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Shaping the Future
Diesel Engine Combustion and
Heat Release
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The Dual Cycle The Dual Cycle traditionally provides best represents the ideal combustion process within a modern diesel
Two parts –
Constant Volume
Constant Pressure
Theoretically for the same swept volume the work area is greater than the SI Cycle – higher BMEP
What is the actual combustion like?
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Compression Ignition Combustion
Into a compressed air charge a rapidly vapourising fuel spray is injected, after an ‘ignition’ delay auto or spontaneous ignition occurs at local centres within the combustion chamber with favourable local fuel preparation, air mixing and temperatures.
Mean air-fuel ratios tend to be much greater than stoichiometric (typically 20 ~ 70:1)
After ignition CI combustion is generally considered to progress in three phases;
• Premix Burning
• Diffusion Burning
• Combustion Tail
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Combustion Visualisation
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CI Combustion – Ignition Delay
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CI Combustion – Ignition Delay
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CI Combustion – Ignition Delay
Ignition Delay
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CI Combustion – Premix The Premix burning phase is the name given to the very rapid initial turbulent combustion of the fuel/air mixture that has been prepared during ignition delay
The longer the ignition delay period the more significant the Premix burning phase – eg at idle
Premix burning causes the very high rates of pressure rise associated with CI systems and is often the primary cause of combustion derived noise and high max cylinder pressures.
During Premix burning anything from 80% to 30% of the charge may be burnt – load depending
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CI Combustion – Premix
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Premix Combustion
CI Combustion – Premix
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Combustion Noise
Premix Combustion causes rapid rise in cylinder pressure which can induce combustion noise (diesel knock)
Desired is reduction of pre-mixed phase Possible through pre-injections
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CI Combustion – Diffusion The Diffusion burning phase is used to define the burning of the fuel spray ‘as it enters the chamber’
The rate of burn of this phase is controlled by the rate of fuel injection and the rate of mixing (diffusion) of the fuel and air at the fuel spray boundaries
In-cylinder bulk air motion is critical to enhance mixing and so promote diffusion burning rates
Diffusion burn tend to be slower than Premix and last longer
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CI Combustion – Diffusion
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Diffusion Combustion
CI Combustion – Diffusion
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Diffusion Combustion
CI Combustion – Dual Cycle
Premix Combustion
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CI Combustion – Tail
As with SI combustion, CI combustion is slowed as it ‘hits’ the chamber walls
Heat transfer effects from the combusting gases to the chamber walls cool the combustion and slow or ‘freeze’ the chemical reactions
The CI combustion tail tends to be significant in length
Incomplete CI combustion is the primary cause of particulates and soot
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Combustion is generally longer than SI (eg ignition delay) this limits the engine max speed (and power) – particularly on small diesels (500 cc per cylinder)
Part load operation is through air-fuel quality (not quantity) and hence operates at very lean AFRs. Compared to SI this reduces Tmax (and hence NOx) and slightly increases γ (ratio of specific heats) during expansion (better efficiency)
Diesel ‘knock’ at light load – due to premix combustion (long ignition delay)
CI combustion benefits from charge compression (turbocharging) – SI combustion does not
CI Combustion - Observations
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Recall – Mass Fraction Burn Diagram
The Mass Fraction Burn can be deduced from Cylinder Pressure Diagrams
Mass Fraction Burn the diagrams can give good information about the progress of the various stages of the combustion
Combustion Burn Duration
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Mass Fraction Burn Diagrams
-20 0 20 40Crankangle (deg)
Mas
s F
ract
ion
Bur
ned
1.0
0.5
0.00.0
0.5
1.0
1.5
2.0
2.5
3.0
-20 0 20 40 60Crankangle (deg)
Rat
e of
Bur
n (%
/deg
)
Rate of Burn or Heat Release
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CI Rate of Burn – Heat Release
This is a rate diagram, its integral will give a cumulative burn diagram – similar to the mass fraction burn diagram of SI Combustion
Diffusion Combustion
Premix Combustion
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Combustion Optimisation
Objectives; • Minimise Ignition Delay • Achieve good fuel air mixing • Combustion completed soon after TDC Note – the higher the engine speed the greater the mixing requirement since
the time for combustion is shorter
Requirements;• Good port entry and incylinder air flow management • Good injection systems
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Engine Operating Parameters
Engine Speed :
Ignition delay increases generally linearly with engine speed, once chemical reaction kinetics dominate over physical mixing effects
Charge Temperature & Pressure
Ignition delay reduces with charge temperature and pressure
EGR
Ignition delay slightly increases with EGR (not a great effect)
Effects on Ignition Delay
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Engine Operating Parameters
Impact of Load and Fuel Cetane Number on Ignition Delay;
Require short ignition delay to control; noise, NOx and overall combustion duration (hence engine speed)
Cetane Number – Ignition quality index (15 to 100), higher the better
High loads > high temps > low ignition delay
(unless engine is cold – reverse applies due to cooling effect of fuel)
Effects on Ignition Delay
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Engine Operating Parameters
Effects on Combustion Rate..
Effect of Charge Temperature on Premix Combustion
Effect of Injection Pressure on Premix & Diffusion
Combustion
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Engine Operating Parameters
Combustion Effects - Summary
PreMix Diffusion
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Combustion “Measurement”
Combustion “Analyser”
Processed Pressure Data
Pressure Transducer
Charge Amplifier
Crankshaft Position Sensor (Encoder)
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Combustion “Measurement”
Combustion “Analyser”
Processed Pressure Data
Pressure Transducer
Charge Amplifier
Crankshaft Position Sensor (Encoder)
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Combustion “Measurement”Combustion “Analyser”
Processed Pressure Data
Pressure Transducer
Charge Amplifier
Crankshaft Position Sensor (Encoder)
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Combustion “Measurement”
Combustion “Analyser”
Processed Pressure Data
Pressure Transducer
Charge Amplifier
Crankshaft Position Sensor (Encoder)
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Thank You for Listening