aezg516 – advances in ic engines
Post on 14-Feb-2022
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Combustion
SI Engine Combustion
Homogenous mixture
Ignited by spark plug
Not an instantaneous process
Calibration parameter
Homogeneous Mixtures
Combustion Process
1. Ignition & flame development
2. Flame propagation
3. Flame termination
Flame Development
Just before the flame front travels
• Ignition delay in Weibe’s function
• Flame development definition varies
though
• Usually considered to be 5% MFB
• Some sources quote 10% MFB
• Very little pressure rise
• Meaning – No actual work done
Flame Development
How is the flame initiated
• Spark plug
• Produces an electric arc
• High temperature plasma ignites AF mixture
• IGNITES only in the immediate vicinity
• Combustion starts slowly – cold mixtures
• 25000 to 40000 volts are applied
• Max current of 200A
• Lasts for 10 nano seconds
• Max energy of 30 to 50 mJ
• Peak temperature of 60000k, avg temperature of
6000k
Flame Development
Temperature and Energy Requirement
• Energy required to ignite and sustain combustion
• Stoichiometric mixture – 0.2 mJ
• Non stoichiometric mixtures – 3 mJ
• Spark plug discharges around 30-50 mJ
• Rest lost in heat transfer
Flame Development
Role of spark plug
• Plug gap
• Heat range
• Location of spark plugs – localized AF Mixture
Flame Propagation
Laminar vs Turbulent Systems
• Propagation is turbulent
• 10x faster than the laminar flame
• Remember the AF mixture is still moving
• Flame propagation affected by air velocity
Flame Propagation
Zones of Interest
• Burnt gases behind the flame front
• The flame front
• Unburnt mixture in-front of the flame front
• Burnt gases are at higher temperature
• P α T – Higher pressures – Unburnt gases expand
Flame Propagation
MFB vs VFB
• 30% MFB – occupies 60% volume
• 70% of unburnt mixture is compressed into 40%
volume
• Compression increases temperature of unburnt
mixture
• Heat is also radiated from burnt gases – conductive
and convective heat transfer is negligible – time is
less
• Unburnt mixture burns rapidly as the combustions
process progresses
Flame Propagation
Nature of Combustion
• Explains the ignition delay
• Rapid rise in pressure consequently
• 90% MFB in 50% time concept
• Ideal cases 2/3rd of the mixture has to be burnt at
TDC
• Almost completely burnt at 15 ATDC
• Max temp and max pressure occurs at 5-10 ATDC
• Burn angle is usually around 25 degrees
• Ideal spark advance? 10 BTDC?
Flame Propagation
Factors influencing flame speed
• Air Fuel Mixture – Rich mixture
burns fastest
• Higher air velocities – higher rpms
– increases flame speed
• Burn angle ( degrees ) as a
function of engine speed ( time )
• Requirement for spark advance to
vary with RPM, air velocity, AFR,
temperatures etc.,
Flame Termination
Flame front comes to a stop
• At 20 ATDC almost 90% of the AF mixture is burnt
• Balance 10% is at extreme corners of the cylinder – burnt gases push the unburnt
mixture
• 10% mixture is in contact with cylinder walls – cooled by coolant
• Turbulence dies out as the volume expands
• Some packets are out of reach due to shape of the flame front
• At times self-ignition occurs – last 10-15% self ignites before the flame can reach
• Max power occurs when there is slight self ignition towards end of combustion –
this self ignition happens in power stroke and is not noticeable or harmful
Summary
Ignition Advance as a Calibration Property
• High turbulence increases flame velocity – less advance is required
• Rich mixtures burn faster – less advance is required
• High compression ratio – burns faster – lesser advance
• Lean mixture – burns slowly – needs more advance
• Lower compression ratio – burns slowly – needs more advance
• Less air velocity – burns slowly – needs more advance
Concept Question
Two similar engines with just minor changes in the intake system and cams are
presented in front of you
• First engine requires 32 degrees of advance to achieve MBT at 5000 rpm
• Second engine achieves MBT at 28 degrees of advance at 5000 rpm
• What is happening here?
Combustion Abnormalities
What happens when conditions are not ideal?
• Surface ignition
• Knock
• Run-on
• After-burn
Combustion Abnormalities
How do you quantify combustion in real world?
• Kistler sensor
• Integrated into spark plugs / glow
plugs or drilled into head if space
permits
• Alternately knock sensors ( poor
man’s version )
Influence of Ignition Angles
How do you quantify combustion in real world?
Early Ignition
• Early pressure rise
• More work done
• High peak P and T
• Higher efficiencies
• More NOX
Late Ignition
• Higher end temperature
• Possibility of exhaust still
burning
• Good for heating catalyst
Influence of AFR and Ignition on NX
Ignition Advance
• Earlier the advance, more the NOX emission
AFR
• Rich – Low NOX since there is not enough O2
• Stoichiometric – High NOX due to high peak temp
• Slightly lean – Max NOX due to excessive O2
• Very lean – Less NOX as temperatures drop due to excess air
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