Editing Multi-valve (section)

== Multi-valve rationale ==

=== Multi-valve engine design ===

A multi-valve engine design has three, four, or five [[poppet valve]]s per cylinder, to achieve greater performance.  In [[automotive engineering]], any four-stroke [[internal combustion engine]] needs at least two valves per cylinder: one for ''intake'' of air (and often fuel<ref name=fuel/>), and another for ''exhaust'' of combustion gases.  Adding more valves increases valve area, which improves the flow of intake and exhaust gases, thereby enhancing [[combustion]], [[volumetric efficiency]], and [[Engine efficiency|power output]]. Multi-valve geometry allows the spark plug to be ideally located within the combustion chamber for optimal flame propagation. Multi-valve engines tend to have smaller valves that have lower [[reciprocating mass]], which can reduce wear on each [[Cam (mechanism)|cam]] lobe, and allow more power from higher [[revolutions per minute|RPM]] without the danger of [[valve float]]. Some engines are designed to open each intake valve at a slightly different time, which increases turbulence, improving the mixing of air and fuel at low engine speeds.{{Citation needed|date=May 2025}} More valves also provide additional cooling to the cylinder head.

Disadvantages of multi-valve engines are a greater parts count, and thus an increase in manufacturing and possibly also maintenance costs, and a potential increase in oil consumption due to the greater number of valve stem seals.

Most multi-valve engines are [[dual overhead camshaft]] (DOHC) designs, but some [[single overhead camshaft]] (SOHC) multi-valve engines exist – for instance the [[Mazda B engine#B8|Mazda B8-ME engine]]) uses fork-shaped rocker arms, so that its single overhead camshaft can drive two valves at once (generally the exhaust valves), so that fewer cam lobes will be needed in order to reduce manufacturing costs.{{citation needed|date=April 2014}}

{{anchor |Three-valve}}
* '''Three-valve cylinder head'''
This has a single large exhaust valve and two smaller intake valves.  A three-valve layout allows better breathing than a two-valve head, but the large exhaust valve results in an RPM limit no higher than a two-valve head. The manufacturing cost for this design can be lower than for a four-valve design. The three-valve design was common in the late 1980s and early 1990s; and from 2004 the main valve arrangement used in [[Ford F-Series]] trucks, and Ford SUVs. The [[Ducati ST3]] [[V-twin]] had 3-valve heads.

{{anchor |Four-valve}}
* '''Four-valve cylinder head'''
This is the most common type of multi-valve head, with two exhaust valves and two similar (or slightly larger) inlet valves.  This design allows similar breathing as compared to a three-valve head, and as the small exhaust valves allow high RPM, this design is very suitable for high power outputs.

{{anchor |Five-valve}}
* '''Five-valve cylinder head'''
Less common is the five-valve head, with two exhaust valves and three inlet valves.  All five valves are similar in size.  This design allows excellent breathing, and, as every valve is small, high RPM and very high power outputs are theoretically available.  Although, compared to a four-valve engine, a five-valve design should have a higher maximum RPM, and the three inlet ports should give efficient cylinder-filling and high gas turbulence (both desirable traits), it has been questioned whether a five-valve configuration gives a cost-effective benefit over four-valve designs. The rise of direct injection may also make five-valve heads more difficult to engineer, as the injector must take up some space on the head. After making five-valve [[Yamaha Genesis engine|Genesis engines]] for several years, [[Yamaha Motor Company|Yamaha]] has since reverted to the cheaper four-valve design.

Examples of the five-valve engines are the various [[List of Volkswagen Group petrol engines#1.8 R4 20vT (EA113/EA827)|1.8 L 20vT engines]] manufactured by AUDI AG, the later versions of the [[Ferrari]] [[Ferrari Dino engine|Dino V8]], and the 1.6 L [[Toyota A engine#4A-GE (20-valve)|20-valve 4A-GE]] engine made by Toyota in collaboration with Yamaha.

{{anchor |Beyond}}
* '''Beyond five valves'''
For a cylindrical bore and equal-area sized valves, increasing the number of valves beyond five ''decreases'' the total valve area. The following table shows the effective areas of differing valve quantities as proportion of cylinder bore.  These percentages are based on simple geometry and do not take into account orifices for spark plugs or injectors, but these voids will usually be sited in the "dead space" unavailable for valves.  Also, in practice, intake valves are often larger than exhaust valves in heads with an even number of valves-per-cylinder{{cn|date=May 2025}}:
* 2 = 50%
* 3 = 64%
* 4 = 68%
* 5 = 68%
* 6 = 66%
* 7 = 64%
* 8 = 61%

=== Alternative technologies ===

[[File:SUBARU EJ25 i-AVLS.jpg|thumb|Cutaway model of Subaru's [[Active Valve Control System|i-AVLS]] variable valve timing system on SOHC 4-valve-per-cylinder [[Subaru EJ engine|EJ25 boxer engine]] at Tokyo Motor Show 2007.]]
[[Turbocharging]] and [[Supercharger|supercharging]] are technologies that also improve engine breathing, and can be used instead of, or in conjunction with, multi-valve engines. The same applies to [[variable valve timing]] and [[variable-length intake manifold]]s. [[Rotary valve#Use in engine design|Rotary valves]] also offer improved engine breathing and high rev performance but these were never very successful. [[Cylinder head porting]], as part of [[engine tuning]], is also used to improve engine performance.