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Dyno
Mind Talks Pulse Charging:
This tech article is going to attempt to explain the basic mysteries behind the
proper
design of 4-stroke exhaust systems. In order to accomplish this, some of
the properties of sound waves must be explained since a truly “tuned”
exhaust must exploit some of these properties. All understanding of
4-stroke exhausts must begin with some explanation of the tuned 2-stroke
exhaust system since no other exhaust system uses sound energy more
effectively to enhance engine performance.
Two stroke pipes are commonly called expansion chambers.
The more
accurate, but lesser known term for two stroke exhausts, is “pulse charger”.
Until the invention of the pulse charger style exhaust, the 2-stroke was
more of a utility power plant used in chainsaws, weed whippers, scooters,
etc for its simplicity and light weight but not because of its high output.
The pulse charger changed all of that. If everything
else were optimized on
a 2-stroke engine, changing from a non-tuned exhaust to a pulse charger
would increase the horsepower by approximately 40%. That change alone was
enough to tip the power scale for competition cycle engines in favor of the
2-stroke. Imagine an exhaust system that so greatly affects the
output on
a 2-stroke, it’s the same as bolting a supercharger on to a 4-stroke! As
a
result, 2-strokes are highly sensitive to pulse charger designs. No other
component can alter the 2-strokes’ performance characteristics, for better
or worse, as much as their exhausts.
A 4-stroke has 4 basically discrete strokes of the piston for
each power
cycle:
1)
Intake
2) Compression
3) Power
4) Exhaust
Whereas, a 2-stroke power stroke has a combination of things happening in
each piston stroke and produces a power stroke on each downward movement of
the piston, instead of every other downward stroke like the 4-stroke. In
this first set of illustrations, the basic function of the pulse charger
style 2-stroke tuned exhaust is demonstrated. Until a basic understanding
is gained, standard physics terms associated with sound waves referring to
negative pressure and sign inversion with increasing or decreasing
confinement area will not be used.
A much more intuitive understanding will come from this
model. Think of
the exhaust pipe as a giant “syringe” and think of the sound wave as being
not a wave but just a single pulse. This pulse is the equivalent of the
rubber plunger inside the syringe, except that this plunger is thin like a
rubber diaphragm and it can change its shape instantly to perfectly seal any
shape of syringe.
A pulse charger has 5 distinct sections:
1) Head pipe - small
diameter tube/ connects to exhaust port
2) Megaphone- 1st cone
section diverging away from head pipe- extends
suction action of sound pulse
3) Belly- Fatter center
section with constant diameter
4) Reverse Megaphone-
Converging cone- reflects sound pulse (think
echo) reverses direction of sound pulse energy
5) Stinger- tailpipe-
regulated pressure bleed
In figure 1, the piston has partially uncovered the exhaust
port and the
sound pulse (plunger) is in the megaphone. As it leads the exhaust charge,
it leaves a partial vacuum (suction) in its wake. This aids greatly in
exhaust removal by adding a “pull” on the other end of the exhaust
pressures’ “push”.
In figure #2, the pulse has traveled through the megaphone
and belly of the
pipe. In doing so, it has not only pulled the burnt gasses into the pipe,
but has also aided in lowering pressure inside the cylinder enough to help
pull the fresh fuel/air mix from the crankcase through the transfer ports.
It has also pulled a significant portion of fresh fuel/air mix into the
head pipe. Not to worry, the sound pulse is about to cause pressure and
flow
in the other direction when the reverse cone reflects the pulse (think
“echo”) back towards the cylinder.
In figure #3, the piston is on its way back up and our
reflected pulse is
causing a backflow of the fresh fuel air mix that made its way into the
head pipe back into the cylinder.
In figure #4, the last of the escaped fuel air mix is pushed
into the
cylinder just as the piston closes the exhaust port.
Since the sound pulse travels at the speed of sound
(naturally) it
completes this pull/push action within the pipe at a relatively constant
rate. There is then only a certain RPM range when the pistons opening and
closing of the exhaust port is synchronous with this sound resonance within
the pipe. This is where the term “on the pipe” came from for
describing the
explosive power band when the pipe is supercharging the motor.
Supercharging
is a generic term that refers to filling a motor’s cylinder to a greater
amount than what normal atmospheric pressure can accomplish. A
turbocharger
uses exhaust gas pressure to spin a turbine (fan) to pressurize the incoming
fuel/air mix. A blower accomplishes the same thing, but uses a mechanical
linkage (pulleys) to drive it. A tuned 2-stroke pipe does it by harnessing
existing sound energy to pull more mix through the motor than would normally
flow through and then pack the charge back into the cylinder at the last
moment to achieve its supercharging effect. Pulse Charger
really is a
better, more accurate description than expansion chamber for a 2-stroke
pipe.
The length determines the RPM range that a pipe will perform
best.
Shorter pipe= less time for pulse to complete its entire journey.
Synchronizes with higher RPM. Thus longer pipe= more time for travel,
works with lower RPM riding.
The cone angles and volumes determine the strength of the
effect. Steeper
angles and larger volumes create a stronger, sharper response.
This was to provide a more intuitive explanation of the power
to be gained
by harnessing the energy of something an engine will make as a byproduct
anyway: SOUND ENERGY.
Side note: a pulse charger would prefer that once the sound
did its work,
the sound would simply vanish so as not to cause any undesirable harmonics.
If the silencer on the tailpipe doesn’t impede flow of exhaust, the quieter
it is, and the more power you will make. It’s a fact. Pass it on.
May the
ghosts of Bernoulli and Kaaden forgive me for this oversimplification of
some very complex processes.
In the 2-stroke engine, the piston does double duty as a
piston and a
valve, since its vertical position in the cylinder determines what passages
for mixture flow are open and closed. The 4-stroke engine has no such
passages in its cylinder walls. Mixture flow in a 4-stroke enters and
exits
through valves located above the piston in the cylinder head.
These valves are opened and closed by the camshaft or cam.
Again, the
4-stroke has four semi- discrete events per power cycle:
1) intake stroke- piston moving
down/intake valve open
2) compression stroke- piston moving
up/ both valves closed
3) power stroke- piston forced down
by burning mixture/ both valves closed
4) exhaust stroke- piston moving up/
exhaust valve open
5) Repeat ad infinitum.......
There is actually a brief period of time between the end of
the exhaust
stroke and the beginning of the intake stroke and when the camshaft has both
the intake and exhaust valves open simultaneously. This period of cam
timing is known as ‘overlap’. The overlap period becomes extremely
important in 4-stroke tuned exhaust design.
Now that a more familiar feel is had on sound energy in
confined spaces, it
is more advantageous to use more correct, technical terms.
By first examining the exhaust flow itself, it has two
components: Exhaust
particulate and sound pressure wave.
The actual exhaust is particulate, has mass and behaves as a
liquid. As
such its flow can be impeded by sharp bends, reduced pipe diameters and all
non-aerodynamic obstacles such as baffles. Anything that would disrupt
water flow also slows exhaust flow.
The sound “pressure wave” flow has no mass and is not
affected by sharp
bends, but as mentioned in the 2-stroke section, it has a property that a
tuned 4-stroke exhaust system must exploit to be as effective as possible.
Any time a confined sound pressure wave encounters an enlargement in its
containment area, a negative pressure wave (vacuum) is sent back through the
pipe towards the origin of the wave.
The intensity of this vacuum is proportional to the
abruptness in change of
its confinement area. An exhaust pulse that exits from a parallel wall
pipe
to atmospheric pressure creates a vacuum of high intensity but of extremely
short duration.
Burnt gasses, upon rushing past the exhaust valve and exiting
the exhaust
port of the cylinder head, enter the first section of the exhaust system.
Head Pipes typically have been parallel wall (constant diameter) tubing
from beginning at the head to its connection to the silencing portion of the
exhaust.
In recent years, some head pipes have been made with stepped
diameters,
usually with one or two increases in pipe diameter (steps) along their
length in an attempt to increase the systems high RPM efficiency without the
drawbacks of merely going to a large diameter tube for the entire length.
As far as exhaust flow within a head pipe is concerned, the
sound and
particulate portions are a single entity. Since there is little or no
change in diameter, there will be little or no effective vacuum produced by
the sound pressure wave.
The force we are concerned with within the head pipe is the
velocity and
mass of the particulate portion of the exhaust: kinetic energy. This is
best understood by thinking of each exhaust event as a single event moving
through the head pipe. Think of it as a high-speed golf ball. Since
this
speedy golf ball fits the diameter of the head pipe perfectly, it leaves a
partial vacuum in its wake much as a plunger through a syringe.
The challenge in head pipe design is in selection of its
diameter. A
smaller diameter with a tighter fit allows for the ‘golf ball’ to
effectively create suction/vacuum for each successive golf ball behind it.
Again, exhaust flows like a fluid and this is how a siphon works. The
problem with the small diameter pipe shows itself at high RPM where there
just is not enough room for all these golf balls to exit efficiently and
they get ‘bunched up’. This is the point where the engine
stops making more
power. (If you can’t get the burned gas out, you can’t bring more
fresh gas
in). High RPM applications are best served by a larger diameter tube where
the golf balls have room for more than just a single file parade out of the
head pipe.
The problem with large diameter head pipes is they can
absolutely destroy
an engines lower RPM power and kill throttle response. Obviously, if
bigger
were better, all exhaust systems would be the diameter of garbage cans and
everyone would live happily ever after.
What happens at low RPM with head pipes is the large diameter
dampens the
golf balls suction to the point of ineffectiveness like this: if the usual
diameter of a gasoline siphon hose was ½ inch , and it was replaced by a
3"
diameter fire truck hose, it would take quite a few talented porno stars to
create enough vacuum in such a large hose to start the siphon process.
Since the volume needed to impart vacuum upon increased in size, while the
suction supplier (porn star) did not, it dampened the effectiveness.
This is where the stepped diameter head pipes come in.
Through cut and try
(cut and dyno), the goal is to proportion the pipe length and diameters
to
retain low speed suctioning properties and diminish high speed “bunching”.
It is a balancing act and not a cure all, but it is very worthwhile to try
this approach when a very broad power band is needed.
Finally, the silencer end of the 4-stroke exhaust, commonly
known as the
‘canister’ among the majority of exhaust manufacturers. Usually they
are
just a larger version of the portion of the 2-stroke exhaust known as the
‘silencer’.
This style of ‘muffler’ has been used for about a century
and has been
applied on all styles of engines such as tractors, autos, lawn mowers, etc.
In the auto industry, it became known a glasspack and was produced under
well known names such as ‘Cherry Bomb’, ‘Thrush’, and ‘Purple Hornies’.
The name glasspack is also a short descriptor of its
construction. This
silencer has an outer shell, perforated inside tube and a sound dampening
material (usually fiberglass) packed in the space between the outer shell
and the outside of the perforated center tube. Exhaust flow is through the
inside of the perforated tube with an unobstructed ‘line of sight’ flow
path. The sound is attenuated as it passes through the perforations into
the fiberglass area and spends some of its energy in the movement of
fiberglass.
The glasspack is light, free flowing, and simple to
manufacture
inexpensively. Unfortunately, it is not the best sound dampener. The
glasspack is the most common high performance silencer. Its straight
through, open core design doesn’t add any more power than a straight tube,
but it doesn’t take any away either.
The part of the 4-stroke cycle to focus on, from an exhaust
design
standpoint, is the area between the end of the exhaust stroke and the
beginning of the intake stroke. This is where the cam has both the
intake
and exhaust valves open simultaneously in ‘overlap’. Power would be
gained
if a 4-stroke exhaust could create a vacuum at the exhaust valve during the
overlap phase.
Intake and exhaust valves are located at opposite sides of
the combustion
chamber. A negative pressure at the exhaust valve during overlap would
assist in pulling burnt gasses from the combustion chamber into the exhaust
port. In doing so it creates an effect known as ‘scavenging’, which is
the
clean sweep effect caused by the pull of exhaust creating a vacuum that
pulls fresh mix from the intake valve and fills the combustion chamber in a
sweeping motion. The fresh charge moves across the chamber following the
burnt gasses to the exhaust valve. Under ideal conditions, scavenging
lowers the combustion chamber below atmospheric and adds an enhanced ‘yank’
on the static column of fresh charge behind the intake valve before the
exhaust valve finishes closing.
The benefit of such negative pressure/overlap period harmony
is two fold.
With less burned gasses in the combustion chamber, the fresh charge, being
less diluted, will burn more efficiently. Also, the lowered chamber
pressure at the exhaust valve closing time gives the intake charge higher
initial kinetic energy to fill the cylinder.
In an exhaust system of fairly constant inner diameter, the
strongest
negative pressure wave is created at the instant the pressure wave exits the
exhaust tip. Since the timing of travel of these pressure fluctuations
happen at the speed of sound (as with the 2-stroke) the synchronization of
negative pressure/overlap timing is fairly RPM specific and depends on the
overall length of the exhaust system.
Another style of 4-stroke exhaust, called a megaphone
exhaust, is one where
the head pipe connects to a diverging cone much like the first cone of a
2-stroke pulse charger exhaust. The purpose is to produce a longer
duration
of negative pressure wave (vacuum) that would be synchronous with the
overlap period for a much wider spread of RPM. These megaphone exhausts
can
provide a wider power band than the conventional constant diameter systems.
Megaphones are however more difficult to manufacture and are
generally
higher in sound output due to their enlarging diameter before the silencing
section.
Sound output is more of an issue now than ever with not only
tighter
decibel limits for state and federal land access, but actual testing is
being done for the first time ever on closed course competition machines.
The aftermarket performance exhaust industry has responded
(?) to these
sound restrictions by making all manner of internal baffles (flow killing)
and aperture reducers (flow murdering) to be retrofitted to their existing
glass packs. All of them reduce noise but also reduce power. There
are
currently two ‘enhanced silencing’ choices to be had in silencers: 1)
‘Butt
plug’ silencing and 2) ‘Fart through a straw’ silencing
A recent industry trend is to make the outer shell and end
caps of their
glass packs out of different shapes i.e. oval, triangle, rounded squares
etc. The materials of the shells are also getting more exotic with
titanium
and carbon fiber. Aesthetics play a huge part in our attraction to
anything
and if an odd shaped glass pack made of titanium is what brings the most joy
then so be it. Any canister can be made of special materials, but it is
the inside that is power producing, and ALL current models of glasspack are
internally the same thing.
Another recent trend is a big emphasis on the use of exhaust
systems not to
increase power (since no manufacturer has any advantage in power output) but
rather to decrease the weight of the bike by several ounces. Are the same
types of requirements on all high performance parts? NO. Example:
A
crankshaft/ carburetor/ tire would not be purchased, no matter how well it
worked, unless it was 8 ounces lighter than stock. Actually, if this issue
were to be traumatized over, try this: A lower center of gravity (mass)
makes for a more stable bike. If the bikes weight goes down, but not the
rider’s, the center of gravity of the bike and rider as a unit is higher.
The extra hundreds of dollars spent to lower the exhausts weight would be
better spent on an exotic light weight helmet which would 1) lower neck
strain 2) Lower center of gravity of entire bike/rider 3) Protect
rider’s
brain 4) Lighten rider and wallet by the same 8 ounces. What does all
of
this mean? Vicious Cycle’s patent pending exhaust system is QUIETER,
provides long duration and high intensity negative pressure waves for an
INCREASE in powerband width as well as peak power, will lead to proprietary
cam profiles to fully exploit its potential, and is strikingly different and
better looking than any currently produced exhaust systems.
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