The Society of Automotive Engineers
Clean Snowmobile Challenge 2002
(SAE CSC 2002) is an engineering
design competition for college and
university student members of the
Society of Automotive Engineers (SAE).
The intent of the competition is to
provide universities with an
intercollegiate competition that allows
them to re-design stock snowmobiles to
reduce emissions and noise, while
maintaining or improving the
performance of the snowmobile. The
emphasis is on low-cost modifications
that are suitable for implementation in
rental sleds. The modified snowmobiles
are expected to be quiet, emit
significantly less unburned hydrocarbons
and carbon monoxide than conventional
snowmobiles (without significantly
increasing oxides of nitrogen emissions),
and maintain or improve the
performance characteristics of
conventional snowmobiles.
OBJECTIVES
The primary goal is to prepare an
existing 2-stroke sled for the
competition. This sled is fuel-injected
and has the ability to be designed for
good mixture control. This project
attempts to tune the exhaust system on
the sled while accommodating the use of
an air injection pump. One of the
primary problems with a 2-stroke engine
is the use of an air-fuel mixture to
scavenge the cylinder. The resulting
exhaust contains a lot of unburned
hydrocarbons.
AVAILABLE RESOURCES
! A 462.8cc Ski-Doo snowmobile
! A snowmobile Dynojet
dynamometer
! 5 gas emissions analyzer
! An installed fuel-injection
system
! Basic tools
! Machine shop
! Composites fabrication
! “Design and Simulation of 2-
Stroke Engines” simulation
software – by Gordon P. Blair
*NOTE: the software is virtually
useless, as it models a 125ccGP
motorcycle with drastic differences
from the Ski-Doo engine. – “Computer
software can help you with the design,
but don’t expect super design from the
software. The software uses
mathematical formulas that are
approximations of reality, and the
software made for home-computers
uses even more simplified formulas
because good simulation of expansion
chambers requires much more
computer-power than your home-PC
can offer.” (5)
! SRW Team baffle-cone
calculator Excel file to size, cut
and construct the exhaust shapes
correctly
! Daqbook DBK19 Thermocouple
card
! Omega Engineering, Inc.
Precision Fine Wire
Thermocouples (Type K&E)
THE 2-STROKE ENGINE CYCLE
The characteristic feature of the twostroke
engine is its means of operation.
In a two-stroke engine, every stroke
leaving top dead center is an expansion
stroke (i.e., a working stroke). In a four
stroke engine, there is only one working
stroke against three negative strokes
(induction, compression, and discharge).
Two-stroke engines are found in small
devices such as chain saws, dirt bikes,
and snowmobiles because they have 3
important advantages over four-stroke
engines:
• Two-stroke engines don’t have
valves, which simplifies their
construction and lowers their
weight.
• Two-stroke engines fire once every
revolution (four-stroke engines fire
once every other revolution) – this
gives the two-stroke engine a
significant power boost.
• Two-stroke engines can work in
any orientation, which can be
important in something like a
chainsaw. A standard four-stroke
engine may have problems with oil
flow unless it is upright, and
solving this problem can add
complexity to the engine.
These advantages make two-stroke
engines lighter, simpler and less
expensive to manufacture. Two-stroke
engines also have the potential to pack
about twice the power into the same
space because there are twice as many
power strokes per revolution. The
combination of light weight and twice
the power potential gives two-stroke
engines a great power-to-weight ratio
compared to many four-stroke engine
designs.
You don’t normally see two-stroke
engines in cars, however. That’s because
two-stroke engines have a couple of
significant disadvantages that will make
more sense once we look at how it
operates. (1)
You can understand a two-stroke engine
by watching each part of the cycle. Start
with the point where the spark plug fires.
Fuel and air in the cylinder have been
compressed, and when the spark plug
fires the mixture ignites. The resulting
explosion drives the piston downward.
Note that as the piston moves
downward, it is compressing the air/fuel
mixture in the crankcase. As the piston
approaches the bottom of its stroke, the
exhaust port is uncovered. The pressure
in the cylinder drives most of the
exhaust gases out of cylinder, as shown
here:
As the piston finally bottoms out, the
intake port is uncovered. The piston’s
movement has pressurized the mixture in
the crankcase, so it rushes into the
cylinder, displacing the remaining
exhaust gases and filling the cylinder
with a fresh charge of fuel, as shown
here:
Note that in many two-stroke engines
that use a cross-flow design, the piston is
shaped so that the incoming fuel mixture
doesn’t simply flow right over the top of
the piston and out the exhaust port.
Now the momentum in the crankshaft
starts driving the piston back toward the
spark plug for the compression stroke.
As the air/fuel mixture in the piston is
compressed, a vacuum is created in the
crankcase. This vacuum opens the reed
valve and sucks air/fuel/oil in from the
carburetor. *Our engine lacks a reed
valve and is fuel injected, not carbureted.
Once the piston makes it to the end of
the compression stroke, the spark plug
fires again to repeat the cycle. It is
called a two-stoke engine because there
is a compression stroke and then a
combustion stroke. In a four-stroke
engine, there are separate intake,
compression, combustion and exhaust
strokes. (1)
You can see that the piston is really
doing three different things in a twostroke
engine:
• On one side of the piston is the
combustion chamber, where the
piston is compressing the air/fuel
mixture and capturing the energy
released by the ignition of the
fuel.
• On the other side of the piston is
the crankcase, where the piston is
creating a vacuum to suck in
air/fuel from the carburetor (ours
is fuel-injected) through the reed
valve and then pressurizing the
crankcase so that air/fuel is
forced into the combustion
chamber.
• Meanwhile, the sides of the
piston are acting like valves,
covering and uncovering the
intake and exhaust ports drilled
into the side of the cylinder wall.
Because the piston alone is doing so
many different things, two-stroke
engines are simple and lightweight.
You must also mix special two-stroke oil
in with the gasoline. In a four-stroke
engine, the crankcase is completely
separate from the combustion chamber,
so you can fill the crankcase with heavy
oil to lubricate the crankshaft bearings,
located on either end of the piston’s
connecting rod and the cylinder wall. In
a two-stroke engine, on the other hand,
the crankcase is serving as a
pressurization chamber to force air/fuel
into the cylinder, so it can’t hold a thick
oil. Instead, you must mix oil in with the
gas to lubricate the crankshaft,
connecting rod and cylinder walls. If you
do not mix in the oil, the engine won’t
last very long. (1)
DISADVANTAGES OF THE 2-
STROKE
• Two-stroke engines don’t last
nearly as long as four-stroke
engines. The lack of a dedicated
lubrication system means that the
parts of a two-stroke engine wear
out a lot faster.
• Two-stroke oil is expensive, and
you need about 4 ounces of it per
gallon of gas. You would burn
about a gallon of oil every 1,000
miles if you used a two-stroke
engine in a car.
• Two-stroke engines do not use
fuel efficiently, so you would get
fewer miles per gallon.
• Two-stroke engines produce a lot
of pollution -- so much, in fact,
that it is speculated that you
won’t see them around too much
longer.
Two-stroke engine pollution comes from
two primary sources. The first is the
combustion of the oil. The oil makes all
two-stroke engines smoky to some
extent, and a badly worn two-stroke
engine can emit huge clouds of oily
smoke. The second reason is that each
time a new charge of air/fuel is loaded
into the combustion chamber, part of it
leaks out through the exhaust port. That’s
why you see a sheen of oil around any
two-stroke boat motor. The emitted
hydrocarbons from the fresh fuel,
combined with the leaking oil is an
obvious hazard to the environment.
These disadvantages mean that twostroke
engines are used generally in
applications where the motor is not used
very often and a fantastic power-toweight
ratio is important. (1)
THE TUNED PIPE
In the 1950’s, an engineer by the name of
Walter Kaadan was consulted by
motorcycle racers, asking him to help
them squeeze more power and speed out
of their motorcycles. After some
experimentation, he found that the 2-
stroke engines in motorcycles were
affected by its exhaust characteristics.
He found that by varying the length of
straight exhaust pipes, the performance
also changed accordingly. After further
experimentation, he found that a
divergent cone instead of a straight pipe
worked better, and this heralded the
arrival of the 2-stroke tuned pipe.
The basic principle of the tuned pipe is
making use of the moving air masses in
the exhaust to assist in the scavenging
(the process whereby the exhaust gases
are removed and replaced with un-burnt
mixture) of a 2-stroke engine. (2)
The exhaust pipe of a two-stroke engine
attempts to harness the energy of the
pressure waves from combustion. The
diameter and length of the five main
sections of a pipe are critical to
producing the desired power band. The
five sections of the pipe are the head
pipe, diffuser cone (divergent), dwell or
belly, baffle cone (convergent), and the
stinger. In general, after market exhaust
pipes shift the power band up the RPM
scale. Most pipes are designed for
original cylinders not tuned cylinders.
(3)
Changing the exhaust pipes on your twostroke
snowmobile can have a marked
effect on the engine’s power
characteristics. Simply put, it’s because
the two-stroke exhaust system,
commonly referred to as an “expansion
chamber”, uses pressure waves
emanating from the combustion chamber
to effectively supercharge your
cylinder.(6)
Each time the exhaust port of a 2-stroke
cylinder is uncovered, exhaust gases
rush out of the opening and through the
exhaust pipe.
This causes a high pressure wave to
radiate out of the pipe towards the
exhaust opening. However, the
momentum of this moving mass of air
also creates a low pressure wave that
follows behind it. If this is carefully
timed, this low pressure wave can be
used to suck in the fresh fuel/air mixture
from the transfer ports.
The pressure wave has now been
reflected at the end of the chamber, and
perfectly timed pushes all fresh mixture
back into the cylinder just as the piston
closes the exhaust-port.
This process repeats itself at the same
frequency at which the engine is running
and thus, if the exhaust pipe can be made
to resonate at the operating RPM of the
engine, this will improve the engine’s
efficiency. It must be noted that at a low
(lower than the resonant frequency)
RPM range, this low pressure pulse
would return too soon, bouncing back
out the exhaust port. The converse is true
for an RPM range higher than the pipe’s
resonant frequency, whereby the low
pressure pulse returns too late such that
the exhaust port is closed. (2) & (5)
An engine’s exhaust port can be thought
of as a sound generator. Each time the
piston uncovers the exhaust port (which
is cut into the side of the cylinder in twostrokes),
the pulse of exhaust gases
rushing out the port creates a positive
pressure wave which radiates from the
exhaust port. The sound will be the same
frequency as the engine is turning, that
is, an engine turning at 8000 RPM
generates an exhaust sound at 8000 RPM
or 133 cycles a second--hence, an
expansion chamber’s total length is
decided by the RPM the engine will
reach, not displacement. (6)
The speed of these waves is more or less
constant, though it’s affected slightly by
the temperature of the air. Higher
temperatures mean that the air molecules
have more energy and move faster, so
sound waves move faster when the air is
warmer.
A complicating factor is that changes in
the shape of the tube cause reflections,
or changes, in the sound waves: Where
the section of the tube grows in
diameter, there will be sound waves
reflected back towards the start of the
tube. These waves will be the opposite
of the original waves that they reflected
from, so they will also be negative
pressure waves. Therefore, by gradually
increasing the diameter of the tube, a
gradual, more useful negative wave can
be generated to help scavenge, or pull
spent gasses out of, the cylinder. (6)
Putting a divergent cone on the end of a
straight pipe lengthens the returning
wave, broadening the power band and
creates a rudimentary expansion
chamber.
To sum up, when the negative wave
reaches the exhaust port at the correct
time, it will pull some of the exhaust
gases out the cylinder, helping the
engine to scavenge its spent exhaust gas.
And putting a divergent cone at the end
of the straight (parallel) "head" pipe
broadens the returning wave. The
returning negative wave isn’t as strong,
but it is longer, so it is more likely to
find the exhaust port open and be able to
pull out the exhaust gases. As with plain,
straight pipes, the total length of the pipe
with a divergent cone welded on
determines the timing of the return
pulses and therefore the engine speed at
which they are effective. The divergent
cone’s critical dimensions are where it
starts (the distance from the exhaust port
to the start of the divergent cone is called
the "head" pipe), while the length of the
megaphone and the rate at which it
diverges from the straight pipe
determine the intensity and length of the
returning wave--A short pipe which
diverges at a sharp angle from the head
pipe gives a stronger, more straight-pipelike
pulse. Conversely, a long, gradual
divergent cone creates a smaller pulse of
longer duration. In addition, the
negative wave is also strong enough to
help pull fresh mixture up through the
transfer ports. (6)
While adding a divergent cone to the
head pipe produces great tuning
advantages, it has its limitations as well:
The broader negative wave from a
megaphone can still arrive too early and
pull fresh mixture out of the cylinder.
However, putting another cone, reversed
to be convergent, on the end of the first
divergent pipe will reflect positive
waves back up the pipe. These positive
waves will follow the negative waves
back to the exhaust port, and if properly
timed will stuff the fresh mixture that
was pulled into the pipe back into the
exhaust port right as the piston closes the
port. (6)
In addition to head pipe length,
divergent and convergent cone lengths,
an expansion chamber has three more
crucial dimensions. The length of the
straight ’belly’ between the divergent and
the convergent cones, the length of the
tailpiece ’stinger’ and the diameter of the
belly section. The stinger acts as a
pressure bleed, allowing pressure to
escape from the pipe. Back pressure in
the pipe, caused by a smaller-diameter or
longer stinger section, helps the wave
action of the pipe, and can increase the
engine’s performance. This, presumably,
happens since the greater pressure
creates a more dense, uniform medium
for the waves to act on--waves travel
better through dense, consistent
mediums. For instance, you can hear a
train from a long way away by putting
you ear to the steel railroad track, which
is much denser and more uniform than
air. But it also causes the engine to run
hotter, usually a very bad characteristic
in two-strokes. (6)
The length of the belly section
determines the relative timing between
the negative and positive waves. The
timing of the waves is determined by the
length of this characteristically straight
pipe. If the belly section is too short,
positive waves have a shorter distance to
travel, and return to the exhaust port
sooner. This is good if the engine is
running at a higher speed, bad if you
want to ride on the street. The diameter
of the belly section is crucial for one
simple reason: ground clearance. It’s
hard to keep big, fat pipes off the
ground, though V-Fours have solved that
for now since two of the pipes exit
directly out the back. (6)
A complete two-stroke pipe has a
properly tuned header, convergent, belly,
divergent and stinger sections--a difficult
process to mesh successfully.
Modern pipes generally have a gently
divergent head pipe to keep gas velocity
high near the port, a second cone of
"medium" divergence, and a third
divergent cone with a strong taper. A
belly section connects to multi-angled
convergent cones, which should exit in a
straight line into the stinger for good
power. As you can see, modern twostroke
expansion chambers create a
complex scenario and are quite difficult
to tune.(6)
To start with, you need a good design of
the chambers you are about to make.
This is absolutely the most important
thing to get a good result. What a good
design is, obviously depends upon what
kind of power you want, what tune you
want the rest of the engine have, etc.
There are many factors that decide how
the chambers should be designed, for
example if you raise the exhaust port,
you will move the powerband to higher
rpm’s, and by lengthening the chambers
you will move the powerband to lower
rpm’s. A good chamber will give lots of
power over a wide rpm-range. A poorly
designed chamber will have a weak
pressure wave that won’t suck the charge
out of the cylinder, and in turn won’t
push much fresh charge back into the
cylinder, producing less horsepower,
wasting fuel and increasing pollution.(5)
Here are a few examples of possible
chamber designs:
or possibly,
THE TUNED PIPE DESIGN
PROCESS
As previously stated, expansion
chambers are shaped as they are so they
reflect sound waves back at the exhaust
port to hold the burnable charge in the
cylinder. Without the expansion
chamber, a large amount of power
producing fuel and air would escape
from the exhaust port because the
exhaust port must be open when the
fresh fuel/air charge rushes into the
combustion chamber. Four-strokes don’t
need two stroke type expansion
chambers because they have valves that
seal their exhaust ports during the intake
cycle. Though each section of an
expansion chamber has its own areas of
influence on power delivery, it is
important to point that no section of an
expansion chamber works entirely
independent of the others. Any change
in length, shape or volume in any part of
the pipe will bring about changes in the
way the pipe affects performance.
Generally, changes that hurt
performance in one area will boost
performance in another, but it is possible
to make changes that only hurt or help
performance.(4)
EXPANSION CHAMBER
COMPONENTS
HEADPIPE: Tapered head pipes are
relatively more difficult and costly to
manufacture, so they are rare on nonrace
machines. Tapered head pipes have
proven to boost performance and ease
pipe tuning in their main area of
influence – low to mid rpm power –
which has proven to be best for most
racing applications. In general, a
relatively longer head pipe will bring
about more bottom-end power at the
expense of peak power. A short head
pipe generally brings on stronger peak
power and subtracts bottom-end.
CONES: The length, volume and taper
of the first cone strongly influence the
amount of peak power the engine will
produce. A relatively short, steeply
tapered, first cone, creates high peak
power with sacrifices at other engine
speeds. Pipes on Open-class bikes
usually have gradually tapered first
cones because smoothness, rather than
peak power, is of more benefit.
BELLY: A pipe’s midsection is where
length or volume adjustments are made
to compensate for less than "ideal" head
pipe, first cone, final cone and
stinger/silencer dimensions that can’t be
used due to the size and shape of the
bike or snowmobile. The pipe’s
midsection or "belly" can be enlarged,
shortened or lengthened to bring about
the same results as most "ideal" designs.
FINAL CONE: What happens after an
engine’s power peaks is nearly as
important as the peak itself. Controlling
power after the peak, the overrev or
overrun is the final cone’s job. A
relatively longer, gently tapered final
cone will give you more overrev. A
short, steep final cone gives you less.
Why not go for lots of overrev? You
will lose too much top-end. It’s pretty
much all give and take – that’s where the
engineering comes in.
STINGER: The tailpipe, or stinger, is
as important as any part of the pipe. Its
size and length influences peak power
and bottom-end, and can even affect an
engine’s resistance to holing pistons.
In general, smaller stinger diameters
create more peak horsepower but
increase the likelihood of melted pistons
because they bottle up the exhaust
heat. Big stinger diameters boost
bottom-end at the expense of peak
power. Excessively large stinger
diameters can hurt performance at all
engine speeds due to insufficient back
pressure. Stingers length is important,
too, because it’s part of the total pipe
length and volume. Generally, longer
stingers help low and midrange power.
Why not run a long, large diameter
stinger? The pipe has to fit on the
bike.(4)
EXHAUST TUNING THEORY
The two-stroke engine can be considered
“atmospheric” when the cylinder filling
(scavenging) is superimposed to the
exhaust phase, i.e., with the exhaust port
opened to the external atmosphere. The
effectiveness of the cylinder filling with
fresh charge depends on small
differences of pressure, and the twostroke
engine can only tolerate small
amounts of exhaust back pressure.
In SI engines, which use the crankcase
as a scavenging pump, the dynamic
effect plays a fundamental role in filling
the cylinder with fresh charge. As
explained earlier, every time the exhaust
flow meets a section increase, a negative
pressure is generated and propagates in
the opposite direction of flow with the
speed of sound. On the other hand, if
the flow finds a restricted section, a
positive pressure wave always
propagates in the opposite direction with
the speed of sound. (8)
According to acoustic theory, the
propagation of waves depends strictly on
the length and section of ducts, volumes,
and logically on the speed of sound.
Therefore, the exact tuning for the
required boost effect is possible only for
a particular range which is usually at the
BMEP condition (brake mean effective
pressure). A tradeoff between maximum
obtainable BMEP and range width is
possible, and different solutions can be
used, depending on the required engine
output. One parameter is fundamental:
the propagation of sonic speed waves.
The speed of sound depends on several
parameters, however if the medium in
which the sound wave propagates is the
same, the most important term is the
absolute temperature as follows:
a = √kRT
or approximately
a = 20√T (m/sec)
Predicting exhaust gas temperature is
difficult because an exact calculation
must be taken into account which
includes the heat transmission from the
exhaust system to the ambient and the
operating air-fuel ratio of the engine. In
Figure 1 below, the reference
temperature into the exhaust system is
reported versus engine maximum power
RPM, and thus a zone corresponding to
the specific output of the appropriate
engine can be defined to assist with
empirical data.
Figure 1
(8)
SHARP RANGE TUNING
With high-performance engines, such as
snowmobiles, the maximum tuning
effects need to be obtained. Therefore,
the dynamic effect of compression and
expansion waves must be carefully
managed. In this case, calculations can
be performed following a method of
solving the differential equations
representing the physics of the actual
engine. Sometimes, an empirical
technique can be used to quickly check
the geometrical dimensions. Using
Figure 2 below with the following
labels, we start with:
Figure 2
S = Total exhaust angle (CA deg)
T = Total scavenge angle (CA deg)
D1 = Exhaust duct internal diameter (m)
N = tuning (RPM)
A negative reflection is requested after
(S-T)/2 = A (CA deg)
The time for the reflection is
tr = A/6N
The value of the sonic speed can be
estimated, if the temperature inside the
muffler is known, as
c1 = 20√T1 (m/sec)
where T1 is the absolute temperature in
ºK. Thus,
2Lcol = c1 tr
Lcol = c1 tr /2
Divergent Part (angle 8º)
D2 = √(6(D1
2))
Ld = (D2 – D1)/ tg 4º
Convergent Part (angle 15º)
A positive reflection (compression
wave) is needed afterward
A + T = B (CA deg)
With similar considerations, using a new
value for c2 if the temperature is
changed (C2 = 20√(T2):
tc = B / 6N
Ltot = c2tc / 2
Lc = (D2 – Du) / 2 tg 7.5º
Lct = D2 / 2 tg 7.5º
Ltot = Lcol + Ld + Lcil +Lct / 2 + s
Thus the length of the cylindrical part
can be derived as:
Lcil = Ltot – s – Lcol – Ld – Lct / 2
The tail has an approximate length of
Ltail = 12Du
*************************(8)
ENGINE MODELING AND
SIMULATION TECHNIQUES
One-Dimensional Methods
The purpose of 1-D methods is to
calculate the response of the different
parts of the engine- namely, the inlet,
crankcase, scavenge and exhaust ducts,
and muffler – to determine, by using
calculations on the entire flow through
the engine, the predicted performance
and the final value of different
parameters such as charging efficiency,
short-circuit ratio, and noise pressure
levels. Usually this method consists of
splitting the engine into several smaller
parts that then can be schematized with
ducts of different sections and lengths
and connecting volumes.
The basic equations can include heat
transfer and friction in the ducts, but
obviously can’t take into account the
turbulence effect in any part of the
engine. These methods have the
advantage of a rapid response on overall
engine performance, making it easy to
compare the influence on engine output
of different geometrical configurations.
The general drawback lies in the fact
that accurate external calibrations,
derived from experimental tests, are
necessary, and then it’s important to
have a solid knowledge of the physical
behavior of every engine family. The
valid use is only to obtain comparative
results for a well-defined class of
engines.
Many methods have been developed for
solving differential equations. These
range from the “characteristic method”,
which could also be solved graphically,
to modern numerical methods made
possible only by the advent of
computers. One of the most effective
known methods, particularly dedicated
to the two-stroke engine, is the “Two-
Stroke Simulation Program” developed
by Queen’s University of Belfast (QUB).
This method includes friction, heat
transfer, and multiple ducts, as well as
catalyst insertion and multi-cylinder
capabilities.
The 3 conservation equations are:
continuity, momentum (without
friction), and constant entropy.
The continuity equation is
∂(ρu) / ∂x = −∂ρ / ∂t
The momentum equation is
− ∂ρ / ∂x = ∂Du / Dt
Where Du/Dt is the substantive
derivative of velocity with respect to
time.
Du / Dt = ∂u / ∂t + u∂u / ∂x
The expansion of the continuity and
momentum equations, respectively,
yields
p x u t u u x
and
p t u u x
∂ ∂ + ∂ ∂ + ∂ ∂
∂ ∂ ∂ + + ∂ ∂ =
1/ ( / ) / /
1/ ( / ) / / 0
ρ
ρ
The sonic speed at the pressure p and
density ρ and entropy is given by
a2 = (∂p / ∂ρ )s
for an ideal gas
a2 = (kp /ρ )
The 3rd conservation condition
(isentropic flow) is
/ = ( / )2k /(k+1)
ref A p p a a
when pref and aA are reference values.
After substitution, the continuity
equation can be expressed in terms of a
and u as
2 /(k −1)a∂a / ∂u + ∂u / ∂t + u∂u / ∂x = 0
and the momentum equation can be
expressed as
2 /(k −1)a∂a / ∂u + ∂u / ∂t + u∂u / ∂x = 0
After some lengthy manipulation of
these equations, the following equation
can be obtained:
da/dt + (k-1) / 2du / dt = 0 (*)
on a line on the x-t field (where x is the
current abscissa) whose slope is
dx / dt = u + a
and
da /dt – (k-1) = 2du /dt (#)
on a line whose slope is dx /dt = u -a
To generalize the results, it’s useful to
write the equations in non-dimensional
form, introducing
A = a/aA; U = u/aA; X = x/L; and
Z = aAt/L
As Riemann variables.
Consider the line of slope dX/dZ = U+A
shown in the Figure 3.
Figure 3
This is known as a “position
characteristic” and is seen to be in the
direction of a disturbance propagating
toward the right through the gas in the
pipe.
Integrating EQN (*)
A + (k-1) / 2U = constant = λ
The constant λ exists at any point along
the position characteristic. A and U can
vary only along a position characteristic
within the restraint imposed by the
relation in this equation.
Therefore, if the properties of the initial
disturbances are known, a family of λ
lines can be drawn, shown in Figure 4.
Figure 4
Disturbances moving toward the left will
propagate along a line of slope dX/dZ =
U –A. Similarly, EQN(#) above gives:
A – (k-1) / 2U = constant = β
The value of β is derived from the initial
values of A and U. These lines are also
shown in Figure 3.
The terms β and λ are known as
Riemann variables. The particle
velocities and fluid properties can be
found at any point of the X-Z plane:
(1+b)/2 = A = a/aA
(1-b)/2 = U = u/aA
Because
a/aA=(p/pref)(k-1)/2k
That is,
((1+b)/2)2k/(k-1) = p/pref
((1+b)/2)2 = T/Tref
((1+b)/2)2/(k-1) = r/rref
The results for the last 6 equations can
be used to produce a numerical solution
to problems of unsteady flow.
Obviously, the real problem is more
complicated, i.e., the duct may have a
variable or a sonic shock wave may be
generated.
Two-Dimensional Methods
Theoretical three-zone model for loopscavenge
engines:
The mathematical model consists of five
conservation laws in somewhat 3-D
form:
( ( / )) / 0
( ( / )) /
( ) / ( ) / ( ) /
∂ Γ ∂ ∂ ∂ + =
− ∂ Γ ∂ ∂ ∂ −
∂ ∂ + ∂ ∂ + ∂ ∂
ψ ψ
ψ
ψ
ψ
ρψ ρψ ρψ
x y S
x x
t u x u y
Any of the five equations can be derived,
where the symbols have the following
significance:
x and y designate the Cartesian
coordinates of a Eulerian frame
Ψ is the general transport property
ΓΨ is the transport coefficient
SΨ is the source term
u and v are the velocity components in
the x and y directions
t designates the time
ρ is the density
μ is the viscosity
k designates the thermal conductivity
cp is the heat capacity
h is the specific stagnation enthalpy
p is the pressure
Dab is the mass diffusion coefficient
X designates the mass fraction of the
fresh charge
The differential equations must be
modified to allow the domain of the
solution to be always within the
boundary of the volume occupied by its
gas with a coordinated transformation.
The cylinder domain is divided into
three zones shown in Figure 5.
Figure 5
For Zone 3, the mesh is confined
through DD’ and CC’. During the
period in which the ports are closed, the
zone collapses to a line. Zone 2 is
bonded by CC’ (inlet port closure line)
and BB” (exhaust port closure line).
This zone also collapses to a line when
both inlet and exhaust ports are closed.
Zone 1 is bonded by BB’and the
cylinder head AA’. This zone contracts
and expands only in the compression and
expansion periods, respectively.
The transformation replaces the
coordinate system (x,y,t) with (x, yt, t)
when the non-dimensional coordinate yt
varies in each zone between 0 and 1.
The final equations are solved with the
finite difference method through a grid.
The results provide a means of
evaluating not only the overall
scavenging efficiency, but also of
mapping the scavenging process versus
crank angle. The results for a loopscavenged
engine appear in good
agreement with the experimental data,
shown in Figure 6.
Figure 6
Figure 7 and Table 1 below show the
comparison of calculated vs.
experimental data for scavenging
profiles.
Figure 7
Table 1
Three-Dimensional Methods
KIVA is the most recent of a long series
of fluid-dynamics codes devoted to the
study of engine applications and
developed at Los Alamos National
Laboratory since the early 1970’s.
KIVA solves the transient 2-D and 3-D
chemically reactive fluid flows with
sprays. It is applicable to laminar or
turbulent flows, subsonic or supersonic,
and single-phase or two-phase flows.
The gas phase solution procedure is
based on a finite volume method called
the Arbitrary Lagrangian-Eulerian
(ALE) method. The grid is a single
block structure. KIVA can simulate
two-stroke engines, allowing connecting
and disconnecting of port volumes with
in-cylinder volume. This is performed
automatically, also controlling the timestep
when opening of the ports results in
a sudden (sonic) acceleration of the
flow. To move the piston from its top
dead center position to bottom dead
center position and back again, a special
piston motion logic has been devised.
The logic is called “snapper.” As shown
in Figure 8, the lowest plane of active
cylinder coincides with the piston crown
and with the piston velocity.
Figure 8
All other vertices remain stationary.
Periodically, however, a new plane of
piston crown vertices replaces the old
one and assumes its role of following the
piston motion. The old plane of piston
vertices is “snapped” back to its original
position, which is now below the piston
crown, and a simple remap onto the new
mesh (redefining cells, vertex, and flags)
is performed. When the piston is
moving downward, the procedure is
reversed. The experimental results
derived from a loop-scavenged twostroke
engine show very good agreement
with predicted velocity field and a
coherent evaluation of two-stroke
characteristic parameters.
(8)
EMISSIONS
BASIC PRINCIPLES
The main source of the generation of
unburned hydrocarbon emissions in SI
(spark ignition) engines is caused by the
short-circuit of the fresh mixture through
the exhaust port during the scavenging
phase. The meaning of the short-circuit
is that part of the incoming fresh mixture
after the inlet in the cylinder is shortcircuited
directly through the exhaust
port. Although the emissions of carbon
monoxide (CO) and nitrogen oxides
(NOx) depend mainly on the combustion
characteristics, unburned hydrocarbon
(HC) emissions in SI engines are caused
by this short-circuit of fresh mixture
through the exhaust port.(8)
BASIC SOLUTION
One of the methods used in production
to reduce hydrocarbon and CO
emissions is air injection into the exhaust
system. Oxides of nitrogen will not
necessarily be reduced; in fact they may
be increased if sufficiently high exhaust
temperature results from the combustion
of the CO and hydrocarbons with the
added air or if the injected air enters the
cylinder during the overlap period,
thereby leaning the mixture in the
cylinder. To achieve a high degree of
exhaust system oxidation of HC and CO,
a high exhaust temperature coupled with
sufficient O2 and residence time to
complete the combustion is needed. If a
flame is established, the heat generated
by the combustion of the CO and
hydrocarbons keeps the reaction
going.(7)
Because of its abundance, the carbon
monoxide in the exhaust provides most
of the combustion-generated heat. The
basic factors governing the combustion
of CO and HC in the exhaust system are:
composition of the reacting mixture,
temperature and pressure of the mixture,
and residence time of the mixture or
time available for reaction.(7)
General laboratory testing has shown
that the minimum HC concentrations
occurred at rich mixtures. When too
much air was injected, especially at lean
mixtures, excessive cooling of the
exhaust increased HC concentrations to
levels above those with no air. Thus, the
normal oxidation process was apparently
inhibited by this cooling. The effect of
air injection on CO concentrations was
somewhat different. Exhaust CO was
uniformly low at most rich-air-fuel
ratios. A small increase in CO occurred
slightly richer than stoichiometric. At
stoichiometric and leaner, CO was very
low. The leanest air-fuel ratio for best
emission reduction was 13.5:1.
Normally, engine operation at such a
rich mixture would reduce fuel economy
by 10%. The very low emissions with
rich mixtures and air injection arose
from a “fire” in the exhaust system. For
mixtures leaner than the small CO peak
in Figure 9, non-luminous oxidation
occurred and CO emission reduction was
relatively poor.
Figure 9 (7)
OPTIMIZATION OF AIR INJECTION
RATE
At each air-fuel ratio there exists one
minimum air injection rate that provides
maximum emission reduction. Minimum
air flow is desired in order to reduce
pump power requirement, size and cost.
Air injection is not highly effective in
reducing CO emission unless a luminous
burning flame occurs.(7)
EXHAUST TEMPERATURE AND
PRESSURE
Exhaust system insulation is necessary
to achieve high reaction rates in engines
with well-cooled exhaust ports.
Insulation also helps to reduce emissions
during warm-up by accelerating the
warm-up rate.
At mixtures significantly leaner than
stoichiometric (in the range of 16 to
17.5:1) air injection is not needed to
supply O2; in fact it would only cool the
exhaust to too low a temperature for any
reaction to occur. On the other hand, at
such lean mixtures only extremely good
heat conservation can produce
temperatures high enough for
appreciable reaction.
For lean mixtures, the following
equation for the concentration of
hydrocarbons leaving the exhaust system
was derived:
where
K T W
C C KrO P V
i
−
= 2
3
2
2
0 exp
Co = conc. of HC leaving exhaust
system
Ci = conc. of HC leaving
cylinders and entering exhaust system
Kr = specific reaction rate,
ft3/lbm*mole/sec
K3 = constant
O2 = oxygen concentration in
exhaust gases, volume %
P = exhaust pressure (psia)
V = exhaust system volume
available for reaction, ft3
T = absolute temperature, °R
W = mass flow rate of air, lb/sec.
Note the importance of the pressure
term. Increasing exhaust back pressure
promotes after-reaction. However,
commercially, the possible back pressure
increase is small. General laboratory
results show that a decrease in exhaust
temperatures from 1100°F to 1000°F
decreases the reaction rate by a factor of
10 – Something to think about.(7)
EXHAUST SYSTEM VOLUMERESIDENCE
TIME EFFECTS
Reactor volume may be viewed as the
volume of the exhaust system which is
insulated and at the high temperature
needed for reaction. General laboratory
testing shows that if the exhaust
temperature were at 1400°F (Ski-Doo
specs show 1330°F), only twice the
convention system volume is required
for virtually complete elimination of the
hydrocarbons. On the other hand, if the
temperature were only 1200°F, eight
times the volume would achieve only a
76% reduction.
Increasing the exhaust system volume
increases the residence time during
which reactions can occur. This will be
a benefit only if the added surface area
does not result in excessive cooling.
Thus, when large volume exhaust
manifolds are used, they must be well
insulated. In conclusion, increasing the
residence time of the exhaust by
increasing volume improves both the CO
and HC oxidation effectiveness of air
injection and reduces the injected air
flow requirement provided that good
heat conservation is maintained. Note:
this applies to both rich and lean fuel
mixtures. The most effective reactors are
those that run rich with air injection.
Rich engine operation produces low NO
and high CO and H2 emissions. Vehicle
experience shows that combustion of the
CO and H2 with the injection air
generates temperatures high enough to
oxidize virtually all the unburned
hydrocarbons.(7)
Table 2 below shows the stock exhaust
emission values taken with the engine
idling at 129°F.
HC CO CO2 O2 NOx
5690ppm 1.81% 3.7% 0 14.1%
Table 2
OUR DESIGN PROCESS
When Jay, Brian and I initially started
the design process, we planned on
quickly “throwing” together a pipe and
then being able to focus the majority of
our attention on decreasing harmful
emissions. NOT EVEN CLOSE. First
and foremost, we realized the need to
have more than just a general idea of
two-stroke engine operation in order to
understand exactly where these so-called
emissions were coming from. During
the process of engine research, we
quickly found just how much of an
effect the exhaust system has on optimal
operation of the two-stroke engine: a
huge one. This realization immediately
altered our design project to something
of much larger scale than we had
originally anticipated. Now we had to
first successfully design a high
performance pipe for this two-stroke
snowmobile (obviously no easy task in
itself) with emissions still in mind of
course, and then deal with the emissions
issues. So…
We decided to look for suitable twostroke
engine simulation software where
we would be able to take engine and
exhaust characteristic values of the Ski-
Doo sled and plug them into the
program, generating values we had
already achieved on the Dyno (values of
horsepower, rpm etc). This would give
us a baseline where we would simply be
able to change the dimensions of the
exhaust (within the simulation program)
and see how it affected our engine
performance, therefore allowing us to
optimize the design by changing values
accordingly. Didn’t happen. Below is a
table of the current engine values.
Feature Dimensions/Ratios
Bore 69.5mm
Stroke 61mm
Total Displacement 462.8cc
Con rod length 130mm
Crankcase
Compression ratio
1.55
Squish Clearance .58mm
Wrist pin-to-crown 25
Wrist pin-to-skirt 25
Wrist pin offset 0
Fly wheel diameter 243mm
No. of exhaust ports 1
Top corner radius 10mm
Bottom corner radius 10
No. of transfer ports 5
Ave max. width 19mm
Top corner radius 6.8mm
Bottom corner radius 6.8mm
Fuel type Gasoline
Air : Fuel ratio 13.5
Table 3 – Note: Values are metric for
compatibility w/ software
We found a simulation program by
Gordon Blair (see Proff. Mick Peterson
for instruction manual) and it was
disastrous. It is not user friendly,
requires much more engine characteristic
values than other simulation software on
the market, is written to simulate a single
cylinder GP motorcycle, and
incorporates the use of a reed valve
(which the sled does not have). Most of
these details were not apparent by
reading the available literature. We
worked around the “single cylinder”
issue by doubling the rpm’s, but when
trying to deal with the reed valve, or lack
thereof, we hit a wall. You can’t just set
the reed valve values to zero or it throws
the entire program off. Not only was it
useless, more importantly, we wasted
valuable time.
At this point we’d already flushed $500
of the University’s money down the
toilet on inapplicable software, so new
software was out of the question. But if
you want make an omlet, you gotta
break a few eggs, right? The only thing
left to do was establish the stock exhaust
temperature gradient using
thermocouples and Daqbook, then
design the new exhaust empirically
(hand calcs and 1,2,3,-Dimensional
analysis as shown above and in lab
notebook), fine-tuning with a trial-anderror
process. We already had stock
horsepower, torque vs. rpm curves
(Appendix A), as well as temperature
profiles (Appendix C) so we could
calculate lengths and diameters from
earlier equations and estimate sizes
based upon where we wanted to see
improvements made.
GETTING STARTED
Because a long, gradual divergent cone
creates a smaller pulse of longer
duration, we manipulated the
calculations to allow for a divergent
cone that was longer and more gradual
than the existing one. This alteration
also strengthens the negative wave
enough to help pull fresh mixture
through the engine transfer ports. This is
how we were able to reach maximum
horsepower shortly after the track was
engaged.
As stated above, if the belly section is
too short, positive waves have a shorter
distance to travel, and return to the
exhaust port sooner. This is good if the
engine is running at a higher speed, bad
if you want to ride on the street. Because
a snowmobile operates at such high
rpm’s, this is the type of design we went
after. Our first test pipe had a belly
length approximately 3” longer than the
stock belly. We started longer because a
pipe’s midsection is where length or
volume adjustments are made to
compensate for less than "ideal" head
pipe, first cone, final cone and
stinger/silencer dimensions that can’t be
used due to the size and shape of the
sled. The pipe’s midsection or "belly"
can be enlarged, shortened or lengthened
to bring about the same results as most
"ideal" designs. It’s much easier to
change a length, cut and weld, than it is
to change multiple lengths and
diameters.
Our convergent (final) cone was made
shorter and steeper than the stock
exhaust to give less overrev, in turn
preserving top-end. Top-end is crucial
to successful snowmobile operation.
A stinger similar to stock diameter was
used to sustain sufficient back pressure
to maintain the wave action of the pipe,
and hopefully maintain the engine’s
performance. Because smaller stinger
diameters deliver more peak
horsepower, but also increase the
likelihood of melted pistons (because of
bottling up the exhaust heat), we opted
to use an approximate stock diameter
and adjust the belly length to increase
performance. A larger stinger diameter
was out of the question because it may
boost bottom end, but only at the
expense of peak power. Stinger length is
also important to help low and midrange
power. This is why we made ours
longer than stock…it worked. Below in
Table 4 and Figure 10 you can see the
dimensions of our first prototype.
L1
L2
D1 D2
D2
7.13°
Head Pipe
Head Pipe Convergent Cone
and stinger
Divergent Cone
& Belly
Divergent Cone &Belly
L3 L4
D2
D3
8°
Figure(s) 10
D1 2.78”
D2 2.39”
D3 5.81”
Du 1.25”
L1 1.575”
L2 6.89”
L3 12.17”
L4 13.54”
L5 8.51”
L6 15.00”
Table 4
MATERIAL SELECTION
We employed carbon steel sheet metal,
.0312” thick. The primary reason(s) for
the selection of this material are it’s
commonplace in the motorcycle
industry, it’s ability to be easily welded
and it is lighter than the stock exhaust.
Stainless steel was found to be more
brittle and therefore more susceptible to
cracking over repeated seasons of use.
METHOD TO THE MADNESS
The SRW Team in Italy was kind
enough to email us their Excel file
calculating arc and cone lengths needed
in order to roll the exhaust up correctly.
We then drew out the flat shapes on
carbon steel sheetmetal and hired
Downeast Sheetmetal in Brewer to roll
and weld. It is crucial that the exhaust is
as close to being perfectly round as
possible for correct flow and wave
propagation. Figure 11 below shows the
process of drawing the correct arcs and
cone lengths on the sheetmetal.
Figure 11
Hiring Downeast Sheetmetal for
welding/rolling future exhaust projects is
highly recommended. They are not only
familiar with the exhaust and do
excellent work, but had an average
turnover time of less than 24hrs. Figure
12 shows the 1st of six Prototype exhaust
pipes built before Note the bend in the
head pipe. Because it is at such a sharp
angle, a substantial amount of power
was lost. The local Mieneke dealer was
able to give us another head pipe, at
nearly exact dimensions, with a gentler
curve.
Convergent Cone & Stinger
L5 L6
D3
Du Du
15°
Figure 12
Shown here in Figure 13 is the Daqbook
DBK19 Thermocouple card with type K
& E thermocouples connected to it.
(Instructions for connection and
operation can be seen in Appendix B)
Figure 13
Figures 14 & 15 show the exhaust-end
connections for the thermocouples, held
in with screws, for the Dynojet testing
procedure. This is the apparatus used to
record both stock and new exhaust
temperature profiles.
Stock Thermocouple Connections
Figure 14
Prototype Thermocouple Connections
Figure 15
When testing either of the exhausts,
stock or new, it is important to note that
a separate radiator should be hooked to
the snowmobile cooling system. This
enables sufficient engine cooling while
allowing the engine and exhaust gases to
heat to normal operating levels. We
used a large radiator in a bucket of water
or snow during Dynojet testing. Figures
16 and 17 show the set-up.
Figure 16
You can see that the external radiator is
simply piped into the snowmobile’s
cooling system.
Figure 17
After dynojet testing 6 pipes with belly
lengths of 13.54”,11.54”, 9.54”, 8.54”, 7
1/8”,6 1/8”, we found that a 7 1/8” belly
provided optimal results. When
comparing this particular prototype (5th
pipe) with the stock exhaust (Appendix
A) we not only exceeded the maximum
torque provided by the stock pipe, but
also produced virtually identical peak
horsepower. More importantly though,
we sustained 10 horsepower more (at
the track) from ~27mph to 52mph AND
hit peak horsepower almost 1000rpm
sooner, boosting low-end as well.
Note: All Dyno numbers provided in
the Appendix are taken at the track. General
experience shows an approximate loss of ½ the
horsepower provided by the engine in the track.
Appendix A shows all significant test
pipe horsepower/torque curves.
Table 5 shows the pipe dimensions for
optimal power and torque output
(Prototype 5).
D1 2.78”
D2 2.39”
D3 5.81”
Du 1.25”
L1 1.575”
L2 5.67”
L3 12.17”
L4 7 1/8”
L5 8.51”
L6 15.00”
Table 5
ANOTHER PITFALL
Once we decided upon which prototype
to use, we returned to Downeast
Sheetmetal and asked them to “bend it
up” so we could put it on the sled.
Using a basic Cartesian (X-Y-Z)
coordinate system, we provided them
with guidelines, dimensions and
locations for fabrication. Unfortunately,
they soon told us that it was impossible
to construct a divergent cone that would
swing through the angle necessary to fit
under the cowling – for them anyway.
We knew there had to be a way to make
the needed cone bend correctly,
maintaining centerline length and end
diameters. We referred back to the SRW
race file and incorporated basic
trigonometry with occasional estimation.
We found the best way to do this was by
constructing a cardboard model to be
used as a welding template, shown in
Figures 18 and 19.
Figure 18
Figure 19
Once the cardboard model was
complete, we made sure it would fit in
the area under the cowling (Figure 20)
and brought it back to Downeast
Sheetmetal to be welded.
Figure 20
The result is shown in Figures 21, 22
and 23.
Figure 21
Figure 22
Figure 23
Now we had to find a way to bend the
stinger because there is no company in
the greater Bangor area that is able to do
it. It’s difficult to find a material that
will bend through such a tight angle
without splitting or crimping, and local
shops had no equipment for the job
either.
FABRICATION PROBLEMS
Fortunately, our exhaust pipe design
proved better than the Ski-doo pipe, but
due to a lack of resources and materials,
there was no way to fabricate it
correctly.
In order to arrive at a stinger that would
bend through such a sharp angle, two
common, plumbing sink pipes were
welded together (courtesy of Home
Depot). Because of area and dimension
restrictions the stinger was not welded in
a direct line with the convergent cone.
As stated above, in order to maintain
good power, the convergent cone should
exit in a straight line into the stinger.
In order for the exhaust pipe to reach the
muffler location, the head pipe and the
belly were also lengthened by 2” and 1”
respectively.
Appendix A shows the drastic effects
of these design changes upon
performance.
Note: For future groups picking up where we
left off, be sure and use the same convergent and
divergent cones and shorten the belly by one
inch. The optimal head pipe and stinger
dimensions are given in Table 5, so you only
have to find away to make them bend correctly.
PROPOSED IDEA FOR EMISSIONS
REDUCTION
In theory, if you add the right amount of
air to the exhaust gases AND keep this
mixture above a certain temperature
(through good insulation), it will
combust inside the exhaust pipe, burning
off the majority of the harmful
emissions. In theory.
As said above, if too much air is added
you will actually cool the exhaust, in
turn increasing HC emissions.
So if we were going to even come close
to making this happen, we wouldn’t be
able to just dump cold air anywhere in
the exhaust flow. We decided to run
3/8” copper tubing (good heat transfer
coefficient) down one side of the exhaust
and back up the other (under the
insulating wrap), to then inject (pointing
downstream) as close to the headers as
possible. This would allow for the air to
pre-heat before it was mixed with the
hottest exhaust gases possible.
AIR PUMP
Finding a suitable air pump is actually a
fairly difficult task; the general market is
quite limited. In order to withstand the
pressure waves created by the exhaust
during operation, a linear piston pump
(positive displacement) is necessary.
The difficulty comes in finding a
variable displacement pump as the sled
will obviously not be running at constant
rpm and therefore, a constant
displacement pump would only suffice
for a very specific rpm value. The only
pump we were able to find that could
withstand exhaust pressures was rated
for 18psi, 2.5ft3 and 12V. It is not
variable displacement and so you can
only hope for a small window for
combustion (if at all). This air pump
accounts for a 2-4% addition to
volumetric flow. Either way, even if it
doesn’t burn, your emission numbers
will still decrease because of dilution.
By the way, this is cheating.
However, when the air pump was turned
on during the dyno runs, the temperature
at each point rose 100°F (see Appendix
C ) immediately. This proves that the air
actually ignited, helping to decrease our
harmful emissions.
EMISSIONS RESULTS
All emissions were recorded at an
engine temperature of 129°F. Table 6
shows the emissions recorded with our
“fitted” pipe with insulation and the air
pump.
HC CO CO2 O2 NOx
1760ppm .65% 1.5% 0 18%
Table 6
HC reduction – 70%
CO reduction – 64%
CO2 reduction – 60%
NOx increase – 27%
MUFFLER
The after-market muffler we chose is
from Precision Performance Products. It
is a "Superlite" performance muffler.
After seemingly endless searches on the
internet and dead-ends at dealers, this
was the only muffler we could find that
would fit the sled. You CANNOT use
an after-market muffler that was not
designed for this sled’s displacement and
expect it to work. Even though the air
pump changes the displacement effect,
the change is negligible to the muffler.
INSULATION
When selecting an insulation material, it
is very important to remember that
immediately after the combusted gases
exit the cylinder, they start to cool down.
As a result of this cooling, they will lose
velocity, in turn reducing the scavenging
effect of the cylinders. If you can
somehow keep the exhaust pipe gases at
a high temperature (close to combustion
temperature), you will maintain a higher
velocity, experience a higher pressure
drop in the system and get a better
efficiency.
After exploring many different options,
Cool IT, Thermo Tec exhaust insulating
wrap appeared to be the best choice for
insulation.
Thermo Tec Claims:
“The exhaust insulating wrap is an
innovative way to create more horsepower and
reduce underhood temperatures (by as much as
70%). Wrapping headers maintains hotter
exhaust gases that exit the system faster through
decreased density. Increased exhaust
scavenging is produced, along with lower intake
temperatures. It withstands continuous heat up
to 2000F, and contains no asbestos. Thermo Tec
exhaust wrap will not over insulate the system
when properly installed due to a proprietary
coating that conducts heat across the wrap’s
surface. This coating controls heat build-up and
dissipation. It is sold with a low profile 1/16”
thickness”
CONCLUSIONS &
RECOMMENDATIONS FOR
FUTURE WORK
As we are sure you can deduce from the
given information, two separate exhaust
pipes were built: one for optimal power
and another to actually fit on the
snowmobile. Because of a lack of local
resources, we were unable to fit the best
pipe to the sled. Hopefully, future
groups will find a way around this
problem.
The air pump increased volumetric flow
by 4% on the optimal pipe design and
approximately 2.5% to the “fitted” pipe,
so the air should ignite using the optimal
pipe dimensions as well. Use of an air
pump that gets you closer to a
stoichiometric reaction may also allow
you to further reduce hydrocarbons.
Beware of NOx though!!!
As said earlier, nitrogen oxides may be
increased if you boost the exhaust
temperature from the combustion of CO
and HC or if the added air enters the
cylinder during the overlap period when
both the intake and exhaust are open, in
turn leaning the mixture.
The belly (of the pipe currently on the
sled) should be shortened by exactly 1”
while the convergent and divergent
cones should not be change at all. Refer
to Table 5 for all other dimensions.
Temperature profiles and data were
taken for both calculation purposes, as
well as finding whether or not the
injected air was ignited.
Appendix A
DynoJet Performance Results:
Horsepower & Torque Graphs for Stock
Pipe & Exhaust Prototypes
Stock exhaust compared with 1st and 2nd Prototype
3rd Prototype compared to Stock
4th Prototype compared with Stock pipe (RPMs)
4th Prototype compared with Stock pipe (MPH)
5th Prototype compared with Stock pipe (RPMs)
5th Prototype compared with Stock pipe (MPH)
6th Prototype compared with Stock pipe (MPH)
6th Prototype compared with Stock pipe (RPMs)
Final Pipe With muffler
Final Pipe with muffler & insulation
Final pipe with muffler, insulation and air pump
Comparison of power for the stock pipe, ideal prototype and final pipe
Comparison of Torque for the stock pipe, ideal prototype and final pipe
Appendix B
Data Acquisition using Daqbook 100s
with DBK-19 thermocouple cards and
specifications
DBK Thermocouple Card Specifications
Appendix C
Temperature Profiles for Snowmobile
Exhaust
Figure 1 shows the temperature profile for the stock snowmobile exhaust. Point 1 is
located at the header. Point 9 is located at the muffler. Point 7 is missing because the
thermocouple was damaged.
Figure 1
Temperature Profile for Stock Exhaust Pipe
0.00E+00
5.00E+01
1.00E+02
1.50E+02
2.00E+02
2.50E+02
3.00E+02
3.50E+02
4.00E+02
4.50E+02
5.00E+02
0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00 5.00E+00 6.00E+00 7.00E+00
Time (seconds)
Temperature (F)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 6
Point 8
Point 9
1
2
3
4 5 68
9
Thermocouple
Locations
Figure 2 shows the temperature profile for the first prototype exhaust with a sharp 60
degree angle at the head pipe.
Figure 2
Temperature Profile for First Prototype
0.00E+00
5.00E+01
1.00E+02
1.50E+02
2.00E+02
2.50E+02
3.00E+02
3.50E+02
4.00E+02
4.50E+02
5.00E+02
0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00 5.00E+00 6.00E+00 7.00E+00
Time (seconds)
Temperature (F)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 6
Point 8
Point 9
Figure 3 shows the temperature profile for the second prototype exhaust with a rounded
60 degree angle at the head pipe.
Figure 3
Temperature Profile for Second Prototype
0.00E+00
1.00E+02
2.00E+02
3.00E+02
4.00E+02
5.00E+02
6.00E+02
0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00 5.00E+00 6.00E+00 7.00E+00
Time (seconds)
Temperature (F)
Point 1
Point 2
Point 3
Point 4
Point 5
Point 6
Point 8
Point 9
Table 1 shows the temperatures of the final fitted pipe with and without the air pump
operating. Temperatures were only taken at the two most important points, the head pipe
and the muffler. The dramatic increase in temperature with the air pump operating shows
the effects of burning more hydrocarbons before they are exhausted to the atmosphere.
Table 1
Without Air Pump With Air Pump
Head Pipe 499 F 601 F
Muffler 277 F 389 F
Bibliography
(1) How 2-Stroke Engines Work - Marshall Bain
(2) The Tuned Exhaust System - Adrian Teo, 1996
(3) Basic 2-Stroke tuning – Eric Gorr
(4) Exhaust Pipe Theory – Stephen Jordan
(5) Building Your Own Expansion Chambers – Skalman
Racing
(6) How 2-Stroke Engines Work, & Why You should Care –
Eric Murray
(7) Emissions From Combustion Engines & Their Control-
Patterson & Henein
(8) Emissions From 2-Stroke Engines – Marco Nuti, 1998
No comments:
Post a Comment