EMISSION CONTROL BY ENGINE VARIABLES AND EGR Notes | EduRev

: EMISSION CONTROL BY ENGINE VARIABLES AND EGR Notes | EduRev

 Page 1


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_1.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
The Lecture Contains:
FUEL INJECTION VARIABLES
ELECTRONIC FUEL INJECTION (EFI) SYSTEMS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 2


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_1.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
The Lecture Contains:
FUEL INJECTION VARIABLES
ELECTRONIC FUEL INJECTION (EFI) SYSTEMS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_2.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
FUEL INJECTION VARIABLES
Demands Made on Injection System
To achieve low soot formation, rates of fuel-air mixing are to be enhanced.  Fuel injection and air motion
in the cylinder are key parameters to achieve rapid fuel-air mixing.  The following strategy is adopted to
improve fuel air mixing and the diesel engine combustion, which leads to reduction both in the soot and
NO
x
 formation:
Use of high fuel injection pressures  and  smaller nozzle hole size to produce very fine fuel
atomization for rapid fuel evaporation and mixing with air.
Fuel spray not to impinge on walls but fuel to be distributed mainly within the air inside the
combustion chamber.
 Matching of injection spray configuration and development with in-cylinder air motion for rapid
fuel-air mixing throughout the injection duration period
Use of variable injection timing, multiple –injection and injection rate shaping technology
High Injection Pressures
The mass flow rate of fuel injected, m
f
 is given by:
 , kg / cycle
(6.1)
where C
d
  is coefficient of discharge, A
n
 is nozzle flow area in m
2
,  ?
f
  is fuel density in kg / m
3
, (P
inj
-
P
cyl
 ) is the pressure drop across nozzle orifice in Pascals, ?? is the injection duration in degrees crank
angle and N is the engine speed in RPM.
Generally, P
inj
 >> P
cyl
.,
Thus, for a given injection rate  and injection duration ?? in crank angles, the injection pressure
should vary with speed as,
(6.2)
The speed of  engines for road vehicles  from  lowest working speed to rated speed may vary by  a
factor of; N
max
/N
min
 = 4 to 5..To achieve similar injection duration and spray penetration from the lowest
to rated engine speed, the injection pressure therefore, is required to vary by a factor of 16 to 25. This
of course is not possible in practice but it has led to use of as high an injection pressure as possible. In
the pre-1990 engines,
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_1.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
The Lecture Contains:
FUEL INJECTION VARIABLES
ELECTRONIC FUEL INJECTION (EFI) SYSTEMS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_2.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
FUEL INJECTION VARIABLES
Demands Made on Injection System
To achieve low soot formation, rates of fuel-air mixing are to be enhanced.  Fuel injection and air motion
in the cylinder are key parameters to achieve rapid fuel-air mixing.  The following strategy is adopted to
improve fuel air mixing and the diesel engine combustion, which leads to reduction both in the soot and
NO
x
 formation:
Use of high fuel injection pressures  and  smaller nozzle hole size to produce very fine fuel
atomization for rapid fuel evaporation and mixing with air.
Fuel spray not to impinge on walls but fuel to be distributed mainly within the air inside the
combustion chamber.
 Matching of injection spray configuration and development with in-cylinder air motion for rapid
fuel-air mixing throughout the injection duration period
Use of variable injection timing, multiple –injection and injection rate shaping technology
High Injection Pressures
The mass flow rate of fuel injected, m
f
 is given by:
 , kg / cycle
(6.1)
where C
d
  is coefficient of discharge, A
n
 is nozzle flow area in m
2
,  ?
f
  is fuel density in kg / m
3
, (P
inj
-
P
cyl
 ) is the pressure drop across nozzle orifice in Pascals, ?? is the injection duration in degrees crank
angle and N is the engine speed in RPM.
Generally, P
inj
 >> P
cyl
.,
Thus, for a given injection rate  and injection duration ?? in crank angles, the injection pressure
should vary with speed as,
(6.2)
The speed of  engines for road vehicles  from  lowest working speed to rated speed may vary by  a
factor of; N
max
/N
min
 = 4 to 5..To achieve similar injection duration and spray penetration from the lowest
to rated engine speed, the injection pressure therefore, is required to vary by a factor of 16 to 25. This
of course is not possible in practice but it has led to use of as high an injection pressure as possible. In
the pre-1990 engines,
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_3.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
contd...
maximum injection pressures were limited to about 700 bars. Since then, the injection pressures have
increased to a value exceeding 2000 bars. Modern electronic fuel injection systems employing unit
injectors, common rail systems etc. provide use of very high injection pressures.
The beneficial effects of high injection pressure are;
Improved fuel atomization producing finer fuel droplets.
The smaller fuel droplets evaporate at a faster rate resulting in rapid fuel-air mixing.
A shorter injection duration
With shorter injection duration injection timing may be retarded. Fuel may now be injected closer
to TDC in hotter air giving shorter ignition delay, resulting in  emission benefits
Higher spray penetration and better air utilization.
The effect of peak injection pressure on PM-NO
x
 trade off is shown on Fig. 6.3. The width of band on
this figure relates to the contribution of lubricating oil to the particulate emissions. With increase in peak
injection pressure, the PM-NO
x
 trade-off curve moves closer to origin indicating reduction both in the
PM and NO
x
 , although the reductions in PM at a given NO
x
 level are more obvious as seen in Fig 6.3
.
Figure 6.3
Effect of peak injection pressure on PM-NOx trade-off for a
turbocharged, inter-cooled, DI diesel engine.
Injection Rate Shaping and Multiple Injection
NO
x
 formation is influenced by
       (i)        duration of ignition delay,
       (ii)       amount of fuel injected during delay period, and
       (iii)      the rate of mixture preparation within the combustion chamber.
The shape of ideal rate of injection curve during ignition delay and the main injection period depends on
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 4


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_1.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
The Lecture Contains:
FUEL INJECTION VARIABLES
ELECTRONIC FUEL INJECTION (EFI) SYSTEMS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_2.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
FUEL INJECTION VARIABLES
Demands Made on Injection System
To achieve low soot formation, rates of fuel-air mixing are to be enhanced.  Fuel injection and air motion
in the cylinder are key parameters to achieve rapid fuel-air mixing.  The following strategy is adopted to
improve fuel air mixing and the diesel engine combustion, which leads to reduction both in the soot and
NO
x
 formation:
Use of high fuel injection pressures  and  smaller nozzle hole size to produce very fine fuel
atomization for rapid fuel evaporation and mixing with air.
Fuel spray not to impinge on walls but fuel to be distributed mainly within the air inside the
combustion chamber.
 Matching of injection spray configuration and development with in-cylinder air motion for rapid
fuel-air mixing throughout the injection duration period
Use of variable injection timing, multiple –injection and injection rate shaping technology
High Injection Pressures
The mass flow rate of fuel injected, m
f
 is given by:
 , kg / cycle
(6.1)
where C
d
  is coefficient of discharge, A
n
 is nozzle flow area in m
2
,  ?
f
  is fuel density in kg / m
3
, (P
inj
-
P
cyl
 ) is the pressure drop across nozzle orifice in Pascals, ?? is the injection duration in degrees crank
angle and N is the engine speed in RPM.
Generally, P
inj
 >> P
cyl
.,
Thus, for a given injection rate  and injection duration ?? in crank angles, the injection pressure
should vary with speed as,
(6.2)
The speed of  engines for road vehicles  from  lowest working speed to rated speed may vary by  a
factor of; N
max
/N
min
 = 4 to 5..To achieve similar injection duration and spray penetration from the lowest
to rated engine speed, the injection pressure therefore, is required to vary by a factor of 16 to 25. This
of course is not possible in practice but it has led to use of as high an injection pressure as possible. In
the pre-1990 engines,
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_3.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
contd...
maximum injection pressures were limited to about 700 bars. Since then, the injection pressures have
increased to a value exceeding 2000 bars. Modern electronic fuel injection systems employing unit
injectors, common rail systems etc. provide use of very high injection pressures.
The beneficial effects of high injection pressure are;
Improved fuel atomization producing finer fuel droplets.
The smaller fuel droplets evaporate at a faster rate resulting in rapid fuel-air mixing.
A shorter injection duration
With shorter injection duration injection timing may be retarded. Fuel may now be injected closer
to TDC in hotter air giving shorter ignition delay, resulting in  emission benefits
Higher spray penetration and better air utilization.
The effect of peak injection pressure on PM-NO
x
 trade off is shown on Fig. 6.3. The width of band on
this figure relates to the contribution of lubricating oil to the particulate emissions. With increase in peak
injection pressure, the PM-NO
x
 trade-off curve moves closer to origin indicating reduction both in the
PM and NO
x
 , although the reductions in PM at a given NO
x
 level are more obvious as seen in Fig 6.3
.
Figure 6.3
Effect of peak injection pressure on PM-NOx trade-off for a
turbocharged, inter-cooled, DI diesel engine.
Injection Rate Shaping and Multiple Injection
NO
x
 formation is influenced by
       (i)        duration of ignition delay,
       (ii)       amount of fuel injected during delay period, and
       (iii)      the rate of mixture preparation within the combustion chamber.
The shape of ideal rate of injection curve during ignition delay and the main injection period depends on
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_3.htm[6/15/2012 3:06:32 PM]
engine load and speed.  General principles of fuel injection scheduling are;
rate of fuel injection within the delay period must be kept small to reduce the amount of fuel
burned during ‘pre-mixed’ combustion phase, and
during the main injection period, rate of injection should be increased steeply to inject fuel within
a short period when the temperature and pressure in the combustion chamber are high for rapid
combustion of the injected fuel. 
 
Page 5


Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_1.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
The Lecture Contains:
FUEL INJECTION VARIABLES
ELECTRONIC FUEL INJECTION (EFI) SYSTEMS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_2.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
EMISSION CONTROL BY ENGINE VARIABLES AND EGR
FUEL INJECTION VARIABLES
Demands Made on Injection System
To achieve low soot formation, rates of fuel-air mixing are to be enhanced.  Fuel injection and air motion
in the cylinder are key parameters to achieve rapid fuel-air mixing.  The following strategy is adopted to
improve fuel air mixing and the diesel engine combustion, which leads to reduction both in the soot and
NO
x
 formation:
Use of high fuel injection pressures  and  smaller nozzle hole size to produce very fine fuel
atomization for rapid fuel evaporation and mixing with air.
Fuel spray not to impinge on walls but fuel to be distributed mainly within the air inside the
combustion chamber.
 Matching of injection spray configuration and development with in-cylinder air motion for rapid
fuel-air mixing throughout the injection duration period
Use of variable injection timing, multiple –injection and injection rate shaping technology
High Injection Pressures
The mass flow rate of fuel injected, m
f
 is given by:
 , kg / cycle
(6.1)
where C
d
  is coefficient of discharge, A
n
 is nozzle flow area in m
2
,  ?
f
  is fuel density in kg / m
3
, (P
inj
-
P
cyl
 ) is the pressure drop across nozzle orifice in Pascals, ?? is the injection duration in degrees crank
angle and N is the engine speed in RPM.
Generally, P
inj
 >> P
cyl
.,
Thus, for a given injection rate  and injection duration ?? in crank angles, the injection pressure
should vary with speed as,
(6.2)
The speed of  engines for road vehicles  from  lowest working speed to rated speed may vary by  a
factor of; N
max
/N
min
 = 4 to 5..To achieve similar injection duration and spray penetration from the lowest
to rated engine speed, the injection pressure therefore, is required to vary by a factor of 16 to 25. This
of course is not possible in practice but it has led to use of as high an injection pressure as possible. In
the pre-1990 engines,
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_3.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
contd...
maximum injection pressures were limited to about 700 bars. Since then, the injection pressures have
increased to a value exceeding 2000 bars. Modern electronic fuel injection systems employing unit
injectors, common rail systems etc. provide use of very high injection pressures.
The beneficial effects of high injection pressure are;
Improved fuel atomization producing finer fuel droplets.
The smaller fuel droplets evaporate at a faster rate resulting in rapid fuel-air mixing.
A shorter injection duration
With shorter injection duration injection timing may be retarded. Fuel may now be injected closer
to TDC in hotter air giving shorter ignition delay, resulting in  emission benefits
Higher spray penetration and better air utilization.
The effect of peak injection pressure on PM-NO
x
 trade off is shown on Fig. 6.3. The width of band on
this figure relates to the contribution of lubricating oil to the particulate emissions. With increase in peak
injection pressure, the PM-NO
x
 trade-off curve moves closer to origin indicating reduction both in the
PM and NO
x
 , although the reductions in PM at a given NO
x
 level are more obvious as seen in Fig 6.3
.
Figure 6.3
Effect of peak injection pressure on PM-NOx trade-off for a
turbocharged, inter-cooled, DI diesel engine.
Injection Rate Shaping and Multiple Injection
NO
x
 formation is influenced by
       (i)        duration of ignition delay,
       (ii)       amount of fuel injected during delay period, and
       (iii)      the rate of mixture preparation within the combustion chamber.
The shape of ideal rate of injection curve during ignition delay and the main injection period depends on
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_3.htm[6/15/2012 3:06:32 PM]
engine load and speed.  General principles of fuel injection scheduling are;
rate of fuel injection within the delay period must be kept small to reduce the amount of fuel
burned during ‘pre-mixed’ combustion phase, and
during the main injection period, rate of injection should be increased steeply to inject fuel within
a short period when the temperature and pressure in the combustion chamber are high for rapid
combustion of the injected fuel. 
 
Objectives_template
file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture27/27_4.htm[6/15/2012 3:06:32 PM]
 Module 6:Emission Control for CI Engines
 Lecture 27:EMISSION CONTROL BY ENGINE VARIABLES AND EGR
 
contd....
The pilot injection is a form of multiple-injection with two injection pulses. In the pilot injection, a small
quantity of fuel (about 10% of fuel per cycle) is injected 3 to 10 crank angle degrees before the main
injection event. The pilot-injected fuel has more time to undergo precombustion reactions .When the
main fuel injection is made, the combustion begins soon after, resulting in a short delay period. As the
pilot fuel quantity injected during delay period is rather small, the peak rate of combustion pressure rise
and peak pressures are reduced.  Typical pilot injection strategy and cylinder pressure are shown on
Fig. 6.4.
Figure 6.4
Pilot injection, injector needle lift and combustion pressure
traces in a naturally aspirated diesel engine at low loads.
With pilot injection, the injection timing of main fuel can be retarded to give low NO
x
 emissions without
adversely affecting the engine power and fuel efficiency.   Also, with pilot injection less fuel burns as
pre-mixed and a larger fraction of fuel burns in diffusion combustion mode. In the normal engine, NO
x
reduction is obtained by retarding the injection timing which results in higher smoke and PM emissions,
and higher BSFC. Results with pilot injection and conventional engine with retarded injection are
compared in Table 6.2. At the same NO
x
 emission levels of 5.5 g/kWh, with pilot injection typically 23 %
lower PM, 50% lower HC and 4 % lower BSFC were obtained.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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