The type of gas distribution mechanism, which provides direct control of raising and lowering the valves, is what is called desmodromic, which ensures their timely opening and closing at all revolutions of the engine crankshaft.
The desmodromic mechanism got its name from the two Greek words desmos (control, communication, control) and dromos (action, movement) – “controlled movement”.
In the 50s of the last century, the desmodromic gas distribution scheme was used on Mercedes-Benz racing cars. Currently, the desmodrome mechanism is used in internal combustion engines mounted on Ducati racing motorcycles and is the hallmark of the company.
The desmodromic mechanism has the generally accepted international name Desmodromic.
The controlled movement of the valve in the desmodromic mechanism is carried out using a special drive, which includes:
- a camshaft with cams of a special shape (according to another scheme – two camshafts, one for closing, the other for opening valves);
- two rocker arms providing opening and closing of each valve;
- connection elements (washers) of the rocker arm with the valve.
Scheme of the desmodromic mechanism
The use of a desmodromic mechanism helps prevent the so-called “Freezing of valves” (incomplete closing of the valve until the piston reaches top dead center), arising from the high engine speed, valve inertia, and spring resonance. Hanging valves can cause serious problems:
- collision of the piston with the valve, and as a result of their destruction;
- overheating of the valves, and as a result, their warping and destruction;
- premature ignition of the fuel-air mixture, and as a result, a decrease in gas pressure, a decrease in engine power, an increase in harmful emissions.
Along with the Desmodromic system, the problem of valve hangs on modern engines can be solved in other ways:
- the use of several (two to three) springs located one in the other and preventing resonant vibrations;
- the use of new materials and alloys for the manufacture of valves and springs, reducing their weight;
- using a pneumatic valve actuator ( on Formula 1 sports cars ).
The gas distribution system has a number of significant drawbacks that inhibit its widespread use:
- high construction costs due to the accuracy of the mating parts;
- the complexity of maintenance due to the presence of a large number of moving parts, prone to wear;
- high noise level;
- the bulkiness of the system and, as a result, application on motorcycle engines.
The Desmodromic system is not an ideal technical solution, but despite this, it continues to be used to this day.
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