The desmodromic (also forced control ) is a special form of valve control in four-stroke engines . The name is derived from the Greek "Desmo Dromos", which means something like controlled movement .
Usually, which are valves of an internal combustion engine by the rocker arms , finger followers or tappets opened and closed by valve springs closed again. The desmodromic or forced valve control works without valve springs and also carries out the closing movement in a controlled manner .
This control requires two rocker arms for each valve and two cams on the camshaft , one of which opens the valve as usual, the other closes the valve. In order to allow the valves to close tightly even when the valve clearance of the cold engine (for the calculated thermal expansion) and to start an engine, a weak valve closing spring is often used in addition to the cam-controlled closing mechanism, which keeps the valve in contact with the seat even when the engine is cold without direct force intervention by the closing cam - because without tightly closing valves and thus without compression, a four-stroke internal combustion engine could not start.
In the early days of motorsport, the quality of spring steels was inadequate for the heavy loads of racing, so that valve flutter at high engine speeds and spring breakage often occurred. Therefore, solutions were sought that allow the valves to close without springs. Many engine manufacturers and designers developed their own, sometimes very complex systems, which often did not survive the test stage. Constructions with gate control , individual cams for the opening and closing phases as well as cam tracks were created.
Well-known constructions with positive control were a part series of Richard Küchen's K-motors from 1924, the Norton Manx from 1949, the Mercedes-Benz W196 and the Mercedes 300 SLR from 1954, which was derived from it. Another interesting thing about Küchen's construction is that the camshaft instead of control disks conventional cam carries, the camshaft assumes the usual position of a vertical shaft .
Although these precise controls theoretically enabled high valve accelerations, they all had the disadvantage that they consist of a large number of small parts that are prone to wear. Due to the resulting high manufacturing and maintenance costs, desmodromic valve control systems were only able to establish themselves in one exception: Fabio Taglioni developed a comparatively simple desmodromic system for Ducati from the mid-1950s , which is still used today in motorcycles of the Italian brand.
Forced control and modern car engine in comparison
An engine with desmodromic valve control has more moving parts that are subject to wear and tear than a conventional one, so that more play is added to the total valve clearance - an indication of higher manufacturing costs and increased maintenance requirements. This speaks against a use in large-scale production, which aims at the lowest possible production costs. Furthermore, the space requirement is very large, so that this design is not suitable for multi-cylinder internal combustion engines (source: Patent DE102006012787; Schaeffler KG , 91074 Herzogenaurach, DE).
The high maintenance effort (with the 4-cylinder, 4-valve engine of the Ducati Desmosedici , around seven hours are required to adjust the valve clearance) and the disadvantage of increased noise development due to possible valve rattling are indeed the small customer group of exclusive enthusiast vehicles, but not the buyers of a large-scale Automobile reasonable. On the other hand, desmodromic valve controls require less time to open and close the valves than conventional valve trains, which has a positive effect on gas exchange. This also makes it easier to meet the increasingly strict emissions regulations.
So far, however, the engine builders have not succeeded in combining the advantages of forced control with the maintenance-free hydraulic valve clearance compensation (HVA for short), which is common in almost all modern internal combustion engines for cars.
Forced controls with hydraulic valve clearance compensation
In the case of a positively controlled valve train, there are two points of movement and three points of play, namely:
- between the cam track for opening the valve and the associated first cam follower element,
- between the cam track for the closed position of the valve and the associated second cam follower element,
- between the end of the intermediate lever and the opening stop on the valve head,
- between the end of the intermediate lever and the closing stop on the valve head,
- and on the bearing axis of the intermediate lever.
On the other hand, there is of course the actual valve clearance between the valve body and the valve seat. The play causes leaks in the combustion chamber, annoying noises, greater wear and tear and changes the transition at the beginning and end of the cam area into the base circle of the cam element, which has disadvantages in terms of closing times, poor exhaust gas values and increased maintenance costs.
Functional description of the priority control with double contour cams
The opening process is analogous to the state of the art with rocker arm, roller tap and HV compensation.
The difference is that the opening lever does not work against the strong valve spring.
The closing process works similarly to the prior art, like a reverse opening mechanism with rocker arm and high-voltage compensation.
The difference is that the closing lever scans an inner contour and pulls on the valve (disk).
The advantages are less friction , lower alternating torque and lower component stress as well as the high speed capability . In addition, the installation space required is significantly less than with existing forced controls. Instead of the roller tap, a sliding tap is also possible.
Functional description of the double hydraulic valve clearance compensation (HVA)
The new positive control with hydraulic valve play compensation comprises an opening lever and a closing lever which each picks up one of the two cam tracks and each has an adjusting device for the bearing or movement play. The pivot centers of the lever supports are identical.
The division of the intermediate link into two individual levers that are independent of each other, i.e. H. are without a fixed connection with each other, and still operate the valve together, since they spatially overlap, it allows the play at the play points per lever to be adjusted in a controlled manner and to effect the play compensation accordingly.
- Base circle position: Both rocker arms are not subjected to external forces (no stroke, no closing force spring, no inertia force ...). In both hydraulic chambers (3; 4) there is no pressure caused by the levers. The non-return valve (1) is therefore open, engine oil can flow under pressure into the compensation elements and press the levers against the cam tracks and stop elements on the valve. The control valve 2 between the hydraulic chambers is in the middle position (preferably by spring force or a special design of the channels) and enables the two hydraulic chambers to be filled and thus to compensate for play.
- Lift position - opening cam: The opening cam presses the opening lever and opens the valve. The resulting (support) force generates pressure (differential pressure) in the upper hydraulic chamber and closes the check valve. The ball valve 2 between the hydraulic chambers is also closed (ball goes down), no oil can flow from the upper into the lower chamber. The closing lever, which is not subjected to force, and the lower hydraulic chamber that interacts with it, are without force or pressure. A subsequent flow of oil and an unintentional inflation of this lower chamber is not possible, since the differential pressure to the upper chamber and to the supply line from the engine closes the central valve 2 and also the check valve 1. The two pistons remain stable in position even under load (inertia force).
- Lift position - closing cam: The opposite is true here. The lower chamber has overpressure compared to the upper chamber and the supply line and thus closes the middle valve (2) and the check valve (1). It is not possible to inflate the upper chamber.
- Backlash compensation valve length / position: Both pistons move in parallel / synchronously, as the distance between the levers at the joint on the valve remains the same. The whole valve (both pistons) moves and acts equally on both levers.
- Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2005, ISBN 3-528-23933-6
- Küchens K-engine from 1924: Desmodromic with control discs on a vertical single-cylinder camshaft
- Page no longer available , search in web archives: The latest Desmodromic: "Desmotronic". Detailed information and videos ) (
- "Desmodromology": for further investigation and visualization. (English)
- Animation for clarification (MPEG1, 4.2 MB; MPG )
- Page no longer available , search in web archives: the first forced control with hydraulic valve clearance compensation ) (