Selective firing of engine cylinders has been used for load control going back at least to the late 19^th century in agricultural engines common at the time regulated by a governor which cut off cylinders to maintain constant speed. In automotive applications, engine torque production is demanded by the vehicle driver in an often highly transient manner. The basic concept of Dynamic Skip Fire technology is to manage torque production in response to the driver command via selectively engaging or disengaging torque production from engine cylinders rather than by throttling. The operation of each firing cylinder can be optimized for best thermal efficiency, subject to possible constraints such as air-fuel mixture ratio, peak pressures or temperatures (durability), and other factors.

—Wilcutts et al.

Instead of relying on fixed cylinder deactivation or switching between fixed patterns as in current multi-cylinder engines, Tula’s DSF technology continuously makes dynamic firing decisions on an individual cylinder basis to deliver the required engine torque for all vehicle speeds and loads.

In their 2013 SAE paper, the Tula engineers described two primary benefits from dynamic skip fire technology:

1. Fuel economy improvement via removal of pumping losses and optimization of combustion. For each engine speed, a sweet-spot of thermal efficiency for operating cylinders exists which is at high load for throttled engines due to minimized pumping loss. With combustion occurring preferentially in this regime, the engine combustion system can potentially be optimized to match this operating area. With intake or exhaust (or both) valve deactivation in place, inactive cylinders are prevented from pumping air through the engine, which enables effective use of three-way catalyst technology.

2. Wide authority over generation of vibrational and acoustic excitations. In normal throttled operation of an engine, excitation spectra are tied to the engine speed, and magnitude is determined by the level of throttling. With dynamic skip fire technology, the spectra are additionally controlled by the number and sequence of firing cylinders. Full dynamic control of firings and non-firings of engine cylinders means that noise, vibration and harshness (NVH) can be dealt with algorithmically, in a flexible and systematic way.

DSF operation tracking engine torque demand. Click to enlarge.		Active NVH management algorithm. Click to enlarge.

Tula’s DSF technology does not rely on fixed cylinder deactivation as in current-production systems, or on switching between fixed patterns (as earlier patented by Daimler-Benz). Rather, it varies the firings and skips continuously with load demand.

Tula licenses its control algorithms to customers via a co-processor solution.

This technology holds the potential to improve fuel economy on select GM vehicles without degrading power capability when it’s required. This joint effort combines software expertise from Silicon Valley with powertrain expertise from General Motors.

—Jon Lauckner, GM chief technology officer, vice president of Global R&D and president of GM Ventures

In addition to the ongoing partnership between the two companies, Ron Yuille, Tula’s Vice President of Powertrain Engineering and Business Development, had held various executive positions within GM, including vice president of powertrain engineering for GM International Operations, vice president of powertrain operations for GM Daewoo and president and CEO of GM Isuzu Diesel Engines Limited.

Since its founding in mid-2010, GM Ventures’ international portfolio includes investments in more than 20 startup companies.

Resources

• Wilcutts, M., Switkes, J., Shost, M., and Tripathi, A. (2013) “Design and Benefits of Dynamic Skip Fire Strategies for Cylinder Deactivated Engines,” SAE Int. J. Engines 6(1):278-288 doi: 10.4271/2013-01-0359