![Ferrari V10 - Galleria Ferrari di Maranello](https://upload.wikimedia.org/wikipedia/commons/4/41/Ferrari_V10_-_Galleria_Ferrari_di_Maranello.jpg)
In an
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
, the geometry of the
exhaust system
An exhaust system is used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system ...
can be optimised ("tuned") to maximise the power output of the engine. Tuned exhausts are designed so that reflected pressure waves arrive at the exhaust port at a particular time in the combustion cycle.
Two-stroke engines
Expansion chambers
![Scooter exhaust](https://upload.wikimedia.org/wikipedia/commons/7/7b/Scooter_exhaust.jpg)
In
two-stroke engine
A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of t ...
s where the exhaust port is opened by being uncovered by the piston (rather than by a separate valve), a tuned exhaust system usually consists of an expansion chamber. The expansion chamber is designed to produce a negative pressure wave to assist in filling the cylinder with the next intake charge, and then to produce a positive pressure wave which reduces the amount of fresh intake charge that escapes through the exhaust port (''port blocking'').
Uniflow scavenging
An alternate design of two-stroke engines is where the exhaust port is opened/closed using a
poppet valve
A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.
It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usual ...
and the intake port is
piston-controlled (opened by being uncovered by the piston). The timing of the exhaust valve closure is designed to assist in filling the cylinder with the next intake charge (as per four-stroke engines).
An
opposed piston engine
An opposed-piston engine is a piston engine in which each cylinder has a piston at both ends, and no cylinder head. Petrol and diesel opposed-piston engines have been used mostly in large-scale applications such as ships, military tanks, and f ...
uses uniflow scavenging, however this design uses piston-controlled cylinder ports with one piston controlling the inlet port and the other the exhaust port. Similarly,
split-single engine
In internal combustion engines, a split-single design is a type of two-stroke where two cylinders share a single combustion chamber.
The first production split-single engine was built in 1918 and the design was used on several motorcycles and ca ...
s use uniflow scavenging, with the piston in one cylinder controlling the transfer port (where the intake mixture enters the cylinder) and the other piston controls the exhaust port.
Four-stroke engines
![Exhaust manifold](https://upload.wikimedia.org/wikipedia/commons/7/77/Exhaust_manifold.jpg)
In a
four-stroke engine
A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either directio ...
, an
exhaust manifold
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'' anyand ''feald'' old and refe ...
which is designed to maximise the power output of an engine is often called "extractors" or "headers". The pipe lengths and merging locations are designed to assist in filling the cylinder with the next intake charge using
exhaust scavenging. Locations where exhaust pipes from individual cylinders merge are called "collectors". The diameters of the exhaust system are designed to minimise back-pressure by optimising the gas velocity.
Extractors/headers usually have equal length pipes for each cylinder, whereas a more basic exhaust manifold may have unequal length pipes.
File:Rotax912sDynAeroMcr01.jpg , Exhaust system for a Rotax 912
The Rotax 912 is a horizontally-opposed four-cylinder, naturally aspirated, four-stroke aircraft engine with a reduction gearbox. It features liquid-cooled cylinder heads and air-cooled cylinders. Originally equipped with carburetors, late ...
s airplane engine
File:Ceramic Coating on Performance Car.jpg , Long tube headers (in white) on a racing car
File:'27 Model T altered.JPG , "Zoomie" headers on a dragster
4-2-1 exhausts
A 4-2-1 exhaust system is a type of
exhaust manifold
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'' anyand ''feald'' old and refe ...
for an engine with four cylinders per bank, such as an
inline-four engine
A straight-four engine (also called an inline-four) is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.
The vast majority of automotive four-cylinder engines use a straight-four layout (with the ...
or a
V8 engine
A V8 engine is an eight-cylinder piston engine in which two banks of four cylinders share a common crankshaft and are arranged in a V configuration.
The first V8 engine was produced by the French Antoinette company in 1904, developed and us ...
. The layout of a 4-2-1 system is as follows: four pipes (primary) come off the
cylinder head
In an internal combustion engine, the cylinder head (often abbreviated to simply "head") sits above the cylinders and forms the roof of the combustion chamber.
In sidevalve engines, the head is a simple sheet of metal; whereas in more modern ov ...
, and merge into two pipes (secondary), which in turn finally link up to form one collector pipe.
Compared with a 4-1 exhaust system, a 4-2-1 often produces more power at mid-range engine speeds (RPM), while a 4-1 exhaust produces more power at high RPM.
Cylinder pairings
The purpose of a 4-2-1 exhaust system is to increase
scavenging
Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding be ...
by merging the exhaust paths of specific pairs of cylinders. Therefore, the cylinder pairings are defined by the intervals between firing events, which is determined by the
firing order
The firing order of an internal combustion engine is the sequence of ignition for the cylinders.
In a spark ignition (e.g. gasoline/petrol) engine, the firing order corresponds to the order in which the spark plugs are operated. In a diesel engi ...
and— for engines with an unevenly spaced firing order— the firing interval.
For an
inline-four engine
A straight-four engine (also called an inline-four) is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.
The vast majority of automotive four-cylinder engines use a straight-four layout (with the ...
with a typical firing order of 1-3-4-2, pairing cylinders 1 & 4 and cylinders 2 & 3 is considered "non-sequential", since the paired cylinders do not follow each other in the firing order. This non-sequential arrangement results in an even spacing of 360 degrees between the firing interval in each cylinder pair. A sequential pairing would result in uneven spacings, such as 180 degrees and 540 degrees for pairings of cylinders 1 & 2 and 3 & 4. This sequential pairing is used by many motorcycle engines.
For a
V8 engine
A V8 engine is an eight-cylinder piston engine in which two banks of four cylinders share a common crankshaft and are arranged in a V configuration.
The first V8 engine was produced by the French Antoinette company in 1904, developed and us ...
with a typical crossplane design, 4-2-1 exhausts are often called "Tri-Y" exhausts. Traditionally, only cylinders within the same bank were paired, resulting in spacings of 90-630 degrees (sequential), 180-540 degrees or 270-450 degrees. Typically, the 270 interval is favoured, requiring different pairings in each bank; e.g. 1 & 2 and 3 & 4 in one bank, but 1 & 3 and 2 & 4 in the other - naturally such exhausts are sensitive to the specific firing order in use.
Even spacings of 360-360 degrees are only possible if a cross-over exhaust manifold is used to pair cylinders from separate banks.
Pipe lengths
The combining of exhaust pressure pulses from each cylinder dictates the lengths of the pipes necessary. Generally, shorter pipes will help produce more power at higher engine rpm, and longer pipes favour low-rpm torque, thereby altering the
power band
The power band of an internal combustion engine or electric motor is the range of operating speeds under which the engine or motor is able to output the most power, that is, the maximum energy per unit of time. This usually means that maximum a ...
.
However, the gases tend to cool as they pass through longer pipes, which reduces the effectiveness of the catalytic converter.
In a
turbocharged
In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...
engine, the key factor in the length of the exhaust pipes is providing evenly-spaced pressure pulses to the turbine of the turbocharger.
See also
Two-stroke engines
*
Expansion chamber
On a two-stroke engine, an expansion chamber or tuned pipe is a tuned exhaust system used to enhance its power output by improving its volumetric efficiency.
History
Expansion chambers were invented and successfully manufactured by Limbach, a ...
*
Exhaust pulse pressure charging Exhaust pulse pressure charging (EPPC) is a system for supercharging two-stroke diesel engines of the loop-scavenge type. Loop-scavenge engines cannot be pressure-charged in the same way as uniflow engines or four-stroke engines because the inle ...
*
Kadenacy effect The Kadenacy effect is an effect of pressure-waves in gases. It is named after Michel Kadenacy who obtained a French patent for an engine utilizing the effect in 1933. There are also European and US patents. In simple terms, the momentum of the ...
Four-stroke engines
*
Exhaust system
An exhaust system is used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system ...
*
Exhaust manifold
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'' anyand ''feald'' old and refe ...
*
Scavenging (automotive)
Scavenging is the process of replacing the exhaust gas in a cylinder of an internal combustion engine with the fresh air/fuel mixture (or fresh air, in the case of direct-injection engines) for the next cycle. If scavenging is incomplete, the ...
*
Pressure wave supercharger
A pressure wave supercharger (also known as a wave rotor) is a type of supercharger technology that harnesses the pressure waves produced by an internal combustion engine exhaust gas pulses to compress the intake air. Its automotive use is not wi ...
References
{{reflist, 30em
Exhaust systems