Adjustable Camshaft(s)

Function

In combination with multi-valve technology the adjustable camshaft is at least a partial solution of the old problem that a piston stroke internal combustion engine features its optimal torque only in a certain speed range. The camshaft adjustment enables the engine to combine good tractive power in the lower speed range with performance development during sporty climbing into high revs. In addition, it reduces the number of non optimum speed ranges, improving the quality of the exhaust gas emissions and the mileage. Further possibilities: improved control of the idle running and the internal exhaust gas re-circulation.

How it works

The camshaft adjustment is arranged between driving wheel and camshaft. The control unit guides the oil pressure onto the adjustment with the help of magnetic valves. This causes the internal piston (see 4th picture to the right) to twist against the external piston (see 4th picture to the left) and thus the camshaft is twisted against its drive.

The result is visible on top in the valve lift diagram. Only the intake camshaft is adjustable in this case. In the lower speed range till approx. 2000 1/min the (petrol) engine does not make use of the adjustment. Due to the small valve overlap a low idle running and better starting-up torque is possible. In the upper speed range the opening and closing of the inlet valve timing is shifted forward by approx. 22°. A larger valve overlap is advantageous in the higher speed range. Nowadays, it is more common, because of the internal exhaust gas re-circulation (with lower NOX emissions) to have a bigger overlap in the lower speed range and a smaller one in the upper speed range.

The camshaft adjustment alters either only the intake camshaft or both camshafts. It can be altered twice in the course of the speed range of the engine. With clocked magnetic valves a continuous movement is possible. The time between opening and closing of a valve does not change. For this you need the variable valve lift control. Another possibility is, e.g., a bevelled cam which operates almost point-shaped on its bucket tappet. This is only possible if there are two camshafts: one for the inlet and one for the exhaust valves. It is shifted lengthwise electronically with hydraulic force, causing an infinitely variable timing setting and valve lift.

Picture 5 displays an adjustment system featuring a chain. The chain is slightly too long, and the chain guides are movable down and upwards (see picture 5). We assume, both camshafts turn clockwise. If the guide of the connecting chain is moved upwards by oil pressure, the chain lengthens itself on top and becomes shorter further down. The right camshaft sprocket is twisted against the rotation direction of the left camshaft sprocket. Consequently, the valves to the right open and close later as compared to the left ones. The reverse takes place if the chain guide is moved down.

06/08