abnormal combustion

1. Abnormal Combustion in Spark Ignition Engines
S K Singh
Centre for Energy Studies
IIT Delhi
A Performance limit due to resources & technology.
11-Aug-22

2. Recall what we’ve discussed on the board in class room…
KNOCK IN SI ENGINES
• Knock is one phenomenon that is most limiting to engine efficiency.
• It occurs when the temperature of the unburned gas in the cylinder
increases to much, and causes the fuel to self ignite.
• This will result in an oscillating pressure wave in the combustion
chamber.
• For the driver, this sounds as a number of low thuds.
• This can be both stressing and agitating, and are therefore
dangerous, since the driver will be less focused on his or her
surroundings.
• Knock is also very strenuous for the engine.
• The high oscillating pressure, can cause damage that will in the end
lead to a shorter lifetime for the engine.
• Really severe knock can damage the engine even after only one or
a few self-ignitions.
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3. Why Knocking is important aspect to discuss
• The reason why knock is so interesting is because the optimal
operating point of the engine is often in the area where knock will
occur.
• Therefore the ability to predict when knock will occur will make it
easier to control the engine towards the optimal operating point.
• Running an engine at its optimum has a lot of advantages.
• The most obvious being that the more efficient the engine runs, the
more you can get out of it for the same input.
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4. 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 auto ignites.
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5. Engine Damage From Severe Knock
Damage to the engine is caused by a combination of high temperature and
high pressure.
Piston Piston crown
Cylinder head gasket Aluminum cylinder head
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6. MECHANICAL DAMAGE DUE TO KNOCK
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7. Observation window
for photography
Spark plug
Intake valve
Exhaust valve
Normal cycle
Knock cycle

8. Engine parameters that effect occurrence of knock are:
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 auto ignition
However at high engine speeds there is less heat loss so the unburned gas
temperature is higher which promotes auto ignition
These are competing effects, some engines show an increase in propensity to
knock at high speeds while others don’t.
PARAMETERS AFFECTING KNOCK
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9. Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
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10. Effect of changing the air-fuel ratio
• Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel
ratio, but was very engine specific.
• Modern engines, with engine management systems, now have their
maximum octane requirement near to 14.5:1.
• For a given engine using gasoline, the relationship between thermal
efficiency, air-fuel ratio, and power is complex.
• Stoichiometric combustion ( air-fuel ratio = 14.7:1 for a typical
gasoline ) is neither maximum power - which occurs around air-fuel
12-13:1 (Rich), nor maximum thermal efficiency - which occurs
around air-fuel 16-18:1 (Lean).
• The air-fuel ratio is controlled at part throttle by a closed loop system
using the oxygen sensor in the exhaust.
• Conventionally, enrichment for maximum power air-fuel ratio is used
during full throttle operation to reduce knocking while providing
better driveability.
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11. iii) Spark timing – maximum compression from the piston advance occurs at
TDC, increasing the spark advance makes the end of combustion crank angle
approach TDC and thus get higher pressure and temperature in the unburned
gas just before burnout.
Factors affecting Knock contd..
P,T
T
Ignition
x
x End of combustion
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12. x
x
x
x
x
x
x
X crank angle corresponding
to borderline knock
Spark advance set to 1% below MBT (Maxm Brake torque) to avoid knock
1% below MBT
Knock Mitigation Using Spark Advance
11-Aug-22

13. Fuel Knock Scale
To provide a standard measure of a fuel’s ability to resist knock, a scale has
been devised by which fuels are assigned an octane number ON (% age by
volume of iso-octane in a mixture of iso-octane and normal heptane, which
Exactly matches the knocking intensity of the fuel in a standard engine under
a set of standard operating conditions ).
The octane number determines whether or not a fuel will knock in a given
engine under given operating conditions.
By definition, normal heptane (n-C7H16) has an octane value of zero and
isooctane (C8H18) has a value of 100.
The higher the octane number, the higher the resistance to knock.
Blends of these two hydrocarbons define the knock resistance of intermediate
octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an
octane number of 90.
A fuel’s octane number is determined by measuring what blend of these two
hydrocarbons matches the test fuel’s knock resistance.

14. Octane Number Measurement
Two methods have been developed to measure ON using a standardized
single-cylinder engine developed under the auspices of the Cooperative Fuel
Research (CFR) Committee in 1931.
The CFR engine is 4-stroke with 3.25” bore and 4.5” stroke, compression
ratio can be varied from 3 to 30.
Research ON (RON) Motor ON (MON)
Inlet temperature (oC) 52 149
Speed (rpm) 600 900
Spark advance (oBTC) 13 19-26 (varies with r)
Coolant temperature (oC) 100
Inlet pressure (atm) 1.0
Humidity (kg water/kg dry air) 0.0036 - 0.0072
Note: In 1931 iso-octane was the most knock resistant HC, now there are
fuels that are more knock resistant than isooctane.
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15. Testing procedure:
• Run the CFR engine on the test fuel at both research and motor conditions.
• Slowly increase the compression ratio until a standard amount of knock
occurs as measured by a magnetostriction knock detector.
• At that compression ratio run the engines on blends of n-hepatane and
isooctane.
• ON is the % by volume of octane in the blend that produces the std. knock
The antiknock index which is displayed at the fuel pump is the average of
the research and motor octane numbers:
Octane Number Measurement
2
MON
RON
index
Antiknock


Note the motor octane number is always lower because it uses more severe
operating conditions: higher inlet temperature and more spark advance.
The automobile manufacturer will specify the minimum fuel ON that will resist
knock throughout the engine’s operating speed and load range.

16. Knock Characteristics of Various Fuels
Formula Name Critical r RON MON
CH4 Methane 12.6 120 120
C3H8 Propane 12.2 112 97
CH4O Methanol - 106 92
C2H6O Ethanol - 107 89
C8H18 Isooctane 7.3 100 100
Blend of HCs Regular gasoline 91 83
n-C7H16 n-heptane 0 0
For fuels with antiknock quality better than octane, the octane number is:
where mT is milliliters of tetraethyl lead per U.S. gallon
  2
/
1
2
035216
.
0
472
.
1
0
.
1
736
.
0
0
.
1
28
.
28
100
T
T
m
m
ON
T
T










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17. Fuel Additives
Chemical additives are used to raise the octane number of gasoline.
The most effective antiknock agents are lead alkyls;
(i) Tetraethyl lead (TEL), (C2H5)4Pb was introduced in 1923
(ii) Tetramethyl lead (TML), (CH3)4Pb was introduced in 1960
In 1959 a manganese antiknock compound known as MMT
(Methylcyclopentadienyl Tricarbonil was introduced to
supplement TEL (used in Canada since 1978 and recently in australia to boost
octane).
About 1970 low-lead and unleaded gasoline were introduced over toxicological
concerns with lead alkyls (TEL contains 64% by weight lead).
Alcohols such as ethanol and methanol have high knock resistance.
Since 1970 another alcohol methyl tertiary butyl ether (MTBE) has been
added to gasoline to increase octane number. MTBE is formed by reacting
methanol and isobutylene (not used in Canada).
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18. Octane Number Requirement of a Vehicle
• The actual octane requirement of a vehicle is called the Octane
Number Requirement (ONR), and is determined by using series of
standard octane fuels that can be blends of iso-octane and normal
heptane ( primary reference ), or commercial gasolines.
• The vehicle is tested under a wide range of conditions and loads,
using decreasing octane fuels from each series until trace knock is
detected.
• The conditions that require maximum octane are not consistent, but
often are full-throttle acceleration from low starting speeds using the
highest gear available.
• They can even be at constant speed conditions, which are usually
performed on chassis dynamometers.
• The maximum ONR is of most interest, as that usually defines the
recommended fuel, however it is recognized that the general public
seldom drive as severely as the testers, and so may be satisfied by
a lower octane fuel.
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19. Engine Management Systems
• Engine management systems are now an important part of the strategy to
reduce automotive pollution.
• The good news for the consumer is their ability to maintain the efficiency
of gasoline combustion, thus improving fuel economy.
• The bad news is their tendency to hinder tuning for power.
• A very basic modern engine system could monitor and control:-
– mass air flow,
– fuel flow,
– ignition timing,
– exhaust oxygen ( lambda oxygen sensor ),
– knock ( vibration sensor ),
– EGR,
– exhaust gas temperature,
– coolant temperature, and
– intake air temperature.
• The knock sensor can be either a nonresonant type installed in the engine
block and capable of measuring a wide range of knock vibrations ( 5-15
kHz ).
• A resonant type that has excellent signal-to-noise ratio between 1000 and
5000 rpm.

20. • A modern engine management system can compensate for altitude, ambient
air temperature, and fuel octane.
• The management system will also control cold start settings, and other
operational parameters.
• There is a new requirement that the engine management system also
contain an on-board diagnostic function that warns of malfunctions such as
engine misfire, exhaust catalyst failure, and evaporative emissions failure.
• The use of fuels with alcohols such as methanol can confuse the engine
management system as they generate more hydrogen which can fool the
oxygen sensor.
• The use of fuel of too low octane can actually result in both a loss of fuel
economy and power, as the management system may have to move the
engine settings to a less efficient part of the performance map.
• The system retards the ignition timing until only trace knock is detected, as
engine damage from knock is of more consequence than power and fuel
economy.
11-Aug-22