How It Works

Cylinder deactivation

Cylinder deactivation technology goes by a number of names, but the principles and objectives are virtually identical. If you selectively shut down one or more cylinders of an engine when the power demands are low, then if carefully managed, it can save fuel, and also reduce emissions. Back in 1981, Cadillac in the USA launched an early interpretation of this fuel-saving theme, with the unforgettable (for the wrong reasons) V8-6-4 engine. Designed to cut out either two- or four-cylinders of the L62 140bhp 6.0-litre V8 powerplant fitted to most of its range, the 8-6-4 was a real disaster. The complex engine mechanics could not keep up with the relatively advanced electronic management system, it was terrible to drive, and the whole idea was dumped after a year on sale. As the years passed, other manufacturers, like Alfa Romeo, Mitsubishi and Honda, flirted with the same concept, but all was pretty quiet until General Motors returned to the fray with Active Fuel Management on their 2005 petrol V8s. In this system, the exhaust valve of a selected cylinder is prevented from opening after the power stroke and the exhaust gas is retained in the cylinder and recompressed, acting like a spring to help retain the balance and minimise any roughness. Updated in 2018, GM’s new Dynamic Fuel Management system is based on Dynamic Skip Fire developed by Tula Technology, and it works very well in the 6.2-litre GM V8 engine.


Europe’s smaller, and mostly four cylinder, engines posed greater challenges though, until the technology was perfected by Volkswagen, first appearing on their 1.4-litre petrol TSI engine in 2012, and continues in today’s 1.5-litre TSI Evo four-cylinder engines. Far less well balanced than any V8, and with only half the cylinders, you would expect the engines to sound and feel ragged. Whilst the designers accepted that some mild unevenness was inescapable, it is barely noticeable when the engine is doing relatively little work, which is when the greatest economy benefits are derived. Some steps can be taken to counter the downsides though, and Ford, who use it on their 1.0- and 1.5-litre EcoBoost three-cylinder engines, fit a special unbalanced vibration-damping dual mass flywheel and pulley, along with a circumferential pendulum damper to neutralise unwanted oscillations. Keeping the valves of the in-operative cylinder closed helps to prevent any critical temperature variations within the engine and helps reduces emissions. 


When this technology is in effect, usually with one less cylinder delivering power, the remaining cylinders are all working harder. In the conditions chosen for operation, those cylinders are then operating more economically than in normal all-cylinders running. Fuel economy gains of up to six per cent are possible, dependent on running conditions, offering around the same level of savings as low rolling resistance economy tyres. It’s interesting that Ford development work revealed that a semi-randomised pattern of cylinder deactivation proved smoother and more effective than cutting the same cylinder every revolution. Their 1.0 litre EcoBoost engine could even run quite smoothly at low load when effectively using “one and a half” cylinders, or three firing strokes in place of the normal six. The benefits weren’t cost effective though, as they required camshaft activation equipment for all three cylinders, as against just one in the production version.


What the technology highlights is that, for a large proportion of driving time, most car engines are well overpowered, and operating in far from optimum conditions for fuel economy and emissions. Cylinder deactivation helps to eliminate some of that inefficiency, and is arguably an alternative to the hybrid car, where engines on light load are made more efficient by drawing extra engine power to charge up the hybrid’s battery. Cylinder deactivation or hybrid power? Either is a supportable choice. But now, with the arrival of low-cost mild hybrids that can offer the bonus of low cost all-wheel-drive, by applying the electric power to the second axle, the days of cylinder deactivation may well be numbered. 

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