Multi-valve

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. It is not practical to simply use two larger valves because of the circular shape of the combustion chamber and the need for valves to also be round, which ensures they can only cover a fraction of the top of each cylinder; three (or more) smaller valves can replace the largest two valves which could be fitted into the space and result in having a greater effective valve area. Adding more valves per cylinder can improve breathing and thus allow an engine to run at a higher RPM, creating more power for a given displacement, though at a greater complexity and cost.

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 showed 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.

Starting in 1922, many of Bugattis engines began using 3 valves per cylinder actuated by a single-ovehead-cam (SOHC). Nissan has produced the 1988-96 KA24E engine with 3 valves per cylinder (two intakes, one exhaust) that are also actuated by (SOHC). Mercedes and Ford are currently producing V6 and V8 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, Audi, Ferrari and Yamaha have introduced engines in the past that had a double overhead cam operating 5 valves per cylinder (three intakes, two exhaust). Toyota's 1991-98 4A-GE 1.6-liter 4-cylinder engine also uses 5-valves-per-cylinder and was co-designed by Yamaha as well.

Maserati has produced a 2.0L turbo-V6 engine with 6 valves-per-cylinder (three intakes, three exhaust) (http://www.maserati-alfieri.co.uk/alfieri26.htm).

Engines with two or four valves per cylinder are by far the most common configurations. Four valve per cylinder engines are typically actuated by DOHC, and are too numerous to list.