IC Engines : Fuel Emission

The major causes of these emissions are non-stoichiometric combustion, dissociation of nitrogen, and impurities in the fuel and air.

Types of Engine Emissions: Engine emissions can be classified into two categories:
(1). Exhaust Emissions
(2). non-exhaust emissions

Exhaust Emissions:

(i). Unburnt hydrocarbons (HC): It mainly consists of hydrogen and carbon which are emitted through the burning of petrol or diesel in an engine.

Cause of HC emissions from SI Engine:
(a). Incomplete combustion: The reasons for the incomplete combustions are improper mixing and Flame quenching.

(b). Crevice volumes and flow in crevices

(c). Leakage past the exhaust valve
(d). Valve overlap
(e). Deposition on walls
(f). Oil on combustion chamber walls

HC variation vs surface to volume (S/V) ratio:

(ii). Carbon Monoxides (CO): 

• Caused by incomplete combustion of fuel and air.
• Maximum CO is generated when an engine runs rich (usually in staring condition or accelerating under load).
• The percentage of CO decreases with speed.

(iii). Oxides of nitrogen {NO_x (NO and NO₂)}:

• At lower temperatures, di-atomic nitrogen N2 is stable and at very high temperatures it will dissociate to monoatomic nitrogen N and will react with oxygen to form nitrogen oxides.
• The maximum NO_x forms at a slightly lean equivalence ratio of about ϕ =0.95.
  

(iv). Photochemical smog: 

• NO_x is the primary cause of Photochemical smog, which has become a major problem in many large cities of the world.
• Smog is formed by the photochemical reaction of automobile exhaust and atmospheric air in the presence of sunlight. NO₂ decomposes into NO and monoatomic oxygen.
  NO₂ +Energy from sunlight ------------> NO +O + Smog

(v). Oxides of Sulphur [SOx (SO2 and SO3)]:

• At high temperatures, Sulphur combines with hydrogen to form H₂S and with oxygen to form SO₂.

(vi). Particulate Matter (PM10):

• PM10 is a general term for tiny airborne particles (under ten microns), e.g., dust, soot, smoke.
• Primary sources are fuel-burning plants and other industrial commercial processes.

Non-exhaust Emissions:

(i). Evaporative losses (both from fuel tank and carburetor):

• These are generally volatile organic compounds.
• Evaporative emissions account for 15 to 25% of total hydrocarbon emissions from a gasoline engine.
• The less the tank fill, the greater is the evaporative loss.

(ii). Crankcase blow-by (from crankcase):

• The blow-by is the phenomenon of leakage past the piston and piston rings from the cylinder of the crankcase.
• The blow-by HC emission is about 20% of the total HC emission from the engine. This is increased to about 35% if the rings are worn.
  

The effect of air-fuel ratio on different emissions: 

(1). In SI Engine: The variation of HC, CO and NOx emissions as a function of equivalence ratio for an SI engine.

(2). In CI Engine: A qualitative picture of HC, CO, and NOx emission with respect to equivalence ratio, ϕ for a four-stroke CI Diesel engine. 

Emission Control Methods:

(i). Modification in the Engine Design and Operating Parameters

(a). Combustion chamber configuration

(b). Lower compression ratio

(c). Modified induction system

(d). Ignition timing

(e). Reduced valve Overlap

(ii). Exhaust Gas Oxidation: The exhaust gas coming out of the exhaust manifold is treated to reduce HC and CO emissions.

(a). After Burner

(b). Exhaust manifold reactor

(c). Catalytic converter: A catalytic converter is a device that uses a catalyst to convert three harmful compounds in car exhaust into harmless compounds. The three harmful compounds are:
(a). Hydrocarbons (in the form of unburned gasoline)
(b). Carbon monoxide (formed by the combustion of gasoline).
(c). Nitrogen oxide (created when the heat in the engine forces nitrogen in the air to combine with oxygen).

Major drawbacks of the catalytic converter are as under:
(a). Owing to the exothermic reactions in the catalyst bed the exhaust systems are hotter than normal.
(b). Cars equipped with such converter should not use leaded fuel as lead destroys complete catalytic activity.

(iii). Blow by control: The basic principle of crankcase blow-by control system is recirculation of vapors back
to the inlet manifold with the help of positive crankcase ventilation (PCV) system.

(iv). Control of Oxides of Nitrogen (NOx): The concentration of oxides of nitrogen in the exhaust is closely related to the peak cycle temperature. The following are the three methods (investigated so far) for reducing peak cycle temperature and thereby reducing NOx emission.
a. Exhaust gas recirculation (EGR):

EGR is defined as the mass percent of total intake flow:

Not only does EGR reduces the maximum temperature in the combustion chamber, but it lowers the overall combustion efficiency.
b. Catalyst: A few types of catalysts have been tested to reduce the emission of NOx; a copper catalyst has been used in the presence of CO for this purpose. 
c. Water injection: It has been observed that the specific fuel consumption decreases a few percent at a medium water injection rate.

Attempts have been made to use water as a device for controlling the NOx. This method, because of Its complexity, is rarely used.

Effects of engine emissions on human health: 

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