Editing Inlet manifold

{{Short description|Automotive technology}}
{{More citations needed|date=February 2019}}[[File:1961 Ferrari 250 TR 61 Spyder Fantuzzi engine.jpg|thumb|Carburetors used as intake runners]]
[[File:Manly 1919 Fig 133 Fordson intake.png|thumb|A cutaway view of the intake of the original Fordson tractor (including the intake manifold, [[carburetor#Vaporizers|vaporizer]], [[carburetor]], and fuel lines)]]

An '''inlet manifold''' or '''intake manifold''' (in [[American English]]) is the part of an [[internal combustion engine]] that supplies the [[fuel]]/[[air]] mixture to the [[cylinder (engine)|cylinder]]s.<ref>{{Cite web|date=2018-11-10|title=What Is an Intake Manifold? • STATE OF SPEED|url=https://stateofspeed.com/2018/11/10/what-is-an-intake-manifold/|access-date=2022-02-03|website=STATE OF SPEED|language=en-US}}</ref>  The word ''[[manifold (engineering)|manifold]]'' comes from the Old English word ''manigfeald'' (from the Anglo-Saxon ''manig'' [many] and ''feald'' [repeatedly]) and refers to the multiplying of one (pipe) into many.<ref>manifold, (adv.) "in the proportion of many to one, by many times". AD1526 ''Oxford English Dictionary'',</ref> <!-- Consider displace it to "Manifold (fluid mechanics)" page -->

In contrast, an [[exhaust manifold]] collects the [[exhaust gas]]es from multiple cylinders into a smaller number of pipes&nbsp;– often down to one pipe.

The primary function of the intake manifold is to ''evenly'' distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s).  Even distribution is important to optimize the efficiency and performance of the engine.  It may also serve as a mount for the carburetor, throttle body, fuel injectors and other components of the engine.

Due to the downward movement of the [[piston]]s and the restriction caused by the throttle valve, in a reciprocating  [[spark ignition]] [[piston engine]], a partial [[vacuum]] (lower than [[atmospheric pressure]]) exists in the intake manifold. This [[manifold vacuum]] can be substantial, and can be used as a source of [[automobile ancillary power]] to drive auxiliary systems: power assisted [[brake]]s, emission control devices, [[cruise control]], [[ignition system|ignition]] advance, [[windshield wiper]]s, [[power window]]s, ventilation system valves, etc.

This vacuum can also be used to draw any piston blow-by gases from the engine's [[crankcase]]. This is known as a positive [[crankcase ventilation system]], in which the gases are burned with the fuel/air mixture.

The intake manifold has historically been manufactured from [[aluminium]] or cast iron, but use of composite plastic materials is gaining popularity (e.g. most Chrysler 4-cylinders, [[Ford Zetec engine|Ford Zetec]] 2.0, Duratec 2.0 and 2.3, and GM's [[GM Ecotec engine|Ecotec]] series).

==Turbulence==
The [[carburetor]] or the [[Fuel injection|fuel injectors]] spray fuel droplets into the air in the manifold. Due to electrostatic forces and condensation from the boundary layer, some of the fuel will form into pools along the walls of the manifold, and due to surface tension of the fuel, small droplets may combine into larger droplets in the airstream. Both actions are undesirable because they create inconsistencies in the [[air-fuel ratio]]. Turbulence in the intake helps to break up fuel droplets, improving the degree of atomization. Better [[atomizer nozzle|atomization]] allows for a more complete burn of all the fuel and helps reduce [[Engine knocking|engine knock]] by enlarging the flame front. To achieve this turbulence it is a common practice to leave the surfaces of the intake and intake ports in the cylinder head rough and unpolished.

Only a certain degree of turbulence is useful in the intake. Once the fuel is sufficiently atomized, additional turbulence causes unneeded pressure drops and a drop in engine performance.

==Volumetric efficiency==
{{Unreferenced section|date=July 2008}}
{{see also|Cylinder head porting}}

[[File:Manifold comparison.jpg|right|thumb|Comparison of a stock intake manifold for a Volkswagen [[List of Volkswagen Group petrol engines#1.8 R4 20vT 110-221kW|1.8T]] engine (top) to a custom-built one used in competition (bottom). In the custom-built manifold, the runners to the intake ports on the cylinder head are much wider and more gently tapered. This difference improves the [[volumetric efficiency]] of the engine's fuel/air intake.]]

The design and orientation of the intake manifold is a major factor in the [[volumetric efficiency]] of an engine. Abrupt contour changes provoke pressure drops, resulting in less air (and/or fuel) entering the combustion chamber; high-performance manifolds have smooth contours and gradual transitions between adjacent segments.

Modern intake manifolds usually employ ''runners'', individual tubes extending to each intake port on the cylinder head which emanate from a central volume or "plenum" beneath the carburetor. The purpose of the runner is to take advantage of the [[Helmholtz resonance]] property of air. Air flows at considerable speed through the open valve. When the valve closes, the air that has not yet entered the valve still has a lot of momentum and compresses against the valve, creating a pocket of high pressure. This high-pressure air begins to equalize with lower-pressure air in the manifold. Due to the air's inertia, the equalization will tend to oscillate: At first the air in the runner will be at a lower pressure than the manifold. The air in the manifold then tries to equalize back into the runner, and the oscillation repeats. This process occurs at the speed of sound, and in most manifolds travels up and down the runner many times before the valve opens again.

The smaller the cross-sectional area of the runner, the higher the pressure changes on resonance for a given airflow. This aspect of Helmholtz resonance reproduces one result of the [[Venturi effect]]. When the piston accelerates downwards, the pressure at the output of the intake runner is reduced. This low pressure pulse runs to the input end, where it is converted into an over-pressure pulse. This pulse travels back through the runner and rams air through the valve. The valve then closes.

To harness the full power of the Helmholtz resonance effect, the opening of the intake valve must be timed correctly, otherwise the pulse could have a negative effect. This poses a very difficult problem for engines, since valve timing is dynamic and based on engine speed, whereas the pulse timing is static and dependent on the length of the intake runner and the speed of sound. The traditional solution has been to tune the length of the intake runner for a specific engine speed where maximum performance is desired. However, modern technology has given rise to a number of solutions involving electronically controlled valve timing (for example [[VANOS]]), and dynamic intake geometry (see below).

As a result of "resonance tuning", some naturally aspirated intake systems operate at a volumetric efficiency above 100%: the air pressure in the combustion chamber before the compression stroke is greater than the atmospheric pressure. In combination with this intake manifold design feature,  the exhaust manifold design, as well as the exhaust valve opening time can be so calibrated as to achieve greater evacuation of the cylinder. The exhaust manifolds achieve a vacuum in the cylinder just before the piston reaches top dead center.{{Citation needed|date=July 2008}} The opening inlet valve can then—at typical compression ratios—fill 10% of the cylinder before beginning downward travel.{{Citation needed|date=July 2008}} Instead of achieving higher pressure in the cylinder, the inlet valve can stay open after the piston reaches bottom dead center while the air still flows in.{{Citation needed|date=July 2008}}{{Vague|date=February 2009}}

In some engines the intake runners are straight for minimal resistance. In most engines, however, the runners have curves, some very convoluted to achieve desired runner length. These turns allow for a more compact manifold, with denser packaging of the whole engine, as a result. Also, these "snaked" runners are needed for some variable length/ split runner designs, and allow the size of the [[Plenum space|plenum]] to be reduced. In an engine with at least six cylinders the averaged intake flow is nearly constant and the plenum volume can be smaller. To avoid standing waves within the plenum it is made as compact as possible. The intake runners each use a smaller part of the plenum surface than the inlet, which supplies air to the plenum, for aerodynamic reasons. Each runner is placed to have nearly the same distance to the main inlet. Runners whose cylinders fire close after each other, are not placed as neighbors.

In '''180-degree intake manifolds''', originally designed for carburetor V8 engines, the two plane, the split plenum intake manifold separates the intake pulses which the manifold experiences by 180 degrees in the firing order. This minimizes interference of one cylinder's pressure waves with those of another, giving better torque from smooth mid-range flow. Such manifolds may have been originally designed for either two- or four-barrel carburetors, but now are used with both throttle-body and [[multi-point fuel injection]]. An example of the latter is the [[Honda J engine]] which converts to a single plane manifold around 3500 rpm for greater peak flow and horsepower.

Older '''heat riser''' manifolds with 'wet runners' for carbureted engines used exhaust gas diversion through the intake manifold to provide vaporizing heat. The amount of exhaust gas flow diversion was controlled by a heat riser valve in the exhaust manifold, and employed a [[Bimetallic strip|bi-metallic spring]] which changed tension according to the heat in the manifold.  Today's fuel-injected engines do not require such devices.

==Variable-length intake manifold==
{{main article|Variable-length intake manifold}}
[[Image:Lower-intake-manifold.jpg|thumb|right|Lower intake manifold on a 1999 Mazda Miata [[Mazda B engine#BP-4W|engine]], showing components of a variable length intake system.]]
A '''variable-length intake manifold''' ('''VLIM''') is an [[internal combustion engine]] manifold technology.
Four common implementations exist. First, two discrete intake runners with different length are employed, and a butterfly valve can close the short path. Second the intake runners can be bent around a common plenum, and a sliding valve separates them from the plenum with a variable length. Straight high-speed runners can receive plugs, which contain small long runner extensions. The plenum of a 6- or 8-cylinder engine can be parted into halves, with the even firing cylinders in one half and the odd firing cylinders in the other part. Both sub-plenums and the air intake are connected to an Y (sort of main plenum). The air oscillates between both sub-plenums, with a large pressure oscillation there, but a constant pressure at the main plenum. Each runner from a sub plenum to the main plenum can be changed in length. For V engines this can be implemented by parting a single large plenum at high engine speed by means of sliding valves into it when speed is reduced.

As the name implies, VLIM can vary the length of the intake tract in order to optimize [[power (physics)|power]] and [[torque]], as well as provide better [[fuel efficiency]].

There are two main effects of variable intake geometry:
* '''Venturi effect''': At low [[Revolutions per minute|rpm]], the speed of the airflow is increased by directing the air through a path with limited capacity (cross-sectional area). The larger path opens when the load increases so that a greater amount of air can enter the chamber. In [[dual overhead cam]] (DOHC) designs, the air paths are often connected to separate [[Poppet valve|intake valves]] so the shorter path can be excluded by deactivating the intake valve itself.
* '''Pressurization''': A [[engine tuning|tuned]] intake path can have a light pressurizing effect similar to a low-pressure [[supercharger]] due to Helmholtz resonance. However, this effect occurs only over a narrow engine speed range which is directly influenced by intake length.  A variable intake can create two or more pressurized "hot spots." When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased.

Many automobile manufacturers use similar technology with different names. Another common term for this technology is '''variable resonance induction system''' ('''VRIS''').
{{anchor |Vehicles using variable intake geometry|Vehicles}}
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* [[Audi]]: 2.8-liter V6 gas engine (1991–98); 3.6- and 4.2-liter V8 engines, 1987–present
* [[Alfa Romeo]]: 2.0 TwinSpark 16v - 155 ps(114&nbsp;kW)
* [[BMW]]:  '''DISA''' and '''DIVA''' systems
*[[Dodge]]: 2.0 A588 – ECH (2001–2005) used in the 2001–2005 model year Dodge Neon R/T
* [[Ferrari]]: [[Ferrari 360 Modena|360 Modena]], [[Ferrari 550 Maranello|550 Maranello]]
* [[Ford Motor Company|Ford]]  '''VIS''' ('''Variable-resonance Intake System'''): on their 2.9-liter 24V Cosworth (BOB) based on the [[Ford Cologne V6 engine]] in the later model [[Ford Scorpio]]
* Ford  '''DSI''' ('''dual-stage intake'''): on their [[Duratec]] 2.5- and 3.0-liter V6s and it was also found on the [[Yamaha Motor Corporation|Yamaha]] V6 in the [[Ford Taurus|Taurus SHO]]
*Ford: The [[Ford Modular engine|Ford Modular V8 engines]] sport either the Intake Manifold Runner Control (IMRC) for 4V engines, or the Charge Motion Control Valve (CMCV) for 3V engines.
*Ford: The [[Ford CVH engine#2.0|2.0L Split Port]] engine in the Ford Escort and Mercury Tracer feature an Intake Manifold Runner Control variable geometry intake manifold.
* [[General Motors]]: 3.9L [[GM High Value Engine|LZ8/LZ9]] V6, 3.2L [[GM 54-Degree V6 engine|LA3]] V6, and the 4.3L [[LF4]] V6 in some second generation S10s and Sonomas
* [[GM Daewoo]]: DOHC versions of  [[E-TEC II]] engines
* [[Holden]]: [[Alloytec]]
* [[Honda]]: [[Honda Integra|Integra]], [[Honda Legend|Legend]], [[Honda NSX|NSX]], [[Honda Prelude|Prelude]]
* [[Hyundai Motor Company|Hyundai]]: [[Hyundai XG|XG]] V6
* [[Isuzu]]: [[Isuzu Rodeo]], used in the second generation V6, 3.2L (6VD1) Rodeos
* [[Jaguar (car)|Jaguar]]: [[Jaguar AJ-V6 engine|AJ-V6]]
* [[Lancia]]: '''VIS'''
* [[Mazda]]:  '''VICS''' ('''variable inertia charging system''') is used on the [[Mazda FE-DOHC engine]] and [[Mazda B engine]] family of [[straight-4]]s, and VRIS (variable resistance induction system) in the [[Mazda K engine]] family of [[V6]] engines. An updated version of this technology is employed on the new [[Mazda Z engine]], which is also used by Ford as the [[Ford Duratec engine|Duratec]].
* [[Mercedes-Benz]]
* [[MG Motor|MG]]: [[MG ZS 180]] [[MG ZT]] 160, 180 and 190
* [[Mitsubishi]]: '''Cyclone''' is used on the 2.0L I4 [[Mitsubishi 4G63 engine|4G63]] engine family.
* [[Nissan]]: I4, V6, V8
* [[Opel]] (or Vauxhall): '''TwinPort''' – modern versions of [[Ecotec Family 1]] and [[Ecotec Family 0]] straight-4 engines; a similar technology is used in [[GM 54-Degree V6 engine|3.2&nbsp;L 54° V6]] engine
* [[Peugeot]]: 2.2&nbsp;L I4, 3.0&nbsp;L V6
* [[Porsche]]: '''VarioRam''' – [[Porsche 964|964]], [[Porsche 993|993]], [[Porsche 996|996]], [[Porsche Boxster|Boxster]]
* [[Proton (carmaker)|Proton]]: '''[[Campro engine#Campro CPS and VIM engine|Campro CPS and VIM]]''' – [[Proton Gen-2|Proton Gen-2 CPS]] and [[Proton Waja|Proton Waja CPS]]; Proton '''[[Campro engine#Campro IAFM engine|Campro IAFM]]''' – 2008 [[Proton Saga]] 1.3
* [[Renault]]: [[Renault Clio|Clio 2.0RS]]
* [[Rover (marque)|Rover]]: [[Rover 623]], [[Rover 825]], [[Rover 75]] v6,  [[Rover 45]] v6
* [[Subaru Legacy (first generation)#Engines|Subaru Legacy]] 1989–1994 [[Japanese Domestic Market|JDM]] EJ20 2.0-liter naturally aspirated DOHC 16-valve flat-4
* [[Subaru SVX#Engines|Subaru SVX]] 1992–1997 EG33 3.3-liter naturally aspirated DOHC 24-valve flat-6
* [[Subaru Legacy (third generation)#Engines|Subaru Legacy]] and [[Subaru Impreza#Engines|Subaru Impreza]] 1999–2001 [[Japanese Domestic Market|JDM]] EJ20 2.0-liter naturally aspirated DOHC 16-valve flat-4
* [[Toyota]]: '''[[T-VIS]]''' – ('''Toyota Variable Induction System''') used in the early versions of the [[Toyota S engine#3S-GE|3S-GE]], [[Toyota M engine#7M-GE|7M-GE]], and [[Toyota 4A-GE|4A-GE]] engines, and '''[[Acoustic Control Induction System|ACIS]]''' (acoustic control induction system)
* [[Volkswagen]]: 1.6&nbsp;L I4, [[VR6]], [[W8 engine|W8]]
* [[Volvo]]: '''VVIS''' ('''Volvo variable induction system''') – [[Volvo Modular engine|Volvo B5254S and B5204S engines]] as found on the [[Volvo 850]] vehicles. Longer inlet ducts used between 1500 and 4100 rpm at 80% load or higher.<ref name=volvoclub>[http://www.volvoclub.org.uk/tech/850GLT-EngineTechInfo.pdf Volvoclub UK: 850GLT Engine Info]</ref>
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==See also==
*[[Cylinder head porting]]
*[[List of auto parts]]
*[[Fusible core injection molding]]

==References==
{{Commons category|Intake manifolds}}
{{reflist}}

{{Piston engine configurations}}
{{Automotive engine}}

[[Category:Engine technology]]
[[Category:Auto parts]]