Multi-valve
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In automotive engineering, an engine is referred to as multi-valve (or multivalve) when each cylinder has more than two valves.
All poppet valve, four-stroke internal combustion engines have at least two valves per cylinder — one for intake of air and fuel, and another for exhaust of combustion products. Adding more valves improves the flow of intake and exhaust gases, potentially improving combustion efficiency, power, and performance. Simply using two larger valves does not work because of the circular shape of the combustion chamber and the need for valves to be round, which gives a lot of unused space; four (or more) smaller valves can replace the largest single valves which could be fitted into the space and have a greater effective area.
Most multivalve engines use an overhead camshaft to actuate the valves, and many use double overhead camshafts (DOHC). However this is not always the case: Chevrolet recently introduced a 3-valve version of its Generation IV V8 which uses pushrods to actuate forked rockers, and Cummins makes a 4-valve pushrod straight-6 Diesel, the Cummins 600. Nissan has produced 3-valve per cylinder engines that were actuated by a single-overhead-cam (SOHC). Mercedes and Ford are currently producing engines using this configuration. Ford claims an 80% improvement in high RPM breathing without the added cost of a second cam per bank of cylinders. The Ford design uses one spark plug per cylinder located in the center, but the Mercedes design uses two spark plugs per cylinder located on opposite sides, leaving the center free to add a direct-to-cylinder fuel injector at a later date. Thus there are many considerations to deciding how many valves an engine should have besides just the added cost verses adding breathing capability.
Some versions of the Honda D-series 4 cylinder engines and all J-series V6 and R-series 4 cylinder engines actuate 4 valves per cylinder with a single overhead cam.
Volkswagen, Ferrari and Yamaha have introduced engines in the past that had 5 valves per cylinder (three intakes, two exhaust) and Maserati has produced an engine with 6 valves per cylinder (three intakes, three exhaust), but engines with two or four valves per cylinder are by far the most common configuration.
