TeorÃa de la perspectiva
The camshaft at the head is a specific measure camshafts above the cylinder head, in order to improve the order of the Soupape S by reduction in the parts in reciprocating motion.
In a valve engine at the head, it is of use to transmit the ordering of opening of the valves by a rotary movement taken on the Vilebrequin and transmitted to a camshaft.
To avoid the Panic of valves to high modes (approximately 4000 t/mn and more), it is advisable to the minimum to reduce the parts subjected to a reciprocating motion, like the push rods, the pushrods and the tumblers themselves. The removal of these parts eliminating from as much the parasitic mechanical plays, the precision of ordering of the valves will be some improved. For that, one places the tree (S) at cams directly above the valves. The transmission of the rotation of the crankshaft towards the camshaft is done by a chain, a Courroie notched, a cascade of pinions, a rod or a tree with conical couples, even exceptionally a rod (NSU). It is simply necessary to maintain a strict synchronization with a ratio of 2 turns of crankshaft for 1 turn of camshaft.
When one has several lines of valves, one can use a camshaft by line of valves (Double camshaft at the head), or only one camshaft, actuating the various valves via small Culbuteur S.
When all the valves are on the same line, a simple camshaft is enough to actuate all the valves without needing tumblers nor to a second tree.
A long time, the industrialists were sulky the camshaft at the head for the cars of great series, because of the costs of maintenance, the problems of lubrication which it posed and of the modifications of the production lines of the engines. In fact, the camshafts at the head having interest only to reach the high modes, it was of use to hold them for sporting engines. From now on, the majority the engines of cars are equipped with camshaft at the head, often double, to actuate the 16 valves current on the modern engines with 4 Cylindre S in line. The V-engine using this technology occupy more space in the driving compartment of the cars than the engines with side valves or Soupapes at the head with tumblers.
The camshaft engines at the head reach their couple and their maximum power with mode-engines higher than those of the valve engines at the head with tumblers. To maximize the output, the car manufacturers must thus twin them with Transmissions having a high number of reports/ratios.
Among the last evolutions it is necessary to announce the engines to variable Distribution, technology Camless, like various devices making it possible to modify the position or the profile of the camshaft during the operation of the engine. This characteristic makes it possible to improve the output of the engine to top as with low mode. Unfortunately, the cost of these devices the reserve with the high-ends and the sporting models.
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