Engine Emission Control
The internal combustion engine produces power by burning fuel and changing the chemical energy of fuel into thermal (heat) energy. The thermal energy is then converted to mechanical power. Not all of the chemical energy of the petrol is converted to useful power and much of it is wasted. Also, the engine’s combustion process produces some harmful products. These are discharged from the engine and become air pollutants. Therefore, an engine’s fuel system, and its emission controls are closely interrelated. Consideration of emission control requirements is extremely important in the design and operation of all parts of the fuel system.
The primary pollutants entering the atmosphere from the internal combustion engine are carbon monoxide (CO), unburned hydrocarbons (HC), and oxides of nitrogen (NOx). They originate from three primary sources such as crankcase blowby gas (HC 20%), exhaust gases (CO 100%, NOx 100% and HC 60%), and fuel evaporation from tank and carburettor (HC 20%). Therefore, automobile emission control can be grouped into following major families:
(a) Crankcase emission control: Positive crankcase ventilation (PCV) systems
control HC emissions from the engine crankcase. (6) Evaporative emission control : Evaporate emission control (EEC) systems control the evaporation of HC vapours from the fuel tank, pump, and carburettor.
(c) Exhaust emission control : Various systems used to control HC, CO, and
NOx emissions from the engine exhaust are (i) air injection system or pulse air system, (ii) engine modification, (Hi) spark timing,
(iv) exhaust gas re-circulation (EGR) system, and (v) catalytic converter system.
In the fight for clean air, and in meeting the emission standards, the prime responsibility rests not only with the manufacturers but with the tune-up technician as well. Strong emphasis must be placed on the need for correct tune-up and maintenance procedure. The chapter presents various engine emission control systems in detail. Some systems presented are obsolete today, but are discussed to provide a total picture of the effort put in the area in various directions.
The major pollutants produced by an internal combustion engine include hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). In addition, an automobile engine
gives off many small liquid or solid particles, such as lead, carbon, sulphur and other particulates, which also contribute to pollution. All these emissions constitute air pollutants, but not smog.
Unburned hydrocarbons, given off by an automobile, are largely from unburned portions of fuel. Car without emission controls produces about 24 kg of hydrocarbon emissions for every cubic meter of petrol use. Hydrocarbons are the only major automotive air pollutant, which comes from other sources in addition to the engine’s exhaust.
More than 200 different varieties of hydrocarbon pollutants are generated from automotive sources. While the fuel system and the engine exhaust contribute maximum to this pollutant, others are oil and petrol fumes come from the crankcase. A car’s tyres, paint, and upholstery also add tiny amounts of hydrocarbons. The three major sources of hydrocarbon emissions in an automobile are presented in Fig. 17.1.
If petrol is burned completely in an automobile engine only water and carbon dioxide are produced and there can be no hydrocarbon in the exhaust. When the vaporized and compressed air-fuel mixture is ignited, combustion occurs so fast that petrol vapour close to the wall of the combustion chamber does not burnt completely. This unburned fuel passes out with the exhaust gases. This problem increases with engines that misfire or are not properly tuned.
Hydrocarbon emission does not cause a direct threat to human health, although there is proof that some compounds are carcinogenic (cause cancer). The major problem associated is that HC reacts with NOx and atmospheric ozone to produce an unpleasant photochemical smog. Such smog has become a problem in large cities, especially where there is a sunny climate.
Carbon Monoxide (CO).
Carbon monoxide is found in automobile exhaust in large amounts. One cubic metre of petrol when burned without emission controls produces 275.5 kg of carbon monoxide, in addition to 24 kg of hydrocarbons. Carbon monoxide is odorless and colourless, and when taken in even in a small quantity it causes headache and vision difficulties, while in larger quantities it is fatal. Larger quantities of carbon monoxide, if collected in the air, push out the oxygen causing breathing difficult. Carbon monoxide combines with haemoglobin in blood and interferes with its oxygen-carrying capability. This is a very dangerous situation, which can cause toxicosis. Mild poisoning effects are also felt when breathing air with a CO concentration as low as 0.03% depending on exposure time.
Since it is produced due to incomplete combustion, the amount of carbon monoxide depends on the method of burning hydrocarbons. During burning of the air-fuel mixture, hydrocarbons combine with oxygen. If the ratio of fuel in the mixture is more and enough oxygen is not
Fig. 17.1. Sources of hydrocarbon emissions.
available for combustion process to complete, then carbon monoxide is formed. When fuel is less in the mixture and supply of oxygen is more, then the production of carbon monoxide reduces due to the formation of harmless carbon dioxide.
Oxides of Nitrogen (NOx).
When the temperature in the combustion chamber reaches 1645 K or above, the nitrogen and oxygen in the air-fuel mixture combine to form large quantities of oxides of nitrogen (NOx). In smaller amounts NOx is also formed at lower temperatures. When the amount of hydrocarbons in the air reaches a certain level, and also the ratio of NOx to HC reaches certain proportion, the two pollutants combine chemically to form smog. NOx combine with haemoglobin in blood in similar manner to that of CO, leading to anoxia. Their acidic properties can lead to the formation of acid rain, and they are known to cause respiratory irritation and asthma.
Lowering the combustion chamber temperature reduces formation of NOx, but results in less efficient burning of the air-fuel mixture. This causes the formation of hydrocarbons and carbon monoxide in large quantities, because of the lower combustion chamber temperatures.
Particulates are fragments of solid or liquid matter such as dust and soot, which remain in the atmosphere for long periods of time, so that they are prime causes of secondary pollution. Absorbed directly or indirectly into the body, particulates can damage the respiratory tract and lungs of both humans and animals. Particulars such as lead and carbon tend to collect in the atmosphere. These are all harmful substances and, in large amounts, can injure health.
Automobiles, however, contributes to a small percentage of the total particulates in’ the atmosphere. Most come from fixed sources such as factories. Particulates produced by automobile can be reduced by eliminating additives such as lead from petrol and by changing other characteristics of the fuel. Keeping this in view, the amount of additives used in petrol has been reduced, and even the types of additives are now carefully controlled.
The sulphur gets into the atmosphere in the form of sulphur oxides, which is produced during processing and burning of petrol and other fossil fuels such as coal and oii. This material combines with water in the air to form corrosive sulphuric acid, which is a secondary pollutant. Recently, such pollution has become a major problem in the north eastern United States, as well as in other parts of the world. Clouds containing sulphuric acid are carried by winds to the Northeast, where they deposit enough acid in the form of rain and snow to kill wildlife.