What is cylinder deactivation? PH Explains

Cylinder deactivation is a technique that is used to boost the efficiency of a petrol engine when it is not under heavy load. As the name suggests, it entails shutting down one or more of the cylinders in an engine in an effort to reduce its fuel consumption.

Many manufacturers employ such systems but they are often referred to in different ways. For example, Volkswagen calls its cylinder deactivation setup 'Active Cylinder Technology'. General Motors, on the other hand, refers to its current system as 'Active Fuel Management'.

Why deactivate cylinders?

If a petrol engine is under light load, or idling, then its throttle will only be partially open. This causes a significant 'throttling loss' as the cylinders have to draw fresh mixture in through a restrictive opening. Each cylinder will also only receive a partial fresh charge during its intake stroke as a result, further decreasing efficiency.

One solution to this issue is to shut down some of the engine's cylinders. The remaining cylinders consequently have to work harder to maintain the desired output but, in order to do so, the throttle needs to be opened further. This larger opening, which is now feeding fewer cylinders, greatly reduces the restriction in the intake system. An improved charge is also delivered to each of the remaining harder-working cylinders, resulting in more efficient operation.

Shutting down cylinders also reduces heat loss from the engine, which again helps to improve efficiency. While the remaining cylinders might be working harder in these part-load conditions, and burning more fuel, the reduction in losses outstrips the increased fuel demands - resulting in an overall improvement in efficiency.

How do cylinder deactivation systems work?

When the engine's control unit detects light load and suitable conditions for deactivation, it will close the intake and exhaust valves of the cylinders no longer required. This avoids excess pumping losses, caused by simply having the now-defunct cylinder drawing air in and pumping it out.

In a cam-in-block engine, the lobes of the cam act on lifters. These engage with the pushrods, which transmit the motion of the cam lobes to the rocker arms in the cylinder head - which then act on the valves, opening them.

To stop this action and deactivate the cylinder, specially designed and hydraulically controlled lifters are used. When the conditions for deactivation are met, pressurised oil is fed into the lifters. This frees locking pins within them, causing them to collapse - so the lobe of the cam ceases to operate its associated valve.

In engines with overhead cams, similar special valvetrain components are employed to shut off the valves. Some systems use sliding cam lobe sections, with one lobe having a 'zero profile' that results in the valve staying closed. Others may use bespoke rocker assemblies, which can be engaged or disengaged to cause the valve to stay closed.

Depending on the engine, exhaust gas or fresh air is trapped in the cylinder during the process of deactivation. This creates a pneumatic spring which helps boost efficiency by cutting the energy required to move the 'dead' pistons - as the gas squashed during the compression stroke then decompresses, minimising load. It also prevents the creation of a vacuum, which would draw oil into the cylinder. This oil would then be burnt, increasing oil consumption and emissions.

The ECU of modern direct- or port-injected engines will also shut off the fuel supply to the deactivated cylinders, and many will cut ignition to the dead cylinder as well. When more power is required, the valves are allowed to open again, injection commences and the cylinder resumes firing. This process takes very little time; Volkswagen's 'Active Cylinder Technology' takes a maximum of 36 milliseconds to make the transition.

Are there any other applications of cylinder deactivation?

The use of cylinder deactivation isn't solely restricted to improving economy. For example, the Cadillac 'Northstar' V8 - introduced in 1992 - featured a special fail-safe mode that made use of cylinder deactivation.

When excessive engine temperatures were detected, the ECU would momentarily shut down one bank of the engine by cutting fuel supply. Fresh air would be pumped through the cylinders, helping cool them and protect against a catastrophic failure, then the cooled bank would be brought back online.

The process would be repeated for the other bank, with the ECU alternating between the two every 80rpm or so. This 'limp home' mode could allow a driver to travel up to 50 miles if necessary, according to the manual, although doing so would degrade the oil to the point where it would need replacing immediately.

Cadillac's engineers even claimed to have driven Northstars 100 miles without coolant, suffering no engine damage in the process.

A brief history of cylinder deactivation

The first manual cylinder deactivation systems appeared in the early 1900s. It took until 1981 for a mass-produced automatic system to arrive, which was used in the Cadillac V8-6-4. This system, which was developed in conjunction with American multinational Eaton, did offer improved economy but it also proved unreliable.

Consequently, the system was ditched - a decision made easier by the fact electronic fuel injection systems and catalytic converters were rapidly improving, helping manufacturers meet economy and emissions targets.

Increasingly stringent regulations and the growing demand for improved economy led to a rise in interest in cylinder deactivation in the early 1990s. Fortunately, technology had advanced sufficiently enough by this point to enable reliable, effective 'displacement on demand' setups.

By the early 2000s, Mercedes-Benz, GM and Honda were all using cylinder deactivation systems - and, today, they remain a vital part of many a manufacturer's engine technology line-up.

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Comments (6) Join the discussion on the forum

  • Riverside Red 10 Jul 2018

    Had cylinder deactivation on my Mercedes R172 SLK55AMG, it worked a treat and would regularly give well over 30mpg on the motorway which wasn't bad for as NA 5.5 litre V8 producing 420hp. The car used to sit on 4 cylinders up to about 75mph and the change between 4 and 8 cylinders was barely noticeable.

    As such I was really disappointed when Mercedes dumped the engine for the pseudo AMG 3 litre V6 turbo in the SLC 43. However it's good to see SLK55 values holding up against the newer SLC43...it shows the market knows better than the manufacturer sometimes.


  • JackReacher 10 Jul 2018

    Works very well in my wife's Seat Leon 1.4 ACT, barely noticeable on change over and you can see from the dash symbol that it deactivates 2 cylinders reguarly. Obviously best when cruising at a constant speed but means it will top 60mpg on the right sort of journey, 50's in day to day driving. Brilliant for a 150bhp petrol engine.

    I don't understand why VAG haven't used it in other cars, even the 2.0t engines could benefit from it.

  • marcg 10 Jul 2018

    These articles need more pictures. I'm fairly mechanically minded but I can't follow all this. I suspect if you can follow the article, you already know the information.

  • unsprung 10 Jul 2018

    here's a video animation of cylinder deactivation (called Active Fuel Management by General Motors) on the current Corvette Stingray:

    and related article here:

    "460 horsepower, 36mpg" (UK mpg)

  • MJ85 10 Jul 2018

    I’ve only seen it on a rental spec Chevy Impala 3.7 over 10 years ago. Always thought it seemed a good idea and was surprised to not see it catch on more.

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