Mazda 323/626/929/GLC/MX-6/RX-7 1978-1989

Composition Of The Exhaust Gases

Print

The exhaust gases emitted into the atmosphere are a combination of burned and unburned fuel. To understand exhaust emission and its composition, review some basic chemistry.

When the air/fuel mixture is introduced into the engine, we are mixing air, composed of nitrogen (78%), oxygen (21%) and other gases (1%) with the fuel, which is 100% hydrocarbons (HC), in a semi-controlled ratio. As the combustion process is accomplished, power is produced to move the vehicle while the heat of combustion is transferred to the cooling system. The exhaust gases are then composed of nitrogen, a diatomic gas (N 2 ), the same as was introduced in the engine, carbon dioxide (CO 2 ), the same gas that is used in beverage carbonation, and water vapor (H 2 O). The nitrogen (N 2 ), for the most part, passes through the engine unchanged, while the oxygen (O 2 ) reacts (burns) with the hydrocarbons (HC) and produces the carbon dioxide (CO 2 ) and water vapor (H 2 O). If this chemical process would be the only process to take place, the exhaust emissions would be harmless. However, during the combustion process, other pollutants are formed and are considered dangerous. These pollutants are carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NOx), oxides of sulfur (SOx) and engine particulates.

Lead (Pb), is considered one of the particulates and is present in the exhaust gases whenever leaded fuels are used. Lead (Pb) does not dissipate easily. Levels can be high along roadways when it is emitted from vehicles and can pose a health threat. However, with the use of unleaded gasoline and the phase out of leaded gasoline for fuel, this pollutant has all but disappeared.

HYDROCARBONS



Hydrocarbons (HC) are essentially unburned fuel that has not been successfully burned during the combustion process or has escaped into the atmosphere through fuel evaporation. The main sources of incomplete combustion are rich air/fuel mixtures, low engine temperatures and improper spark timing. The main sources of hydrocarbon emission through fuel evaporation come from the vehicle's fuel tank and carburetor bowl.

To reduce combustion hydrocarbon emissions, engine modifications were made to minimize dead space and surface area in the combustion chamber. In addition, the air/fuel mixture was made more lean through improved carburetion and fuel injection. External controls were also added to further the combustion of hydrocarbons outside the engine. Two such methods were the addition of an air injection system, to inject fresh air into the exhaust manifolds and the installation of a catalytic converter, a unit that is able to burn traces of hydrocarbons without significantly affecting the internal combustion process or fuel economy.

To control hydrocarbon emissions through fuel evaporation, modifications were made to the fuel tank and carburetor bowl (if equipped) to allow storage of the fuel vapors during periods of engine shut-down. During specific engine operating conditions, these modifications will purge and burn the vapors by blending them with the air/fuel mixture.

CARBON MONOXIDE



Carbon monoxide is formed when not enough oxygen is present during the combustion process to convert carbon (C) to carbon dioxide (CO 2 ). An increase in the carbon monoxide (CO) emission is normally accompanied by an increase in the hydrocarbon (HC) emission because of the lack of oxygen to completely burn all of the fuel mixture.

Carbon monoxide (CO) also increases the rate at which the photo-chemical smog is formed by speeding up the conversion of nitric oxide (NO) to nitrogen dioxide (NO 2 ). The carbon monoxide (CO) combines with oxygen (O 2 ) and nitric oxide (NO) to produce carbon dioxide (CO 2 ) and nitrogen dioxide (NO 2 ). (CO + O 2 + NO = CO 2 + NO 2 ).

The dangers of carbon monoxide, which is an odorless, colorless and toxic gas, are many. When carbon monoxide is inhaled into the lungs and passed into the blood stream, oxygen is replaced by the carbon monoxide in the red blood cells, causing a reduction in the amount of oxygen supplied to the many parts of the body. This lack of oxygen causes headaches, lack of coordination, reduced mental alertness and, should the carbon monoxide concentration be high enough, death could result.

NITROGEN



Normally, nitrogen is an inert gas. When heated to approximately 2500°F (1371°C) through the combustion process, this gas becomes active and causes an increase in the nitric oxide (NOx) emission.

Oxides of nitrogen (NOx) are composed of approximately 97-98% nitric oxide (NO). Nitric oxide is a colorless gas but when it is passed into the atmosphere, it combines with oxygen and forms nitrogen dioxide (NO 2 ). The nitrogen dioxide then combines with chemically active hydrocarbons (HC) and, when in the presence of sunlight, causes the formation of photo-chemical smog.

OZONE



To further complicate matters, some of the nitrogen dioxide (NO 2 ) is broken apart by the sunlight to form nitric oxide and oxygen. (NO 2 + sunlight = NO + O). This single atom of oxygen then combines with diatomic (meaning 2 atoms) oxygen (O 2 ) to form ozone (O 3 ). Ozone is one of the smells associated with smog. It has a pungent and offensive odor, irritates the eyes and lung tissues, affects the growth of plant life and causes rapid deterioration of rubber products. Ozone can be formed by sunlight as well as electrical discharge into the air.

The most common discharge area on the automobile engine is the secondary ignition electrical system, especially when inferior quality spark plug cables are used. As the surge of high voltage is routed through a secondary cable, the circuit builds up an electrical field around the wire, and acts upon the oxygen in the surrounding air to form the ozone. The faint glow along the cable with the engine running that may be visible on a dark night, is called the "corona discharge." The combination of corona and ozone has been a major cause of cable deterioration. Recently, different types and better quality insulating materials have lengthened the life of the electrical cables.

Although ozone at ground level can be harmful, ozone is beneficial to the earth's inhabitants. By having a concentrated ozone layer called the "ozonosphere," between 10-20 miles (16-32km) up in the atmosphere, much of the ultraviolet radiation from the sun's rays is absorbed and screened. If this ozone layer was not present, much of the earth's surface would be burned, dried and unfit for human life.

There is much discussion concerning the ozone layer and its density. A feeling exists that this protective layer of ozone is slowly diminishing and corrective action must be directed to this problem. Much experimenting is presently being conducted to determine if a problem exists and, if so, the short and long-term effects of the problem and how it can be remedied.

OXIDES OF SULFUR



Oxides of sulfur (SOx) were initially ignored in the exhaust system emissions, since the sulfur content of gasoline as a fuel is less than 1 / 10 of 1%. Because of this small amount, it was felt that it contributed very little to the overall pollution problem. However, because of the difficulty in solving the sulfur emissions in industrial pollution, as well as the introduction of catalytic converters to automobile exhaust systems, a change was mandated. The automobile exhaust system, when equipped with a catalytic converter, changes the sulfur dioxide (SO 2 ) into sulfur trioxide (SO 3 ).

When sulfur trioxide (SO 3 ) combines with water vapor (H 2 O), a sulfuric acid mist (H 2 SO 4 ) is formed. This sulfuric acid mist is a very difficult pollutant to handle and is extremely corrosive. It is the same mist that rises from the vents of an automobile storage battery when an active chemical reaction takes place within the battery cells.

When a large concentration of vehicles equipped with catalytic converters are operating in an area, this acid mist will rise and be distributed over a large ground area causing land, plant, crop, paint and building damage.

PARTICULATE MATTER



A certain amount of particulate matter is present in the burning of any fuel, with carbon constituting the largest percentage of the particulates. In gasoline, the remaining percentage of particulates is the burned remains of various other compounds used in its manufacture. When a gasoline engine is in good internal condition, the particulate emissions are low but as the engine wears internally, the particulate emissions increase. By visually inspecting the tailpipe emissions, a determination can be made as to where an engine defect may exist. An engine with light gray smoke emitting from the tail pipe normally indicates an increase in the oil consumption through burning due to internal engine wear. Black smoke would indicate a defective fuel delivery system, causing the engine to operate in a rich mode. Regardless of the color of the smoke, the internal part of the engine or the fuel delivery system should be repaired to a "like new" condition to prevent excessive particulate emissions.

Diesel and turbine engines emit a darkened plume of smoke from the exhaust system because of the type of fuel used. Emission control regulations are mandated for this type of emission and more stringent measures are being used to prevent excessive emission of the particulate matter. Electronic components are being introduced to control the injection of the fuel at precisely the proper time of piston travel, to achieve the optimum in fuel ignition and fuel usage. Other particulate after-burning components are being tested to achieve lower particulate emissions.

Good grades of engine lubricating oils should be used, meeting the vehicle manufacturer's specification. "Cut-rate" oils can contribute to the particulate emission problem because of their low "flash" or ignition temperature point. Such oils burn prematurely during the combustion process causing emissions of particulate matter.

The cooling system is an important factor in the reduction of particulate matter. With the cooling system operating at a temperature specified by the manufacturer, optimal combustion will occur. The cooling system must be maintained in the same manner as the engine oiling system, since each system must perform properly in order for the engine to operate efficiently for a long time.

 
label.common.footer.alt.autozoneLogo