This is a part of Lecture Series on Automotive Fuels & Emissions for M. Tech Students at ARAI ACADEMY.

1. EMISSION FORMATION
IN
S.I. ENGINES
Dr. K. C. Vora
Dy Director & Head
ARAI Academy,
ARAI, Pune
Copyright © 2006 ARAI EDL

2. Four-stroke Engines
2

3. INTRODUCTION TO SI ENGINE
 Air Fuel Mixture:
 In traditional SI engines, the fuel and air are mixed together in the
intake system using a low pressure (circa 2 to 3 bar) fuel injection
system (carburettors no longer used).
 Fuel injection system is normally multi-point port injection, which
means that there is one fuel injector (sometimes two) in each inlet
port.
 Multi-point injectors normally inject fuel onto the back of the closed
inlet valve using sequential timing with the required amount o f fuel
quantity being updated by the ECU every engine event.
 Air/fuel Ratio, AFR:
 The AFR has a very significant effect on the power output, thermal
efficiency and exhaust emissions and has to be controlled
precisely over the whole operating range.
 All modern engines use an electronic control unit (ECU) and
various sensors and actuators to control the AFR.
 The air to fuel ratio by mass (AFR) is typically 14.3 to 14.7 for
gasoline fuels.

4.  Spark Ignition Combustion
 Homogeneous mixture of air, fuel and
residual gas.
 Spark ignition shortly before TDC.
 Flame propagation.
 The combustion typically takes 50
degrees of crank angle
 The products of combustion: N2, CO2,
Figure 1.19 Idealised SI engine flame propagation
H2O vapour, O2, CO, H2, HCs, NOx.
 Cycle to cycle variation
 knock

5. THERMODYNAMIC GAS CYCLES
Otto Cycle
1 – 2: isentropic compression
2 – 3: constant-volume heat addition p 3
3 – 4: isentropic expansion
4 – 1: constant-volume heat rejection
 Compression ratio V V
r= 1 = 4
V V 2
2 3 4
 Heat addition Qin=mcv(T3-T2) 1
 Heat rejection Qout=mcv(T4-T1) V
5

6.  Isentropic compression
 Perfect gas pV = mRT
 Isentropic process pVγ = constant
Isentropic expansion p T
 2 = rγ 2 = rγ −1
p T
1 1
γ γ −1
Cycle efficiency
p4 =  1 
  T4 1

p3  r  = 
 
  T3 r
Wout Qin − Qout 1
η = = = 1−
Otto Q
in Qin γ −1 r
6

7. Fuel & Air
Gasoline Kerosene Diesel Heavy fuel
 Fuels C 85.5 86.3 86.3 86.1
• Gasoline: assume iso-octane (C8H18)
H 14.4 13.6 12.8 11.8
• Diesel: assume deodecane (C12H26)
S 0.1 0.1 <0.9 2.1
 Air
• Molar mass of air = 0.21 x 32 + 0.79 x 28 = 28.8 (kg/kgmol)
O2 N2
 Fuel and Air Mixture
% by volume 21.0 79.0
• Stoichiometric air/fuel ratio
• Rich mixture % by mass 23.3 76.7
• Weak (or lean) mixture
Stoichiometric air/fuel ratio
Fuel/air equivalent ratio φ: φ=
Actual air/fuel ratio
Air/fuel equivalent Actual air/fuel ratio 1
ratio λ: λ= =
Stoichiometric air/fuel ratio φ
7

8. Gasoline Air
C7 H13 + 10 O2 + 39 N2
The
Combustion
Process Energy!!
(theoretical) 7 CO 2 + 6.5 H2 O + 39N 2
Carbon Water
Nitrogen
Dioxide (Steam)
Gasoline : CnH1.87n

9. Gasoline Air
The C7 H13 + 10 O2 + 39 N2
Combustion
Process Energy!!
(Actual) x1.CO2 + x2.H2 O + x3.N2
Carbon Water Nitrogen
Dioxide (Steam)
x4.O2 + x5.CO + x6.NOx + x7.CXHY
Oxygen Carbon Oxides of Hydro-carbon
Monoxide Nitrogen

10. Automotive Emissions
Fuel + Air →
Combustion
CO 2 + O 2 + N 2 + H 2 + ... +
   
Products
CO + HCs + PMs
 NOx + 
 
Pollutants
10

11. Today's Air Real Fuel
The
Combustion
Process
Pollutants:
Unburned
(actual) Exhaust:
• Nitrogen
Hydrocarbons
Carbon Monoxide
• Water (steam) Oxides of
• Carbon Dioxide Nitrogen
• Pollutants Other elements
or compounds

12. Refueling Evaporative
Losses Emissions
The
Motor
Vehicle as
a Source
of Air
Pollution
Exhaust Crankcase
Emissions Losses,
etc.

13. • In the engine
-incomplete
combustion
How -"wall quench"
-high pressure
Emission and temp
s -"Blowby"
• Due to
are evaporation of
Formed fuel
-"breathing"
-hot engine and
fuel
-displacement of
vapors

14. TYPICAL ENIGNE OUT
EMISSION
NOx : 100 to 1000 ppm or 10g/kg fuel
CO : 1 to 2 percent or 200g/kg fuel
HC : 1500 ppm (as C1) or 10g/kg fuel

15. MAJOR CAUSES OF HC EMISSIONS
1. Evaporative losses from fuel tank, fuel lines
and carburetor.
2. Fuel composition.
3. Air/Fuel (A/F) ratio deviation from stoichiometry.
Fuel air mixture is too lean to burn. Lower temperature reduces
evaporation.
Fuel air mixture is too rich to burn resulting in-complete combustion.
4. Incomplete combustion.
5. Flame quenching at walls.
6. Absorption and desorption in lubricating oils
and deposits.
7. Crevices in combustion chamber and piston
rings.
8. Short-circuiting of fresh charge.

16. HOW HC EMISSIONS ARE FORMED?

17. 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.

18. 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.

19. 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.

20. 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.

21. HYDRO-CARBON COMPOSITION OF
SPARK-IGNITION ENIGNE EXHAUST
(BY CLASS)
Carbon, Percent of total HC
Paraffins Olefins Acetylee Aromatics
Without
catalyst 33 27 8 32
With
catalyst 57 15 2 26

22. HOW CO EMISSIONS ARE FORMED?
Carbon monoxide is formed due to in-homogenity of
fuel distribution with rich A/F mixture. This is an
intermediate product in the combustion of hydrocarbon
fuels.
CO is formed when-
• Oxygen is not available in adequate quantity.
• Cycle temperatures are low.
• Primarily dependent on the Air/Fuel Ratio.
• Levels of exhaust manifold CO are lower than the
maximum values measured within the combustion chamber
• 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.

23. HOW NOx EMISSIONS ARE FORMED?
• 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.

24. THE MAJOR CAUSES OF NOx
EMISSIONS
• Higher Combustion Temperature.
• Higher oxygen content.
• Ample Resident / reaction time
NO = Nitric Oxide (Predominant), NO2 = Nitrogen Dioxide
Extended Zeldovich mechanism:
O + N2 = NO + N
N + O2 = NO + O
N + OH = NO + H
Zeldovich was the first to suggest the importance of
first two reactions and
Lavoice added 3rd reaction to the mechanism.

25. Effect of
Air-Fuel
Ratio on
Emission
(General)

26. Effect of
Air-Fuel
Ratio on
Emission
(Typical)
Fig. 1-1 Spark ignition engine emissions for different fuel/air equivalent ratios
26

27. Effect Air-Fuel Ratio on Engine Performance
Fig. 1-8 Response of specific fuel consumption and power output to
27 changes in air/fuel ratio

28. CRITICAL FACTORS & ENGINE VARIABLES
IN HC EMISSION MECHANISMS
(a) Crevices
(1) Crevice volumes
(2) Crevice Location (relative to spark Plug)
(3) Load
(4) Crevice wall temperature
(5) Mixture composition
1) Formation of (b) Oil layers
(1) Oil consumption
HC (2) Wall temperature
(3) Speed
(c) Incomplete combustion
(1) Burn rate and variability
(2) Mixture composition†
(3) Load
(4) Spark timing‡
(d) Combustion chamber walls
(1) Deposits
(2) Wall roughness

29. CRITICAL FACTORS & ENGINE VARIABLES
IN HC EMISSION MECHANISMS
(a) Mixing rate with bulk gas
(1) Speed
(2) Swirl ratio
(3) Combustion chamber shape
(b) Bulk gas temperature during expansion and
2) In-cylinder exhaust
mixing and (1) Speed
(2) Spark timing‡
oxidation
(3) Mixture composition†
(4) Compression ratio
(5) Heat losses to walls
(C) Bulk gas oxygen concentration
(1) Equivalence ratio
(D) Wall temperature
(1) Important if HC source near wall
(2) For crevice: importance depends on
geometry

30. CRITICAL FACTORS & ENGINE VARIABLES
IN HC EMISSION MECHANISMS
(a) Residual fraction
(1) Load
(2) Exhaust Pressure
(3) Valve overlap
3) Fraction HC (4) Compression ratio
Flowing out (5) Speed
of cylinder (b) In Cylinder flow during exhaust stroke
(1) Valve overlap
(2) Exhaust valve size and location
(3) Combustion chamber shape
(4) Compression ratio
(5) Speed

31. CRITICAL FACTORS & ENGINE VARIABLES
IN HC EMISSION MECHANISMS
(a) Exhaust gas temperature
(1) Speeds
(2) Spark timing
(3) Mixture composition
(4) Compression ratio
(5) Secondary air flow
4) Oxidation in (6) Heat losses in cylinder and exhaust
(b) Oxygen Concentration
exhaust (1) A/F ratio
system (2) Secondary air flow and addition point
(c) Residence time
(1) Speed
(2) Load
(3) Volume of critical exhaust system
component
(d) Exhaust reactors:
(1) Oxidation catalyst
(2) Three-way catalyst

32. Thank You
….