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
Cylinder Deactivation – Why and How?
Cylinder deactivation is used to reduce the fuel consumption and emissions of
an engine during light load operation.
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
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
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?
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.
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?
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.
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
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.
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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