Expansion chamber
Encyclopedia
An expansion chamber is an exhaust system
used on a two-stroke cycle
engine to enhance its power
output by improving its volumetric efficiency
. It makes use of the energy left in the burnt exhaust exiting the cylinder to aid the filling of the cylinder for the next cycle. It is the two-stroke equivalent of the tuned pipe
s (or headers) used on four-stroke cycle engines.
during the cold war
. They first appeared in the west on Japanese motorcycles after East German motorcycle racer Ernst Degner
defected to the west while racing for MZ
in the 1961 Swedish Grand Prix. He hid the blueprints under his racing leathers and defected during the race by riding off the track and claiming asylum. He did not finish the race. He later provided the blueprints to Japan's Suzuki
.
gas exiting the cylinder initially flows in the form of a "wave
front" as all disturbances in fluids do. The exhaust gas pushes its way into the pipe which is already occupied by gas from previous cycles, pushing that gas ahead and causing a wave front. Once the gas flow itself stops, the wave continues on by passing the energy to the next gas down stream and so on to the end of the pipe. If this wave encounters any change in cross section
or temperature
it will reflect a portion of its strength in the opposite direction to its travel. For example a high pressure wave encountering an increase in area will reflect back a low pressure wave in the opposite direction. A high pressure wave encountering a decrease in area will reflect back a high pressure wave in the opposite direction. The basic principle is described in wave dynamics.
An expansion chamber makes use of this phenomenon by varying its diameter (cross section) and length to cause these reflections to arrive back at the cylinder at the desired times in the cycle.
There are three main parts to the expansion cycle.
first exposes the exhaust port on the cylinder wall, the exhaust flows out powerfully due to its own pressure without assistance from the expansion chamber and so the diameter/area over the length of the first portion of the pipe is constant or near constant with a divergence of 0 to 2 degrees which preserves wave energy. This section of the system is called the "head pipe" (the exhaust port length is considered part of the head pipe for measurement purposes). By keeping the head pipe diameter near constant, the energy in the wave is preserved because there is no expansion until needed later in the cycle. In any case the flow leaving the cylinder during most of the blowdown process is sonic or supersonic and therefore no wave could travel back into the cylinder against that flow
Combined with the high pressure wave there is a general rise in pressure in the chamber caused by deliberately restricting the outlet with a small tube called the stinger. The stinger restricts flow out of the chamber to cause higher pressure during the compression cycle and empties the chamber during the compression/power stroke to ready it for the next cycle. The stingers length and inside diameter are selected to match the engine's requirements. (The inside diameter has the greatest effect and so is the most sensitive of the two.)
. Sometimes these secondary wave reflections can inhibit the desired goal of more power.
It is useful to keep in mind that although the waves traverse the entire expansion chamber over each cycle, the actual gases leaving the cylinder during a particular cycle do not. The gas flows and stops intermittently and the wave continues on to the end of the pipe. The hot gases leaving the port form a "slug" which fills the header pipe and remains there for the duration of that cycle. This causes a high temperature zone in the head pipe which is always filled with the most recent and hottest gas. Because this area is hotter, the speed of sound and thus the speed of the waves that travel through it are increased. During the next cycle that slug of gas will be pushed down the pipe by the next slug to occupy the next zone and so on. The volume this "slug" occupies constantly varies according to throttle position and engine speed. It is only the wave energy itself that traverses the whole pipe during a single cycle. The actual gas leaving the pipe during a particular cycle was created two or three cycles earlier. (This is why exhaust gas sampling on two stroke engines is done with a special valve right in the exhaust port. The gas exiting the stinger has had too much resident time and mixing with gas from other cycles causing errors in analysis.)
Expansion chambers almost always have turns and curves built into them to accommodate their fit within the engine bay. Gases and waves do not behave in the same way when encountering turns. Waves travel by reflecting and spherical radiation. Turns causes a loss in the sharpness of the wave forms and therefore must be kept to a minimum to avoid unpredictable losses.
Calculations used to design expansion chambers take into account only the primary wave actions. This is usually fairly close but errors can occur due to these complicating factors.
is rolled into cones and round sections, which are then welded together section by section. Although time consuming, it is usually the method chosen for development of a new design due to its flexibility, accuracy and low tooling costs.
(Note-Functionally, expansion chambers need not be round in cross section but in practice a round shape is the best acoustically and is the only shape which (at a reasonable weight) can withstand the intense vibration and pounding without cracking.)
The first, and most widely used, is to seal off both ends of the expansion chamber with automotive freeze plugs. These consist of a thick rubber donut with a washer on both ends and a bolt running through the middle. As the bolt is tightened, the rubber expands providing an air-tight seal. At one end, the bolt is replaced with a long Schrader valve
to allow the pipe to be filled with compressed air. The dented areas are then heated with an oxyacetylene torch and the dents are pushed out by the air pressure in a controlled manner.
The second approach is to fill the expansion chamber with water and put it in a freezer. The water expands as it freezes causing the dents to be pushed out. This approach involves considerable trial and error and in some cases, can result in damage to the pipe.
(RPM) will not deliver the proper wave timings at 4,000 or 11,000 RPM. In fact it is likely to incur a power loss outside its "tuned" range.
The length of the pipe determines at what time the waves arrive back at the cylinder. Longer pipes require more time for the waves to traverse and so will be tuned to a lower RPM than a shorter pipe. The shorter the pipe the higher the RPM it is tuned to.
The rate of convergence/divergence of the cones determines the duration of the wave returned. A gentle taper give a long duration but weaker return wave while a steeper taper gives a short but strong return wave. The longer the wave, the broader the RPM range at which it is useful. This extra power band
width is at the sacrifice of peak torque.
The diameter of the center or dwell section determines the ratio of scavenging suction to port blocking pressure as well as the over all energy recovery. The resulting volume determines the maximum pressure rise with large volumes giving less pressure rise. The fatter the pipe the harder it sucks but the weaker the blocking pressure. Thinner pipes will scavenge less but block the port very strongly. The optimum diameter is related to compression ratio, the quality of the transfer port layout and its scavenging efficiency.
A variety of devices are used to try to extend the tuned range of the expansion chamber. Pipes that slide like a trombone
adjust the timing to match the RPM changes of the running engine. Devices that control the exhaust port timing to vary blowdown duration as well as extending the tuned range of the expansion chamber. Valves that open at certain speeds to absorb or dump waves arriving at undesirable times.
Another approach to altering the tuned RPM of an expansion chamber is to alter the speed of the pressure waves inside the exhaust pipe. The speed at which pressure waves travel is greatly affected by temperature: higher temperature means faster wave speed. As a result, expansion chambers can be retuned for higher-than-design RPM resonance, by increasing the average temperature of the exhaust gases inside the pipe. Techniques to achieve this increase in gas temperature can include: insulating the pipe (thermal wrap), restricting flow from the pipe (smaller stinger diameter), or by retarding the ignition timing at the correct RPM (a later burn allows more heat to escape into the pipe).
Conversely, a pipe can be retuned to work at a lower-than-design RPM range by reducing the temperature of the exhaust gases. Injecting water or a water-alcohol mix into the headpipe of an expansion chamber can reduce temperatures significantly, enough to lower the tuned RPM of an exhaust system by as much as 1500-2000 RPM. The heat absorbed as the liquid changes into a gas is responsible for the drop in temperature. As a result, the two stroke exhaust can be tuned to stay "on the pipe" over a remarkably wide RPM range, if the designer takes advantage of all the tools available.
. By adjusting the exhaust pipe's total length, one could "tune" the pipe. For model engine
s this can be achieved by cutting the coupler that connects exhaust manifold
and tuned pipe little by little and testing until improvements are seen at desirable RPM band.
For full sized motorcycle
s, various manufacturers already have calculated the length of the exhaust pipe to suit the need of the particular model. However, one could obtain different length pipe to suit one's need as an aftermarket part. Generally longer pipe moves the effective band of tuned pipe to lower RPM range, and shorter length moves the effectiveness band to higher RPM.
Exhaust system
An exhaust system is usually tubing 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...
used on a two-stroke cycle
Two-stroke cycle
A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft...
engine to enhance its power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
output by improving its volumetric efficiency
Volumetric efficiency
Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More specifically, volumetric efficiency is a ratio of what quantity of fuel and air actually enters the cylinder during induction to the...
. It makes use of the energy left in the burnt exhaust exiting the cylinder to aid the filling of the cylinder for the next cycle. It is the two-stroke equivalent of the tuned pipe
Tuned pipe
A tuned pipe is a part of a two-stroke engine's exhaust system.It should be distinguished from a muffler as a tuned exhaust pipe does more than muffling the sound...
s (or headers) used on four-stroke cycle engines.
History
Expansion chambers were first designed by East German Walter KaadenWalter Kaaden
Walter Kaaden was a German engineer who improved the performance of two-stroke engines by understanding the role of resonance waves in the exhaust system. Working for the MZ Motorrad- und Zweiradwerk part of the Industrieverband Fahrzeugbau , he laid the foundations of the modern two-stroke engine...
during the cold war
Cold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...
. They first appeared in the west on Japanese motorcycles after East German motorcycle racer Ernst Degner
Ernst Degner
Ernst Degner was a German Grand Prix motorcycle road racer....
defected to the west while racing for MZ
MZ
-In geography:* Mozambique, ISO 3166-1 country code** .mz, the country code top level domain for Mozambique* Mizoram , a state in northeast India-In media:* Mitteldeutsche Zeitung, a German newspaper* Marvel Zombies a Marvel Comics limited series...
in the 1961 Swedish Grand Prix. He hid the blueprints under his racing leathers and defected during the race by riding off the track and claiming asylum. He did not finish the race. He later provided the blueprints to Japan's Suzuki
Suzuki
is a Japanese multinational corporation headquartered in Hamamatsu, Japan that specializes in manufacturing compact automobiles and 4x4 vehicles, a full range of motorcycles, all-terrain vehicles , outboard marine engines, wheelchairs and a variety of other small internal combustion engines...
.
How it works
The high pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
gas exiting the cylinder initially flows in the form of a "wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
front" as all disturbances in fluids do. The exhaust gas pushes its way into the pipe which is already occupied by gas from previous cycles, pushing that gas ahead and causing a wave front. Once the gas flow itself stops, the wave continues on by passing the energy to the next gas down stream and so on to the end of the pipe. If this wave encounters any change in cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
or temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
it will reflect a portion of its strength in the opposite direction to its travel. For example a high pressure wave encountering an increase in area will reflect back a low pressure wave in the opposite direction. A high pressure wave encountering a decrease in area will reflect back a high pressure wave in the opposite direction. The basic principle is described in wave dynamics.
An expansion chamber makes use of this phenomenon by varying its diameter (cross section) and length to cause these reflections to arrive back at the cylinder at the desired times in the cycle.
Blowdown
When the descending pistonPiston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
first exposes the exhaust port on the cylinder wall, the exhaust flows out powerfully due to its own pressure without assistance from the expansion chamber and so the diameter/area over the length of the first portion of the pipe is constant or near constant with a divergence of 0 to 2 degrees which preserves wave energy. This section of the system is called the "head pipe" (the exhaust port length is considered part of the head pipe for measurement purposes). By keeping the head pipe diameter near constant, the energy in the wave is preserved because there is no expansion until needed later in the cycle. In any case the flow leaving the cylinder during most of the blowdown process is sonic or supersonic and therefore no wave could travel back into the cylinder against that flow
Transfer
Once the exhaust pressure has fallen to near atmospheric level the piston uncovers the transfer ports. At this point energy from the expansion chamber can be used to aid the flow of fresh mixture into the cylinder. To do this the expansion chamber is increased in diameter so that the out going high pressure wave reflects a negative pressure wave back toward the cylinder. This negative pressure arrives in the cylinder during the transfer cycle and greatly increases the flow of fresh mixture into the cylinder (and can even suck fresh mixture out into the exhaust port). This part of the pipe is called the divergent (or diffuser) section and it diverges at 6 to 12 degrees. It may be made up of more than one diverging cone depending on requirements.Port blocking
When the transfer is complete the piston is on the way back up on its compression stroke but the exhaust port is still open, an unavoidable problem with the two stroke design. To help prevent the piston pushing fresh mixture out the open exhaust port a strong high pressure wave from the expansion chamber is timed to arrive during the compression stroke. The port blocking wave is created by reducing the diameter of the chamber. This is called the convergent section (a.k.a baffle cone or section). The outgoing high pressure wave hits the narrowing convergent section and reflects back a high pressure wave to the cylinder which arrives in time to block the port during the compression stroke and can push back into the cylinder any fresh mixture drawn out into the head pipe. The convergent section is made to converge at 8 to 90 degrees depending on requirements.Combined with the high pressure wave there is a general rise in pressure in the chamber caused by deliberately restricting the outlet with a small tube called the stinger. The stinger restricts flow out of the chamber to cause higher pressure during the compression cycle and empties the chamber during the compression/power stroke to ready it for the next cycle. The stingers length and inside diameter are selected to match the engine's requirements. (The inside diameter has the greatest effect and so is the most sensitive of the two.)
Complicating Factors
The detailed operation of expansion chambers in practice is not as straightforward as the fundamental process described above. Waves traveling back up the pipe encounter the divergent section in reverse and reflect a portion of their energy back out. Temperature variations in different parts of the pipe cause reflections and changes in the local speed of soundSpeed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
. Sometimes these secondary wave reflections can inhibit the desired goal of more power.
It is useful to keep in mind that although the waves traverse the entire expansion chamber over each cycle, the actual gases leaving the cylinder during a particular cycle do not. The gas flows and stops intermittently and the wave continues on to the end of the pipe. The hot gases leaving the port form a "slug" which fills the header pipe and remains there for the duration of that cycle. This causes a high temperature zone in the head pipe which is always filled with the most recent and hottest gas. Because this area is hotter, the speed of sound and thus the speed of the waves that travel through it are increased. During the next cycle that slug of gas will be pushed down the pipe by the next slug to occupy the next zone and so on. The volume this "slug" occupies constantly varies according to throttle position and engine speed. It is only the wave energy itself that traverses the whole pipe during a single cycle. The actual gas leaving the pipe during a particular cycle was created two or three cycles earlier. (This is why exhaust gas sampling on two stroke engines is done with a special valve right in the exhaust port. The gas exiting the stinger has had too much resident time and mixing with gas from other cycles causing errors in analysis.)
Expansion chambers almost always have turns and curves built into them to accommodate their fit within the engine bay. Gases and waves do not behave in the same way when encountering turns. Waves travel by reflecting and spherical radiation. Turns causes a loss in the sharpness of the wave forms and therefore must be kept to a minimum to avoid unpredictable losses.
Calculations used to design expansion chambers take into account only the primary wave actions. This is usually fairly close but errors can occur due to these complicating factors.
Hand forming
Flat sheet metalSheet metal
Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material...
is rolled into cones and round sections, which are then welded together section by section. Although time consuming, it is usually the method chosen for development of a new design due to its flexibility, accuracy and low tooling costs.
Hydroforming
Two flat representations of the required finished pipe are cut out of sheet metal. The edges of the two identical flat cutouts are welded together forming a sandwich. On one end of the pipe a fitting is welded and high-pressure water is pumped into the cavity between the sheets. The pressure inflates the flat sheet into its final rounded shape. This method can be quicker than hand forming and only slightly more costly in tooling, however it requires a number of trials before a finished design as accurate as hand formed or stamped can be produced. All curves must be made in a single plane so cutting apart and re-welding is often required but the final product can be as good as a stamped pipe if enough care is taken to be precise.Stamping
Flat sheet metal is pressed between a male and female mold in the shape of the required pipe. Each half of the pipe is stamped this way and the two halves are welded together. Stamping requires expensive tooling and machinery and is used only for mass production.(Note-Functionally, expansion chambers need not be round in cross section but in practice a round shape is the best acoustically and is the only shape which (at a reasonable weight) can withstand the intense vibration and pounding without cracking.)
Repair
Due to their large size, expansion chambers on two-stroke off-road motorcycles are prone to damage and denting from rocks, logs and other obstacles. Large dents can change the dimensions of the pipe sufficiently to detract from performance and reduce efficiency. There are two main strategies for repairing this type of damage. Both are intended to push out dents and restore the pipe to near original dimensions.The first, and most widely used, is to seal off both ends of the expansion chamber with automotive freeze plugs. These consist of a thick rubber donut with a washer on both ends and a bolt running through the middle. As the bolt is tightened, the rubber expands providing an air-tight seal. At one end, the bolt is replaced with a long Schrader valve
Schrader valve
The Schrader valve is a brand of pneumatic tire valve used on virtually every motor vehicle in the world today. The Schrader company, for which it was named, was founded in 1844 by August Schrader...
to allow the pipe to be filled with compressed air. The dented areas are then heated with an oxyacetylene torch and the dents are pushed out by the air pressure in a controlled manner.
The second approach is to fill the expansion chamber with water and put it in a freezer. The water expands as it freezes causing the dents to be pushed out. This approach involves considerable trial and error and in some cases, can result in damage to the pipe.
Summary
All these events need to be synchronized with the engine port timings and speed. An expansion chamber "tuned" for 8,000 revolutions per minuteRevolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
(RPM) will not deliver the proper wave timings at 4,000 or 11,000 RPM. In fact it is likely to incur a power loss outside its "tuned" range.
The length of the pipe determines at what time the waves arrive back at the cylinder. Longer pipes require more time for the waves to traverse and so will be tuned to a lower RPM than a shorter pipe. The shorter the pipe the higher the RPM it is tuned to.
The rate of convergence/divergence of the cones determines the duration of the wave returned. A gentle taper give a long duration but weaker return wave while a steeper taper gives a short but strong return wave. The longer the wave, the broader the RPM range at which it is useful. This extra power band
Power band
The power band of an engine or electric motor refers to the range of operating speeds under which an the engine or motor is able to operate efficiently...
width is at the sacrifice of peak torque.
The diameter of the center or dwell section determines the ratio of scavenging suction to port blocking pressure as well as the over all energy recovery. The resulting volume determines the maximum pressure rise with large volumes giving less pressure rise. The fatter the pipe the harder it sucks but the weaker the blocking pressure. Thinner pipes will scavenge less but block the port very strongly. The optimum diameter is related to compression ratio, the quality of the transfer port layout and its scavenging efficiency.
A variety of devices are used to try to extend the tuned range of the expansion chamber. Pipes that slide like a trombone
Trombone
The trombone is a musical instrument in the brass family. Like all brass instruments, sound is produced when the player’s vibrating lips cause the air column inside the instrument to vibrate...
adjust the timing to match the RPM changes of the running engine. Devices that control the exhaust port timing to vary blowdown duration as well as extending the tuned range of the expansion chamber. Valves that open at certain speeds to absorb or dump waves arriving at undesirable times.
Another approach to altering the tuned RPM of an expansion chamber is to alter the speed of the pressure waves inside the exhaust pipe. The speed at which pressure waves travel is greatly affected by temperature: higher temperature means faster wave speed. As a result, expansion chambers can be retuned for higher-than-design RPM resonance, by increasing the average temperature of the exhaust gases inside the pipe. Techniques to achieve this increase in gas temperature can include: insulating the pipe (thermal wrap), restricting flow from the pipe (smaller stinger diameter), or by retarding the ignition timing at the correct RPM (a later burn allows more heat to escape into the pipe).
Conversely, a pipe can be retuned to work at a lower-than-design RPM range by reducing the temperature of the exhaust gases. Injecting water or a water-alcohol mix into the headpipe of an expansion chamber can reduce temperatures significantly, enough to lower the tuned RPM of an exhaust system by as much as 1500-2000 RPM. The heat absorbed as the liquid changes into a gas is responsible for the drop in temperature. As a result, the two stroke exhaust can be tuned to stay "on the pipe" over a remarkably wide RPM range, if the designer takes advantage of all the tools available.
Limitations
The timing of the return pressure wave depends solely on the length of the exhaust to the point where the pressure wave is reflected. However, the length of time the exhaust port is open alters with RPM. Since the timing of the pressure wave is unaltered its return will no longer coincide with the exhaust port's closure. This means that tuned pipes only reach maximum effectiveness over a fairly narrow RPM range.Tuning the pipe
Because a tuned pipe cannot be effective over the full spectrum of the RPM, it has to be "tuned" for a certain RPM range, just as a musical instrument is tuned. Usually, it is tuned for the lower RPM because that's where most engines have weakest horse power and torqueTorque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
. By adjusting the exhaust pipe's total length, one could "tune" the pipe. For model engine
Model engine
In radio-controlled modeling, a model engine is an internal combustion engine used to power a radio-controlled aircraft, radio-controlled car, radio-controlled boat, free flight and control line aircraft, and tether car models also use these engines....
s this can be achieved by cutting the coupler that connects exhaust manifold
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 and refers to the folding together of multiple inputs and outputs.In contrast, an inlet manifold is the part of an engine...
and tuned pipe little by little and testing until improvements are seen at desirable RPM band.
For full sized motorcycle
Motorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...
s, various manufacturers already have calculated the length of the exhaust pipe to suit the need of the particular model. However, one could obtain different length pipe to suit one's need as an aftermarket part. Generally longer pipe moves the effective band of tuned pipe to lower RPM range, and shorter length moves the effectiveness band to higher RPM.