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Variable valve timing (VVT) is a system for varying the valve opening of an internal combustion engine. This allows the engine to deliver high power, but also to work tractably and efficiently at low power.〔Volkswagen, Fluted variator, pp. 4-5〕 There are many systems for VVT, which involve changing either the relative timing, duration or opening of the engine's inlet and exhaust valves. One of the first practical VVT systems used a variator to change the phase〔Phase refers to the relative timing between the inlet and exhaust camshafts, expressed as an angular measure.〕 of the camshaft and valves. This simple system cannot change the duration of the valve opening, or their lift.〔'Lift' is the distance by which the valves open.〕 Later VVT systems, such as the helical camshaft or the movable fulcrum systems, could change these factors too. Despite this limitation, the variator is a relatively simple device to add to an existing engine and so they remain in service today. As the benefit of the variator relies on changing the relative timing between inlet and exhaust, variator systems are only applied to double overhead camshaft engines. A variator system that moved a single camshaft for both inlet and exhaust would be possible, but would have no performance benefit. == Alfa Romeo system == In 1970, Fiat patented a variable valve timing system, however it was Alfa Romeo who first applied VVT in 1980 to the long-established Twin Cam engine used in the Alfa Romeo Spider. The technique derives from work carried in the 1970s by Ing. Giampaolo Garcea and in Italian the device is termed ''variatore di fase''.〔 The Alfa Romeo Twin Spark engine, introduced in the 1987 Alfa Romeo 75, also uses variable valve timing.〔http://www.italiaspeed.com/2005/cars/alfa_romeo/10/75/1610.html〕 The Alfa system varies the phase (not the duration) of the cam timing and operates on the inlet camshaft.〔http://www.austincc.edu/wkibbe/vvt.htm〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Variator (variable valve timing)」の詳細全文を読む スポンサード リンク
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