Desmodromic valve

Desmodromic (including positive control ) is a special type of valve control for four-stroke engines. Derived is the name from the Greek " dromos Desmo " which means as much as controlled motion.

Operation

Usually, the valves of an internal combustion engine can be opened by rocker arms, rocker arms or tappets and closed by valve springs again. The desmodromic valve control or forcibly operates without valve springs and also performs the closing movement controlled from.

This control requires two rocker arms for each valve on the camshaft and two cams, of which, as usual, the valve opens the other closes the valve. To suggest tight even with valve clearance cold engine ( for the calculated thermal expansion ) and to start an engine, the valves, weak valve closing spring is often in addition to the cam-controlled locking mechanism used, the installation of the valve to the seat even when the engine is cold without direct force engagement ensures the closing cam - because without tight valve closure and thus without compression could not start a four-stroke internal combustion engine.

Development

The quality of the spring steels was inadequate for the heavy load in racing in the early days of motor racing, so it often came to valve float at high speeds and spring breaks. Therefore solutions have been sought which permit a closing of the valves without springs. Many engine manufacturers and designers developed their own, often very complex systems, which often did not survive the experimental stage. There were constructions with slide control, the individual cams for opening and closing phase, as well as curved paths.

Known designs with forced control were a part series by Richard Küchens K engines from 1924, the Norton Manx 1949, the Mercedes -Benz W196 and derived from him Mercedes 300 SLR of 1954. At Küchens construction is also interesting that the camshaft timing pulleys instead conventional cam wears, the camshaft occupies the usual position of a vertical shaft.

Although these precise controls enabled theoretically high valve accelerations, but all had the disadvantage to consist of a large number of small parts susceptible to wear. Due to the resulting high production and maintenance costs, desmodromic valve control systems were able to achieve in only one exception: Fabio Taglioni developed from the mid-1950s for Ducati Desmodromic a relatively simple, which is to this day used in the motorcycles, the Italian brand.

Override control and modern car engine compared

An engine with desmodromic valve control has more moving and Wearing parts than a conventional, so that adding more individual games to the entire valve game -. Evidence of higher production costs and increased maintenance This argues against use in Großserienbau aiming at the lowest possible manufacturing cost. Furthermore, the space required is very large, so that this construction for multi-cylinder internal combustion engine is not suitable (Source: Patent DE102006012787; Schaeffler KG, 91074 Herzogenaurach, Germany).

The high costs of maintenance ( the 4- cylinder 4- valve engine of the Ducati Desmosedici are about seven hours to adjust the valve clearance necessary) and the disadvantage of increased noise due to the possible valve clatter are indeed the small number of customers an exclusive collectors' cars, but not the buyers of a large series automobile reasonable. On the other hand, need desmodromic valve controls less time for the opening and closing of the valves than conventional valve trains, which has positive effects on gas exchange. This means that the increasingly stringent emissions regulations are easier to fulfill.

So far, it is the engine builders but have been unable to take advantage of a coercive control with the maintenance-free hydraulic valve clearance compensation (short: HVA ) to combine, which is common in almost all modern internal combustion engines for cars.

Override control with hydraulic valve clearance compensation

In a positive-controlled valve train, there are two movement points and three clearance points, namely:

• Between the cam track for opening the valve and the associated first cam follower,
• Between the cam track for the closed position of the valve and the associated second cam follower,
• Between the end of the intermediate lever and the opening stop on the valve head,
• Between the end of the intermediate lever and the lock stop on the valve head,
• And the bearing axis of the intermediate lever.

On the other hand, there are also still the actual valve clearance between the valve body and the valve seat. The games causing leakage of the combustion chamber, disturbing noise, increased wear and change the transition at the beginning and end of the cam area in the base circle of the cam element, which causes disadvantages in the closing hours, poor exhaust values ​​and increased maintenance costs.

Functional description of the positive control with double contour cam

The opening process is analogous to the prior art with rocker arms, roller actuation and HV compensation. The difference is that the opening lever does not operate against the strong valve spring.

The closing operation works similarly to the prior art, such as a reverse orifice mechanism with cam follower and HV compensation. The difference is that the lock lever senses an inner contour and pulls the valve ( plate ).

The advantages are less friction, lower alternating torque and reduced component stress and the high- speed capability. In addition, the space requirement is significantly less than existing forced control. Instead of Rollenabgriffs a sliding actuation is possible.

Functional description of the dual hydraulic lash adjuster ( HVA )

The new Override control with hydraulic valve clearance compensation includes an opening lever and a locking lever that sends one of the two cam tracks and each has an adjustment for the storage or movement. The pivot centers of the lever pads are identical.

The division of the intermediate member into two separate levers, independently of each other, that are without a fixed connection with each other, and yet together actuate the valve, since they overlap spatially enables controlled the game at the gaming sites per lever to adjust and to effect the game balance accordingly.

• Base circle position: Both rocker arms are not acted upon by external forces with (no hub, no clamping force spring, no mass force ... ). In the two hydraulic chambers (3, 4) there is no damage caused by the lever. The check valve ( 1) is therefore open, it can flow engine oil pressure in the compensating elements and press the lever on the cam tracks and stop elements at the valve. 2, the control valve between the hydraulic chambers is in the middle position (preferably by a spring force, or special shaping of the channels ) and allows the filling of the two hydraulic chambers, and thus a clearance compensation.
• Stroke position - opening cam: The opening cam presses on the opening lever and the valve opens. The resultant ( bearing ) force is generated pressure ( differential pressure ) in the upper hydraulic chamber, and the check valve. Also, the ball valve is between the two hydraulic chambers is closed ( ball goes down ), there can be no oil flow from the upper to the lower chamber. The not acted upon with force closing lever and cooperating lower hydraulic chamber without force or pressure. A subsequent flow of oil and an undesired inflation of the lower chamber is not possible, since the differential pressure to the upper chamber and for supplying the motor includes the central valve 2 and the check valve 1. The two pistons remain positionally stable even under load ( inertia ).
• Lift position - closing cam: Here it has the opposite effect. The lower chamber has opposite to the upper chamber and the inlet pressure and closes as the central valve (2) and the check valve (1). An inflation of the upper chamber is not possible.
• Play compensation valve length / location: Both pistons moved parallel / synchronized, since the distance of the lever at the hinge point of the valve remains unchanged. The all valve (both piston) moves and acts equally on both levers.