Automobile Emissions:
The rise in civilization is closely related to improvements in transportation. In the development of transport the internal combustion engines, both petrol and diesel engines, occupy a very important position. The petrol and diesel engines, occupy a very important position. The petrol engine has provided reliable small power units for personalized transport and in this way revolutionized the living habits of people to a great extent. The diesel engine has provided the power units for transportation system, i.e. buses, and goods transportation system, i.e., trucks. Indeed the petrol engine powered automobile and diesel engine powered buses and trucks are the symbols of our modern technological society.
However, in recent times the internal combustion engine powered vehicles have come under heavy attack due to various problems created by them. The most serious of these problems is air pollution. Air pollution can be defined as addition to our atmosphere of any material which will have a deleterious effect on life upon our planet.
The engine emissions can be classified into two categories:
• Exhaust emissions and
• Non-exhaust emissions
The major exhaust emissions are
• Unburnt hydrocarbons, (HC)
• Oxides of carbon, (CO & CO2) 1
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• Oxides of Nitrogen, (NO & NO2)
• Oxides of sulphur, (SO2 & SO3)
• Particulates
• Soot and smoke
The first four are common to both SI and CI engines and the last two are mainly from CI engines.
The main non-exhaust emissions are:
• Unburnt hydrocarbons from fuel tank
• Unburnt hydrocarbons from the crankcase blow by.
HYDROCARBON EMISSIONS
Exhaust gases leaving the combustion chamber of an SI engine contain up to 6000 ppm of hydrocarbon components, the equivalent of 1-1.5% of the fuel. About 40% of these constitute the unburnt components of the fuel. The other 60% consists of partially reacted components that were not present in the original fuel. These consist of small non-equilibrium molecules, which are formed when large fuel molecules break up (thermal cracking) during the combustion reaction. It is often convenient to treat these molecules as if they contained one carbon atom, as CH1
Hydrocarbon emissions will be different for each gasoline blend depending on the original fuel components. Combustion chamber geometry and engine operating parameters also influence the HC components spectrum.
HYDROCARBON FORMATION
The causes for hydrocarbon emissions from SI engine are:
• Incomplete combustion
• Crevice volumes and flow in crevices
• Leakage past the exhaust valve
• Valve overlap
• Deposits on walls
• Oil on combustion chamber walls
i) INCOMPLETE COMBUSTION
Even when the fuel and air entering an engine are at the ideal stoichiometric condition, perfect combustion does not occur and some HC ends up in the exhaust. The reasons are:
• Improper mixing: Due to incomplete mixing of the air and fuel some fuel particles do not find oxygen to react with. This causes HC emissions.
• Flame quenching: As the flame goes very close to the walls it gets quenched at the walls leaving a small volume of unreacted air-fuel mixture. However, this mixture near the wall that does not originally get burned as the flame front passes will burn later in the combustion process due to additional mixing, swirl and turbulence.
ii) CREVICE VOLUME AND FLOW IN CREVICES
The crevice volume includes the area between piston, piston rings, and the cylinder walls. These volumes consist of a series of volumes connected by flow restrictions such as the ring side clearance and ring gap. The geometry changes as each ring moves up and down in its ring groove, sealing either at the top or bottom surface.
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During the compression stroke and early part of the combustion process, air and fuel are compressed into the crevice volume of the combustion chamber at high pressure. As much as 3.5% of the fuel in the chamber can be forced into this crevice volume. Later in the cycle during the expansion stroke, pressure in the cylinder is reduced below crevice volume pressure, and reverse blow-by occurs. Fuel and air flow back into the combustion chamber, where most of the mixture is consumed in the flame. However, before the last element of reverse blow-by occurs flame will be quenched and unreacted fuel particles come out in the exhaust.
iii) LEAKAGE PAST THE EXHAUST VALVE
As pressure increase during compression and combustion, some amount of air-fuel mixture is forced into the crevice volume around the edges of the exhaust valve and between the valve and valve seat. A small amount even leaks past the valve into the exhaust manifold. When the exhaust valve opens, the air fuel mixture which is still in the crevice volume
4
gets carried into the exhaust manifold. This causes a momentary increase in HC concentration at the start of the blow down process.
iv) VALVE OVERLAP
Valve overlap is a must to obtain satisfactory performance from the engine. During valve overlap, both the exhaust and intake valves are open, simultaneously creating a path where the fresh air-fuel mixture can flow directly into the exhaust. A well-designed engine minimizes this flow, but a small amount of fresh fuel-air mixture escape is inevitable. The worst condition for this is at idle and low speed, when the overlap in terms of time (milliseconds) is the largest.
v) DEPOSITS ON WALL
Gas particles, including those of fuel vapour, are absorbed by the deposits on the walls of the combustion chamber. The amount of absorption is a function of gas pressure. The maximum absorption occurs during compression and combustion. Later in the cycle, when the exhaust valve opens and cylinder pressure gets reduced, absorption capacity of the deposit material becomes lower. Gas particles are desorbed back into the cylinder. These particles, including some HC, come out from the cylinder during the exhaust stroke. This problem is greater in engines with higher compression ratios due to the higher pressure these engines generate. More gas adsorption occurs as pressure goes up. Clean combustion chamber walls with minimum deposits will reduce HC emissions in the exhaust.
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Old engines will typically have a greater amount of wall deposit buildup. This increases HC emissions correspondingly. This is due to age and to fewer swirls that was generally found in earlier engine design.
vi) OIL ON COMBUSTION CHAMBER WALLS
A very thin layer of oil gets deposited on the cylinder walls to provide lubrication between the walls and the moving piston. During the intake and compression strokes, the incoming air and fuel comes in contact with this oil film. In the same way as wall deposit, this oil film absorbs and desorbs gas particles, depending on gas pressure.
During compression and combustion, when cylinder pressure is high, gas particles, including fuel vapour, are absorbed into the oil film. When particles are later reduced during expansion and blow down, the absorption capability of the oil is reduced and fuel particles are desorbed back into the cylinder. Some of the fuel ends up in the exhaust.
As an engine ages, the clearance between piston rings and cylinder walls becomes greater, and a thicker film of oil is left on the walls. Some of this oil film is scraped off the walls during the compression stroke and gets burned during combustion. Oil is a high-molecular weigh hydrocarbon compound that does not burn as readily as gasoline. Some of it comes out as HC emissions. This happens at a very slow rate with a new engine but increases with engine age and wear.
EFFECTS OF HYDROCARBON
Hydrocarbons are a precursor to ground-level ozone, a serious air pollutant in cities across the United States. A key component of smog, ground-level ozone is formed by reactions involving hydrocarbons and nitrogen oxides in the presence of sunlight. Hydrocarbon emissions result from incomplete fuel combustion and from fuel evaporation. Today’s cars are equipped with emission controls designed to reduce both exhaust and evaporative hydrocarbon emissions.
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Ground-level ozone causes health problems such as difficulty breathing, lung damage, and reduced cardiovascular functioning. A number of hydrocarbons are also considered toxic, meaning they can cause cancer or other health problems.
CARBON MONOXIDE
Carbon monoxide is a colourless and odorless but a poisonous gas. It is generated in an engine when it is operated with a fuel rich equivalent ratio. When there is not enough oxygen to convert all carbon to CO2, some fuel does not get burned and some carbon ends up as CO. Typically the exhaust of an SI engine will be about 0.2 to 5% carbon monoxide. Not only is CO considered an undesirable emission, but it also represents lost chemical energy. CO is a fuel that can be combusted to supply additional thermal energy.
CO + 0.5 O2 CO2 + Heat
Maximum CO is generated when an engine runs rich. Rich mixture is required during starting or when accelerating under load. Even when the intake air-fuel mixture is stoichiometric or lean, some CO will be generated in the engine. Poor mixing, local rich regions, and incomplete combustion will also be the source for CO emissions.
A well designed SI engine operating under ideal conditions can have an exhaust mole fraction of CO as low as 0.001. CI engines that operate overall lean generally have very low CO emissions.
EFFECTS OF CARBON MONOXIDE
It doesn’t take much CO to cause problems. Below is a table outlining the general effects of carbon monoxide on healthy adults. Individual susceptibility will vary. At lower levels, people sometimes mistake the symptoms of CO exposure for the flu, or do not associate their severe headache and nausea with carbon monoxide exposure.
People with heart or lung conditions or other health problems can be more sensitive to the effects of carbon monoxide. In addition the fetus of a pregnant woman can be adversely affected by carbon monoxide she inhales. For this reason WISHA Permissible limits for carbon monoxide are 35 ppm averaged over 8 hours with a 200 ppm ceiling limit.
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OXIDES OF NITROGEN
Exhaust gases of an engine can have up to 2000 ppm of oxides of nitrogen. Most of this will be nitrogen oxide (NO), with a small amount of nitrogen dioxide (NO2).
There will also be traces of other nitrogen-oxygen combinations. These are all grouped together NOx, with x representing some suitable number. NOx is very undesirable. Regulations to reduce NOx emissions continue to become more and more stringent year by year. Released NOx reacts in the atmosphere to form ozone and is one of the major causes of photochemical smog.
NOx is created mostly from the nitrogen in the air. Nitrogen can also be found in fuel blends. Further, fuel may contain trace amounts of NH3, NC, and HCN, but this would contribute only to a minor degree. There are a number of possible reactions that form NO. All the restrictions are probably occurring during the combustion process and immediately after. These include but are not limited to
O + N2 NO + N
N + O2 NO + O
N + OH NO + H
NO, in turn, can further react to from NO2 by various means, including
NO + H20 NO2 + H2
NO + O2 NO2 + O
At low temperature, atmospheric nitrogen exists as a stable diatomic molecule. Therefore, only very small trace amounts of oxides of nitrogen are found. However, at very high temperatures that occurs in the combustion chamber of an engine, some diatomic nitrogen (N2) breaks down to monatomic nitrogen (N) which is reactive.
If one goes a little deep into combustion chemistry it can be understood that chemical equations all react much further to the right as high combustion chamber temperatures are reached. The higher the combustion reaction temperature, the more diatomic nitrogen, N2, will dissociate to monatomic nitrogen, N, and the more NOx will be formed. At low temperature very little NOx is created.
Although maximum flame temperature will occur at a stoichiometric air-fuel ratio (φ = 1), the maximum NOx is formed at a slightly lean equivalence ratio of about φ = 0.95. At this condition flame temperature is still very high, and in addition, there is an excess of oxygen that can combine with the nitrogen to form various oxides.
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In addition to temperature, the formation of NOx, depend on the pressure and air-fuel ratio. Combustion duration plays a significant role in NOx formation within the cylinder. The amount of NOx generated also depends on the location of the spark plug within the combustion chamber. The highest concentration is formed around the spark plug, where the highest temperature occurs.
EFFECTS OF NITROGEN OXIDES (NOX)
NOx causes a wide variety of health and environmental impacts because of various compounds and derivatives in the family of nitrogen oxides, including nitrogen dioxide, nitric acid, nitrous oxide, nitrates, and nitric oxide.
Ground-level Ozone (Smog) – is formed when NOx and volatile organic compounds (VOCs) react in the presence of sunlight. Children, people with lung diseases such as asthma, and people who work or exercise outside are susceptible to adverse effects such as damage to lung tissue and reduction in lung function. Ozone can be transported by wind currents and cause health impacts far from original sources. Millions of Americans live in areas that do not meet the health standards for ozone. Other impacts from ozone include damaged vegetation and reduced crop yields
Acid Rain – NOx and sulfur dioxide react with other substances in the air to form acids which fall to earth as rain, fog, snow or dry particles. Some may be carried by wind for hundreds of miles. Acid rain damages; causes deterioration of cars, buildings and historical monuments; and causes lakes and streams to become acidic and unsuitable for many fish.
Particles – NOx reacts with ammonia, moisture, and other compounds to form nitric acid and related particles. Human health concerns include effects on breathing and the respiratory system, damage to lung tissue, and premature death. Small particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease such as emphysema and bronchitis, and aggravate existing heart disease.
Water Quality Deterioration – Increased nitrogen loading in water bodies, particularly coastal estuaries, upsets the chemical balance of nutrients used by aquatic plants and animals. Additional nitrogen accelerates “eutrophication,” which leads to oxygen depletion and reduces fish and shellfish populations. NOx emissions in the air are one of the largest sources of nitrogen pollution in the Chesapeake Bay.
Climate Change – One member of the NOx, nitrous oxide or N2O, is a greenhouse gas. It accumulates in the atmosphere with other greenhouse gasses causing a gradual rise in the earth’s temperature. This will lead to increased risks to human health, a rise in the sea level, and other adverse changes to plant and animal habitat.
Toxic Chemicals – In the air, NOx reacts readily with common organic chemicals and even ozone, to form a wide variety of toxic products, some of which may cause biological mutations. Examples of these chemicals include the nitrate radical, nitroarenes, and nitrosamines.
Visibility Impairment – Nitrate particles and nitrogen dioxide can block the transmission of light, reducing visibility in urban areas and on a regional scale in our national parks.
