Camless
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Most piston engines today employ a camshaft to operate poppet valves. This consists of a cylindrical rod running the length of the cylinder bank with a number of oblong lobes or cams protruding from it, one for each valve. The cams force the valves open by pressing on the valve, or on some intermediate mechanism, as they rotate.
Sliding friction between the surface of the cam and the cam follower which rides upon it is considerable. In addition to mechanical friction, considerable force is required to overcome the valve springs used to close the engine's valves. This can amount to an estimated 25% of an engine's total output at idle, reducing overall efficiency. A roller follower valvetrain has significantly less friction and allows for concave cam lobes. Two approaches have been tried to reclaim this "wasted" energy of the valve spring, but have proven difficult to implement:
- Springless valves, like the desmodromic system employed today by Ducati
- Camless valvetrains using solenoids or magnetic systems have long been investigated by BMW and Fiat, and are currently being prototyped by Valeo and Ricardo. New Fiat 500 is supposed to have this kind of engine.
Camless engines would not only be more efficient in terms of mechanical energy, they would also be more flexible, as the valves could be computer-controlled. Infinitely variable valve timing would be possible, though variable valve lift would be more difficult. Valeo estimates that the efficiency of a camless engine would be 20% greater than a comparable camshaft-operated engine, though Ricardo is more conservative. These companies are positioning this technology as an alternative to hybrid engines currently used to increase fuel economy.
