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camless
Most four-stroke piston engines today employ one or more camshafts to operate poppet valves. The lobes on the camshafts operate cam followers which in turn open the poppet valves. A camless (or, free valve engine) uses electromagnetic, hydraulic, or pneumatic actuators to open the poppet valves instead. Actuators can be used to both open and close the valves, or an actuator opens the valve while a spring closes it.
As a camshaft normally has only one lobe per valve, the valve duration and lift is fixed. The camshaft runs at half the engine speed. Although many modern engines use camshaft phasing, adjusting the lift and valve duration in a working engine is more difficult. Some manufacturers use systems with more than one cam lobe, but this is still a compromise as only a few profiles can be in operation at once. This is not the case with the camless engine, where lift and valve timing can be adjusted freely from valve to valve and from cycle to cycle. It also allows multiple lift events per cycle and, indeed, no events per cycle—switching off the cylinder entirely.
==Camless development==
Camless engines are not without their problems though. Common problems include high power consumption, accuracy at high speed, temperature sensitivity, weight and packaging issues, high noise, high cost, and unsafe operation in case of electrical problems.
Camless valve trains have long been investigated by several companies, including Renault, BMW, Fiat, Valeo, General Motors, Ricardo, Lotus Engineering, Ford, Gongda, and Koenigsegg's subsidiary FreeValve. Camless systems are commercially available, although not yet in production road vehicle engines. In the spring of 2015 Christian von Koenigsegg told reporters that the technology pursued by his company is "getting ready for fruition", but said nothing specific about the time-table.〔http://www.autoblog.com/2015/03/06/koenigsegg-four-door-camless-engine-report/〕〔http://www.topgear.com/uk/car-news/four-door-koenigsegg-2015-03-04〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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