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Cylinder Deactivation Reborn - Part 1

We look at cylinder deactivation and its earliest application in a mass-produced car...

By Michael Knowling

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At a glance...

  • First of 2-part series
  • The principle of cylinder deactivation
  • The 1981 Cadillac with V8-6-4 technology
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The idea of cylinder deactivation is becoming increasing popular as car manufacturers strive to reduce fuel consumption. Cylinder deactivation effectively creates a variable displacement engine, which means you can enjoy the on-demand power of a large capacity engine together with the fuel economy of a smaller engine.

In this article, the first of a 2-part series, we’ll look at the principle of cylinder deactivation and the first mass produced car to put the idea into action.

Cylinder Deactivation – Why and How?

Cylinder deactivation is used to reduce the fuel consumption and emissions of an engine during light load operation.

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In typical light load driving you use only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed and the engine needs to work to draw air. This causes an inefficiency known as pumping loss.

Mercedes says that some large capacity engines need to be throttled so much at light load that the cylinder pressure at Top Dead Centre is approximately half that of a small 4 cylinder engine. Low cylinder pressure means low fuel efficiency.

The use of cylinder deactivation at light load means the throttle valve can be opened further to provide the same power output. This reduces pumping losses and increases pressure in each cylinder. Fuel consumption can be improved by around 20 percent in highway conditions.

So how is cylinder deactivation achieved?

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Well, put simply, it involves keeping the intake and exhaust valves closed for a particular cylinder. We will examine ways to keep the valves closed as we look at each manufacturer’s system.

By keeping the intake and exhaust valves closed, it creates an ‘air spring’ in the combustion chamber – the trapped exhaust gasses (kept from the previous charge burn) are compressed during the piston’s upstroke and push down on the piston during its downstroke. The compression and decompression of the trapped exhaust gasses have an equalising effect – overall, there is virtually no extra load on the engine.

In the latest breed of cylinder deactivation systems, the engine management system is also used to cut fuel delivery to the disabled cylinders. The transition between normal engine operation and cylinder deactivation is also smoothed using changes in ignition timing, cam timing and throttle position (thanks to electronic throttle control).

In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. In the case of a V12, up to 6 cylinders can be disabled.

So now let’s look at cylinder deactivation in action...

Cadillac L62 V8-6-4

The first mass-produced vehicle to employ cylinder deactivation was the 1981 Cadillac.

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Through the late 1970s, Cadillac (the luxury division of GM) was under immense pressure to improve the fuel consumption of its V8-powered saloons and limousines. The introduction of US Corporate Average Fuel Economy (CAFE) regulations forced the issue.

Cadillac’s solution was to develop a cylinder deactivation system in conjunction with the Eaton Corporation.

The existing 368ci (6.0 litre) pushrod V8 was used as the platform and an all-new valve control system allowed the sequential deactivation of two pairs of cylinders. In other words, it could run as an 8, 6 or 4 cylinder engine.

So how did Cadillac deactivate the cylinders?

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Well, a series of solenoids were used to release the fulcrum on the intake and exhaust valves rocker arms – a cut-away of the system can be seen here. The lifters and pushrods continued to operate as normal but the rocker arms sat motionless and the valves remained closed due to valve spring tension. Interestingly, Cadillac chose to deactivate opposing pairs of cylinders rather than a bank of cylinders (as is done today). When increased engine power was required, the solenoids returned the rocker fulcrums to their normal operating position and full valve operation resumed.

Coordinating the activation and deactivation of cylinders was an electronic control unit (ECU). The ECU controlled the engine’s throttle-body fuel injection system as well as the cylinder deactivation solenoids.

The engine was set to run on all 8 cylinders during starting, heavy acceleration and at all speeds up to 27 mph (43 km/h). At light to moderate engine load (such as highway cruising) the system would deactivate pairs of cylinders as required. It is said that the V8-6-4 engine could drive interstate using 4 cylinders for the majority of the journey (given a relatively flat road and economical driving style).

A dashboard display showed the number of active cylinders and the system relied on the driver applying more throttle to maintain speed upon cylinder deactivation – a trait that some drivers had trouble accepting.

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The engine produced 140hp (104kW) at 3800 rpm and 265lb-ft (360Nm) at 1400 rpm. However, highway fuel consumption was claimed to improve by around 30 percent with cylinder deactivation. Urban/city driving was improved to a lesser extent.

But despite being seen as a technological marvel, the L62 V8-6-4 engine had numerous problems and Cadillac found itself faced with many legal battles.

The throttle-body fuel injection system (essentially a carby with an injector) was a major cause of the problems. The throttle-body injector would continuously deliver fuel to all cylinders irrespective of cylinder deactivation. Fuel would accumulate in the intake ports of deactivated cylinders until full engine operation was resumed – at which point the deactivated cylinder would receive an extremely rich air-fuel ratio and splutter. Matters weren’t helped by the relatively poor resolution of the ECU – it updated just 10 times a second.

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The V8-6-4 engine was fitted to the 1981 Cadillac DeVille, Brougham, Eldorado and as an option in the Seville . Amazingly, it was discontinued after just one year of service (though it was retained for use in some 1982 Cadillac limousines). Many V8-6-4 Cadillacs have since been converted to run continuously on all 8 cylinders.

The L62 V8-6-4 was an interesting development that pushed the boundaries of contemporary technology. Unfortunately, it smeared the reputation of cylinder deactivation for many years to come...

In the second part of this series we’ll check out the latest breed of cylinder deactivation systems.

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