Abnormal Combustion in SI Engines

Abnormal Combustion
Combustion of fuel should be done near the end
compression stroke almost 15 to 20° before TDC.
But some time it takes earlier or later:
o Pre-ignition
o Detonation
o Knocking

Normal Combustion
In a conventional SI engine, the fuel is completely
vaporized and homogeneously premixed with
air and residual gas, when combustion occurs. Under
normal operating conditions, the mixture is
ignited by the spark plug in a optimized moment of the
cycle (usually 20-40° before TDC).
The combustion is normal when 2 conditions are
respected:
1) the ignition of combustible mixture is controlled by
the spark plug, with a present timing
2) after the ignition, the flame propagates regularly to
the whole mixture, without any sudden
increase of velocity.

Self Ignition/ Auto-ignition temperature
The auto-ignition temperature or kindling point of
a substance is the lowest temperature in which
it spontaneously ignites in a normal atmosphere without
an external source of ignition, such as a flame or spark.
This temperature is required to supply the activation
energy needed for combustion. The temperature at
which a chemical ignites decreases as the pressure is
increased.
Ignition delay or ignition lag
In the internal combustion engine, the time interval
between the passage of the spark and the inflammation
of the air-fuel mixture. Also known as ignition delay.

Flame Front
A flame front generator is a system in which a
gas/air mixture is introduced at ground level and
flows up a one-inch line to the pilot burner. After filling
this line with the mixture it is ignited by a spark. The
resulting flame travels to the top of the flare where the
pilot burner is ignited.

Abnormal Combustion in SI Engine
Knock is the term used to describe a pinging noise emitted from a SI engine
undergoing abnormal combustion.
The noise is generated by shock waves produced in the cylinder when
unburned gas ahead of the flame auto-ignites.

Knock
As the flame propagates away from the spark plug the pressure and
temperature of the unburned gas increases.
Under certain conditions the end-gas can autoignite and burn very rapidly
producing a shock wave
The end-gas autoignites after a certain induction time which is dictated by
the chemical kinetics of the fuel-air mixture.
If the flame burns all the fresh gas before autoignition in the end-gas can
occur then knock is avoided.
Therefore knock is a potential problem when the burn time is long!
shock
P,T
time time
P,T
end-gas
flame

i) Compression ratio – at high compression ratios, even before spark ignition,
the fuel-air mixture is compressed to a high pressure and temperature which
promotes autoignition
ii) Engine speed – At low engine speeds the flame velocity is slow and thus
the burn time is long, this results in more time for autoignition
However at high engine speeds there is less heat loss so the unburned gas
temperature is higher which promotes autoignition
These are competing effects, some engines show an increase in propensity to
knock at high speeds while others don’t.
iii) Spark timing – maximum compression from the piston advance occurs at
TC, increasing the spark advance makes the end of combustion crank angle
approach TC and thus get higher pressure and temperature in the unburned
gas just before burnout.
Parameters Influencing Knock

Pre-Ignition
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only
 Pre-ignition is the ignition of the air-fuel charge
prior to the spark plug firing.
 Anytime something causes the mixture in the
chamber to ignite before the spark plugs fire, it
is classified as pre- ignition.
 This occurs when a point within the combustion
chamber becomes so hot that it becomes a
source of ignition and causes the fuel to ignite
before the spark plug fires.

Causes of Pre-ignition
 Carbon deposits form a heat source, causes ignition.
 An overheated spark plug
 Glowing carbon deposits on a hot exhaust valve
 A sharp edge in the combustion chamber or on top of a piston
Any of these things can act as a glow plug to ignite the charge
prematurely (early).
• The fuel ignites prematurely (early) causing a momentarily backlash
as the piston tries to move piston in reverse direction.
• This can be very damaging because of the stresses it creates.
• It can also localize heat to such an extent that it canpartially melt or
burn a hole through the top of a piston!

Detonation-induced pre-ignition
 Detonation causes a very rapid pressure spike near the
peak pressure point for a very brief period of time.
 Pre-ignition causes tremendous pressure that is present
for a very long time - possibly the entire compression
stroke.
 It is possible for heavy detonation to induce pre-ignition.
 If the engine is operating in heavy detonation for a
significant period of time, the excessive temperatures and
pressure spikes (which disturb the usual protective
boundary layer)
 can cause spark plug electrodes and other things in the
combustion chamber to overheat to the point where they
start to glow red hot.
 At that point, the glowing item can cause pre-ignition and
rapid destruction of the cylinder.

Detonation
 Detonation is the combustion of fuel after the air-fuel
charge has been ignited normally by the spark plugs or
before starting the normal combustion.
 Detonation is the spontaneous combustion of the end-gas
(remaining fuel/air mixture) in the chamber.
 the result can be excessive pressure, excessive
temperatures, and unstable pressure pulses known as
"detonation."
 .

Cont.……
 Detonation, or engine knock, occurs simply
 when fuel pre-ignites before the piston reaches scheduled
spark ignition.
 This means that a powerful explosion is trying to expand a
cylinder chamber that is shrinking in size, attempting to
reverse the direction of the piston and the engine.
 Causing sudden pressure changes in the cylinder and
 extreme temperature spikes that can be very damaging on
engine pistons, rings, rods, gaskets, bearings, and even the
cylinder heads.

 If there is enough heat and pressure in the combustion
chamber, the fuel points may burn due to high
temperature gas in cylinder before the working of spark
plug
 Under these circumstances, known as "pre-ignition", the
piston may be travelling up towards for compression
stroke, a but high pressure wave of burnt gas may
resist the forward movement of piston.
 These are the worst kinds of detonation conditions, and
can bend connecting rods and destroy pistons.

Detonation Causes
 Ignition Timing
 Lean Air/Fuel ratio
 Fuel Octane
 Exhaust gas back pressure
 Intercooler
 Ambient heat

How to Remove Detonation
1. use higher octane fuel. Since Higher octane fuel is
more stable burns more controllably and is not
as likely to combust before the flame front.
2. retard the ignition timing: Retarding the ignition timing
will delay the timing of the spark.
 Cool the incoming air charge to lower the temperature
inside the combustion chamber.
 Many supercharger manufacturers will recommend
cooler plugs for your supercharged engine.
 Use lean air fuel mixture
 It is important to make sure that the fuel system (pump,
injectors, etc.) is working properly.

Factors Affecting the knocking/ Detonation
SI Engines CI Engines
Self Ignition
Temperature High Less
Ignition Lag High Less
Compression
Ratio Less High
Intial
Temperature and
Pressure Less High
Wall Temperature Less High
Speed rpm High Less
Stroke length Less High

Thanks

Abnormal Combustion in SI Engines

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