Modified (Autosaved)

CONTENTS

CHAPTER 01 INTRODUCTION TO EXPANSION CHAMBERS .......................................... 1

CHAPTER 02 HISTORY ............................................................................................................... 2

CHAPTER 03 WORKING OF EXPANSION CHAMBERS ...................................................... 3

3.1. BLOWDOWN ...................................................................................................................... 3

3.2. TRANSFER .......................................................................................................................... 3

3.3. PORT BLOCKING .............................................................................................................. 4

CHAPTER 04 HOW ARE EXPANSION CHAMBERS MADE ................................................ 5

4.1. HAND FORMED ................................................................................................................. 5

4.2. HYDRO FORMING ............................................................................................................ 6

4.3. STAMPING .......................................................................................................................... 6

CHAPTER 05 EXHAUSTEC ........................................................................................................ 7

5.1. HISTORY OF EXHAUSTEC ............................................................................................. 8

5.2. WHY EXHAUSTEC? ......................................................................................................... 9

5.3. WORKING OF EXHAUSTEC......................................................................................... 10

5.4. ADVANTAGES AND DISADVANTAGES ................................................................ 11

5.4.1. ADVANTAGES.............................................................................................................. 11

5.4.2. DISADVANTAGES ....................................................................................................... 11

5.5. COMPLICATING FACTORS......................................................................................... 12

CHAPTER 06 CONCLUSION .................................................................................................... 13

CHAPTER 07 REFERENCES ..................................................................................................... 15

ABSTRACT

In this seminar the different types of expansion chambers, working of the expansion chambers

are explained and the special type of the expansion chamber used in the Bajaj patented

exhausted. Which is basically a small chamber fixed to the exhaust pipe which can increase the

power and torque at lower revs. It increases the volumetric efficiency also. . This can be partly

addressed by use of a tuned exhaust system to deliver a pulse of positive pressure prior to the

port closing, to retain the charge. In recent development the exhaustec is used in the four stroke

engines.

INTRODUCTION TO EXPANSION CHAMBERS 2014

DEPT. AUTOMOBILE ENGG., DSCE PAGE 1

CHAPTER 01

INTRODUCTION TO EXPANSION CHAMBERS

The expansion chamber is one of the more significant tools which are used to attenuate

the noise emitted from the exhaust system of the vehicle engine. The first attempted to modeling

the simple expansion chamber was reported by Davies et al (1954). They used transmission line

theory by assuming both continuity of pressure and volume velocity at discontinuities. The

exhaust port is opened and closed directly by the position of the piston rather than by a separate

valve, which restricts the timing of its operation; typically, the port remains open long after is

optimum, allowing some of the incoming charge to escape. This can be partly addressed by use

of a tuned exhaust system to deliver a pulse of positive pressure prior to the port closing, to

retain the charge. In recent development the exhaustec is used in the four stroke engines. Then,

in the early seventies, other design techniques gradually evolved for one dimensional analysis

of muffler. Alfredson and Davies (1971) developed equation from an energy balance point of

view of acoustic pressure in simple area expansion and determine the attenuation in such

silencer. This is one of the first worker to consider mean flow in silencer. The cascading

property makes the 4-pole approach more convenient in modeling mechanical systems, because

it allows formulating different models independently and combining them by simply

multiplying their 4-pole matrices. Since, the 4-pole method formulates the system equation in

two terminal variables, typically the pressure and volume flow in acoustics, it has been applied

to systems composed of acoustic elements. Therefore, applications are mainly found in duct

acoustics.



CHAPTER 02

HISTORY

Expansion chambers were invented and successfully manufactured by Limbach, a

German engineer, in 1938, to economize fuel in two stroke engines. Germany was running short

of petrol, which was at that stage produced using coal and sewerage transformation. An

unexpected bonus was that the two stroke engines using tuned exhausts produced far more

power than if running with a normal silencer. After the end of the Second World War, some

time passed before the concept was re-developed by East German Walter Kaaden 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 later passed his knowledge to Japan's Suzuki. The expansion chamber is one of

the more significant tools which are used to attenuate the noise emitted from the exhaust system

of the vehicle engine. The first attempted to modeling the simple expansion chamber was

reported by Davies et al (1954). They used transmission line theory by assuming both continuity

of pressure and volume velocity at discontinuities. Igarashi and his co. workers (Igarashi and

Toyama 1958, Miwa and Igarashi 1959 and Igarashi and Arai 1960) in a series of reports

determined the transmission characteristic of expansion chamber and resonator. They used an

electrical analogy to 4-pole parameters and determine the transmission losses of muffler. Then,

in the early seventies, other design techniques gradually evolved for one dimensional analysis

of muffler. Alfredson and Davies (1971) developed equation from an energy balance point of

view of acoustic pressure in simple area expansion and determine the attenuation in such

silencer. This is one of the first worker to consider mean flow in silencer. The cascading

property makes the 4-pole approach more convenient in modeling mechanical systems, because

it allows formulating different models independently and combining them by simply

multiplying their 4-pole matrices. Since, the 4-pole method formulates the system equation in

two terminal variables, typically the pressure and volume flow in acoustics, it has been applied

to systems composed of acoustic elements. Therefore, applications are mainly found in duct

acoustics.



CHAPTER 03

WORKING OF EXPANSION CHAMBERS

Figure 3.1. Working of Expansion chamber (courtesy Masesa)

There are three main parts of the expansion cycle.

3.1. BLOWDOWN

When the descending piston 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 blow down process is sonic or supersonic and

therefore no wave could travel back into the cylinder against that flow.

3.2. 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 outgoing 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 head pipe. 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.

3.3. 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 engines requirements.

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



CHAPTER 04

HOW ARE EXPANSION CHAMBERS MADE

There are three main methods of fabricating expansion chambers.

4.1. 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.

Figure 4.1. Showing a hand formed EC

Figure 4.2. Hand formed EC



4.2. HYDRO FORMING

Figure 4.2.1. Showing the Hydro Forming EC

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.

4.3. 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.)



CHAPTER 05

EXHAUSTEC

Figure 5.1. Exhaustec Muffler( courtesy Bajaj Ltd.)

ExhausTEC stands for Exhaust Torque Expansion Chamber, a Bajaj Auto trademark.

The technology involves use of a small chamber connected to the exhaust pipe of the engine to

modify the back-pressure and the swirl characteristics, with an aim to improve the low-end

performance of the bikes. The unique ExhausTEC technology allows you to rev up when your

heart feels like and the engine will pick up instantaneously irrespective of the gear you are in.

It improves engine torque even at low rpms and is optimized to get maximum performance from

the engine. Gives a feeling of abundant latent power at any stage of riding, which ensures

effortless pulling for any load conditions.

Figure 5.2. Showing the torque expansion chamber (courtesy Bajaj ltd.)



5.1. HISTORY OF EXHAUSTEC

Bajaj gets patent for EshausTEC technology The Indian Patent Office granted Bajaj

Auto a patent for its 'ExhausTEC' invention vide Patent No 231498 dated March 5, 2009. This

grant was published in Patent Gazette, dated March 27, 2009 ExhausTEC stands for Exhaust

Torque Expansion Chamber, a Bajaj Auto trademark. The technology involves use of a small

chamber connected to the exhaust pipe of the engine to modify the back-pressure and the swirl

characteristics, with an aim to improve the low-end performance of the bikes. This was

attempted in response to the issue of a reported lack of low-end response in Bajaj's single-

cylinder four-stroke engines. The ExhausTEC technology is claimed to be highly effective in

improving the overall engine response, especially the low-end torque characteristics. This

enhanced performance is claimed to come at no loss of top-end performance or engine

smoothness. Earlier, BAL had applied for patent of this technology in 2004. Later, TVS

unveiled a series of new products, which Bajaj alleged of patent infringement of its ExhausTEC

technology and served notice to the Chennai-based firm in December, 2007. Bajaj Auto claimed

that ExhausTEC significantly improves low- and mid-range torque (tendency of a force to rotate

an object about an axis) in a single cylinder four stroke engine, employing a chamber of

predetermined volume attached to the exhaust pipe. TRADEMARK: Bajaj has used its logo as

a Trademark.



5.2. WHY EXHAUSTEC?

As the expansion chambers are bulk and heavy the new modern expansion chamber that

is fitted on the exhaust pipe of the tail pipe. As the name implies, an expansion chamber exhaust

consists of a chamber where gases from the exhaust phase expand into. However, the change of

shape of the chamber, as it reduces in size, sets up a pulse that returns towards the exhaust port.

If the returning pulse arrives at just the right time, it will push the unburnt gases back into the

cylinder.

Figure 5.2.1. Showing the Exhaustec (courtesy Bajaj ltd)



5.3. WORKING OF EXHAUSTEC

Figure 5.3.1. Showing the working of Exhaustec (Masesa courtesy)

The high pressure 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 downstream 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.



5.4. ADVANTAGES AND DISADVANTAGES

5.4.1. ADVANTAGES Improved volumetric efficiency.

Improved & optimized low end torque.

Improved Scavenging.

Less frequent gear shifting in city driving conditions.

Less frequent clutch operation.

Subsequent increase in fuel efficiency (Mileage).

Better engine performance - Power & Pick-up.

Improved drive-ability especially at low and mid-range engine R.P.M.

5.4.2. DISADVANTAGES

The operation of expansion chambers in practice is not as straightforward as described

above.

Temperature variations in different parts of the pipe cause reflections and changes in the

local speed of sound.

The hot gasses leaving the port form a "slug" which fills the header pipe and remains

there for the duration of that cycle.

Because this area is hotter, the speed of sound and thus the speed of the waves that travel

through it are increased.

The actual gas leaving the pipe during a particular cycle was created two or three cycles

earlier.

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.



5.5. COMPLICATING FACTORS

The operation of expansion chambers in practice is not as straightforward as 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 sound. 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 gasses 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 gasses 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. Expansion chambers almost always have turns and curves built into

them to accommodate their fit within the engine bay. Gasses 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.



CHAPTER 06

CONCLUSION

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. 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 head pipe 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.



CHAPTER 07

REFERENCES

1. ExhausTEC- http://www.royalauto.in/?tag=exhaustec

2. http://www.bajajauto.com/dis135_technology06.asp

3. Motorcycle.com Archived 2 February 2011 at Website

4. Oxley, Mat (2010), Stealing Speed: The Biggest Spy Scandal in Motorsport

History, Haynes Publishing Group, ISBN 1-84425-975-7

5. Working of exhaust -http://www.masesa.com/tecnologias/exhaustec/

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