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Valve timing diagram for a 4-stroke engine. With traditional fixed valve timing, an engine will have a period of "valve overlap" at the end of the exhaust stroke, when both the intake and exhaust valves are open. The intake valve is opened before the exhaust gases have completely left the cylinder, and their considerable velocity assists in ...
The valves are typically operated by a camshaft rotating at half the speed of the crankshaft. It has a series of cams along its length, each designed to open a valve during the appropriate part of an intake or exhaust stroke. A tappet between valve and cam is a contact surface on which the cam slides to open the valve. Many engines use one or ...
Four-stroke engines must also time the valve openings relative to the firing order, as the valves do not open and close on every stroke. Firing order affects the vibration, sound and evenness of power output from the engine and heavily influences crankshaft design.
The valves in a typical four-stroke engine allow the air/fuel mixture into the cylinder at the beginning of the cycle and exhaust spent gases at the end of the cycle. In a conventional four-stroke engine, valves are opened by a cam and closed by return spring.
In a 4-stroke ICE, each piston experiences 2 strokes per crankshaft revolution in the following order. Starting the description at TDC, these are: [29] [30] Intake, induction or suction: The intake valves are open as a result of the cam lobe pressing down on the valve stem.
Late intake valve closing (LIVC) The first variation of continuous variable valve timing involves holding the intake valve open slightly longer than a traditional engine. This results in the piston actually pushing air out of the cylinder and back into the intake manifold during the compression stroke.
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A four-stroke power valve is a device fitted to four-stroke engines that constantly adjusts the internal diameter of the exhaust system to better suit the speed of the engine. Yamaha was the first to develop such a system, called the Exhaust Ultimate Power Valve (EXUP). [1] It first appeared on the Yamaha FZR EXUP and was later used on many ...