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All Tests

Engine Control 2

No, the opening of a valve did not always take place via such a cam. On the first Daimler and Maybach engine, there are grooves milled into a disk to open and close a valve. close. After that, the intake valve had no such mechanism at all for some time, but opened by the vacuum of the intake take.

Of course, the cam does not touch the valve directly at the end of the shaft. A so-called bucket tappet like an upside down cup belongs on it. By the way, 'tappets' are components in the valve train that pass on the impact of the cam. The tappet is a small distance from the valve stem, the pushrod a larger distance.

A bucket tappet can be thought of as a small cylinder without a bottom. If the valve is driven directly via it, an adjusting device is required. These are small plates of a of a certain thickness that fit exactly into the slightly retracted remaining base.

With the camshaft in front of you and the tappets with the plates underneath, measure the distance with different blades of the feeler gauge when the respective cam is opposite the tappet. opposite the bucket tappet. Correction is made by means of a tool that presses down the plunger together with the valve, thus allowing a change to the plate with the correct thickness.

The pressure is maintained by a Non-return valve in the oil supply line. Only a tiny amount may be lost while the valve is actuated, and this is replenished afterwards, but only up to the pressure of the oil. This can compensate for any differences in the actuation mechanism that may arise. The valve closes securely.

Tappets, whether hydraulic or rigid, are not always found only directly on the valve. On a camshaft next to the crankshaft, they can also be located between the cam and the pushrod. At a cam, everything except the area with zero elevation is important. On engines with valve adjustment, there may even be a pre-cam that reduces this elevation.

Cams determine not only when the valve opens and when it closes, but also how fast it opens and closes and what dimension has the largest opening. That is why camshafts are rightly regarded as the heart of engine control. They virtually determine the operating behaviour of the internal combustion engine.

Even as a naturally aspirated engine with a not very large overall displacement, it can demonstrate satisfactory pulling power, which is needed, for example, if you suddenly want to overtake without downshifting after cruising.. But this may also be absolutely necessary with an engine that is noticeably more comfortable at high revs.

In engines of the first category, the timing is such that at relatively low revs the combustion chambers are relatively well filled. It is not so much the opening of the intake valve (Io) as its closing (Ic) that is of rather importance, and the opening of the exhaust valve (Oo) rather than its closing (Oc).

A very important value, the difference in the angle between Oo and Ic, also called 'valve overlap'. This shows the extremely dynamic conditions, because it is a time period in which both valves types are open. From a static point of view, the angle would have to be zero so that old and fresh gases do not mix.

As you can see, one aspect of the charge exchange is the possibility that outflowing old gases set the somewhat sluggish fresh gas column in motion very early on. In the case of supercharging, this can combustion chamber can be ventilated. In general, the speed of the best exchange is also considered to be that of the highest torque.

More modern camshafts, which are hollow to save weight, consist of a tube in which, after the cams have been picked up in exactly the right position, they are fixed in the tube by enormous short-term water pressure. Solid shafts are made of machine steel for rather small series and cast iron for larger ones.

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