Manifold (automotive)

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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.

Contents

[edit] 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 accessories to 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 design and orientation of the intake manifold is a major factor in the volumetric efficiency of an engine. High performing manifolds contain smooth contours and transitions between different segments. Manifolds that are restrictive and contain abrupt changes in contour produce pressure drops at these points. This reduction in manifold pressure results in less air (and fuel) actually entering the combustion chamber.

Modern intake manifolds usually contain intake runners. These are individual tubes extending to each intake port on the cylinder head. The purpose of the intake runner is to take advantage of the Helmholtz resonance property of air. When the valve is open, air is flowing through the valve at considerable speed. When this 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 air. This high pressure air begins to equalize with the lower pressure air in the manifold. Due to the inertia of the air, this equalization will tend to overcompensate, leaving the air in the runner at a lower pressure than the manifold. The air in the manifold then tries to equalize back into the runner, and the process repeats. This process occurs at the speed of sound, and in most intakes travels up and down the intake runner many times before the valve opens again.

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 ingenious solutions involving electronically controlled valve timing, and dynamic intake geometry.

Some naturally aspirated intake systems operate at a volumetric efficiency above 100%. In other words the air pressure in the combustion chamber, before the compression stroke is greater than the atmospheric pressure. Some mechanics quickly dismiss this as impossible and a violation of the law of conservation of energy. It is important to understand that the additional energy required to compress the air above atmospheric pressure comes from the momentum of the piston.

[edit] Tunnel Ram

The tunnel ram intake manifold was invented in 1959 and was first seen on the 354-inch Hemi.[1]

[edit] Variable length intake manifold

Variable Length Intake Manifold (VLIM) is an automobile 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.

Lower intake manifold on a 1999 Mazda Miata engine, showing components of a variable length intake system.
Lower intake manifold on a 1999 Mazda Miata engine, showing components of a variable length intake system.

There are two main effects of variable intake geometry:

  • Swirl - Variable geometry can create a beneficial air swirl pattern in the combustion chamber. The swirls help distribute the fuel and form a homogeneous air-fuel mixture which ignites without engine knocking. At low rpm, the speed of the airflow is increased by directing the air through a longer path with limited capacity (i.e., cross-sectional area), but the shorter and larger path opens when the load increases so that a greater amount of air can enter the chamber. In DOHC designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by de-activating 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 engine speed band. A variable intake can create two or more pressurized "hot spots", increasing engine output. 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).

[edit] Exhaust manifold

Left side of a Ford Cologne V6 engine, clearly showing a (rusty) cast iron exhaust manifold - three exhaust ports into one pipe.
Left side of a Ford Cologne V6 engine, clearly showing a (rusty) cast iron exhaust manifold - three exhaust ports into one pipe.
Headers on a 440 Max Wedge
Headers on a 440 Max Wedge

Exhaust manifolds are generally simple cast iron units which collect engine exhaust 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 mechanism by which a header does this is called exhaust scavenging. 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 a loss of low-end torque 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.

The state of an amount of gas is determined by its pressure, volume, and temperature according to the equation:
\ PV = nRT

where

\ P is the absolute pressure,
\ V is the volume of the vessel,
\ n is the number of moles of gas,
\ R is the universal gas constant,
\ T is the absolute temperature.


If we analyze the formula for the ideal gas law, we can easily see that if the volume decreases, and all the variables on the right side of the equation remain constant, that P, pressure must increase to satisfy the equation. Thus, P is inversely proportionate to V. In layman's terms, the pressure in the combustion chamber will increase if we decrease the volume in the chamber (move the piston towards the cylinder head). This concept is intuitive for most people, however it is important to understand the underlying processes involved.

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.

[edit] Dynamic Exhaust Geometry

Today's understanding of exhaust systems and aerodynamics has given rise to a number of mechanical improvements. One such improvement can been seen is the EXUP (EXhaust Ultimate Power valve) fitted onto many Yamaha motorcycles. This valve is akin to a butterfly valve, and is placed inside the collector. The valve is almost fully closed during low RPM operation, and opens as engine RPM increases. The valve works by restricting the flow of exhaust gas, thereby creating a venturi effect. When the valve is closed it limits the pressure decrease in the exhaust system, on the engine side of the valve. By maintaining this pressure exhaust gas exiting the combustion chamber does not expand or slow down as much. One determent of this apparatus is the additional labor placed on the engine when the valve is closed. For high performance engines the benefits clearly outweigh the downside, since torque is typically desired in low RPMs and horsepower in high RPMs.

[edit] See also

[edit] References

  1. ^ Carcraft.com: Tunnel-Ram vs. Single-Quad
  2. ^ Volvoclub UK: 850GLT Engine Info


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