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SI Engine Pollution and Control _ppt_ - PowerPoint Presentation

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SI Engine Pollution and Control _ppt_ - PowerPoint Presentation Powered By Docstoc
					POLLUTION FROM SI ENGINES
     & THEIR CONTROL
         Pollution from S.I. Engine


Products of Complete         Products of Incomplete
   Combustion                    Combustion


NOx   SOx    Lead       CO         HC      Parti-     Lead
                                           culates
                              I.C. Engine & Environment

   COx               HC                   NOx                     Lead                  SOx
Particulates


CO       CO2   CH4        Others    N2O     NO      NO2                       SO2         SO3   Particles   Smoke
                                                               Poison                           Aerosols    Soot

Poison   GHG   GHG    Carcinogens   GHG   P C Smog P C Smog    Visibility   Acid Rain   Acid Rain
                      P C Smog      OD    GHG      Acid Rain

                                                                                                      Visibility
                                                                                                      Irritation
                   S.I. ENGINE EMISSIONS



     EVAPORATIVE           CRANKCASE               EXHAUST



FUEL             CARB.             CO, HC, NOX, PART.
TANK          FLOAT BOWL
UBHC             UBHC



FOR THE S.I. ENGINE WITH CARBURETOR:

EVAPORATIVE EMISSIONS ACCOUNT FOR APPROXIMATELY 20%

CRANKCASE EMISSIONS ACCOUNT FOR APPROXIMATELY            20%

EXHAUST EMISSIONS ACCOUNT FOR THE BALANCE                60%
Vehicular Emissions
               The Internal Combustion Engine and Atmospheric Pollution

Type of Pollution        Principal Sources                 Relevance of the I.C. Engine

Lead                     Anti-knock compounds              A (for the SI Engine)

Carcinogens              Diesel exhaust                    A

Acid Rain                Sulfur dioxide                    B (for the CI Engine)
                         Oxides of nitrogen                A
                         Unburned hydrocarbons             A (for the SI Engine)
                         Carbon monoxide                   A (for the SI Engine)

Global warming           CFCs                              B (for car with A/c)
                                                           (or else not involved)
                         Carbon dioxide                    B (may be even A)
                         Methane                           B (may be A if CNG used)

Photochemical smog       Carbon monoxide                   A (for the SI Engine)
                         Unburned hydrocarbons             A (for the SI Engine)
                         Sulfur dioxide                    B (for the CI Engine)
                         Oxides of nitrogen                A

Ozone depletion          CFCs                              B (for car with A/c)
                                                           (or else not involved)
                         Unburned hydrocarbons             A (for the SI Engine)
                         Oxides of nitrogen                A

A: Major contributor
B: Secondary influence
EVAPORATIVE EMISSIONS
Major Sources:
Dirunal Emissions

   Take place from fuel tanks and carburetor float bowls
   (in engines fitted with carburetors) of parked vehicles.

   It draws in air at night as it cools down

   Expels air and gasoline vapour as it heats up during the day.

   These could be up to 50g per day on hot days.
Hot Soak Emissions

           This occurs after an engine is shut down.

           The residual thermal energy of the engine heats up
           the fuel system leading to release of fuel vapours.
Running Losses

       Gasoline vapours are expelled from the tank (or float bowl)
       when the car is driven and the fuel tank becomes hot.

       This can be high if the ambient temperature is high.
Filling Losses (Refueling Losses)

        Gasoline vapours can escape
        when the vehicle is being refueled in the service station.
“Evaporative emissions increase significantly
      if the fuel volatility increases”
• Evaporative emissions are tested in the
  “Sealed Housing Evaporative Determination – SHED” test procedure
   evolved in the US.
• Vehicle is placed in the enclosure and emissions are measured as
  the temperature in the fuel tank is increased.
• This gives diurnal emissions.
• Running losses are determined by running the vehicle on a chassis dynamometer
  with absorbent charcoal canisters attached at various possible emission sources.

• The latest procedure involves running the vehicle through
  3 standard driving cycles in the SHED.
• The hot soak test measures emissions for one hour immediately following
  the hot soak test.
• Acceptable losses from the complete procedure are 2g of fuel per test
  for US, Europe and India.
Evaporative Emission Control:

1. Positive Crankcase Ventilation (PCV) System
   (for crankcase emissions)

2. Charcoal Canister System
   (for Fuel tank and carburetor float bowl emissions)
Exhaust Emissions:

         1. CO

         1. NO

         1. HC
CO Formation

• Primarily dependent on the equivalence ratio.

• Levels of CO observed are lower than the maximum values
  measured within the combustion chamber

• but are significantly higher than equilibrium values
  for the exhaust conditions

• The processes which govern CO exhaust levels are
  kinetically controlled

• The rate of re-conversion from CO to CO2 is slower than
  the rate of cooling.

• This explains why CO is formed even with
  stoichiometric and lean mixtures.
NO Formation:

• There is a temperature distribution across the chamber due to passage
  of flame.

• Mixture that burns early is compressed to higher temperatures after
  combustion, as the cylinder pressure continues to rise.

• Mixture that burns later is compressed primarily as unburned mixture
  and ends up after combustion at a lower burned gas temperature.

• Using the NO formation kinetic model based on the extended
  Zeldovich mechanism:

         O + N2  NO + N

         N + O2  NO + O

         N + OH  NO + H
• Assuming equilibrium concentrations for O, O2, N2, OH and H
  corresponding to the equivalence ratio and burned gas fraction of the mixture
  we obtain the rate-limited concentration profile. The NO concentration
  corresponding to chemical equilibrium can also be obtained.

• The rate-controlled concentrations arise from the residual gas NO concentration,
  lagging the equilibrium levels, then cross the equilibrium levels and
  “freeze” well above the equilibrium values corresponding to exhaust conditions.

• Depending on details of engine operating conditions, the rate limited
  concentrations may or may not come close to equilibrium levels at
  peak cylinder pressure and gas temperature.

• The amount of decomposition from peak NO levels, which occurs
  during expansion depends on engine conditions as well as whether
  the mixture element burned early or late.

• The earlier burning fractions of the charge contribute much more to
  the exhausted NO than do later burning fractions of the charge.
•   Frozen NO concentrations in these early-burning elements can be
    an order of magnitude higher than concentrations in late burning elements.

•   In the absence of vigorous bulk gas motion, the highest NO
    concentrations occur nearest the spark plug.

•   These descriptions of NO formation in the SI engine have been confirmed
    by experimental observations.
Among the major engine variables that affect NO emissions are

1.   Equivalence Ratio
2.   Burned gas fraction (Residual gas plus EGR if any)
3.   Excess air
4.   Spark Timing
HC Formation:
The sequence of processes involved in the engine out HC emissions is:

1.   Storage
2.   In-cylinder post-flame oxidation
3.   Residual gas retention
4.   Exhaust oxidation

HC Sources

1.   Quench Layers

     •   Quenching contributes to only about 5-10% of total HC. However, bulk
         quenching or misfire due to operation under dilute or lean conditions
         can lead to high HC.
     •   Quench layer thickness has been measured and found to be in the range
         of 0.05 to 0.4 mm (thinnest at high load) when using propane as fuel.
     •   Diffusion of HC from the quench layer into the burned gas and
         subsequent oxidation occurs, especially with smooth clean combustion
         chamber walls.
2.   Crevices

     •   These are narrow volumes present around the surface of the combustion
         chamber, having high surface-to-volume ratio into which flame will not
         propagate.

     •   They are present between the piston crown and cylinder liner, along the
         gasket joints between cylinder head and block, along the seats of the intake
         and exhaust valves, space around the plug center electrode and between spark
         plug threads.

     •   During compression and combustion, these crevice volumes are filled with
         unburned charge. During expansion, a part of the UBHC-air mixture leaves
         the crevices and is oxidized by the hot burned gas mixture.

     •   The final contribution of each crevice to the overall HC emissions depends on
         its volume and location relative to the spark plug and exhaust valve.
3.       Lubricant Oil Layer

     •   The presence of lubricating oil in the fuel or on the walls of the combustion
         chamber is known to result in an increase in exhaust HC levels.

     •   The exhaust HC was primarily unreacted fuel and not oil or oil-derived compounds.

     •   It has been proposed that fuel vapor absorption into and desorption from
          oil layers on the walls of the combustion chamber could explain
          the presence of HC in the exhaust.

4.       Deposits

     •   Deposit buildup on the combustion chamber walls (which occurs in vehicles
         over several thousand kilometers) is known to increase UBHC emissions.

     •   Deposit buildup rates depend on fuel and operating conditions.

     •   Olefinic and aromatic compounds tend to have faster buildup
         than do paraffinic compounds.
5.    Liquid Fuel and Mixture Preparation – Cold Start

 •    The largest contribution (>90%) to HC emissions from the SI engine during
      a standard test occurs during the first minute of operation.

This is due to the following reasons:

 •   The catalytic converter is not yet warmed up

 •   A substantially larger amount of fuel is injected than the stoichiometric
     proportion in order to guarantee prompt vaporization and starting



6.   Poor Combustion Quality

     Flame extinction in the bulk gas before the flame front reaches the wall is a
     source of HC emissions under certain engine operating conditions.
Exhaust Emission Control:

Four basic methods are used to control engine emissions:

              1. Engineering of the combustion process

              2. Optimizing the choice of the operating parameters and

              3. Using after-treatment devices in the exhaust system.

              4. Using reformulated fuels, for example, oxygenated gasoline in winter to
                 reduce CO and low volatility gasoline in summer to reduce
                 evaporative HC.

         This requires advances in,

                   1. Fuel injector design
.                  2. Oxygen sensors
                   3. on-board computers
Two NOx control measures have been used since the 1970s, namely,

       1. Spark retard and
       2. Exhaust gas recirculation (EGR).

              Both methods reduce peak temperatures and hence NOx emissions.

              If EGR is used, spark timing has to be advanced to maintain
               optimal thermal efficiency.

              EGR fraction increases with engine load up to the lean limit – about 15-20%
               of the fuel-air mixture.

Currently, the most important after-treatment device is the Three-way catalyst (TWC),
which was first installed in the US in 1975.
Three-way catalyst consists of:

    • Rhodium – the principal metal used to remove NO

    • Platinum – the principal metal used to remove HC and CO

NO reacts with CO, HC and H2 via reduction reactions on the surface of the catalyst.

Remaining CO and HC are removed through an oxidation reaction
forming CO2 and H2O in the products.


Light-off temperature: The temperature at which the catalytic converter becomes
                       50% efficient. It is approximately 270oC for oxidation of HC
                       and about 220oC for oxidation of CO.

Conversion efficiency at fully warmed up condition is 98-99% for CO and 95% for HC,
depending on the HC components.
Catalytic Converter:
    •    Consists of an active catalytic material in a specially designed metal
         casing, which directs the exhaust gas through the catalyst bed

    •    Active material (noble metals like platinum, palladium and rhodium or
         base metals like copper and chromium)

Two types of configurations are commonly used,

    •     Ceramic honeycomb or matrix structure- also called monolith

    •     A bed of spherical ceramic pellets
Catalyst poisoning/degradation may be due the following causes:

1.   Overheating due to engine malfunction. About 20s of ignition failure
     in one cylinder at 4000 rev/min or above may provide sufficient temperature
     to destroy the catalyst.

2.   Presence of sulfur, phosphorus or lead in the fuel, especially lead, can poison
     the catalyst.

     With 0.75g Pb/liter, the efficiency drops to 40% in 10h of operation.

3.   Sintering is promoted by exposure of catalyst to high operating temperatures.
     Involves the migration and agglomeration of sites, thus determining their
     active surface area.
Oxidation Catalysts:

          The oxidation catalyst oxidizes CO and HC to CO2 and H2O.

          Sufficient oxygen must be present to oxidize CO and HC.

          Because of their higher intrinsic (inherent) activity, noble metals are
most suitable as catalytic material.

          A mixture of platinum (Pt) and palladium (Pd) is most commonly
used.

          For oxidation of CO, olefins, and methane: specific activity of Pd is
higher than that of Pt.

          For oxidation of aromatics: Pt and Pd have similar activity.

          For oxidation of paraffins (molecular weight greater than C3): Pt
is more active than Pd.
Three-way Catalysts

• If the engine is operated at all times with an air-fuel ratio at or close to
  stoichiometric then both NO reduction and HC/CO oxidation can be done in a
  single catalyst bed.

• The catalyst effectively brings the exhaust gas composition to a near-equilibrium
  state at their exhaust conditions, that is, a composition of CO2, H2O and N2.

• Enough reducing gases will be present to reduce NO and enough oxygen to oxidize
  CO and HC. Such a catalyst is called a Three Way Catalyst (TWC).

• It requires an electronic carburetor or a fuel injection system (FIS), through closed
  loop control of Φ.

• An oxygen sensor in the exhaust is used to indicate whether the engine is
  operating rich or in the lean side of stoichiometric and provide a signal for
  adjusting the fuel system to achieve the desired A/F.
• Commercial TWC contain Pt & Rh (Pt/Rh = 2 to 17), with some alumina,
  NiO and CeO2. Alumina is the preferred support material.

• Catalyst must be quickly warmed up (20–30s) - current system takes 2 min.

• Catalytic reactors must have low thermal inertia, that is, it must be constructed
  of material, which have low specific heat but high thermal conductivity. Hence
  warm up time to operating temperature will be less.

• Methods for decreasing warm up time are:

      1. Use of an after burner
      2. Locating the converter or use of a start up converter closer to the exhaust
                 valve/manifold.
        3. Electric heating - Additional cost plus a major drain in the battery;
required
         for starting the engine. Up to 1.5 kW for short period may be required.
   4. Absorb the UBHC during cold start and release it after warming up.

				
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