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

: Effect of CI 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/lecture15/15_1.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Effect of CI Engine Design and Operating Variables on Emissions
The Lecture Contains:
Engine Design Variables
Operating Variables
Compression Ratio
Direct Injection (DI) versus Indirect Injection (IDI) Engines
Combustion Chamber Design
Fuel Injection Timing and Injection Pressure
Engine load and Speed
Exhaust Gas Recirculation
Fuel Quality
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 2


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_1.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Effect of CI Engine Design and Operating Variables on Emissions
The Lecture Contains:
Engine Design Variables
Operating Variables
Compression Ratio
Direct Injection (DI) versus Indirect Injection (IDI) Engines
Combustion Chamber Design
Fuel Injection Timing and Injection Pressure
Engine load and Speed
Exhaust Gas Recirculation
Fuel Quality
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_2.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
CI Engine Design and Operating Variables and Emissions
Important diesel engine variables that influence emissions are:
Engine Design Variables
Compression ratio
Combustion chamber type
Combustion chamber design
Injection system: injection pressure and timing, nozzle holes, nozzle sac volume
 
Operating Variables:
EGR
Engine speed
Engine load
Fuel quality
Compression Ratio
  In the diesel engines the minimum compression ratio that can be used is governed by the ease
of engine cold starting ability. For the high speed direct injection engines CR of around 16 to
17.5:1 is used. The turbocharged heavy duty engines employ CR in the range of 13 to 14:1. Cold
starting requirements prevents further reduction in the compression ratio.
  Use of higher compression ratio results in a shorter ignition delay period. A shorter delay would
result in less ‘overmixing' of fuel and air and hence, lower HC emissions. Further, the higher
combustion temperatures obtained at a higher compression ratios tend to increase oxidation of
the unburned HC.
  At a low compression ratio, a longer delay increases the fraction of fuel burned during the
premixed phase resulting in higher peak pressures and temperatures which cause an increase in
NO x formation. On the other hand, increase in compression ratio due to higher combustion
temperatures would tend to increase formation of NOx. If the ignition delay is too long the
combustion may begin in the expansion stroke reducing combustion pressure and temperature.
Too long an ignition delay leads to lower NOx emissions along with poor fuel efficiency.
  Use of a low compression ratio results in too long a delay during engine warm up under cold
conditions, and it causes high emissions of unburned fuel which due to its appearance is called
‘white smoke'.
A high compression ratio leading to high combustion temperatures would increase soot formation
while on the other hand it increases soot oxidation.
  For obtaining low particulate and NOx emissions simultaneously, an optimum compression ratio
is to be used. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_1.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Effect of CI Engine Design and Operating Variables on Emissions
The Lecture Contains:
Engine Design Variables
Operating Variables
Compression Ratio
Direct Injection (DI) versus Indirect Injection (IDI) Engines
Combustion Chamber Design
Fuel Injection Timing and Injection Pressure
Engine load and Speed
Exhaust Gas Recirculation
Fuel Quality
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_2.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
CI Engine Design and Operating Variables and Emissions
Important diesel engine variables that influence emissions are:
Engine Design Variables
Compression ratio
Combustion chamber type
Combustion chamber design
Injection system: injection pressure and timing, nozzle holes, nozzle sac volume
 
Operating Variables:
EGR
Engine speed
Engine load
Fuel quality
Compression Ratio
  In the diesel engines the minimum compression ratio that can be used is governed by the ease
of engine cold starting ability. For the high speed direct injection engines CR of around 16 to
17.5:1 is used. The turbocharged heavy duty engines employ CR in the range of 13 to 14:1. Cold
starting requirements prevents further reduction in the compression ratio.
  Use of higher compression ratio results in a shorter ignition delay period. A shorter delay would
result in less ‘overmixing' of fuel and air and hence, lower HC emissions. Further, the higher
combustion temperatures obtained at a higher compression ratios tend to increase oxidation of
the unburned HC.
  At a low compression ratio, a longer delay increases the fraction of fuel burned during the
premixed phase resulting in higher peak pressures and temperatures which cause an increase in
NO x formation. On the other hand, increase in compression ratio due to higher combustion
temperatures would tend to increase formation of NOx. If the ignition delay is too long the
combustion may begin in the expansion stroke reducing combustion pressure and temperature.
Too long an ignition delay leads to lower NOx emissions along with poor fuel efficiency.
  Use of a low compression ratio results in too long a delay during engine warm up under cold
conditions, and it causes high emissions of unburned fuel which due to its appearance is called
‘white smoke'.
A high compression ratio leading to high combustion temperatures would increase soot formation
while on the other hand it increases soot oxidation.
  For obtaining low particulate and NOx emissions simultaneously, an optimum compression ratio
is to be used. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_3.htm[6/15/2012 3:01:41 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Direct Injection (DI) versus Indirect Injection (IDI) Engines
The DI and IDI engines have been in use for many years. The IDI engines were mostly employed in
high speed small engine applications. Now, after the development of high speed direct injection (HSDI)
engines the IDI engines due to their poor fuel economy are being phased out of production. The
combustion in IDI engines takes place in two stages; a rich mixture burns in the pre-chamber where all
the fuel is injected and the partially burned rich fuel- air mixture from the pre-chamber is transported to
the main chamber where due to presence of excess air combustion is completed. The jet of high
pressurepartially burned gases from the pre-chamber enters the main chamber generating high
turbulence that causes rapid mixing and most of the fuel burns as lean mixture.
Figure
3.4
Comparison of NO x concentrations in DI andDI diesel engines
A comparison of NO
x
 emissions with DI and IDI engines is shown on Fig. 3.4. At light loads, most of NO
may form in the pre-chamber. But, at higher loads additional NO formation would occur in the main
chamber. Although temperatures are higher in the pre-chamber than the main chamber, but except at
light loads mixture is overall rich and hence, the lower formation of NO. In the DI engines, at the end of
premixed combustion higher peak pressures and temperatures are obtained compared to the IDI
engines and NO is formed in near stoichiometric mixtures during mixing controlled phase and post
combustion gases. Due to these factors overall, the indirect injection engines emit lower NOx. In the DI
engines due to low turbulence levels some of CO formed in the rich spray regions may not find the
required oxygen for complete combustion while thetemperatures are still high. It results in .higher CO
emissions than the IDI engines even though more excess air is present in the DI engines. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 4


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_1.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Effect of CI Engine Design and Operating Variables on Emissions
The Lecture Contains:
Engine Design Variables
Operating Variables
Compression Ratio
Direct Injection (DI) versus Indirect Injection (IDI) Engines
Combustion Chamber Design
Fuel Injection Timing and Injection Pressure
Engine load and Speed
Exhaust Gas Recirculation
Fuel Quality
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_2.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
CI Engine Design and Operating Variables and Emissions
Important diesel engine variables that influence emissions are:
Engine Design Variables
Compression ratio
Combustion chamber type
Combustion chamber design
Injection system: injection pressure and timing, nozzle holes, nozzle sac volume
 
Operating Variables:
EGR
Engine speed
Engine load
Fuel quality
Compression Ratio
  In the diesel engines the minimum compression ratio that can be used is governed by the ease
of engine cold starting ability. For the high speed direct injection engines CR of around 16 to
17.5:1 is used. The turbocharged heavy duty engines employ CR in the range of 13 to 14:1. Cold
starting requirements prevents further reduction in the compression ratio.
  Use of higher compression ratio results in a shorter ignition delay period. A shorter delay would
result in less ‘overmixing' of fuel and air and hence, lower HC emissions. Further, the higher
combustion temperatures obtained at a higher compression ratios tend to increase oxidation of
the unburned HC.
  At a low compression ratio, a longer delay increases the fraction of fuel burned during the
premixed phase resulting in higher peak pressures and temperatures which cause an increase in
NO x formation. On the other hand, increase in compression ratio due to higher combustion
temperatures would tend to increase formation of NOx. If the ignition delay is too long the
combustion may begin in the expansion stroke reducing combustion pressure and temperature.
Too long an ignition delay leads to lower NOx emissions along with poor fuel efficiency.
  Use of a low compression ratio results in too long a delay during engine warm up under cold
conditions, and it causes high emissions of unburned fuel which due to its appearance is called
‘white smoke'.
A high compression ratio leading to high combustion temperatures would increase soot formation
while on the other hand it increases soot oxidation.
  For obtaining low particulate and NOx emissions simultaneously, an optimum compression ratio
is to be used. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_3.htm[6/15/2012 3:01:41 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Direct Injection (DI) versus Indirect Injection (IDI) Engines
The DI and IDI engines have been in use for many years. The IDI engines were mostly employed in
high speed small engine applications. Now, after the development of high speed direct injection (HSDI)
engines the IDI engines due to their poor fuel economy are being phased out of production. The
combustion in IDI engines takes place in two stages; a rich mixture burns in the pre-chamber where all
the fuel is injected and the partially burned rich fuel- air mixture from the pre-chamber is transported to
the main chamber where due to presence of excess air combustion is completed. The jet of high
pressurepartially burned gases from the pre-chamber enters the main chamber generating high
turbulence that causes rapid mixing and most of the fuel burns as lean mixture.
Figure
3.4
Comparison of NO x concentrations in DI andDI diesel engines
A comparison of NO
x
 emissions with DI and IDI engines is shown on Fig. 3.4. At light loads, most of NO
may form in the pre-chamber. But, at higher loads additional NO formation would occur in the main
chamber. Although temperatures are higher in the pre-chamber than the main chamber, but except at
light loads mixture is overall rich and hence, the lower formation of NO. In the DI engines, at the end of
premixed combustion higher peak pressures and temperatures are obtained compared to the IDI
engines and NO is formed in near stoichiometric mixtures during mixing controlled phase and post
combustion gases. Due to these factors overall, the indirect injection engines emit lower NOx. In the DI
engines due to low turbulence levels some of CO formed in the rich spray regions may not find the
required oxygen for complete combustion while thetemperatures are still high. It results in .higher CO
emissions than the IDI engines even though more excess air is present in the DI engines. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_4.htm[6/15/2012 3:01:41 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Combustion Chamber Design
a. Dead Volumes
Air in the combustion chamber is contained in several different volumes like piston bowl, top land
crevice, piston – cylinder head clearance, valve recess and head gasket clearance. Typical distribution
of clearance volume at tdc among different components for a DI diesel engine is shown in Table 3.2.
The piston bowl in DI diesel engines contains slightly more than 50% of total clearance volume at tdc.
The air contained in top land crevice, head gasket clearance and valve recess is nearly 15 % and is
poorly utilized during combustion. Even the air contained in the volume between piston crown and
cylinder head at tdc is poorly utilized. Piston-cylinder crevice volumes store morethan proportionate air
due to lower temperature in the crevice region compared to the temperature of air in the cylinder. A
reduction in crevice volume therefore, increases air utilization. Similarly, a lower clearance between
piston and cylinder head increases air utilization and reduces the possibility of fuel entering the crevices.
Reduction in ‘poor air utilization' volumes results in lower particulate emission and fuel consumption..
Table 3.2
Volume Distribution of Combustion Chamber at
TDC
Piston bowl 55%
Piston – Cylinder head 30%
Clearance Valve recess 6%
Top land crevice 7%
Head gasket 2%
b. Multi-Valves and Air Motion
Use of multiple valves (3 or 4) per cylinder increases flow area and hence, the volumetric efficiency of
the engine. Four valves per cylinder are now common in gasoline engines. In the direct injection diesel
engines use of four valves enables a centralized location of injector and combustion bowl in the piston.
The injector can be placed more centrally and vertically. Typical PM-NO
x
 characteristics for 4- and 2-
valve per cylinder passenger car diesel engines are compared on Fig. 3.5.With two valves designs, the
injector is always offset and inclined. Injector inclination of 20 and 10 degrees from vertical has been
observed to give an increase of about 25 and 5 %, respectively in PM emissions due to poor fuel
distribution in the cylinder compared to a vertically located injector. Reduction of up to 4 to 7 % in
specific fuel consumption is also obtained at the same NO
x
 emission level in a multi-valve engine with
centrally placed combustion bowl and injector.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 5


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_1.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Effect of CI Engine Design and Operating Variables on Emissions
The Lecture Contains:
Engine Design Variables
Operating Variables
Compression Ratio
Direct Injection (DI) versus Indirect Injection (IDI) Engines
Combustion Chamber Design
Fuel Injection Timing and Injection Pressure
Engine load and Speed
Exhaust Gas Recirculation
Fuel Quality
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_2.htm[6/15/2012 3:01:41 PM]
 Module 3: Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
CI Engine Design and Operating Variables and Emissions
Important diesel engine variables that influence emissions are:
Engine Design Variables
Compression ratio
Combustion chamber type
Combustion chamber design
Injection system: injection pressure and timing, nozzle holes, nozzle sac volume
 
Operating Variables:
EGR
Engine speed
Engine load
Fuel quality
Compression Ratio
  In the diesel engines the minimum compression ratio that can be used is governed by the ease
of engine cold starting ability. For the high speed direct injection engines CR of around 16 to
17.5:1 is used. The turbocharged heavy duty engines employ CR in the range of 13 to 14:1. Cold
starting requirements prevents further reduction in the compression ratio.
  Use of higher compression ratio results in a shorter ignition delay period. A shorter delay would
result in less ‘overmixing' of fuel and air and hence, lower HC emissions. Further, the higher
combustion temperatures obtained at a higher compression ratios tend to increase oxidation of
the unburned HC.
  At a low compression ratio, a longer delay increases the fraction of fuel burned during the
premixed phase resulting in higher peak pressures and temperatures which cause an increase in
NO x formation. On the other hand, increase in compression ratio due to higher combustion
temperatures would tend to increase formation of NOx. If the ignition delay is too long the
combustion may begin in the expansion stroke reducing combustion pressure and temperature.
Too long an ignition delay leads to lower NOx emissions along with poor fuel efficiency.
  Use of a low compression ratio results in too long a delay during engine warm up under cold
conditions, and it causes high emissions of unburned fuel which due to its appearance is called
‘white smoke'.
A high compression ratio leading to high combustion temperatures would increase soot formation
while on the other hand it increases soot oxidation.
  For obtaining low particulate and NOx emissions simultaneously, an optimum compression ratio
is to be used. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_3.htm[6/15/2012 3:01:41 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Direct Injection (DI) versus Indirect Injection (IDI) Engines
The DI and IDI engines have been in use for many years. The IDI engines were mostly employed in
high speed small engine applications. Now, after the development of high speed direct injection (HSDI)
engines the IDI engines due to their poor fuel economy are being phased out of production. The
combustion in IDI engines takes place in two stages; a rich mixture burns in the pre-chamber where all
the fuel is injected and the partially burned rich fuel- air mixture from the pre-chamber is transported to
the main chamber where due to presence of excess air combustion is completed. The jet of high
pressurepartially burned gases from the pre-chamber enters the main chamber generating high
turbulence that causes rapid mixing and most of the fuel burns as lean mixture.
Figure
3.4
Comparison of NO x concentrations in DI andDI diesel engines
A comparison of NO
x
 emissions with DI and IDI engines is shown on Fig. 3.4. At light loads, most of NO
may form in the pre-chamber. But, at higher loads additional NO formation would occur in the main
chamber. Although temperatures are higher in the pre-chamber than the main chamber, but except at
light loads mixture is overall rich and hence, the lower formation of NO. In the DI engines, at the end of
premixed combustion higher peak pressures and temperatures are obtained compared to the IDI
engines and NO is formed in near stoichiometric mixtures during mixing controlled phase and post
combustion gases. Due to these factors overall, the indirect injection engines emit lower NOx. In the DI
engines due to low turbulence levels some of CO formed in the rich spray regions may not find the
required oxygen for complete combustion while thetemperatures are still high. It results in .higher CO
emissions than the IDI engines even though more excess air is present in the DI engines. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_4.htm[6/15/2012 3:01:41 PM]
 Module 3:Influence of Engine Design and Operating Parameters on Emissions
 Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions
 
Combustion Chamber Design
a. Dead Volumes
Air in the combustion chamber is contained in several different volumes like piston bowl, top land
crevice, piston – cylinder head clearance, valve recess and head gasket clearance. Typical distribution
of clearance volume at tdc among different components for a DI diesel engine is shown in Table 3.2.
The piston bowl in DI diesel engines contains slightly more than 50% of total clearance volume at tdc.
The air contained in top land crevice, head gasket clearance and valve recess is nearly 15 % and is
poorly utilized during combustion. Even the air contained in the volume between piston crown and
cylinder head at tdc is poorly utilized. Piston-cylinder crevice volumes store morethan proportionate air
due to lower temperature in the crevice region compared to the temperature of air in the cylinder. A
reduction in crevice volume therefore, increases air utilization. Similarly, a lower clearance between
piston and cylinder head increases air utilization and reduces the possibility of fuel entering the crevices.
Reduction in ‘poor air utilization' volumes results in lower particulate emission and fuel consumption..
Table 3.2
Volume Distribution of Combustion Chamber at
TDC
Piston bowl 55%
Piston – Cylinder head 30%
Clearance Valve recess 6%
Top land crevice 7%
Head gasket 2%
b. Multi-Valves and Air Motion
Use of multiple valves (3 or 4) per cylinder increases flow area and hence, the volumetric efficiency of
the engine. Four valves per cylinder are now common in gasoline engines. In the direct injection diesel
engines use of four valves enables a centralized location of injector and combustion bowl in the piston.
The injector can be placed more centrally and vertically. Typical PM-NO
x
 characteristics for 4- and 2-
valve per cylinder passenger car diesel engines are compared on Fig. 3.5.With two valves designs, the
injector is always offset and inclined. Injector inclination of 20 and 10 degrees from vertical has been
observed to give an increase of about 25 and 5 %, respectively in PM emissions due to poor fuel
distribution in the cylinder compared to a vertically located injector. Reduction of up to 4 to 7 % in
specific fuel consumption is also obtained at the same NO
x
 emission level in a multi-valve engine with
centrally placed combustion bowl and injector.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_4.htm[6/15/2012 3:01:41 PM]
Figure 3.5
PM – NO 
x
 trade-off for two valve and four-valve
passenger car DI diesel engines.
A centrally placed combustion bowl has lower swirl requirements and results in more equal fuel
distribution and availability of equal air to each spray for mixing. In four-valve engines, symmetrical air
motion in the piston bowl and equal fuel distribution between different sprays lead to optimum mixture
formation and combustion with very low smoke levels.
Use of lower air swirl in 4-valve engines compared to 2-valve engines for the same PM and NOx
emission levels, results in reduction of fuel -air ‘overmixing' during premixed phase of combustion. As
‘overmixing' of fuel is an important source of HC emissions, lower HC emissions in 4-valve engines are
obtained. 
 
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