AEZG516 – Advances in IC Engines

Sajeeth Kumar

Lecture 07 – Gasoline Combustion

Scope

Points for Discussion

Combustion in gasoline engines

Flame propagation

Flame properties

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

Analytical

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

Have you seen this happen?

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 )

Combustion Abnormalities

Engine Knock

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

Thank You!

Next Lecture : Diesel Combustion