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Video: Skipping benefits
Efficiency is king. Every manufacturer is looking to improve its vehicles' ability to use less fuel. Whether you’re buying a V12 sports car or a three-cylinder city run around, during the car’s development engineers have been poring over every option to make it use less fuel and emit lower levels of carbon.
That’s why we have stop-start technology, more and more engines are now turbocharged and firms have introduced engines that can shut down cylinders when you aren’t smashing the throttle into the floor.
But many of the current systems can be expensive, so even if they improve efficiency by as much as 6%, it’s a big decision to include them on a car. There is another way however.
Supplier Delphi and Tula Technology have developed a cylinder deactivation system that will deliver an 8-15% reduction in CO2 emissions, depending on the engine, but at a much smaller cost.
Called dynamic skip fire, the software-based system controls which and how many cylinders in the engine fire depending on the needs of the driver. So whereas current systems will shut-down pre-designated cylinders, this system can work on any combination of cylinders as long as the correct amount of torque is being delivered to complete a task. And it’s relatively cheap. For four-cylinder turbocharged engines, improvements are an estimated €40 per percent CO2 reduction.
And if the system is integrated along with mild electrification Delphi and Tula suggest it could deliver an additional CO2 reduction of up to 3%, bringing the total benefits to a projected 11% on four cylinder engines.
So how does it work? During operation, the decision to fire or skip a cylinder is made immediately prior to each firing event, with each event considered independently in sequence. When a cylinder is skipped, both intake and exhaust valves are held closed using Delphi’s cylinder deactivation hardware. A wide range of factors is considered when deciding which cylinders to fire, including torque demand, the NVH characteristics of the engine and the frequencies and amplitudes that must be avoided to eliminate any impact on occupant comfort. The technology re-evaluates the situation up to 32,000 times a minute to find the most appropriate cylinders to fire.
By selecting the sequence of combustion events dynamically, engine operation occurs near peak efficiency, helping to eliminate pumping losses and improving thermal efficiency. The control methodology also allows new approaches to combustion strategies, opening the possibility of further significant efficiency gains, and optimisation of other powertrain systems. For example, selecting combustion on a per-event basis enables efficient, millisecond torque modulation, while high manifold pressure provides faster torque response, with better combustion stability at low engine loads.
“Another significant advantage of DSF is that it allows all-cylinder cutoff during deceleration which eliminates the catalyst refueling penalties associated with more limited fuel shut-off strategies. It also enables longer deceleration events by further reducing pumping losses,” says Delphi.
Delphi and Tula have fitted the technology to a 1.8 litre, four-cylinder, turbocharged vehicle as part of its development. The engine has been refitted with a Delphi gasoline direct injection fuel system, an engine controller that uses Tula’s dynamic skip fire algorithms, and a Delphi deactivation roller finger follower (DRFF) valve control system.
The DRFF is a variable valve actuation technology that uses a rocker arm mechanism working with a three-lobe cam that switches between full- and zero-lift profiles. The system can deactivate a valve in only a few milliseconds. With the DRFF, full cylinder deactivation can be achieved in a single cam revolution at speeds up to 3,000 rpm, without the need for high valve spring loads that increase stress and friction.
Delphi and Tula says that with dynamic skip fire activated, CO2 reductions of up to 10% are achievable for the WLTC, NEDC, US Combined and JC08 drive cycles.
“Dynamic skip fire is one of those simple ideas that releases a remarkable range of benefits simply by allowing other vehicle systems to work more efficiently. It ensures a smooth driving experience while delivering exceptional fuel consumption benefits with no impact on regulated toxic emissions,” says Tula.
But the savings could get much better: “We are also working on a four-cylinder turbo-GDi demonstration vehicle leveraging dynamic skip fire and 48V mild hybridisation on a single platform. When that vehicle is fully optimised, we expect a total CO2 reduction of up to 20%” says Delphi.
Delphi and Tula’s system could be used on just about any engine, so in the future a lot more cars could benefit from cylinder deactivation whether they're eight-, six-, four- or even three-cylinder units.