POLLUTION FROM SI ENGINES

1
POLLUTION FROM SI ENGINES THEIR CONTROL
2
  Pollution from S.I. Engine  


Products of Complete
Products of Incomplete
Combustion
Combustion


NOx SOx Lead CO
HC Parti- Lead


culates  
3
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

4
 
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
5
Vehicular Emissions
6
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
7
EVAPORATIVE EMISSIONS
8

• 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.

9
      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.
10
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.
11
Filling Losses (Refueling Losses) Gasoline
vapours can escape when the vehicle is being
refueled in the service station.
12
Evaporative emissions increase significantly if
the fuel volatility increases
13

• 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

14

• Evaporative Emission Control
• Positive Crankcase Ventilation (PCV) System
• (for crankcase emissions)
• Charcoal Canister System
• (for Fuel tank and carburetor float bowl
  emissions)

15
(No Transcript)
16
(No Transcript)
17

• Exhaust Emissions
• CO
• NO
• HC

18
(No Transcript)
19
(No Transcript)
20

• 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.

21
(No Transcript)
22

• 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

23

• 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.

24

• 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.

25
(No Transcript)
26

• Among the major engine variables that affect NO
  emissions are
•  
• Equivalence Ratio
• Burned gas fraction (Residual gas plus EGR if
  any)
• Excess air
• Spark Timing

27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30

• HC Formation
• The sequence of processes involved in the engine
  out HC emissions is
•  
• Storage
• In-cylinder post-flame oxidation 
• Residual gas retention 
• Exhaust oxidation
• HC Sources
•  
• 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.

31

• 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.

32

• 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

33

• 5.        Liquid Fuel and Mixture Preparation
  Cold Start
• The largest contribution (gt90) 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

• 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.

34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37

• 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

38
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.
39

•  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.
•  

40

• 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

41

• Catalyst poisoning/degradation may be due the
  following causes
•  
• 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.
•  
• 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.
• Sintering is promoted by exposure of catalyst to
  high operating temperatures.
• Involves the migration and agglomeration of
  sites, thus determining their
• active surface area.

42
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.
43

• 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 F.
• 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.

44

• 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 (2030s) -
  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.

45
(No Transcript)
46
(No Transcript)
47
(No Transcript)
48
(No Transcript)
49
(No Transcript)
50
(No Transcript)