Meaning, DSF reduces fuel consumption by 14-18% over V8.

—Eisazadeh-Far and Younkins

In the study reported at the World Congress, Tula engineers tested the DSF technology in an independent certified lab on a 2010 GMC Denali over a wide ranges of engine speeds, loads and DSF strategies. All tests were conducted at optimum camshaft position and spark timing for the best BSFC considering the knock limits.

BSFC (g/kWh) contour plots of V8 (top) and DSF (bottom) operations. Eisazadeh-Far and Younkins (2016). Click to enlarge.

Tula Technology used a production General Motors L94 6.2L V8 engine, modified by Tula to be compatible with Dynamic Skip Fire. The base engine is used by General Motors in many applications, among them, the 2010 GMC Yukon Denali.

DSF implementation does not require major modifications to the engine architecture; it only requires valve deactivation. The hardware modifications required for DSF were:

1. Addition of production lost-motion lifters for cylinder deactivation, required for both intake and exhaust valves for the four cylinders not equipped with these lifters from General Motors.

2. Modification to the engine block to allow oil routing to each of the lost-motion lifters.

3. Fabrication of a new lifter oil manifold assembly, including a solenoid system to direct oil to the control port of the added lost-motion lifters.

Among their other findings, the Tula team determined that:

• DSF operation has a direct impact on combustion stability of the engine. Low loads at V8 require very low air mass in each cylinder. Low charge mass in the cylinders will cause combustion instability. With an intelligent calibration, DSF enhances combustion stability by increasing the mass of air in active cylinders. DSF improves the burning rate of the mixture which will boost the thermal efficiency.

• DSF reduces CO, CO₂, HC, and at some conditions NO_x emissions. The reduction is achieved through higher thermal efficiency and increased in-cylinder loading when in DSF operation. In addition, DSF increases the temperature of the catalyst which will result higher conversion efficiency.

Resources

• Eisazadeh-Far, K. and Younkins, M. (2016), “Fuel Economy Gains through Dynamic-Skip-Fire in Spark Ignition Engines,” SAE Technical Paper 2016-01-0672 doi: 10.4271/2016-01-0672