ME401_part_8

Internal Combustion Engines Dr. Md. Ehsan 2009 ME 401

Emission from IC engines Expected Products : CO2 and H2O Undesirable Products : CO Unburned HC NOx PM SOx Pb Major Emission Concerns :

SI Engines : CO, Unburned HC, NOx, Pb

CI Engines : PM, NOx, Unburned HC, SOx

Unburned HC :

Major Concern for SI engines. Could be as high as 6000 ppm, 1.5% of fuel Constitutes of fuel itself and partially reacted components.

Causes of Formation : Non-Stoichiometric AF ratios of combustion Incomplete combustion, EGR Crevice volumes Valve Overlap Oil or deposition on combustion chamber wall


Carbon Monoxide (CO) :

Major Concern for SI engines. Could be as high as 5% of exhaust volume.

Causes of Formation : Non-Stoichiometric AF ratios of combustion Incomplete combustion, EGR Nitrogen Oxides :

Major Concern for both SI and CI engines. Could be as high as 2000 ppm Constitutes of No and NO2

Causes of Formation : High temperature of combustion Availability of excess Oxygen at high temp Highest rate at slightly lean mixture (=0.95) Photo-Chemical Smog : NO2 + Energy from Sunlight NO + O O + O2 O3 (Ground level) Particulate Matter (PM) :

Major Concern for CI engines, may be in SI engines at high loads. High concentration effects the opacity of exhaust (smoke) Constitutes of Carbon Soot Particle, coated with SO3 and Soluble Organic Fractions (SOF) of fuel. Average mean diameter 10 m (PM 10), 2.5 m (PM 2.5)

Causes of Formation : Near-Stoichiometric AF ratios of combustion Presence of heavy components in the fuel Richer burning at high loads At higher temperatures : PM but NOX


Sulfur Oxides (SOx) :

Major Concern for CI engines.

Formation : Sulfur in fuel SI (150-600 ppm), CI (5000 ppm-1%) LSD (low sulfur diesel), ULSD (Ultra low sulfur diesel < 50ppm)


Emission Standards : Emission Standards may vary in different countries. The standards are getting stringent day by day. EURO stand. CARB stand. FTP stand. EPA stand. Mode stand.


Vehicle Emission test on Chassis Dynamometer


Driving Cycles


Measurement of Engine Emissions

ORSAT APPARATUS In the Orsat apparatus the analysis is determined volumetrically appears on a dry basis. It is convenient for measurement of CO2, CO and O2. The exhaust sample is exposed to a number of chemicals which absorb one of the constituent components.


Gas Analyzers 3-Gas Analyzers : CO2 , CO and HC 4-Gas Analyzers : CO2 , CO, HC and remaining Oxygen 5-Gas Analyzers : CO2 , CO, HC, remaining Oxygen and NOx


NDIRA : Mostly used for measuring gases like CO, CO2 and Total HC CO2 absorbs radiation 4-4.5 m, CO absorbs radiation 4.5-5 m


FID is mainly used for unburned HC measurement. Pure Ar-H2 flames use very little ionization but this changes with the presence of HC in exhaust.

NOx and O2 measurement is generally done using electro-chemical cells.


Smoke Meters :


Emission Control Techniques Pre-Engine System : Hot Air Intake System In-Engine System : MPFI, EGR, -sensor Post-Engine System : Thermal reactor, Catalytic Converter, DPF Manufacture needs to attain the tail pipe emission as per the standard, which may be done involving one or more of the systems.

Typical Sources of Emission from an Automobile


Emission as a function of Equivalence ratio In a SI engine

Emission as a function of Equivalence ratio In a CI engine


Thermal Reactors :

They provide more time for the exhaust at about 600C, allowing more complete combustion of some exhaust components. Mainly involves oxidation of HC and CO.

Exhaust gas Recirculation (EGR) EGR % = {mEGR/(mf+ma)} x 100 EGR ensures lower temperature and lower NOx formation, at the cost of lower volumetric efficiency, thermal efficiency and higher HC formation. EGR is typically done at the rate of 10-15% of the intake, not exceeding 30%. EGR is done on top of the normal residual fraction of exhaust gas. EGR is not done at idling, EGR is not done at WOT. In CI engines EGR is less common as abrasive soot particles may re-enter the engine.


CATALYTIC CONVERTERS (CC) Typically used for SI engine emission control. 2-WAY Catalytic Converter : Removes CO and HC 3-WAY Catalytic Converter : Removes NOx, CO and HC Catalytic Converter Efficiency = {1 (mexhaust out / mexhaust in)} x 100 % A CC may have different removal efficiencies for different components.

The Alumina wash coat ( about 20 m) containing the noble materials is used to enhance the surface area ( 100-200 m2/g) of chemical reaction.


Catalysts (Noble metals) : Platinum(Pt), Rhodium (Rd), Palladium(Pd); About 4-5 g/cat converter. CC could be either Monolith(Honey-comb) or Pellet(Bead) type. 2-Way Catalytic Converter : Catalysts - Pt, Pd Oxidation reaction : CO + O2 CO2 HC + O2 CO2 +H2O


3-Way Catalytic Converter : Catalysts - Pt, Rd, (Pd) Reducing reaction : NO + CO N2 + CO2 2 NO + 5H2 2NH3 + 2H2O NO + H2 N2 + H2O 2NO + 5CO + 3H2O 2NH3 + 5CO2 Oxidation reaction : CO + O2 CO2 HC + O2 CO2 +H2O

3-way catalytic converters work best with engines having precise control of air-fuel ratio. Many of them use O2 sensors incorporated at the exhaust, communicating with the engine management system. Specially the NOx reduction reaction requires very accurate air-fuel ratio, regular carburetors and simple fuel-injection are simply too inaccurate to keep up to the requirement.

Catalytic converters stars working effectively as they get warm above

250-300C. The reactions taking place are exothermic. Converter material should be able to with stand up to 1000C. Generally heat shields are used to protect other parts of the vehicle body.

Extra oxygen is needed to support the reactions, that might be

provided by lean air-fuel ratio or pump-type air injection.


Unleaded (Pb free) fuel is essential to prevent Fouling of Catalytic converters.

Most common cause of failure is an engine that pumps too much unburned fuel, which can overheat or carbon-clog the catalyst.

Fuel specification like Sulpher content should be maintained to prevent catalyst poisoning detoriating performance.

Fouling, clogging, meltdown, breakage of ceramic substance may cause a converter to stop doing its job, and/or plug it and raise back pressure.

In most automobiles you need to use a silencer in addition the catalytic converter to achieve desired low noise level.


DIESEL PARTICULATE FILTER (DPF)


Mitsubishi DPF System


Selective Catalytic Reduction (SCR) is the process whereby a reductant such as ammonia or urea is mixed with NOx emissions then passed through a special flow-through catalyst to create a reduction process. During the process, the ammonia-NOx mixture is converted into harmless nitrogen and water.

Comparative performance of various emission control systems

for diesel engines

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