Fig 1


Fig 2


Fig 3


Fig 4


Fig 5


Fig 6


ACT (Active Cylinder Technology) explained

If your 4 cylinder engine started running on just 2, you'd normally be parked at the side of the road soon after, waiting for the nice man from the recovery company. Not so with the new Volkswagen TSi ACT engine. In fact, it's meant to. Let's explain how.

The 4 cylinder engine is a petrol TSi where by 2 cylinders can be switched off (automatically) according to power requirements. They come in or out of play in the 1400-4000 rpm rev range. Fuel saving wise you can expect 0.4 Litre per 100km or, in other words, 1 Litre in 155 miles. In real money that'll get you roughly 50 miles extra to the tank. Not bad when you have to do nothing and you're not even aware it's happening.

How it works

Fig 2. Actuators are attached to the cylinder head while two identical camshafts are run via a tooth belt and a 'phase adjuster' (camshaft adjuster).

Fig 3 & 4. The splines on the camshaft make it possible for the cam sections over cylinder 2 and 3 to shift side ways of the inlet and exhaust valves. There are two cam 'contours' for each valve, one of which has a continuous root circle. This contour is transferred to the valve by a roller finger.

The shift device is initialised by one double actuator per cam section. This is the start of the switch off process.

Fig 5. It only takes 11 milliseconds for the pin of the double actuator to engage in the worm gear. As the gate is connected to the cam module this action moves it from the full to the zero stroke in the root circle phrase.

Fig 6. The valves remain closed whilst fuel injection to the 2nd and 3rd cylinder is shut down. The pistons then 'free wheel' with combustion. This can result in fuel savings of up to 20% against a tradional engine combustion cycle.

When increased engine power is needed the cylinders re-activate within one and a half camshaft turns and therefore completely indistinguishable by the driver.

Related Links: ASG (Automatic Shift Gearbox) | Tyre labelling EU legislation changes | Tyres - Safety & buying guide | Tyres - Under pressure

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