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Cylinder Head 3 - Design

The design of a cylinder head depends first of all on the location of the engine in the vehicle, more specifically from the position above the cylinder head. Expressed even simpler: For example, Can the intake pipes preferably run straight further after an unavoidable bend past the respective inlet valve or is another, more generous as possible bending inevitable?

We have elsewhere pointed out already that the operations around the charge exchange under no circumstances must be regarded only static. Because that would mean, the piston going from BDC to TDC pushes out the old gases and brings in the fresh gases in return. However, if we look at the dynamic side, we ignore in the following effects which arise by the interaction of several cylinders.

For an optimum exchange of the old by fresh gases is attempted, the cylinder chamber (Vh + Vc) to charge in a way that at the end it contains more fresh gas, as would fit in based on the pure cubic content. Mind you, we are talking about neither of charging by compressor or turbocharger, nor from the so-called dynamic charging, for example, the is useful for the resonances in the intake manifold.

For this purpose it is useful to look at the processes around the intake and exhaust cam, the two classic, central control elements of an internal combustion engine. Because these operations when petrol engine maybe have a little more importance than at the diesel, we will restrict ourselves to the petrol engine. Whether for that matter pure air is sucked (direct injection) or a fuel-air mixture, is only of secondary importance.

Next to the heigth of the valve lift, the course of the cam is of great importance. Already Keith Duckworth by the company Cosworth, the later succeeded one of the most popular F1 racing engines, has struggled with these problems during his first works for enhanced performance. Subsequently he has rejected much of what he had read and reflected its own considerations and tested in practice (e.g. at the dynamometer). When later distinguished companies such as Mercedes consulted him, the results of his work can not have been so bad.

Thus, if a cam opens a valve, then it does this first of all via a 'ramp'. The first cam lobe is thus not particularly strong, because otherwise the opening acceleration of the valve would reach unacceptably high values. Previously it was used simultaneously to overcome the valve clearance, what practically is to find only in motorcycle and truck engines today. One calls this first part of the increase also 'pre cam'.

The partner of the cam, for example, a bucket tappet, is also called cam followers. Synchronous movements with the cam are forced by one or two springs. Rarely there are also override controls. A cam can control more than one valve, e.g. at fork levers of the same cylinder or to different sites different cylinders. Famous example is the VW Beetle. Less frequently the possibility was used to actuate inlet and outlet valves with a cam.

The control times still form the core of the engine management system to recognize by the fact that meanwhile even the vast majority of engines are operated with charging in four-valve technology. However, the purely mechanical opening and closing of valves requires much attention, even in the fulfillment of CO2 regulations. Here rolling friction has enforced largely opposite sliding friction. The purely electrical operation of valves seems no longer to be an issue for the time being. 12/15

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