DOHC
A double overhead cam (also called a dual overhead cam or DOHC) engine is a type of internal combustion engine where the camshafts that operate the intake and exhaust valves are mounted above the cylinders, and where there are separate camshafts for inlet and exhaust valves. Double camshafts are required in order to have multiple inlet or exhaust valves that are directly actuated.
Usually the cams operate the valves directly or by a short rocker, as opposed to pushrod engines which have long rods to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head.
Most recent internal combustion engine designs use a DOHC valvetrain, since it allows for greater efficiency. In engines with more than one cylinder bank, such as the V engine, this designation means two camshafts per bank.
When the DOHC concept was introduced, it was common for the first such engine from a manufacturer to be known as the DOHC engine; for instance, Ford's new DOHC straight-4 in the late 1980s. Ford now uses the Duratec name on all its North American 4 cylinder and V6 DOHC engines.
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