inlet manifold

Manifold (automotive)

In automotive engineering, an intake manifold or inlet manifold is the part of an engine that supplies the fuel/air mixture to the cylinders. An exhaust manifold or header collects the exhaust gases from multiple cylinders into one pipe. The word manifold may come from the Old English word manigfeald (from the Anglo-Saxon manig [many] and feald [fold]) and refers to the folding together of multiple inputs and outputs.

Intake manifold

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 pistons 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: ignition advance, power assisted brakes, cruise control, windshield wipers, power windows, 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 closed crankcase ventilation or positive crankcase ventilation (PCV) system. This way the gases are burned with the fuel/air mixture.

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


The carburetor or the fuel injectors spray fuel droplets into the air in the manifold. Due to electrostatic forces some of the fuel will form into pools along the walls of the manifold, or may converge into larger droplets in the air. Both actions are undesirable because they create inconsistencies in the air-fuel ratio. Turbulence in the intake causes forces of uneven proportions in varying vectors to be applied to the fuel, aiding in atomization. Better atomization allows for a more complete burn of all the fuel and helps reduce 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

This section explains the aerodynamics of the intake manifold and thus relies upon a lot of technical terminology.

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. 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 manifold 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 Helmholz 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 RPM, 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 RPM where maximum performance is desired. However, modern technology has given rise to a number of solutions involving electronically-controlled valve timing (for example Valvetronic), and dynamic intake geometry (see below).

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. The additional energy required to compress the air above atmospheric pressure comes from the momentum of the piston. In combination with the exhaust manifold the valve opening time can be prolonged and friction losses reduced. The exhaust manifolds achieves a vacuum in the cylinder just before the piston reaches top dead center. The opening inlet valve can then—at typical compression ratios—fill 10% of the cylinder before beginning downward travel. 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.

See also cylinder head porting

Variable length intake manifold

Variable Length Intake Manifold (VLIM) is an internal combustion engine manifold technology. As the name implies, VLIM can vary the length of the intake tract in order to optimize power and torque, as well as provide better fuel efficiency.

There are two main effects of variable intake geometry:

  • Venturi effect - At low 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 designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by deactivating the intake valve itself.
  • Pressurization - A 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 RPM 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).

Exhaust manifold

Exhaust manifolds are generally simple cast iron units which collect engine exhaust from multiple cylinders and deliver it to the exhaust pipe. For many engines after market high performance exhaust headers (also known as extractors in Australia) are available. These headers consist of individual primary tubes for each cylinder, which then usually converege into one tube called a collector. Headers that do not have collectors are called zoomie headers, and are used exclusively on race cars.

The goal of performance exhaust headers is mainly to decrease flow resistance (also know as back pressure), and to increase the volumetric efficiency of an engine, resulting in a gain in power output. The processes occurring can be explained by the gas laws, specifically the ideal gas law and the combined gas law.

It is a common myth among drag racers and motor-enthusiasts that not enough back pressure in the exhaust will cause a loss of torque. This myth stems from the phenomena associated with exhaust scavenging. A diminished scavenging effect can result from lower velocity exhaust flow when using headers with large primary tubes. Most enthusiasts incorrectly conclude that their restrictive OEM exhaust provided more torque because of the back pressure it creates. The correct reason for the loss in torque is explained below.

When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. At some point during the exhaust stroke the exhaust valve will open. The high pressure exhaust gas escapes into the exhaust header, creating an exhaust pulse. An exhaust pulse is a release of exhaust gas, containing three main parts, a high pressure "head", a medium pressure "body" and a low pressure "tail". The high pressure "head" is created from the huge pressure difference between the exhaust in the combustion chamber and the atmospheric pressure outside of the exhaust system. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the velocity at which the exhaust is leaving the engine decreases. This forms the medium pressure "body" component of the exhaust pulse. The remaining exhaust gases form the "tail" component. This tail component may initially match in pressure to that of the atmosphere, however, the pressure is further reduced by the siphoning effect created by the momentum of the high and medium pressure components. The end result may be a pressure at the low end of the exhaust pulse that is less than the atmospheric pressure. This creates a greater pressure difference between the intake manifold and the combustion chamber, which increases the velocity in which air is brought into the engine. This increase in intake air velocity leads to an increase in the amount of air in the combustion chamber, which allows the engine to add more fuel and thus make more power.

Modern naturally aspirated four-stroke engines usually feature valve-overlap where the benefit of exhaust scavenging is further increased by opening the intake valve while the exhaust valve is also open. This overlap helps purge the combustion chamber of any remaining exhaust gas, and may allow a small amount of intake air to escape out the exhaust port.

The magnitude of the exhaust scavenging effect is a direct function of the velocity of the high and medium pressure components of the exhaust pulse. Performance headers work to increase the exhaust velocity as much as possible. One technique is tuned length primary tubes. This technique attempts to time the occurrence of each exhaust pulse, to occur one after the other in succession while still in the exhaust system. The lower pressure tail of an exhaust pulse then serves to create a greater pressure difference between the high pressure head of the next exhaust pulse, thus increasing the velocity of that exhaust pulse. In V6 and V8 engines where there is more than one exhaust bank, Y-pipes and X-pipes work on the same principle of using the low pressure component of an exhaust pulse to increase the velocity of the next exhaust pulse.

Great care must be used when selecting the length and diameter of the primary tubes. Tubes that are too large will cause the exhaust gas to expand and slow down, decreasing the scavenging effect. Tubes that are too small will require additional force to expel the exhaust gas from the chamber, causing unneeded labor on the engine and ultimately a loss of power. This is true for all parts of the exhaust system. In competitive environments it's often required to select the header based on the specific application of the engine. Since engines produce more exhaust gas at higher RPMs the header will respond differently across the RPM range. Typically, large primary tubes offer the best gains in power and torque at higher RPMs, while smaller tubes offer the best gains at lower RPMs. Many people who put race headers on their vehicle experience a noticeable low-end torque loss. This is a result of insufficient exhaust gas output at lower RPMs. The exhaust expands once it enters the primary tube and slows down, reducing the scavenging effect. Many automotive mechanics and enthusiasts erroneously conclude the loss in torque was due to a lack of back pressure, when in fact the real cause was the expansion of the exhaust and resulting decrease in velocity. Despite the low-end torque loss, at higher RPMs the engine will produce more power and in race situations, the vehicle should be faster.

Many headers are also resonance tuned, to utilize the low-pressure reflected wave rarefaction pulse which can help scavenging during valve overlap. This pulse is created in all exhaust systems each time a change in density occurs, such as when exhaust merges into the collector. For clarification, the rarefaction pulse is the technical term for the same process that was described above in the "head, body, tail" description. By tuning the length of the primary tubes, usually by means of resonance tuning, the rarefaction pulse can be timed to coincide with the exact moment valve overlap occurs.

Some modern exhaust headers are available with a ceramic coating. This coating serves to prohibit rust and to reduce the amount of heat radiated into the engine bay. The heat reduction will help prevent intake manifold heat soak, which will decrease the temperature of the air entering the engine.

Dynamic Exhaust Geometry

Today's understanding of exhaust systems and fluiddynamics has given rise to a number of mechanical improvements. One such improvement can been seen is the EXUP (EXhaust Ultimate Power valve) fitted to selected Yamaha motorcycles (FZR,YZF,XV1900,R series) that constantly adjusts the back pressure within the collector of the exhaust system to enhance pressure wave formation as a function of engine rpm. This ensures good low to mid-range performance.

At low engine speeds the wave pressure within the pipe network is low and hence to increase low RPM torque large amplitude exhaust pressure waves are artificially induced. This is achieved by partial closing of an internal valve within the exhaust (EXUP valve) at the point where the four primary pipes from the cylinders join. This junction point essentially behaves as an artificial atmosphere, hence the alteration of the pressure at this point controls the behavior of reflected waves at this sudden increase in area discontinuity. Closing the valve increases the local pressure, hence inducing the formation of larger amplitude negative reflected expansion waves. This thus enhances low speed torque upto an RPM where the loss due to increased back pressure outweighs the EXUP tuning effect. At higher RPM’s the EXUP valve is fully opened and the exhaust is allowed to flow freely.

See also


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