Engine Emissions & Their Cotrol

Problems Caused by Engine Emissions:

Internal combustion engine generates undesirable emissions which can cause serious problems:

• Global Warming 
• Acid Rain
• Smog
• Odors
• Respiratory Problems 
• Health Problems

Classification of Engine Emissions:

Engine emissions are further classified into two types:

1. Exhaust Emissions:

These emissions will result after the combustion of fuel. These emissions include:

• Unburnt Hydrocarbon (HC)
• Oxides of Carbon (Carbon Monoxide & Carbon Dioxide)
• Oxides of Nitrogen(NO & Nitrogen Dioxide)
• Oxides of Sulphur(SO & Sulphur Dioxide)
• Particulates 
• Soot and Smoke

2. Non-Exhaust Emissions:

These emissions result before the combustion of fuel. These emissions include:

1. Fuel Tank  ↣  The fuel tank emits fuel vapor into the atmosphere.
2. Carburetor  ↣  It also gives fuel vapor.
3. Crankcase  ↣  It emits blow-by gases and fuel vapor into the atmosphere.

Causes of Engine Emissions:

These emissions results from:

• Non-Stoichiometric Combustion
• Dissociation of Nitrogen
• Impurities in Fuel and Air 

Causes Of Hydrocarbon Emissions:

The causes of hydrocarbon emissions are as follows:

1. Incomplete Combustion:

When fuel gets into the combustion chamber and mixed with air and combust, some hydrocarbon emitted. The reason of this emissions or incomplete combustion of fuel is due to:

• Improper Mixing  ↣  When fuel and air is mixed in combustion chamber, some fuel particles might not get oxygen molecules and result in emissions.
• Flame Quenching  ↣  As the flame gets close to the walls, it is quenched at the walls and result in small volume of unreacted A/F mixture.

2. Crevice Volume & Flow in Crevices:

During compression stroke, fuel gets into the crevice volume(Spaces between walls and piston or piston rings indentation) and not combusted and result in hydrocarbon emissions.

3. Valve Overlap:

During intake fuel into the combustion chamber, a time will come when both the valves open simultaneously and fuel coming into the C/C directly goes out of the C/C and result in HC emissions.

4. Deposits on Walls:

Fuel gets into the unreacted deposited volume of F/A mixture and not get combusted. During exhuast stroke, it goes out of the combustion chamber.

Carbon Monoxide (CO) Emissions:

• Odorless, colorless but poisonous gas.
• Maximum carbon monoxide is generated when an engine runs on rich F/A mixture.
• Carbon monoxide should be converted into carbon dioxide for desirable emissions.

Nitrogen Oxides (NOx) Emissions:

• Noxes are very undesirable because they react with air to from Ozone which then produce Photochemical Smog.
• Dissociation of nitrogen molecule is at high temperature.
• To control noxes, you should control flame temperature, advance spark (loss in power), exhaust gas recirculation(EGR, circulate little amount if exhaust gases in cylinder to absorb heat to decrease flame temperature).   

Other Emissions:

• Photochemical Smog  ➽  Reaction of nitrogen dioxide with sunlight produce nascent oxygen when can form ozone by reacting with free oxygen.

• Particulates  ➽  The exhaust of CI engine contain soot particles(cluster of solid carbon) and produce pollution.
• Aldehydes  ➽  When alcohol fuel is used and engine is running at high compression ratio, it release aldehyde which then reacts with water to from carboxylic acid producing irritation in eyes.
• Lead(Pb)  ➽  Mostly emitted from gasoline additives, battery factories and non-ferrous smelters. It can increase octane number but is a worst toxic.
• Sulphur Oxides  ➽  They reacts with air to from sulfite and sulfate acids. It is usually an emission of Diesel engines.

Emissions Control of SI Engines:

We control emissions by three methods which are::

1. Modify Engine
2. Modify fuel (Not to be discussed)
3. After Treatment

1. Modify Engine:

Modification in engine depends upon following engine components or variables which are:

• Combustion Chamber Configuration  ➽  to reduce flame quenching zones, we deal with combustion chamber's surface area to volume ratio and space around piston rings.
• Lower Compression Ratio  ➽  If flame temperature is low, reduction in quenching zones and reduce noxes.
• Modified Induction System  ➽  Injection should done at high velocities. You should use injection system rather multi-choke carburetors.
• Ignition Timing  ➽  Advance spark which can reduce emissions of noxes.
• Reduce Valve Overlap  ➽  Reduce valve opening time, by Variable Valve Timing(VVT).

 2. After Treatment or Emission Control By Exhaust Gas Oxidation:

This treatment is done by Exhaust gas oxidation which includes:

 A. Thermal Converters:

• It only deals with Hydrocarbon and carbon monoxide.
• The exhaust gases are further send to a converter in which it is heated with air to form desirable emissions.
• Exhaust gases are heated at 200-700℃ with secondary air to produce water and carbon dioxide.

B. Catalytic Converters:

• It deals with Hydrocarbon, carbon monoxide and noxes.
• Catalytic converters consist of two plates: reducing and oxidizing plate.
• Exhaust gases are first passed through reducing porous plate where it is converted into nitrogen gas.
• And then pass through oxidizing porous plate where it oxidize into carbon dioxide and water.

C. After Burner:

• It deals with hydrocarbon and carbon monoxide emissions.
• It is a burner in which secondary air is introduced and is heated by ignition source to produce desirable emissions.
• After-burner is located just after the exhaust manifold.

D. Exhaust Manifold Reactor(EMR):

• It deals with hydrocarbon and carbon monoxide emissions.
• They are enlarged and insulated exhaust manifolds into which secondary air is injected and continues burning to produce desirable emissions.
• The efficiency of this unit depends upon the amount of heat generated and the time, the gas is within the manifold.

E. Exhaust Gas Re-Circulation(EGR):

• It only deals with noxes.
• 10-15% of exhaust gases are recirculated into the combustion chamber.
• These injected exhaust gases act as heat absorber and reduce temperature inside the combustion chamber which reduce the dissociation of nitrogen molecule and reduce noxes emissions.
• The exhaust gas recirculation will dilute the A/F mixture, volumetric and thermal efficiency decreases which reduce flame temperature, reduction in nitrogen molecule dissociation, emissions reduced.

F. Particulate Trap(Solid Particles):

• Only in CI engines because of fuel's high molecular weight.
• It removes 60-90% of particles from exhaust flow.
• As the particulate trap is fills with soot and restrict flow, the exhaust temperature rises but it is still not high enough to ignite the soot and regenerate the trap.

Crankcase Blowby:

Flow of F/A mixture into crevices or crankcase or quenching zones and left un-combusted and release into atmosphere is called Crankcase Blowby.

• Blowby is proportional to Engine speed.
• For new engines it is 0.1 cubic meter/min.
• For old engines, it is greater then that.

Note: 

• If you want to know about Selective Catalytic reduction, you should watch this video:

• How exhaust gas system works? Watch the video given below:

References:

• University notes + my knowledge.
• Videos from YouTube.
• Images from Google Images.
• Book: IC Engines by V Ganesan (4th Edition).
• Class Lectures.