In automotive
engineering,
an exhaust manifold collects
the exhaust gases from multiple cylinders 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 or intake manifold supplies air to the
cylinders).
Exhaust
manifolds are generally simple cast iron or stainless steel units which collect engine exhaust
gas 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 extractor manifolds in British and Australian English, and simply as "tubular
manifolds" in UK English. These consist of individual exhaust head pipes for
each cylinder, which then usually converge into one tube called a collector. Headers that do not have
collectors are called zoomie headers.
The most
common types of aftermarket headers are made of mild steel or stainless steel
tubing for the primary tubes along with flat flanges and possibly a larger
diameter collector made of a similar material as the primaries. They may be
coated with a ceramic-type finish (sometimes both inside and outside), or
painted with a heat-resistant finish, or bare. Chrome plated headers are
available but these tend to blue after use. Polished stainless steel will also
color (usually a yellow tint), but less than chrome in most cases.
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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 are 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, they reduce engine bay heating by
lessening the heat output via radiation.
·
A
ceramic mixture is bonded to the manifold via thermal spraying to give a tough ceramic coating with very good thermal
insulation. This is often used on performance production cars and track-only
racers.
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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), 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.
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:
1. 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
2. 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
3. The remaining exhaust gas forms the
low-pressure tail component.
This tail component may initially match ambient atmospheric pressure, but
the momentum 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 pipe works 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.
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 it close in 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 for this decrease in pressure is that the fluid
(liquid, air or gas) will travel along a pipe and when it comes at a crossing
the fluid will take the path of least resistance and some will bleed off, thus
lowering the pressure slightly. Without an X-H pipe the flow of exhaust would
be jerky or inconsistent, and the engine would not run at its highest
efficiency. 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 a lean air-fuel ratio (AFR)), or a misfire due to a rich AFR, 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 dynamics 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.
WORKING