22-04-2011, 12:16 PM
Automotive emission control systems.pptx (Size: 84.75 KB / Downloads: 126)
Automotive Emission Control Systems
SI Engine Emissions
There are three main types/ sources from which pollutants are emitted from the SI Engines
Crank Case : where the piston blow-by fumes and oil mist are vented to the atmosphere
The fuel system: where Evaporative emissions from the carburettor or petrol injection air intake and fuel tank are vented out to the atmosphere
The Exhaust system : where the products of incomplete combustion are expelled from the tail pipe into the atmosphere.
Crank Case Emissions
The piston and its rings are designed to form a gas tight seal between the sliding piston and the Cylinder walls
However, in practice there is always a certain amount of gases escaping leaking through the piston rings. These gases may be unburnt hydrocarbons, products of combustion, CO2, H2O, or CO. this phenomenon is known as Blow-by.
The blow-by increases with the engine speed, and particularly, when the cylinder walls wear out, it becomes more noticeable in the upper speed range
Crank Case Emissions
The blow- by takes place through the piston ring gap, between the piston ring and grooves, and also when the shape of the piston does not match with that of the cylinder(near the TDC)
It reduces the engine compression ratio and Power Developed
It may lead to a high concentration of the unburnt gases in the crank case, leading to the chances of explosion
The air fuel mixture (fully or partially burnt), contaminate the lubricating oil.
Crank Case Emissions
Since it is impossible to eliminate the blowby completely, the crank case, and camshaft case is deliberately connected, to carry the blow by fumes.
The removal of blow by gases and the vapour fumes from the crank case is obtained by creating a partial depression at the outlet location so that high pressure blow-by is attracted by lower pressure region of crank case, where the gases are expelled
Crank Case Emissions
Following are the two methods of creating extraction depression.
The road draught crank case ventilation system
The induction manifold vacuum positive crankcase ventilation system.
Evaporative Emissions
Evaporative emissions account for about 15 to 25% of total hydrocarbon emission from gasoline
The two main sources are
Fuel tank,
Carburettor
Fuel Tank: the main factors governing the tank emissions are
fuel volatility and the
ambient temperature.
Tank design and location – affects the temperature
Evaporative Emissions
Carburettor: two categories:
Running Losses
Parking losses
Exhaust Emissions
Hydrocarbons: The emission amount of HC (due to incomplete combustion) is closely related to :
Design variables(induction system, combustion chamber design)
Operating variables(A/F ratio, speed, load)
Mode of operation(idling, running, accelerating)
Exhaust Emissions
Factors affecting the HC Emissions
Surface-volume ratio
Wall quenching
Incomplete Combustion: when the air fuel ratio is rich or lean, the flame propagation is affected giving HC emissions. The incomplete combustion may be due to:
Low charge temperatures
Too rich or too lean mixtures
Poor condition of ignition systems
Non uniform fuel in the mixture supplied to the engine
Residual gas in the cylinder.
Spark plug timing
Compression ratio
Exhaust Emissions
Carbon Monoxide (CO) :
Incomplete oxidation of CO to CO2
Improper air fuel ratio: (ideal 15)
More CO Percentage during engine idling, and reduced emissions at high speeds
Emissions are lowest during acceleration and at steady speeds
Exhaust Emissions
Oxides of Nitrogen (NOx)
NO, NO2 are generally formed at higher temperature.
Max NOx levels are found at 10 % higher mixture than stochiometric. Then it decreases because of lower temperature attained
Factors affecting NOx emissions
A/F ratio
R.p.m.
Angle of advance
It is also observed that NOx increases with increasing manifold pressure, compression ratio, and engine load
Evaporative Loss Control Device(ELCD)
This device consists of a
Adsorbent chamber
Purge control valve, actuated by exhaust back pressure
Pressure control valve
Cleaning the exhaust gas
After Burner:
A device which consists of a chamber and an ignition device., present after the exhaust manifold
Cleaning the exhaust gas
Exhaust manifold reactor:
Fresh air, mixed with the high temperature Hydrocarbons react to complete the oxidationthis system consists of an oxidation zone/ reaction zone very near to the exhaust manifold. It consists of an air pump, which pumps the filtered air. The degree of oxidation depends on the temperature of the exhaust , time and mixing provided
Cleaning the exhaust gas
Catalytic Converter:
It is a device, which induces oxidation reactions in the presence of catalyst.
It consists of two separate elements: one for NOx and the other for CO/HC emission.
Secondary air is injected ahead of first element.
Controlling the Air-Fuel mixture
Reduction of emissions can be obtained by operating the engine with leaner air:fuel ratio.
This can be achieved by modifying the carburettor
The leaner mixtures may be operated for idling and cruise operation.
This requires the increase in idle speeds to prevent stalling and rough idle, usually associated with leaner mixtures.
Controlling the Air-Fuel mixture
The fuel distribution can be improved by
better manifold design,
inlet air heating,
raising of coolant temperature
Use of electronic fuel injection system
Controlling the Combustion Process
Use of leaner air-fuel mixtures: (as stated above)
Retarding the ignition timing:
This reduces the NOx , as it decreases the maximum temperatures.
Reduces HC emissions by causing higher exhaust temperatures
Results in greater requirement of cooling, and loss in power
However, normal spark advance during acceleration may be provided.
Controlling the Combustion Process
Modification of Combustion chamber design:
To avoid flame quenching zones, where otherwise, the combustion may be incomplete which results in high HC emissions.
This includes:
Reduction in surface to volume ratio
Reduced squish area
Reduced deal space around the piston ring
Reduced distance of top surface of piston from the top piston ring
Controlling the Combustion Process
Lower compression ratio:
Reduces the quenching space and area, thus reducing the HC emissions
Reduces NOx emissions due to lower maximum temperatures
However it also results in loss of power and fuel economy
Controlling the Combustion Process
Reduced valve overlap:
Valve overlap results in escaping out of fresh charge directly out of the exhaust valve as it enters thro the inlet valve.
This reduction in valve overlap reduces HC emissions
Controlling the Combustion Process
Induction system modification: The hydrocarbon loss, and their emission can be reduced by accurate metering of fuel and air at the induction system/carburetor.
Inlet air heating
Use of carburetors having closer carburetion tolerances
Special type of carburetors like ‘high speed carburetors’ and ‘multi choke carburetors’