Exhaust manifold
Encyclopedia
In automotive engineering
, an exhaust manifold collects the exhaust gas
es from multiple cylinder
s into one pipe. The word manifold comes 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.
In contrast, an inlet manifold
is the part of an engine that supplies the air to the cylinders.
Exhaust manifolds are generally simple cast iron
or stainless steel units which collect engine exhaust from multiple cylinders and deliver it to the exhaust pipe. For many engines, there are aftermarket tubular exhaust manifolds known as headers in US English, as extractors in Australian English
, and simply as "tubular manifolds" in UK English. These consist of individual exhaust headpipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers, and are used exclusively on race cars.
The most common types of aftermarket headers are made of either ceramic, or stainless steel. Ceramic headers are lighter in weight than stainless steel, however, under extreme temperatures they can crack - something stainless steel is not prone to.
Another form of modification used is to insulate a standard or aftermarket manifold. This decreases the amount of heat given off into the engine bay, therefore reducing the intake manifold temperature. There a few types of thermal insulation but three are particularly common:
The goal of performance exhaust headers is mainly to decrease flow resistance (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
.
of the high- and medium- pressure components reduces the pressure in the combustion chamber to a lower-than-atmospheric level. This relatively low pressure helps to extract all the combustion products from the cylinder and induct the intake charge during the overlap period when both intake and exhaust valves are partially open. The effect is known as scavenging. Length, cross-sectional area, and shaping of the exhaust ports and pipeworks influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.
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 create exhaust flow resistance which the engine must work to expel the exhaust gas from the chamber, reducing power and leaving exhaust in the chamber to dilute the incoming intake charge. Since engines produce more exhaust gas at higher speeds, the header(s) are tuned to a particular engine speed range according to the intended application. Typically, wide primary tubes offer the best gains in power and torque at higher engine speeds, while narrow tubes offer the best gains at lower speeds.
Many headers are also resonance
tuned, to utilize the low-pressure reflected wave rarefaction
pulse which can help scavenging the combustion chamber 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. Typically, long primary tubes resonate at a lower engine speed than short primary tubes.
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.
has given rise to a number of mechanical improvements. One such improvement can be seen in the exhaust ultimate power valve ("EXUP") fitted to some Yamaha motorcycles. It constantly adjusts the back pressure within the collector of the exhaust system to enhance pressure wave formation as a function of engine speed. This ensures good low to mid-range performance.
At low engine speeds the wave pressure within the pipe network is low. A full oscillation of the Helmholtz resonance occurs before the exhaust valve is closed, and to increase low-speed torque, large amplitude exhaust pressure waves are artificially induced. This is achieved by partial closing of an internal valve within the exhaust — the 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, thus inducing the formation of larger amplitude negative reflected expansion waves. This enhances low speed torque up to a speed at which the loss due to increased back pressure outweighs the EXUP tuning effect. At higher speeds the EXUP valve is fully opened and the exhaust is allowed to flow freely.
Automotive engineering
Modern automotive engineering, along with aerospace engineering and marine engineering, is a branch of vehicle engineering, incorporating elements of mechanical, electrical, electronic, software and safety engineering as applied to the design, manufacture and operation of motorcycles, automobiles,...
, an exhaust manifold collects the exhaust gas
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
es from multiple cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
s into one pipe. The word manifold comes 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.
In contrast, an inlet manifold
Inlet manifold
In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...
is the part of an engine that supplies the air to the cylinders.
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
or stainless steel units which collect engine exhaust from multiple cylinders and deliver it to the exhaust pipe. For many engines, there are aftermarket tubular exhaust manifolds known as headers in US English, as extractors in Australian English
Australian English
Australian English is the name given to the group of dialects spoken in Australia that form a major variety of the English language....
, and simply as "tubular manifolds" in UK English. These consist of individual exhaust headpipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers, and are used exclusively on race cars.
The most common types of aftermarket headers are made of either ceramic, or stainless steel. Ceramic headers are lighter in weight than stainless steel, however, under extreme temperatures they can crack - something stainless steel is not prone to.
Another form of modification used is to insulate a standard or aftermarket manifold. This decreases the amount of heat given off into the engine bay, therefore reducing the intake manifold temperature. There a few types of thermal insulation but three are particularly common:
- Ceramic paint is sprayed or brushed onto the manifold and then cured in an oven. These are usually thin, so have little insulatory properties however reduce engine bay heating by lessening the heat output via radiation.
- A ceramic mixture is bonded to the manifold via thermal sprayingThermal sprayingThermal spraying techniques are coating processes in which melted materials are sprayed onto a surface. The "feedstock" is heated by electrical or chemical means ....
to give a tough ceramic coating with very good thermal insulation. This is often used on performance production cars and track-only racers. - Exhaust wrap is wrapped completely around the manifold. Although this is cheap and fairly simple, it can lead to premature degradation of the manifold.
The goal of performance exhaust headers is mainly to decrease flow resistance (back pressure
Back pressure
Back pressure refers to the resistance to a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents, against its direction of flow....
), 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
Ideal gas law
The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...
and the combined gas law
Combined gas law
The combined gas law is a gas law which combines Charles's law, Boyle's law, and Gay-Lussac's law. These laws each relate one thermodynamic variable to another mathematically while holding everything else constant. Charles's law states that volume and temperature are directly proportional to each...
.
Exhaust Scavenging
When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. When the exhaust valve opens, the high pressure exhaust gas escapes into the exhaust manifold or header, creating an exhaust pulse comprising three main parts: The high-pressure head is created by the large 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 exhaust velocity decreases. This forms the medium-pressure body component of the exhaust pulse. The remaining exhaust gas forms the low-pressure tail component. This tail component may initially match ambient atmospheric pressure, but the momentumMomentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
of the high- and medium- pressure components reduces the pressure in the combustion chamber to a lower-than-atmospheric level. This relatively low pressure helps to extract all the combustion products from the cylinder and induct the intake charge during the overlap period when both intake and exhaust valves are partially open. The effect is known as scavenging. Length, cross-sectional area, and shaping of the exhaust ports and pipeworks influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.
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 create exhaust flow resistance which the engine must work to expel the exhaust gas from the chamber, reducing power and leaving exhaust in the chamber to dilute the incoming intake charge. Since engines produce more exhaust gas at higher speeds, the header(s) are tuned to a particular engine speed range according to the intended application. Typically, wide primary tubes offer the best gains in power and torque at higher engine speeds, while narrow tubes offer the best gains at lower speeds.
Many headers are also resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
tuned, to utilize the low-pressure reflected wave rarefaction
Rarefaction
Rarefaction is the reduction of a medium's density, or the opposite of compression.A natural example of this is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.Another...
pulse which can help scavenging the combustion chamber 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. Typically, long primary tubes resonate at a lower engine speed than short primary tubes.
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.
Why a cross plane V8 needs an H or X exhaust pipe
Cross plane V8 engines have a left and right bank each containing 4 cylinders. When the engine is running pistons are firing according to the engine firing order. If a bank has two consecutive piston firings it will create a high pressure area in the exhaust pipe, because two exhaust pulses are moving through at close to the same time. As the two pulses move in the exhaust pipe they should encounter either an X or H pipe. When they encounter the pipe, part of the pulse diverts into the X-H pipe which lowers the total pressure by a small amount. The reason there is a lowering is because liquid or air or exhaust gas will travel along a pipe and when it comes to a cross road the liquid/air/exhaust will take the path of least resistance and some will bleed off, thus lowering the pressure slightly. Without a X-H pipe the flow of exhaust would be jerky or inconsistent and the engine would not run at its highest efficiency because the double exhaust pulse would cause part of the next exhaust pulse in that bank to not exit that cylinder completely and cause either a detonation (because of high air fuel ratio), or a miss fire due to low air fuel ration, depending on how much of the double pulse was left and what the mixture of that pulse was.Dynamic exhaust geometry
Today's understanding of exhaust systems and fluid dynamicsFluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
has given rise to a number of mechanical improvements. One such improvement can be seen in the exhaust ultimate power valve ("EXUP") fitted to some Yamaha motorcycles. It constantly adjusts the back pressure within the collector of the exhaust system to enhance pressure wave formation as a function of engine speed. This ensures good low to mid-range performance.
At low engine speeds the wave pressure within the pipe network is low. A full oscillation of the Helmholtz resonance occurs before the exhaust valve is closed, and to increase low-speed torque, large amplitude exhaust pressure waves are artificially induced. This is achieved by partial closing of an internal valve within the exhaust — the 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, thus inducing the formation of larger amplitude negative reflected expansion waves. This enhances low speed torque up to a speed at which the loss due to increased back pressure outweighs the EXUP tuning effect. At higher speeds the EXUP valve is fully opened and the exhaust is allowed to flow freely.