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Expansion chamber

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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 pipes (or headers) used on four-stroke cycle engines.

How it works

The high pressure gas exiting the cylinder flows in the form of a "waves" as all disturbances in fluids do. 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.

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.

Image:Arbeitsweise_Zweitakt.gif

There are three main parts to the expansion cycle.

Blowdown

When the descending piston first exposes the exhaust port, the exhaust flows out powerfully due to its own pressure without assistance from the expansion chamber and so the first portion of the pipe is constant or near constant with a divergence of 0 to 2 degrees. 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 near constant, the energy in the wave is preserved until needed later in the cycle.

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 headpipe. 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. 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 engines requirements. (The inside diameter has the greatest effect and so is the most sensitive of the two.)

How Expansion chambers are made

There are three main methods of fabricating expansion chambers.

Hand formed

Flat sheet metal 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.

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.)

Summary

All these events need to be synchronized with the engine port timings and speed. An expansion chamber “tuned” for 8,000 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.

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.

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