crx304

This will be an ongoing thread of Information that you can use to your liking
PLEASE DO NOT RESPOND TO THIS THREAD!!!!



thanks Jim:

The engine is like an air pump; the more air that is
allowed to flow through it, the more horsepower that you get out of it. In other words,
if you have a free-flowing air intake and exhaust system in your average vehicle,
you’ll get more horsepower because of the efficient flow of air into and out of the
engine. Fuel requires air to burn and thus to produce energy. The more air that is
available for combustion will also improve efficiency otherwise known as gas
mileage.

Relation Between Performance and Air Flow

Components that
influence airflow into the engine are the:

air filter
intake air piping
mass
air sensor (if applicable)
throttle body or carburetor
intake
manifold
camshaft
intake port and valve of cylinder heads
turbo's
compression, section, and supercharger (if applicable)

Components that
influence airflow out of the engine are the:

exhaust valve and exhaust ports of
the cylinder heads
camshafts
exhaust manifolds
turbo's turbine (if
applicable)
exhaust tubing
catalytic converters
muffler

When these
components are modified to increase flow out of the engine, pumping losses are
reduced. Pumping losses refer to the amount of horsepower (HP) used to push the
exhaust gases out of the cylinders on the engine's exhaust stroke. Since less HP is
used to get the exhaust out of the engine, more horsepower is available at the
flywheel. An added benefit of reducing pumping losses is that fuel mileage will also
increase.

No matter how much additional air is forced into the engine, no
additional HP will be made unless additional fuel is also added. The energy that makes
HP in an engine comes from the combustion of the fuel, not only the air. In general,
every two HP produced requires one pound of fuel per hour. When modifications are
performed that increase airflow into the engine, more air is available for the
combustion of fuel. The combustion of the additional fuel is what translates into
additional HP.

Air flow is not just influenced by the size (area) of the paths it
takes into and out of the engine. It is also influenced by the speed at which it
moves.

Specific Port Flow (cubic meter/sec) = Flow Velocity (m/s) x Average Path
Area (m2)

Whenever an engine modification increases the average area of the
airflow paths into and out of an engine, there is a chance the velocity of the flow will
decrease. Most of the time the factor of velocity decrease is very small compared to
the area increased, so flow is generally increased. If modifications are taken too
far, the velocity will decrease more than the area increases, so flow is adversely
affected (example - four inch exhaust system on a 1.6 liter engine).

In the
following section, we will analyze the components of an exhaust system in a car and how
air flows from the engine to the outside environment.



Analysis of the
Exhaust System in an Average Car

Exhaust system components are designed for a
specific engine. The pipe diameter, component length, catalytic converter size,
muffler size, and exhaust manifold design are engineered to provide proper exhaust
flow, silencing, and emission levels on a particular engine. In this section, I will
go over the function and specifics of each component.



The Exhaust
Manifold

The exhaust manifold is a pipe that conducts the exhaust gases from
the combustion chambers to the exhaust pipe. Many exhaust manifolds are made from
cast iron or nodular iron. Some are made from stainless steel or heavy-gauge steel. The
exhaust manifold contains an exhaust port for each exhaust port in the cylinder head,
and a flat machined surface on this manifold fits against a matching surface on the
exhaust port area in the cylinder head.

Some exhaust manifolds have a gasket
between the manifold and the cylinder head, as can be seen in the diagram
below:



Exhaust manifold and gasket on an in-line engine

Gaskets are
meant to prevent leakage of air/gases between the manifold and cylinder heads. The
gaskets are usually made out of copper, asbestos-type material, or paper. In other
applications, the machined surface fits directly against the matching surface on
the cylinder head. The exhaust passages from each port in the manifold join into a
common single passage before they reach the manifold flange. An exhaust pipe is
connected to the exhaust manifold flange. On a V-type engine an exhaust manifold is
bolted to each cylinder head.



The Exhaust Pipe (In-line)/ "Y" Pipe (V-
type)

The exhaust pipe is connected from the exhaust manifold to the
catalytic converter. On in-line engines the exhaust pipe is a single pipe, but on V-type
engines the exhaust pipe is connected to each manifold flange, and these two pipes are
connected into a single pipe under the rear of the engine. This single "Y" pipe is then
attached to the catalytic converter. Exhaust pipes may be made from stainless steel
or zinc-plated steel, and some exhaust pipes are double-walled. In some exhaust
systems, an intermediate pipe is connected between the exhaust pipe and the
catalytic converter. Some have a heavy tapered steel or steel composition sealing
washer positioned between the exhaust pipe flange and the exhaust manifold flange.
Other exhaust pipes have a tapered end that fits against a ball-shaped surface on the
exhaust manifold flange. Bolts or studs and nuts retain the exhaust pipe to the
exhaust manifold, as shown in the diagram below.



Some V-type engines have
dual exhaust systems with separate exhaust pipes and exhaust systems connected to
each exhaust manifold.



The Catalytic Converter



Three major
automotive pollutants are carbon monoxide (CO), unburned hydrocarbons (HC), and
oxides of nitrogen (NOx). When air and gasoline are mixed and burned in the combustion
chambers, the by-products of combustion are carbon, carbon dioxide (CO2), CO, and
water vapor. Gasoline is a hydrocarbon fuel containing hydrogen and carbon. Since
the combustion process in the cylinders is never 100% complete, some unburned HC are
left over in the exhaust. Some HC emissions occur from evaporative sources, such as
gasoline tanks and carburetors.

Oxides of nitrogen (NOx) are caused by high
cylinder temperature. Nitrogen and oxygen are both present in air. If the combustion
chamber temperatures are above 1,371 degrees Celsius, some of the oxygen and
nitrogen combine to form NOx. In the presence of sunlight, HC and NOx join to form
smog.

Catalytic converters may be pellet-type or monolithic-type. A pellet-type
converter contains a bed made from hundreds of small beads, and the exhaust gas passes
over this bed (see Fig 1). In a monolithic-type converter, the exhaust gas passes
through a honeycomb ceramic block (Fig 2). The converter beads, or ceramic block, are
coated with a thin coating of platinum, palladium, or rhodium, and mounted in a
stainless steel container. An oxidation catalyst changes HC and CO to CO2 and water
vapor (H20). The oxidation catalyst may be referred to as a two-way catalytic
converter

In a three-
way catalytic converter, the converter is positioned in front of the oxidation
catalyst. A three-way catalytic converter reduces NOx emissions as well as CO and HC.
The three-way catalyst reduces NOx into nitrogen and oxygen (Fig 4).

Some
catalytic converters contain a thermo-sensor that illuminates a light on the
instrument panel if the converter begins to overheat. Unleaded gasoline must be used
in engines with catalytic converters. If leaded gasoline is used, the lead in the
gasoline coats the catalyst and makes it ineffective. Under this condition, tail
pipe emissions become very high. An engine that is improperly tuned would also cause
severe overheating of the catalytic converter. Examples of improper tuning would be
a rich air-fuel mixture or cylinder misfiring.

Many catalytic converters have an
air hose connected from the belt-driven air pump to the oxidation catalyst. This
converter must have a supply of oxygen to operate efficiently. On some engines, a mini-
catalytic converter is built into the exhaust manifold or bolted to the manifold
flange.



The Resonator, Muffler, and Tailpipe

Since the
resonator and muffler perform basically the same functions, I decided to write about
them under one heading. Firstly, the main function of the muffler is to reduce the
sound of the engine’s outcoming exhaust gases through the exhaust pipes to a minimal
level. Since the muffler cannot reduce the noise of the engine by itself, some (if not
most) exhaust systems also have a resonator between the catalytic converter and the
muffler. Resonators are basically the second muffler, and are usually the "straight
through" type.

The muffler quiets the noise of the exhaust by "muffling" the
sound waves created by the opening and closing of the exhaust valves. When an exhaust
valve opens, it discharges the burned gases at high pressures into the exhaust pipe,
which is at low pressure. This type of action creates sound waves that travel through
the flowing gas, moving much faster than the gas itself (up to 1400 mph = 625.8m/s),
that the resonator and muffler must silence. It generally does this by converting the
sound wave energy into heat by passing the exhaust gas and its accompanying wave
pattern, through perforated tubes and tuning chambers. Passing into perforations
and reflectors within the chamber forces the sound waves to dissipate their
energy.



The above described and pictured muffler design is the most common
type, the reverse-flow design, which changes the direction of exhaust flow inside the
muffler. Exhaust gases are directed to the third chamber, forced forward to the first
chamber, from where they travel the length of the muffler and are exhausted into the
tailpipe.

Some mufflers are a straight through design in which the exhaust
passes through a single perforated pipe into a outside chamber packed with metal,
fiberglass, packed glass, or other sound absorbing (or insulating) material. As the
exhaust gases expand from the perforated inner pipe into the outer chamber, they come
in contact with the insulator and escape to the atmosphere under constant pressure.
Because of this, the expanding chamber tends to equalize or ------ the pressure peaks
throughout the exhaust from each individual cylinder of the engine. This type of
muffler is thus freer flowing and designed for the purpose of reducing back pressure
and, consequently, makes slightly more noise.

The tail pipe basically carries
the flow of exhaust from the muffler to the rear of the vehicle. Some vehicles have an
integral resonator in the tail pipe. Like the resonator mentioned earlier, this
resonator is similar to a small muffler, and it provides additional exhaust
silencing. In some exhaust systems, the resonator is clamped into the tail pipe. Tail
pipes have many different bends to fit around the chassis and driveline components.
In general, all exhaust systems components must be positioned away from the chassis
and driveline to prevent rattling. The tail pipe usually extends under the rear
bumper, and the end of this pipe is cut at an angle to deflect the exhaust
downward.



Methods on How to Improve Efficiency and Power

After the
above discussion of the components in an automotive exhaust system, it is obvious
that the principle of the engine as a pump is not being utilized to the fullest. Air is
not allowed to flow too freely because of restrictions in the form of the catalytic
converter, the resonator, and the muffler. However, these components are necessary
by regulations to maintain safe exhaust gas emissions and minimal sound levels
(noise suppression). Also, in part, it takes time and money to design an excellent
performing and free flowing exhaust system; something that car manufacturers just
can’t afford to waste resources on. This is where aftermarket companies come in to
create cost effective options for performance minded car owners. Of course, a free
flowing exhaust would be expected to make more noise than a normal one. But a good
manufactured system has a deep throaty tone, while yielding increases in horse power
and also passing emission tests. I will now go through some of the modifications of the
exhaust system that would "unleash" some horsepower and efficiency, while still
being street-legal.

Replacing the Exhaust Manifold with a Tuned
Header

A header is a different type of manifold; it is made of separate equal-
length cylindrical tubes with smooth curves in it for improving the flow of
exhaust.

Each time a power stroke occurs and an exhaust valve opens, a positive
pressure occurs in the exhaust manifold. A negative pressure occurs in the exhaust
manifolds between the positive pressure pulses, especially at lower engine speeds.
Some exhaust headers are tuned so the exhaust pulses enter the exhaust manifold
between the exhaust pulses from other cylinders, preventing interference between
the exhaust pulses. If the exhaust pressure pulses interfere with each other, the
exhaust flow is slowed, causing a decrease in volumetric efficiency (and thus
decrease in horsepower). Proper exhaust manifold/header tuning actually creates a
vacuum, which helps to draw exhaust out of the cylinders and improve volumetric
efficiency, resulting in an increase in horsepower.

Dual Exhaust
Systems

For engines with the "V" type configurations, it would be more efficient
to use a dual exhaust system than the "Y" pipe. In other words, two pipes (instead of
one) connect the exhaust manifold/header to two catalytic converters, two
resonators, and two mufflers. Thereby each manifold will have their own resonators,
catalytic converters, exhaust pipes, mufflers, and tailpipes. The advantage of a
dual exhaust system is that the engine exhausts air and gases more freely, thereby
lowering the back pressure, which is inherent in an exhaust system. With a dual
exhaust system, a sizable increase in engine horsepower can be obtained because the
"breathing" capacity of the engine is improved, leaving less exhaust gases in the
engine at the end of each exhaust stroke. This, in turn, leaves more room for an extra
intake of the air-fuel mixture. The disadvantage of a dual exhaust system is that it
would be costly due to the additional components. No doubt the addition of another
exhaust system adds more weight to the car, but the increase in horsepower is
substantial enough to outweigh the horsepower losses through additional
weight.

Removing the Resonator

The resonator does not function also as
emissions control device, so removing it and putting a straight pipe connecting the
catalytic converter and the exhaust pipe will not cause the car to fail emissions
test. Instead, some horsepower can be realized and not to mention the loudness of the
exhaust. However, with a tuned muffler, the sound can be toned down to a deep throaty
sound that is not irritable.

Upgrading to Larger Pipe Diameter

The factory
exhaust pipes have diameters around 1.5" to 2" (some 2.25" for newer larger engine
cars). Increasing the diameter of the piping will also increase the average
path/cross- sectional area that the air can pass with a minute decrease in velocity. As
mentioned before, if the diameter (and hence cross-sectional area) of the pipe is
increased too much, the velocity of the air flow will decrease more than the area
increases, so flow would be adversely affected and power would be lost.

So,
depending on the size of the engine, the optimal size pipe to upgrade to varies from 2"
to 2.5". On average, a naturally aspirated 2.5 liter engine would suffice with 2.25"
exhaust piping from the catalytic converter back to the muffler inlet.

Mandrel
Bent Versus Crush Bent Piping

Another way to upgrade the exhaust piping from the
catalytic converter back to the muffler is to have the exhaust piping mandrel (heat)
bent instead of the conventional crush bending. As the name suggests, mandrel bends
are achieved through the heating of the piping before bending whereas crush bent just
literally mean that the piping is bent entirely by force. However, the main
difference between mandrel bent and crush bent piping is the ease of flow. Mandrel
piping keeps the pipe at a constant cross-sectional area throughout a bend which makes
exhaust flow easier. On the other hand, crush bending deforms the pipe at the bend(s),
which can restrict the exhaust flow. The disadvantage of mandrel bending is that it is
relatively expensive, because of the costs involved in operating a mandrel bending
heat machine. A popular alternative is to get piping with larger diameter and then
have it crush bent. This way, it kind of evens out the differences in air flow ease,
especially if that particular exhaust pipe configuration has a lot of bends and 90
degree bends.

Straight Through Versus Reverse Flow Mufflers

Having a
optimally free flowing exhaust all the way from the manifold would not do much good if
the restrictive stock muffler is still used. The inlet and outlet diameters of the
pipe in the muffler should also be as large as feasible, so as to allow free flow of
exhaust gases. A straight through muffler would be preferred to a reverse flow
muffler mainly because the process of air re- direction in the reverse flow muffler is
too restrictive. A straight through muffler design would allow exhaust gases to be
expedited out as efficiently as possible, although the muffling abilities would not
be as efficient as that of the reverse flow design. Therefore it will be inevitable
that the exhaust will sound louder than before, but as mentioned before a couple of
times, an aftermarket straight through muffler uses noise suppressing material
that tones down the sound to that of one that’s deep and throaty and not irritating.
However, as will be discussed in the next section, a new generation of mufflers may be
able to tackle this.



The Future

These days, you can’t think of exhaust
system as just some crude plumbing hung on as an afterthought to pipe away air, heat,
and to keep those decibels down. It’s become an integral part of the powertrain and
under-car architecture critical to performance, fuel efficiency, and emissions
reduction. There has already been development of low or zero emission vehicles
already in the recent auto shows by major automobile manufacturers like Honda
(Natural Gas vehicle) and Ford (Electric car). A oncoming development I would like to
discuss about in this section is the Electronic Muffler.

As an executive with
Walker, one of the major muffler makers involved in developing the concept puts it,
"After the introduction of the catalytic converter in 1975, this is probably the most
revolutionary technology that’s happened to exhaust systems in the entire history
of the automobile."

While the idea is surprising, the basic principle isn’t hard
to grasp. From a microphone and a crankshaft speed/position sensor, the computer
receives input on the pattern of pressure waves (that’s what sound is, after all) the
engine is emitting at its tail pipe. This data is processed using patented
algorithms, which produce mirror-image pulses that are sent to speakers mounted near
the exhaust outlet, creating contra-waves that cancel out the noise. In other words,
the sensors trap the waveform signature of the engine, and the speakers generate anti-
noise waves 180 degrees out of phase with the gas waves. This destructive
interference idea is sort of like fighting fire with fire. The sound waves collide,
wiping each other out. It doesn’t just mask the noise, it actually removes sound
energy from the environment, and from the law of conservation of energy where the
energy has to turn up someplace, all that is left is low-level heat.

Although
electronic mufflers are not widely used (if ever) at present, they may be installed on
vehicles in the near future. In 1989, a joint Electronic Muffler System development
program was started and the University of Michigan’s Delphi study predicts that 20%
of the cars produced in North America will have electronic mufflers by the turn of the
century.

Well, if the electronic mufflers are really as effective as they claim
to be and they were available now, we could build a perfect exhaust system using the
setup described earlier with the addition of an electronic muffler then the problem
of loud exhaust wouldn’t exist. But then again, by the time the electronic muffler is
out in the market, technology might have other improvements of the exhaust system and
we will again try to match components to produce more horsepower and attain better gas
mileage (efficiency).



In conclusion, we go back to the basic analogy of the
engine as a pump; the more air that can flow freely, the more horsepower that can be
optimally achieved from the engine. This research paper has only dealt with how to get
air OUT of the engine. It is important to note that the INFLOW of air also influences the
output performance of the engine. As a matter of fact, we need the inflow of air before
the outflow process starts. In brief, the inflow of air can be modified by removing the
intake resonator, or even removing the entire airbox and installing a pipe with a cone-
shaped filter at the end. There are many other ways to improve air inflow, but I shall
not discuss about them as it would be outside he scope of this paper if I’m primarily
interested in the outflow of exhaust gases.

Also, it is important to note that
"horsepower" is a unit of energy over time. So the more energy it requires to do
something, the less power you will get out of it. In other words, it is because
motorcycles are lighter than cars that they can achieve similar if not higher
horsepower. That is why race cars are stripped of the interior, air conditioning, and
any other unnecessary weight. This way, there will be less weight to move, meaning
less energy required and thus more power produced. That is why automotive engineers
are trying to use materials of lighter weight, like plastics and carbon fiber.