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COMBUSTIONOMBUSTIONIN SI ENGINESN SI ENGINESBy-Mahendra Prasad MeenaBy-Mahendra Prasad Meena
Ravi MeenaRavi Meena
Piyush SehgalPiyush Sehgal
Jitendra MeenaJitendra Meena
Mechanical EngineeringMechanical EngineeringDepartmentDepartment
http://var/www/apps/conversion/current/tmp/Downloads/00000015.AVIhttp://var/www/apps/conversion/current/tmp/Downloads/00000015.AVI -
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onditions of combustion in SInditions of combustion in SIenginesnginesThe angular speed in SI engines is in the range of 500-5000 min-1,The angular speed in SI engines is in the range of 500-5000 min-1,
so the whole cycle is 10-100 ms. During this time the mixture mustso the whole cycle is 10-100 ms. During this time the mixture must
be ignited, burnt and reburnt.be ignited, burnt and reburnt.
Laminar flame is too slow, however the mixture in thecylinder is turbulized and flame is turbulent, which makes
the rate of combustion very high.
In the process of combustion in SI can be divided into threephases
a) Ignition lag
b) Flame propagation
c) afterburning.
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Idle 4-stroke S I engineIdle 4-stroke S I engine
0 360
a
b
c
d
Compre
ssion
180
Cranke angle(deg)
Pressure
Expansion
combustion
TDC
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Stages of combustion in SItages of combustion in SIenginesngines
Changes of the pressureChanges of the pressurein the cylinder of SIin the cylinder of SIengineengine
Phases of combustion:
I ignition lag
II Flame Propagation
III Afterburning
AB- Ignition lag
B C- Flame propagation
CD- After burning
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Rate OF Pressure RISERate OF Pressure RISE
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Optimisation of combustion inptimisation of combustion inSI enginesI engines
There is a dominating tendency to burn leanThere is a dominating tendency to burn lean
mixturesmixtures
((>>1), because it improves efficiency of the>>1), because it improves efficiency of theengine and reduces pollutant emission.engine and reduces pollutant emission.
Combustion of lean mixtures in SI enginesCombustion of lean mixtures in SI engines
causes some problems:causes some problems:
a) Speed of laminar flame propagation is lower,a) Speed of laminar flame propagation is lower,
b) Ignition is more difficult.b) Ignition is more difficult.
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Fuel Knock Scaleuel Knock Scale To provide a standard measure of a fuels ability to resistTo provide a standard measure of a fuels ability to resist
knock, a scale has been devised by which fuels areknock, a scale has been devised by which fuels areassigned anassigned an octane numberctane number ON.ON.
The octane number determines whether or not a fuel willThe octane number determines whether or not a fuel willknock in a given engine under given operating conditions.knock in a given engine under given operating conditions.
By definition, normal heptane (n-C7H16) has an octaneBy definition, normal heptane (n-C7H16) has an octanevalue of zero andisooctane (C8H18) has a value of 100.value of zero andisooctane (C8H18) has a value of 100.
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Fuel Knock Scaleuel Knock Scale The higher the octane number, the higher the resistanceThe higher the octane number, the higher the resistance
to knock.to knock.
Blends of these two hydrocarbons define the knockBlends of these two hydrocarbons define the knockresistance of intermediate octane numbers: e.g.,resistance of intermediate octane numbers: e.g.,
a blend of 10% n-heptane and 90% isooctane has ana blend of 10% n-heptane and 90% isooctane has anoctane number of 90.octane number of 90.
A fuels octane number is determined by measuring whatA fuels octane number is determined by measuring whatblend of these two hydrocarbons matches the test fuelsblend of these two hydrocarbons matches the test fuelsknock resistance.knock resistance.
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