Shaping the Future

Diesel Engine Combustion and

Heat Release

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?

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

Combustion Visualisation

CI Combustion – Ignition Delay

CI Combustion – Ignition Delay

CI Combustion – Ignition Delay

Ignition Delay

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

CI Combustion – Premix

Premix Combustion

CI Combustion – Premix

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

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

CI Combustion – Diffusion

Diffusion Combustion

CI Combustion – Diffusion

Diffusion Combustion

CI Combustion – Dual Cycle

Premix Combustion

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

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

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

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

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

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

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

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

Engine Operating Parameters

Effects on Combustion Rate..

Effect of Charge Temperature on Premix Combustion

Effect of Injection Pressure on Premix & Diffusion

Combustion

Engine Operating Parameters

Combustion Effects - Summary

PreMix Diffusion

Combustion “Measurement”

Combustion “Analyser”

Processed Pressure Data

Pressure Transducer

Charge Amplifier

Crankshaft Position Sensor (Encoder)

Combustion “Measurement”

Combustion “Analyser”

Processed Pressure Data

Pressure Transducer

Charge Amplifier

Crankshaft Position Sensor (Encoder)

Combustion “Measurement”Combustion “Analyser”

Processed Pressure Data

Pressure Transducer

Charge Amplifier

Crankshaft Position Sensor (Encoder)

Combustion “Measurement”

Combustion “Analyser”

Processed Pressure Data

Pressure Transducer

Charge Amplifier

Crankshaft Position Sensor (Encoder)

Thank You for Listening