Cylinder deactivation technology has been around for a long time. As far back as 1982, this writer drove a Volvo 760 with a prototype engine that could switch off some cylinders in certain conditions so you can have ‘two engines in one’. Also called variable displacement, this generally involves shutting off half an engine’s cylinders during light usage in order to save fuel and cut emissions. However, the technology has taken a long time to be reliable enough, perhaps due to the need for advanced electronic systems to manage the operations.
General Motors was the first to introduce cylinder deactivation in the mid-1980s but their system was so problematic that they had to withdraw it and it wouldn’t be many years later before the technology was not flawed, refined and could be introduced without owners facing problems. Cylinder deactivation technology is now available from GM, Chrysler, Honda and Mercedes-Benz, and Mazda also has the technology in its latest 2.5-litre SKYACTIV-G engine introduced this year.
There are various companies that have developed the technology and provide it to the carmakers and one of them is Delphi Technologies which has a system known as Dynamic Skip Fire (DSF). Presented as the industry’s first fully variable engine cylinder deactivation technology, it is an advanced valvetrain system that combines Delphi Technologies’ cylinder deactivation hardware with proprietary software developed by Tula to fire the exact number of cylinders needed to meet the need for power and torque at a given speed and driving condition. The on-demand system can constantly adapt to changing driving conditions.
DSF determines which cylinders to fire in real-time, at each firing opportunity. This is possible because it’s an integral part of the engine control strategy, where valve activation, throttle opening and ignition are optimized together. The strategy centres on a deactivation device called a roller finger follower, which essentially functions as a virtual on-off switch. The electronic control system sends a signal, via oil pressure, to this switch. When it gets the signal, it locks down the air intake and exhaust valves, preventing air flow in and exhaust out. The pistons continue to work with the captured energy, but without air there’s no combustion. When the cylinder needs to fire again, the switch re-opens the valves.
The selection of which cylinders to ‘skip’ is made based on avoiding known resonance patterns within the engine, providing refinement. This means the problems you generally see with noise, vibration and harshness are solved.
By making a fire/no fire decision for every cylinder on every cycle, DSF takes cylinder deactivation technology to the theoretical limit. The system can vary the combination of cylinders firing as often as 6000 times per minute (at 3000 rpm) on a 4-cylinder engine and up to twice that on an 8-cylinder unit.
The benefits of such technology are said to be 7% – 15% reduction in carbon dioxide emissions and 10% – 20% improvement in fuel economy compared with conventional engines (depending on the engine and driving conditions).
Since late last year, Delphi Technologies has been developing a system which can be integrated with the company’s 48V mild hybrid system. This will be used in a turbocharged direct-injection petrol engine and with integrated control of the two systems, it will be possible to achieve smooth torque delivery even at low engine speeds and firing densities.