How much can cylinder deactivation increase efficiency?

Sometimes less is more - cylinder deactivation

Switching off cylinders is nothing new in and of itself. Because as early as the 80s and 90s, Mercedes made sure that breaks were prescribed for the combustion chambers in the S-Class and BMW in the 7 and 8 series. In the USA, the deactivation of cylinders, e.g. in the Chrysler Group (Hemi V8 engines with 5.7 liter displacement) was and is very common, but in Germany it was abandoned again (Mercedes until 2005 in the 5l V8). Now the temporary deactivation of cylinders at Audi, VW and Mercedes is getting a little renaissance.

In spring 2012, the Audi S8, S7 as well as S6 and S6 Avant will make their debut with engines that switch off half of their cylinders when necessary

But why has the technology, known for short as ZAS, not really established itself so far?

There were several reasons for this: On the one hand, the technical implementation was very complex, but the benefit from lower fuel consumption in practice was not as clear as hoped, and on the other hand the customer could not and did not want to be enthusiastic about it. After all, he pays a lot of money for the power of his 8 cylinders and he wants the horsepower available at all times, whether necessary or not. He didn't want to miss the sonorous sound of the engine either.

The new systems confidently display the pausing of individual cylinders on the instrument panel. Because “saving” or better said “efficiency” has become socially acceptable. At the IAA, Audi promoted the new generation of the S8 with the 4.0l TFSI engine for cylinder deactivation very clearly and brought the technology under the name in spring 2012 cylinder on demand on the market.


The new V8 4.0 TFSI from Audi - with start-stop and cylinder deactivation, it saves at least 10% fuel

While the old S8 was still equipped with a powerful and powerful ten-cylinder engine with a displacement of 5.2 liters, the new generation manages with eight combustion chambers and “only” four liters of volume. In the partial load range, the biturbo V8 switches off four cylinders, which reduces its consumption in the NEDC cycle by around five percent, which corresponds to around 10 to 12 grams of CO2 per km. If you add the start-stop system used in the Audi S models, which deactivates the engine when the vehicle is stationary, the savings amount to up to 24 grams of CO2 per km.

Dynamic downsizing

But what exactly is cylinder deactivation?

Cylinder deactivation is also known as dynamic downsizing and is a system for reducing the fuel consumption of internal combustion engines. An engine rarely needs all of its power. High-horsepower cars, in particular, should only need more fuel when they also need more power. In the other situations, cylinders can simply be switched off, which means that the fuel supply is interrupted, or rather, no fuel is injected into the relevant cylinders. However, if you only switched off the injection and the ignition, the cylinders would work like air pumps and the lambda control would be properly out of step. Therefore, and to avoid gas exchange losses, the inlet and outlet valves of these cylinders must also be closed. Depending on the system, a complete cylinder bank can be switched off (V-engine) or, in the case of in-line engines, individual cylinders (individual cylinder deactivation).

Use of ZAS

ZAS are used mainly because of a certain minimum running smoothness, mainly in engines with at least eight cylinders. In addition to the aforementioned AUDI S8 with 382 kW (520 PS) and a 23% reduction in standard consumption from 13.2 l to 10.2 l, the smaller models S6, S6 Avant and S7 are also equipped with the 8-cylinder which can be switched off . The double turbocharged engines still have 309 kW (420 hp) with a consumption of less than 10 liters.


Power and torque diagram of the 4.0 TFSI. The cylinder deactivation works with a torque requirement of 120 - 150 Nm within 960 to 3500 rpm


The new four-liter V8 will also be used in the Bentley Continental GT in 2012, where consumption is reduced by up to 40 percent in the NEDC cycle compared to the previous V12. Bentley was involved in the development process of this technology and already uses a similar system in its "large" V8 with 6.75 liter displacement and over 1000 Nm in the Mulsanne.

The Mercedes sister AMG is trying to reduce the consumption of its models by 30 percent and therefore brought a new gasoline direct injection V8 engine onto the market at the 2011 IAA with the SLK 55 AMG, which also features cylinder deactivation (AMG cylinder management) uses.

The V8 engine of the Mercedes SLK 55 AMG also uses cylinder deactivation

When the driver of the naturally aspirated V8 with an output of 310 kW (422 hp) and 540 Nm torque selects the "Controlled Efficiency" mode at the push of a button on the center console, cylinders two, three, five are activated in the partial load range between 800 and 3600 tours and eight. In four-cylinder mode, 230 Newton meters of torque are then available. The transition from four to eight-cylinder mode should go completely unnoticed by the pilot at 3600 rpm and within 30 milliseconds. The display shows the status of the engine with the help of the symbols "Eco4" and "Eco8". In sport mode and in manual mode, cylinder deactivation does not work.

In Formula 1, cylinder deactivation is also known. Here the electronics switch off the fuel injection during pit stops or in the safety car phase.

Make two out of four

But it doesn't always have to be large-displacement engines: VW will in future switch off two cylinders in the Golf 1.4 TSI with its 103 kW (140 PS). The four-cylinder turbo direct injection shuts down two combustion chambers in the lower and medium speed range between 1400 and 4000 rpm. To do this, the system closes the inlet and outlet valves of cylinders 2 and 3; the engine only ignites once per crankshaft revolution. The valves are closed using complex actuators with sliding sleeves on the inlet and outlet camshafts


The valves can be completely closed by switching the cam lift


The technology in the VW is basically comparable to that in the Audi and will be explained below. The two-cylinder status is only displayed if the driver asks about the current consumption in the multifunction display. Otherwise he would hardly notice it - even with two cylinders, the excellently balanced 1.4 TSI still runs very quietly and with little vibration.

In the official driving cycle, this reduces consumption by 0.4 liters per 100 kilometers, according to VW. With the automatic start-stop, this adds up to a fuel saving of 0.6 liters. At 50 km / h in third or fourth gear, a saving of almost one liter per 100 kilometers should result.


Volkswagen states that at 50 km / h in third or fourth gear there should be a saving of almost one liter per 100 kilometers (source: VW)

ZAS working method

The more detailed operation of a modern cylinder deactivation is to be explained using the example of the Audi V8. The cylinder deactivation works in the 4.0 TFSI at low load. The upper limit for this is, depending on the speed, between about 25 and 40 percent of the maximum torque, about 120 to 250 Nm; the effective mean pressure here is up to eight bar. The speed range begins above idle speed at 960 rpm and extends up to 3500 rpm. The cylinder deactivation is active from third to eighth gear as soon as the cooling water is heated to over 30 degrees. If these conditions are met, the system closes the inlet and outlet valves of two cylinders per bank. The V8 now works as a V4, still with a regular firing order, but only fires two instead of four times per crankshaft revolution. Instead of 1 - 5 - 4 - 8 - 6 - 3 - 7 - 2 the firing order is now 1 - 4 - 6 - 7; The efficiency increases in the active cylinders because the operating points shift towards higher loads.

That evolvedAudi valvelift system (AVS) uses electromagnetically displaceable sleeves on the camshafts

The further developed Audi valvelift system (AVS) uses a zero lift cam that runs in the base circle. If this is switched on electromagnetically, the valves remain closed (source: Audi)

In addition to the conventional full profile, these cam pieces also have a zero lift cam as an additional profile. While the full profiles in eight-cylinder operation operate the roller finger followers and, via them, the valves, i.e. behave like conventional cams, the zero lift cams rotate over the rocker arms, they do not operate them, the valve springs keep the valves closed. Spiral grooves are milled into the outside of the rotating cam pieces, via which the sleeves can be shifted a few millimeters on the shafts in a flash: When electromagnetic actuators in the cylinder head cover receive a signal from the engine control, two integrated metal pins engage in the grooves from the outside and bring them into place them in final position. The cam pieces are finally locked in place by spring-loaded balls. At the same time, the engine management shuts down the injection. All mechanical switching processes take place within half a camshaft revolution; Depending on the speed, they take 9 to 31 milliseconds. Flanking interventions on the ignition and throttle valve smooth them out.

Immediately before the valves are closed, the combustion chambers are filled again with fresh air. This inclusion of fresh gas means a lower pressure in the cylinder and a correspondingly low energy consumption - an important factor for increasing efficiency. The fuel injection and ignition are shut down by the engine management. In the deactivated cylinders, the pistons move by the drive from the crankshaft, so they are dragged.

The FIS display of the instrument cluster shows the driver the four-cylinder status - the bar on the consumption indicator turns green and a text message appears at the same time.

If the driver is driving with 4 instead of 8 cylinders, this is indicated to him by a green bar and text

As soon as the driver accelerates vigorously, the deactivated cylinders switch on again. The return to eight-cylinder operation, like the shutdown, is so smooth and quick that it cannot, in fact, be felt; it takes an average of just 300 milliseconds. In principle, however, the switchovers are associated with short-term efficiency disadvantages, so that a consumption advantage only arises from switch-off times of more than three seconds. Audi has therefore developed a control logic that monitors the driver's accelerator, brake and steering movements. If it detects an uneven pattern from them, it refrains from switching it off in certain situations, for example when driving in a roundabout or when walking at a sporty pace on the country road. Deactivating it for just a few seconds would increase rather than decrease consumption.

The cylinder on demand system is permanently operational, even in S mode of the automatic transmission and in the dynamic map of Audi drive select. The greatest savings are achieved with constant travel at moderate speed, as is common on many expressways. At 80 km / h, the efficiency gain for all S models is twelve percent, and even at 130 km / h, seven percent are saved. In the NEDC cycle, as already mentioned, the increase in efficiency is approx. Five percent, with a start stop approx. 10%.

ZAS without sacrificing comfort

Unfortunately, cylinder deactivation also has a few comfort-related disadvantages. Eight-cylinder engines are not only known for their pulling power and harmonious power delivery, but also for their smooth running behavior. Where a four-cylinder needs balancer shafts, an eight-cylinder can usually do without them. And four cylinders are known to run rougher than eight. When the V8 works as a V4, its crankshaft drive produces higher torsional vibrations, depending on the load and the speed; these also penetrate the interior as airborne sound. The large exhaust system also emits certain humming noises that cannot be avoided despite an intelligent flap control. Of course, the driver and passengers should not hear any of these interfering noises. When the 4.0 TFSI is running in four-cylinder mode, that's why they take care Active noise control (ANC) and on the other active engine mounts to ensure that unwanted noises in the interior or vibrations do not spoil the customer's journey.


When it comes to saving fuel and CO2, everyone is talking about downsizing. If you only do this temporarily by switching off the cylinders (dynamic downsizing), then you seem to kill two birds with one stone. Power and torque are available on demand thanks to the waiting cylinders (cylinder on demand) and fuel is saved in the partial load range. However, it should not be forgotten that the cylinders still have to be dragged along! It seems that the time is now ripe for cylinder deactivation, not least due to the fact that appropriate comfort measures such as Active Noise Control and Active Engine Mounts make this attractive to customers because they may not even notice when they are missing cylinders.

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by Johannes Wiesinger

edited: 02/19/2015
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