How a cylinder deactivation system works and why it affects fuel consumption.
Cylinder deactivation technology is also called cylinder-on-demand (CoD), and is a clever use of technology that stops some of the cylinders of an internal combustion engine (ICE) from taking in fuel and air while it's running. In order to understand what cylinder deactivation is, a brief breakdown of how an ICE works is required. For an in-depth explanation, read our post on how combustion engines work here.
The basic idea is that this engine burns gasoline and converts it into energy to power the wheels. It does this by sucking a mixture of fuel and air into cylinders in the engine where a moving piston then compresses the mixture in each cylinder. Once compressed, a spark from the spark plug ignites the mix which causes the piston to be pushed back down again. Exhaust gasses are pushed out through the valves while the up-and-down motion of the pistons is converted to rotary motion by the crankshaft.
Now that you understand the basics of internal combustion, cylinder deactivation technology may make more sense - during cruising, you don't necessarily need all the power the engine can provide, and you can save fuel by deactivating some of the cylinders. But how does such a cylinder deactivation system work?
When the computer brain in your vehicle detects that you aren't in need of all the power the engine can provide, a cylinder deactivation solenoid valve kicks in to prevent fuel and air from entering the cylinder. This means that the combustion process in some of those cylinders can't occur, and less fuel is used overall.
Engines that have this capability are sometimes referred to as a variable displacement engine, as the engine cylinder deactivation (CDA) system allows the engine displacement to change temporarily. For example, if half of a 4.0-liter V8's cylinders are temporarily turned off, the engine basically functions like a 2.0-liter V4 for the duration of the deactivation. A large engine running on all cylinders in low-power conditions like cruising at highway speed is inefficient. Turning it into a small engine temporarily can boost fuel economy by as much as 25 percent under certain driving conditions.
The benefits of a system such as this include:
Between the 80s and early 2000s, several manufacturers toyed with the concept. Daimler, Mercedes-Benz, and General Motors all trialed the technology in the early aughts. Nowadays, several automakers use variable displacement technology. Some of the most notable ones are:
Today's cylinder deactivation systems are largely reliable, but they do introduce another layer of complexity that could go wrong. The systems are also limited in terms of what combinations of cylinders can be shut off without upsetting the engine's balance or causing undesirable noise output or vibrations. Engine mountings, flywheels, and vibration dampers can address some of these. The VW Group incorporates noise-canceling technology in the audio system to cancel the sound of its 4.0-liter V8 engine when it is in four-cylinder mode in some of its cars using the engine, such as the Audi A8.
Cylinder deactivation technology has been used with great success to improve the fuel efficiency and emissions of internal combustion engines, but development on this front is likely to stall as manufacturers divert development resources for their newest cars to EVs. Still, it has assisted us in reaching a level of engine efficiency that those brave '80s Cadillac engineers that tried and failed would hardly have thought possible today.
One of the most obvious cylinder deactivation problems, especially in V8 vehicles, is that it can change the V8 engine sound when in four-cylinder mode. The signature V8 sound is a selling point on such a vehicle and for this reason, Jeep has actually developed a dual exhaust system that operates another set of exhaust pipes in four-cylinder mode to retain the Hemi V8's sound. For a better understanding of all the different V engine configurations, read this article.
The system is controlled by the vehicle's engine control unit that determines the fuel distribution and mixture strength - and only activated when various parameters are met. It is switched on and off seamlessly, mostly without the driving being unaware unless looking at the indication on the dashboard. It cannot be manually switched off by the driver.
Some engines can suck in oil from the crankshaft housing through the cylinder deactivation system's pressure-relief valve. Check the positive crankcase ventilation (PCV) system for oil deposits - evidence your engine might have this problem. Fouled spark plugs that cause poor running and make the engine difficult to start could also be symptoms. Ask your dealership whether there have been any service bulletins or redesigned parts to address the problem.
Because these systems are hydraulically powered by pressurized engine oil, poor maintenance that causes dirty or contaminated oil can affect the operation and free movement of such systems' solenoids and locking pins, causing poor running. Here is a handy maintenance schedule to help you keep track.