Walker Support

Exhaust 101

Exhaust System Basics •
Basics of Combustion •
Controlling the Products of Combustion •
Exhaust System Components •
On Board Diagnostics •
Inspections

Controlling The Products of Combustion

NOISE

The increasing noise level present in urban, suburban, and rural areas is a serious concern of environmentalists and the public at large. As a result, local, state, and federal legislators have enacted laws to reduce the level of noise pollution produced by motor vehicles. This type of regulation has placed added emphasis on research into sound and the study of acoustics.

Acoustics is a branch of physics that deals with sound and sound waves. In the automotive exhaust industry, the concern is with a particular kind of sound as it comes from the engine and through the exhaust system - the offensive sound called exhaust noise. Noise is normally defined as sound that is unpleasant or offensive to the listener. Rock music, for example, may be music to some individuals, while to others it is noise. As it applies to automobiles, some may enjoy the sound of open exhaust headers, while others (most people) feel that the quieter the vehicle the better.

The determination of a noise being offensive or inoffensive is based on our individual subjective perception of the sound being heard; thus, subjective sound is a term that applies to one of the design criteria of exhaust systems. In addition to sound intensity, many other characteristics play a part in subjective sound quality.

Noise legislation puts strict limits on the amount or intensity of noise people can be exposed to. For these purposes, determination of noise intensity is accomplished with a meter, and the result is termed objective noise value. It is important to note that mufflers designed for acceptable objective noise levels will not necessarily have acceptable "subjective sound" quality and vice versa. Both objective and subjective characteristics must be independently designed into the muffler.

THE PHYSICS OF SOUND

Sound is generated when molecules of air are disturbed in waves of a given size. The molecules do not travel with the sound - they just bump into each other and bounce back again, causing rhythmic waves of pressure to spread out from the source of the sound. The movement of sound can be measured in several ways:

Amplitude - A measure of the depth and height of pressure waves. The more intense the sound, the greater the amplitude.
Wave Length - The distance between the beginning and the end of individual sound pressure waves.
Frequency - The number of sound pressure waves that pass a given point every second. Frequency is expressed in cycles/second or Hertz (Hz).

Noise Level - A sound pressure level is measured in logarithmic units called decibels, or "dB." There are several different decibel scales that weigh sound pressure levels in different ways. Of these, the "A" scale comes closest to measuring sounds the way people hear them. Hence the term dB(A).

To people's ears, loudness doubles about every 6 to 10 dB(A); for example, a 70 dB(A) sound seems twice as loud as 60, and half as loud as 80. Normal conversation is about 65 dB(A), heavy traffic is about 85 dB(A), and a close thunderclap might hit as high as 105 dB(A).

Because the dB(A) is a logarithmic measurement, when two equal sound sources are combined, the loudness does NOT double; it increases by 3 dB(A).

For example: two voices that register at 65 dB(A) each will produce 68 dB(A) when combined.

The Effect of Distance

Another important factor in sound measurement is the distance between the source and the measuring instrument. A sound that measures 80 dB(A) at twenty- five feet would measure only 74 dB(A) at 50 feet. We will not go over the mathematical equations to explain the effect of distance on noise. Suffice it to say that the sound level of a noise decreases as distance increases.

Resonance

One other term that is important to understand is resonance. Resonance plays a major part in noise control. Every surface of every object has a fundamental frequency at which it begins to vibrate or radiate sound. Musical instruments do this and so do mufflers. When sound waves at this fundamental frequency strike the surface, they are amplified, that is, increased. This is called resonance. With resonance, small sounds can be amplified to become very loud.

Containing Sound

There are three basic approaches to controlling noises:

Containment
Absorptive Devices
Reactive Devices

Containment reduces the transmission of sound waves by confining them. Almost any material can help contain sound; the denser it is, the better it works. Many economy mufflers silence exhaust noise by containing the flow of exhaust gases. This is typically not desirable. When exhaust gases are contained, the backpressure increases and vehicle power, performance and fuel economy suffer.

Absorptive devices convert sound energy to heat energy. Sound waves collide with material that flexes and vibrates easily. The material absorbs the sound rather than passing it on as resonance, or bouncing it back toward the source. Sound is reduced by a loss of energy when the absorptive material is moved. This device is more effective on high frequencies. Glasspack type, or glass fiber filled mufflers are an example of sound reduction by absorptive control.

Reactive devices reduce, change, or eliminate noise by reflecting sound waves back toward the source.

One common application of a reactive sound device is the louvered tubes found on the inside of most mufflers. The main purpose of these tubes is to steer sound waves against each other and use their energy to cancel each other out.

There is one variation of reactive sound control that provides us with a device that reduces low frequency exhaust noise. This device is referred to as the Hemholtz tuner. The Hemholtz tuner operates on the theory that when sound waves pulse through a constricted area into a large closed area, the sound energy will be reduced. This action causes a reduction in the low frequency noise of the exhaust gas pulsations.

In a muffler, the Hemholtz tuner is the large empty "chamber" that typically does not have an outlet. This "room" is there to reduce the low frequency noise from the exhaust system.

A pinch can is another example of a reactive device used to control sound. This device works on a theory that is the opposite of the Hemholtz tuner theory. A pinch can takes sound waves and forces them into a confined area. As the sound pulsation is exposed to the restricted area, the pressure level of the pulsation decreases. This act of "pinching down" the sound energy, serves to eliminate high frequency noise from the exhaust. Most mufflers utilize some type of pinch can within the muffler.

THE CHEMISTRY OF EXHAUST GASES

Gasoline engines are fueled with two chemical compounds, gasoline and air. Basically, gasoline is made of two elements, hydrogen (H₂) and carbon (C). In the form of gasoline, these two chemicals combine to make what we call a hydrocarbon (HC). Air is made up of basically two elements, oxygen (0₂) and nitrogen (N₂).

If gasoline engines burned their fuel as efficiently as possible, they would produce three by-products: water vapor (H₂O), carbon dioxide (CO₂) and nitrogen (N₂).

For the most part, none of these chemical compounds are harmful; however, environmentalists argue that excessive amounts of carbon dioxide promote the formation of the greenhouse effect. Nonetheless, H₂O, CO₂ and N₂ are the most desirable by-products of combustion, and automotive engineers strive to create emission control systems that allow a vehicle to produce only these three chemical groups.

Unfortunately, engines do not run perfectly, and as a result they also produce three by-products commonly referred to as the "terrible trio" of automotive pollutants. This trio includes the following:

Carbon monoxide (CO) - is an odorless, tasteless, poisonous gas that can cause a variety of health problems and even death. Many urban areas experience critically high levels of carbon monoxide, especially during the cold winter months when engines take longer to warm up and run cleanly.
Unburned hydrocarbons (HC) - cause a variety of respiratory problems, crop damage, and promote the formation of smog.
Oxides of nitrogen (NOx) - like unburned hydrocarbons, cause respiratory problems, and promote the formation of smog.

Automobiles and Carbon Monoxide

Carbon monoxide (CO) consists of a carbon atom and an oxygen atom linked together.

Carbon monoxide results from incomplete combustion of fuel and is emitted directly from the tail pipe. Incomplete combustion is most likely to occur at low air-to-fuel ratios in the engine. These conditions are most common during cold engine starts:

When the air supply is restricted
When the fuel supply is enriched on fuel injected vehicles
When an O₂ sensor has failed.

Also, carbon monoxide formation is common at higher altitudes where the "thin" air effectively reduces the amount of oxygen available for combustion (except in vehicles with engines that are designed or adjusted to compensate for altitude).

Two-thirds of the nationwide carbon monoxide emissions come from transportation sources, with the largest contribution coming from highway motor vehicles. In urban areas, the contribution from motor vehicles is often greater. In Los Angeles, a city with severe carbon monoxide pollution, motor vehicles account for 87 percent of carbon monoxide emissions.

Carbon monoxide enters the bloodstream through the lungs and forms carboxyhemoglobin, a compound that inhibits the blood's capacity to carry oxygen to organs and tissues. Persons with heart disease are especially sensitive to carbon monoxide poisoning, and may experience chest pain if they breathe the gas while exercising. Infants, fetuses, elderly persons, and individuals with respiratory diseases are also particularly sensitive. Carbon monoxide can affect healthy individuals, impairing exercise capacity, visual perception, manual dexterity, learning functions, and the ability to perform complex tasks. As stated earlier, carbon monoxide emissions from automobiles increase dramatically in cold weather. This is because cars need more fuel to start at cold temperatures, and because some emission control devices (such as oxygen sensors and catalytic converters) operate less efficiently when they are cold. Because of the concern over carbon monoxide and other pollutants, automobile manufacturers have taken significant steps to improve vehicle performance in all operating environments.

Automobiles And Hydrocarbons

Ozone is a form of molecular oxygen that consists of three oxygen atoms linked together. Ozone in the upper atmosphere (the "ozone layer") occurs naturally and protects life on earth by filtering out ultraviolet radiation from the sun. But ozone at ground level is a noxious pollutant. It is the major component of smog and represents this country's most serious urban air quality problem.

Ozone is not emitted directly but is formed in the atmosphere through a complex set of chemical reactions involving hydrocarbons, oxides of nitrogen, and sunlight. The rate at which the reactions proceed is related to both temperature and the intensity of sunlight. Because of this, ozone levels rise most frequently on sultry summer afternoons.

Cars are getting cleaner, but people are driving more, offsetting progress in ozone pollution control.

Ozone is a severe irritant. It is responsible for the choking, coughing, and stinging eyes associated with smog. Ozone damages lung tissue, aggravates respiratory disease, and makes people more susceptible to respiratory infections. Children are especially vulnerable to ozone's harmful effects. Elevated ozone levels also inhibit plant growth and can cause widespread damage to crops and forests.

Automobiles and Oxides of Nitrogen
Nitric oxide (NO) is the principal oxide of nitrogen produced in the combustion process: it is readily oxidized in the atmosphere to nitrogen dioxide (NO²). Collectively, NO and NO² are referred to as NO^X or oxides of nitrogen. NO^X can irritate the lungs and lower resistance to respiratory infection. Nitrogen oxides are an important precursor to both ozone and acid rain.

More than half of the nationwide NO_X emissions come from transportation sources, with the largest contribution coming from Light Duty Gas Vehicles (LDGV). Like carbon monoxide, proximity to urban areas increases the contribution from motor vehicles.

Possible Clean Fuels

Changes in fuel have resulted in cleaner engine operation. Some cities with unhealthy carbon monoxide levels require the use of "oxygenated" fuels to reduce carbon monoxide emissions. Oxygenated fuels are gasolines blended with oxygen-containing additives such as alcohols or their derivatives. "Gasohol," for example, is an oxygenated blend that contains 10 percent ethanol. The extra oxygen in the fuel decreases carbon monoxide emissions by effectively increasing the air-to-fuel ratio.

The result is that air quality has improved significantly over the last several years, even though the number of vehicle miles traveled has also increased. In recent measurements the number of days with poor air quality has decreased in the nation's urban areas.

Some fuels are inherently cleaner than gasoline because they emit less unburned hydrocarbons, and because the hydrocarbons they do emit are less likely to react in the atmosphere to form ozone. These fuels include:

Alcohols - Methanol (made from natural gas, coal, or biomass) and ethanol (made from grains or sugar) are high- octane liquid fuels. Vehicles designed to run on pure alcohol fuels have the potential to emit 80 to 90 percent less reactive hydrocarbons than advanced- technology gasoline cars.
Electricity - Battery powered vehicles have the potential for zero tailpipe or evaporative hydrocarbon emissions, though power plant emissions must be accounted for. Today's electric vehicle technology is limited, primarily by battery capacity, but recent developments indicate much promise for more widespread use in the future.
Natural Gas - Compressed natural gas is also an excellent automotive fuel, particularly for fleet vehicles where long driving range is not important. Natural gas vehicles have the potential to emit 85 to 95 percent less reactive hydrocarbons than advanced-technology gasoline vehicles.
Liquid Petroleum Gas (Propane) - Propane is a by-product of petroleum refining. Propane vehicles emit considerably less ozone-forming hydrocarbons than do vehicles fueled with conventional gasoline.
Reformulated Gasoline - The petroleum industry is studying ways to change refinery procedures to make cleaner- burning gasoline. A number of "clean" gasolines have recently been introduced into the marketplace, and research is continuing to develop even cleaner fuels.