Effect of SI Engine Design and Operating Variables on Emissions Notes | EduRev

: Effect of SI Engine Design and Operating Variables on Emissions Notes | EduRev

 Page 1


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_1.htm[6/15/2012 3:01:19 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Effect of SI Engine Design and Operating Variables on Emissions
The Lecture Contains:
SI Engine Variables and Emissions
Compression Ratio
Ignition Timing
Air –Fuel Ratio
Residual Gas and EGR
Engine Speed
Cold Start and Warm-up Phase
Coolant Temperature
Summary
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 2


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_1.htm[6/15/2012 3:01:19 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Effect of SI Engine Design and Operating Variables on Emissions
The Lecture Contains:
SI Engine Variables and Emissions
Compression Ratio
Ignition Timing
Air –Fuel Ratio
Residual Gas and EGR
Engine Speed
Cold Start and Warm-up Phase
Coolant Temperature
Summary
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2.htm[6/15/2012 3:01:19 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
SI Engine Variables and Emissions
Any engine variable that affects oxygen availability during combustion would influence CO emissions.
The factors which influence flame quenching, quench layer thickness and post flame oxidation control
engine out HC emissions. The burned gas temperature-time history and oxygen concentration control
NO formation and emission. Hence the engine variables that influence burned gas temperature and
oxygen concentration would affect the NO emissions.
Principal design and operating variables affecting engine emissions are:
Design Variables :
Compression Ratio
Combustion chamber surface to volume ratio
Ignition timing
Valve timings and valve overlap
Air motion, swirl tumble etc
Charge stratification
Operating Variables :
Air-fuel Ratio
Charge dilution and exhaust gas recirculation (EGR)
Speed
Load
Coolant temperature
Transient engine operation: acceleration, deceleration etc.
The effect of some variables discussed below is typical in nature and variations in the trends with
specific engine design change are observed.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_1.htm[6/15/2012 3:01:19 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Effect of SI Engine Design and Operating Variables on Emissions
The Lecture Contains:
SI Engine Variables and Emissions
Compression Ratio
Ignition Timing
Air –Fuel Ratio
Residual Gas and EGR
Engine Speed
Cold Start and Warm-up Phase
Coolant Temperature
Summary
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2.htm[6/15/2012 3:01:19 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
SI Engine Variables and Emissions
Any engine variable that affects oxygen availability during combustion would influence CO emissions.
The factors which influence flame quenching, quench layer thickness and post flame oxidation control
engine out HC emissions. The burned gas temperature-time history and oxygen concentration control
NO formation and emission. Hence the engine variables that influence burned gas temperature and
oxygen concentration would affect the NO emissions.
Principal design and operating variables affecting engine emissions are:
Design Variables :
Compression Ratio
Combustion chamber surface to volume ratio
Ignition timing
Valve timings and valve overlap
Air motion, swirl tumble etc
Charge stratification
Operating Variables :
Air-fuel Ratio
Charge dilution and exhaust gas recirculation (EGR)
Speed
Load
Coolant temperature
Transient engine operation: acceleration, deceleration etc.
The effect of some variables discussed below is typical in nature and variations in the trends with
specific engine design change are observed.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2_1.htm[6/15/2012 3:01:19 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Compression Ratio
The effect of compression ratio on engine emissions is shown on Fig. 3.1. The typical effect observed
when the engine CR was reduced from 10:1 (CR used on high performance engines during pre-
emission control period) to 8.5 and 7.0:1 are given on this figure.
Use of high CR results in
(i) Higher burned gas temperature
(ii) Lower residual gas content
These lead to higher  NO emissions on volume basis. However, as engine efficiency increases with
increase in compression ratio, brake specific NO emissions decrease. High CR combustion chambers
result in
(i) High surface to volume ratio and
(ii) A proportionately higher crevice volume.
(iii) Lower exhaust gas temperatures
Thus the volume of flame quenching regions increases resulting in higher HC  emissions. The problem
is further enhanced as due to lower exhaust gas temperatures oxidation of  the  unburned HC is
reduced  during exhaust process. These factors result in an increase in HC emissions with increase in
engine CR.  At lower CR% fuel efficiency is also reduced thus increasing  specific CO emissions.
Figure
3.1
Effect of reduction in compression ratio from 10:1 to 8.5:1 and
7.0:1 on SI engine emissions
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 4


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_1.htm[6/15/2012 3:01:19 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Effect of SI Engine Design and Operating Variables on Emissions
The Lecture Contains:
SI Engine Variables and Emissions
Compression Ratio
Ignition Timing
Air –Fuel Ratio
Residual Gas and EGR
Engine Speed
Cold Start and Warm-up Phase
Coolant Temperature
Summary
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2.htm[6/15/2012 3:01:19 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
SI Engine Variables and Emissions
Any engine variable that affects oxygen availability during combustion would influence CO emissions.
The factors which influence flame quenching, quench layer thickness and post flame oxidation control
engine out HC emissions. The burned gas temperature-time history and oxygen concentration control
NO formation and emission. Hence the engine variables that influence burned gas temperature and
oxygen concentration would affect the NO emissions.
Principal design and operating variables affecting engine emissions are:
Design Variables :
Compression Ratio
Combustion chamber surface to volume ratio
Ignition timing
Valve timings and valve overlap
Air motion, swirl tumble etc
Charge stratification
Operating Variables :
Air-fuel Ratio
Charge dilution and exhaust gas recirculation (EGR)
Speed
Load
Coolant temperature
Transient engine operation: acceleration, deceleration etc.
The effect of some variables discussed below is typical in nature and variations in the trends with
specific engine design change are observed.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2_1.htm[6/15/2012 3:01:19 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Compression Ratio
The effect of compression ratio on engine emissions is shown on Fig. 3.1. The typical effect observed
when the engine CR was reduced from 10:1 (CR used on high performance engines during pre-
emission control period) to 8.5 and 7.0:1 are given on this figure.
Use of high CR results in
(i) Higher burned gas temperature
(ii) Lower residual gas content
These lead to higher  NO emissions on volume basis. However, as engine efficiency increases with
increase in compression ratio, brake specific NO emissions decrease. High CR combustion chambers
result in
(i) High surface to volume ratio and
(ii) A proportionately higher crevice volume.
(iii) Lower exhaust gas temperatures
Thus the volume of flame quenching regions increases resulting in higher HC  emissions. The problem
is further enhanced as due to lower exhaust gas temperatures oxidation of  the  unburned HC is
reduced  during exhaust process. These factors result in an increase in HC emissions with increase in
engine CR.  At lower CR% fuel efficiency is also reduced thus increasing  specific CO emissions.
Figure
3.1
Effect of reduction in compression ratio from 10:1 to 8.5:1 and
7.0:1 on SI engine emissions
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2_1.htm[6/15/2012 3:01:19 PM]
 
Page 5


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_1.htm[6/15/2012 3:01:19 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Effect of SI Engine Design and Operating Variables on Emissions
The Lecture Contains:
SI Engine Variables and Emissions
Compression Ratio
Ignition Timing
Air –Fuel Ratio
Residual Gas and EGR
Engine Speed
Cold Start and Warm-up Phase
Coolant Temperature
Summary
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2.htm[6/15/2012 3:01:19 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
SI Engine Variables and Emissions
Any engine variable that affects oxygen availability during combustion would influence CO emissions.
The factors which influence flame quenching, quench layer thickness and post flame oxidation control
engine out HC emissions. The burned gas temperature-time history and oxygen concentration control
NO formation and emission. Hence the engine variables that influence burned gas temperature and
oxygen concentration would affect the NO emissions.
Principal design and operating variables affecting engine emissions are:
Design Variables :
Compression Ratio
Combustion chamber surface to volume ratio
Ignition timing
Valve timings and valve overlap
Air motion, swirl tumble etc
Charge stratification
Operating Variables :
Air-fuel Ratio
Charge dilution and exhaust gas recirculation (EGR)
Speed
Load
Coolant temperature
Transient engine operation: acceleration, deceleration etc.
The effect of some variables discussed below is typical in nature and variations in the trends with
specific engine design change are observed.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2_1.htm[6/15/2012 3:01:19 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
 
Compression Ratio
The effect of compression ratio on engine emissions is shown on Fig. 3.1. The typical effect observed
when the engine CR was reduced from 10:1 (CR used on high performance engines during pre-
emission control period) to 8.5 and 7.0:1 are given on this figure.
Use of high CR results in
(i) Higher burned gas temperature
(ii) Lower residual gas content
These lead to higher  NO emissions on volume basis. However, as engine efficiency increases with
increase in compression ratio, brake specific NO emissions decrease. High CR combustion chambers
result in
(i) High surface to volume ratio and
(ii) A proportionately higher crevice volume.
(iii) Lower exhaust gas temperatures
Thus the volume of flame quenching regions increases resulting in higher HC  emissions. The problem
is further enhanced as due to lower exhaust gas temperatures oxidation of  the  unburned HC is
reduced  during exhaust process. These factors result in an increase in HC emissions with increase in
engine CR.  At lower CR% fuel efficiency is also reduced thus increasing  specific CO emissions.
Figure
3.1
Effect of reduction in compression ratio from 10:1 to 8.5:1 and
7.0:1 on SI engine emissions
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2_1.htm[6/15/2012 3:01:19 PM]
 
Objectives_template
file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2_2.htm[6/15/2012 3:01:20 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions
  
Ignition Timing
The effect of ignition timing on NO and HC emissions is shown on Fig 3.2 When ignition occurs earlier
in the cycle more heat is released before and around the top dead center. Thus, with advanced ignition
timings higher peak cylinder pressures and temperatures result. As has been discussed lecture 5 with
increase in combution temperatures NO formation increases. Hence , higher NO emissions are obtained
as the ignition timing is advanced.
As the ignition timing is retarded more burning takes place during expansion stroke resulting in lower
peak combustion pressures and a lower of mass of charge is pushed into crevice volume.  Also, at the
retarded ignition timings exhaust gas temperature increases as the engine thermal efficiency is
reduced.  In the hotter  exhaust gas with the retarded ignition timing higher oxidation rates of the HC
and CO in the exhaust system are obtained.. Due to these reasons, lower HC emissions are obtained
with retarded ignition timings. The disadvantage of the  retarded ignition timing is lower engine
efficiency, lower power and a poorer fuel economy.
When the emission control legislation was introduced for the first time around 1970 in the USA and
Europe, ignition timing versus speed and manifold vacuum curves were among the first engine
parameters that were modified for control of NO
x
 emissions due to ease of their adjustment.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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