Pumping loss, air springs and oil-fed lifters: what happens inside an engine that shuts down cylinders at light load, from Cadillac's 1981 flop to Dynamic Fuel Management.
Cylinder deactivation keeps the valves of some cylinders shut at light load, so they stop burning fuel and the engine breathes with less throttling. Reported fuel savings range from 5.5% in EPA tests of GM's system to 25 percent on the highway, and about 15% of US light vehicles used it in 2020.
In 1981 Cadillac sold a V8 that could run on eight, six or four cylinders, with an engine computer deciding which. Customers did not understand it, failures arrived in unpredictable clusters, and the company retired the technology almost at once. Four decades on, about 15% of new light cars and trucks in the United States for the 2020 model year used some form of cylinder deactivation.
What is cylinder deactivation, and why does it save fuel?
Cylinder deactivation, also called variable displacement, lets an engine change its effective size by shutting down some of its cylinders when little power is needed. It is mostly used on large multi-cylinder engines, and it is aimed at light-load driving, where those engines waste the most fuel.
Light-load driving is the norm. When you cruise or crawl through traffic, you are typically using only around 30 percent of the engine's maximum power. To deliver that modest output, the engine's throttle valve is almost shut, and the engine must labour to pull air past it. This is called pumping loss. Picture sucking a drink through a pinched straw: the harder you restrict the flow, the more effort each sip takes. A large engine can be throttled so far at light load that the pressure in its cylinders at top dead centre is about half that of a small four-cylinder.
Low cylinder pressure hurts efficiency, and there is a second waste on top of it. Without deactivation, fuel is delivered to and burnt in every cylinder even though full performance is not required. Shut down half the cylinders and two things improve together. The cylinders still working pull air from an intake manifold that now has fewer mouths to feed, so pressure rises and the pumping effort falls. And with fewer cylinders burning fuel, consumption drops.
Shut down half the cylinders and the ones still working breathe more easily, which is the whole trick.
How does the throttle create pumping loss?
On a petrol engine the throttle is usually a butterfly valve, a hinged plate in the air passage. It sits at the entrance of the intake manifold or in the throttle body. With the throttle wide open, the manifold sits near outside air pressure. Part-close it and manifold vacuum drops further below ambient, which is the vacuum the engine fights against.
Most modern engines are drive-by-wire. A sensor reads the accelerator pedal and sends a signal to the engine control unit, which decides the throttle opening in light of other sensor inputs. That matters for deactivation, because it lets the computer adjust throttle position by itself during the switch from full to partial operation, smoothing it out.
Diesels sidestep this particular loss because they control power by the quantity of fuel injected and usually have no butterfly valve in the intake. A few petrol engines also manage airflow with variable intake valve timing in place of a conventional throttle, which gives the same control with less pumping loss. Cylinder deactivation attacks the same waste from another direction.
How do cars switch cylinders off?
The basic trick is to keep a chosen cylinder's intake and exhaust valves closed. That traps the exhaust gas left from the previous burn, and the trapped gas behaves as an air spring: the piston compresses it on the way up and it pushes back on the way down. Think of a bicycle pump with a finger over the nozzle. You feel resistance as you push, but you get the energy back on the return, so the net extra load on the engine is close to nothing.
The newest systems also cut fuel to the idle cylinders through the engine management system. To keep the changeover smooth, the computer alters ignition timing, cam timing and throttle position together as the engine moves between full and partial operation. A V12 can have as many as six cylinders disabled. Two problems remain for every variable-displacement engine: unbalanced cooling and vibration.
There are two main ways to hold the valves shut, and which is used depends on the valvetrain. In a pushrod engine, solenoids alter the oil pressure delivered to lock pins inside the lifters. With the pin out of place, the lifter collapses and cannot raise its pushrod, so the cam does its work on a part that simply gives way and the valve stays closed. In an overhead-cam engine, each valve gets a pair of rocker arms locked together. One follows the cam profile and the other opens the valve. Oil pressure, controlled by a solenoid, releases the locking pin; one arm keeps tracking the camshaft while the other sits still. With computer control, the system switches off and on again almost instantly.
How much fuel does it actually save?
The answers depend on who is measuring, and the figures do not agree. One general account puts the reduction at 8 to 25 percent in highway conditions. General Motors' Active Fuel Management, judged on EPA tests, shows an estimated improvement of 5.5-7.5%. Alfa Romeo reported 12% against its own fuel-injected engine without deactivation and almost 25% against the carburetted production 2.0-litre. Mitsubishi's first system gave about 20 percent over the standard 4G12, while the later MIVEC-MD claimed 10-20 percent, although some of that came from variable valve timing rather than from deactivation.
The spread makes sense. The savings depend on the base engine, the test and how much of a cycle is spent at light load, which is why a lazy motorway run with a big V8 offers the most room.
Why did the first attempts fail?
The oldest ancestor is the hit-and-miss engine of the late 19th century, a single-cylinder stationary engine whose centrifugal governor held the exhaust valve open above a set speed, taking the cylinder out of action. The motoring versions began with Cadillac's L62 V8-6-4 of 1981, developed with Eaton Corporation. It used the industry's first engine control unit for the purpose and switched off opposite pairs of cylinders to give three configurations. The cars carried an elaborate diagnostic routine, even displaying trouble codes on the air conditioning panel. It was standard on every Cadillac except the Seville, whose base engine was a 350 diesel V8. Troublesome and misunderstood, it gave way to a rash of failures and was withdrawn quickly.
Alfa Romeo tried a more modest idea the same year. With the University of Genoa it built the Alfetta CEM, showing it at the Frankfurt Motor Show. Its 130 PS (96 kW; 128 bhp) 2.0-litre engine could shut off two of its four cylinders. Ten cars went to taxi drivers in Milan for real-world checking, and in 1983 a small series of 1,000 was offered to selected customers, of which 991 were built. There was no follow-up.
Mitsubishi's Modulated Displacement followed in 1982 on the 1.4-litre 4G12 straight-four. Because Cadillac's effort had been a failure and this was a four-cylinder, Mitsubishi presented its own as a world first. It disabled the valves of cylinders 1 and 4 below 70 km/h (43.5 mph), at idle and on deceleration. Contemporary reports complained that the engine ran very roughly on two cylinders even with hydraulically damped engine mounts. Mitsubishi tried a flexible section of exhaust pipe, held the system back until the coolant reached 70C, and fitted a flywheel 70 percent heavier. Buyers showed little interest, and the first generation was short-lived. A second generation, MIVEC-MD, arrived in 1993 with better electronic control that made the switch from four cylinders to two almost imperceptible, but Modulated Displacement was dropped around 1996.
Aftermarket kits fared worse. In 1979 the EPA tested an aftermarket kit, the Automotive Cylinder Deactivation System, that let eight-cylinder engines run on four, and found carbon monoxide and nitrogen oxide emissions above the legal limits of the time. Economy improved but acceleration suffered badly, and the loss of engine vacuum caused a dangerous loss of braking assistance in four-cylinder mode. The version actually sold had to be switched by hand with tools under the bonnet.
Who builds it now?
| Maker and system | Year | What it does |
|---|---|---|
| Cadillac V8-6-4 | 1981 | Eight, six or four cylinders; withdrawn quickly |
| Mitsubishi Modulated Displacement | 1982 | Cylinders 1 and 4 off on a 1.4-litre four |
| Daimler Active Cylinder Control | 2001 | 5.8-litre V12 in the CL600 and S600 |
| Honda Variable Cylinder Management | 2003 | Six to four to three cylinders, by bank |
| Chrysler Multi-Displacement System | 2004 | Every other cylinder in firing order on pushrod V8s |
| GM Active Fuel Management | 2005 | Half the cylinders of a V6 or V8 |
| Volkswagen Active Cylinder Technology | 2012 | First on four-cylinder engines |
| Alfa Romeo Quadrifoglio V6 | 2015 | Runs as an inline-three under light load |
| GM Dynamic Fuel Management | 2018 | Any number of cylinders in varied combinations |
Daimler AG's Active Cylinder Control debuted in 2001, and Mercedes-Benz had developed a twelve-cylinder Multi-Displacement System in the late 1990s, which skips alternate cylinders in the firing order. The same idea reached pushrod V8s on a wide scale with the 2004 DaimlerChrysler Hemi. Honda introduced Variable Cylinder Management in 2003 on its J family, working by deactivating whole banks. Bentley unveiled an updated L-Series engine with variable displacement in 2015.
Smaller engines followed. Volkswagen was the first to do it on four-cylinder engines in 2012. Alfa Romeo's Giulia and Stelvio Quadrifoglio switch their V6 to inline-three operation under light loads. In November 2016 Ford announced a three-cylinder Ecoboost that can deactivate one cylinder, the smallest engine yet to use the technique and a sign that small cars can benefit too. Mazda said in November 2017 that cylinder deactivation would be standard on every 2018 CX-5 and available on the Mazda6.
Among US manufacturers in 2020, the leading users were Mazda at 64%, GM at 44%, Honda at 24% and FCA at 23%.
Active Fuel Management and the oil question
General Motors' version has the longest paper trail. Active Fuel Management, formerly displacement on demand, lets a V6 or V8 turn off half its cylinders under light load. It arrived commercially in 2005 on the Chevrolet TrailBlazer EXT, GMC Envoy XL and Envoy XUV with the optional Vortec 5300 V8, announced in April 2003, and spread to the Chevrolet Impala and Monte Carlo and to the Pontiac Grand Prix. The 2010 Chevrolet Camaro SS, with the L99 engine, ran on four cylinders at light load.
The second-generation system, dating from 2004, depended on improved electronics: electronic throttle control, electronically controlled transmissions and more computing power. A solenoid control valve assembly in the engine valley cover sends a pressurised oil signal to special hydraulic roller lifters from Eaton and Delphi, and these lifters disable and re-enable the intake and exhaust valves. Unlike the first-generation system, only half the cylinders can be deactivated.
Here is the catch. Because the system uses engine oil to modulate the valves hydraulically, it depends on the condition of that oil. As the anti-foaming agents in the oil are used up, air can become entrained or dissolved in it, which delays the timing of the hydraulic control signals. Viscosity and cleanliness matter too. Using the wrong grade, such as SAE 10W40 where SAE 5W30 is specified, or skipping oil and filter changes at the factory-recommended intervals, can significantly impair performance of the system.
The dependence is built into the design: the same oil that lubricates the engine also carries the control signal to the lifters, so oil that has aged or been mismatched can change how promptly cylinders drop out and return.
What this means for buyers and drivers
For a buyer, the practical lessons are modest. On any engine that uses oil pressure to lock or collapse its lifters, follow the maker's oil grade and service interval precisely, because the system relies on it. Expect the engine to manage cooling and vibration itself; these are the two hurdles all variable-displacement designs must clear, and Mitsubishi's flywheel and mounting fixes show how hard they can be.
The savings you can expect also depend on how you drive. Economy figures vary widely, and a driver spending most of the time at light load benefits most. Honda's system, which steps from six to four to three cylinders, illustrates the idea: the engine adapts its size to the demand.
Some engines carry a related protection. Cadillac's Northstar series and Ford's Modular and Duratec engines have a fail-safe mode: should the engine run hot or lose its coolant, the controller cuts fuel and spark to half the cylinders. With the valves left to operate normally, the idle cylinders pump cool air through and help to cool the engine.
Where this goes next
General Motors announced Dynamic Fuel Management in January 2018, first for the Chevrolet Silverado. It can drop out whichever cylinders are needed, in many different patterns, and avoids switching between banks, using oil pressure solenoids to collapse each lifter individually, so every cylinder is controlled independently. It builds on Dynamic Skip Fire, developed by the California company Tula Technology, and the 6.2L V8 that carries it earned a place on Ward's 10 Best Engines list for 2019. GM has also shown concept engines that switch between 4, 8 and 16 cylinders, such as the XV16 in the 2003 Cadillac Sixteen.
Other routes to the same goal remain experimental. Saab built a variable compression engine with a hinged block, the Hefley engine uses a sliding crank race, and the Scalzo Piston Deactivation Engine can stop individual pistons entirely. No production vehicle uses any of these designs.
