Knocking

1. Analysis & Control of Knock in SI Engines
P M V Subbarao
Professor
Mechanical Engineering Department
Measures to AvoidUncontrollable Combustion….

2. The Knock in SI Engines
• Knock in gasoline engines is one of the major challenges to
design an engine with higher thermal efficiencies.
• Without knock, an engine can be designed to have a higher
compression ratio, giving higher efficiency and power output.
• The demand to design engines closer to the allowable knock
limit with consequential reductions in safety limits is highly
appreciated.
• The phenomenon is characterized by excessively high pressure
amplitudes with stochastic occurrence.
• The phenomenon occurs independent of the mixture formation
process in both natural aspirated and turbo charged engines.
• This phenomenon is a fundamental issue of modern SI engine
design methods.

3. The Reason for the Birth of Knock
• 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 auto-ignition in the end-
gas can occur then knock is avoided.
• Therefore knock is a potential problem when the burn time is
long.

4. A Geometrical Problem turned into A Chemistry
……..
Compression Quality Brake Thermal Efficiency
Ratio Requirement
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 %

5. Fuel : The Resource is the Culprit : 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.
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.

6. • 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

7. 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.
•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.

8. 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 Motor
Inlet temperature (oC) 52 149
Speed (rpm) 600 900
Spark advance (oBTC) 13 19-26
(varies with CR)
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.

9. 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 stand.
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.

10. 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:
  2
/
1
2
035216
.
0
472
.
1
0
.
1
736
.
0
0
.
1
28
.
28
100
T
T
m
m
ON
T
T










where mT is milliliters of tetraethyl lead per U.S. gallon

11. 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 was introduced to
supplement TEL (used in Canada since 1978).
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).

12. The aromatics, toluene and xylene are the most likely candidates for a good
solvent to use as an antiknock additive/octane booster.
They are already present in gasoline and no adverse effects due to adding more
are apparent.
Organo Silicon Compounds – Under Study
Future Antiknocking Additives

13. Octane Number Requirement of a Vehicle
• The actual octane requirement of a vehicle is called the Octane
Number Requirement (ONR).
• This 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 full-throttle
acceleration from low starting speeds using the highest gear
available.

14. • The end-gas temperature and the time available before flame
arrival are the two fundamental symptoms that determine
whether or not knock will occur.
• Engine parameters that effect these two fundamental
variables are:
• Compression ratio, spark advance, speed, inlet pressure and
temperature, coolant temperature, fuel/air ratio.
Engine Design Parameters Causing the Knock

15. Important Engine Variables
• 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.

16. The Combustible Domain of IC Engine

17. Knock limit as a function of CR and ON for moderate
and high turbulence combustion chambers.

18. Effect of Initial Mixture Temperature on Available
Combustion Time to Avoid Knocking

19. Spark timing – maximum compression from the piston 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.
Most Useful Engine Parameter to Control Knocking
P,T
T
Ignition
x
x End of combustion

20. x
x
x
x
x
x
x
X crank angle corresponding
to borderline knock
Spark advance set to 1% below MBT to avoid knock
1% below MBT
Knock Mitigation Using Spark Advance

21. Auto Sparking Strategy

22. Set spark timing for MBT, leaner mixture needs more spark advance since
burn time longer.
Along MBT curve as you increase excess air reach partial burn limit (not all
cycles result in complete burn) and then ignition limit (misfires start to
occur).
Effect of Fuel-air Dilution
Partial burn limit
Complete burns in all cycles
Partial
burn regime
MBT spark timing
Ignition
limit

23. Why Damage due to Knocking
• There are several theories about what it is that causes the damage
on the engine during knocking conditions.
• The most accepted is that it is caused by heat transfer .
• When knocking conditions occur, the piston and the walls of the
combustion chamber are exposed to a great deal of additional heat
which results in overheating of these parts.
• As a result, the thermal boundary layer at the combustion chamber
wall can be destroyed.
• This causes increased heat transfer which might lead to certain
surfaces causing pre-ignition .
• Substantial knock can damage the engine and is stressful to the
driver and is therefore the most important limitation for SI
engines.
• In order to control the knock it is sometimes necessary to regulate
away from the most efficient operating point.

24. Knock Behavior and Conceptual Formulation
• The knock phenomenon to be investigated is characterized by
excessively high pressure amplitudes nearby or direct at the
knock limit.
• Due to this damaging knocking cycles, an efficient engine
operation at the knock limit is impossible, due to the risk of
severe engine damage.
• To characterize the knock behavior of an SI engine with wide
open throttle (WOT) the control range (CR) of a knock control
system will subsequently be introduced as an index.
• The CR is defined as the advance ignition angle between the
knock limit (KL) and the damage limit (DL) of a specific
engine operation point
KL
DL
CR 
 


25. 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.

26. Knock Sensor
• Knock Sensors generate a voltage
when vibration is applied to them
utilizing the piezoelectric effect.
• Generated voltage is proportional
to the acceleration .
• Due to the vibration, a counter
weight inside the sensor is
applying pressure on the piezo
element, this pressure creates an
electric charge in the piezo element
which is the output signal of the
sensor.
• Tuned to engine knock frequency
(typically 6-8kHz).

27. Location of Knocking Sensor
The knock sensor is located on the engine block, cylinder head, or the intake
manifold.
This is because the function of this sensor is to sense vibrations an engine
creates.
The PCM uses this signal to alter the ignition timing and prevent detonation.
It will compare this information with its preset tables to identify an engine
knock or ping.
If a ping is sensed it will retard the timing to protect the engine from this
damaging pre-ignition.

28. Knock Sensor Voltage Generation

29. Knock Sensor Circuit
• Once signs of detonation are detected (i.e. knocking), the
knock sensor sends a voltage signal to the engine management
computer which retards the spark timing slightly to avoid
detonation.

30. Knock Control

31. Benefits
• Vehicle engines work more efficiently and produce more
power when operating near the detonation limit.
• Although simple, knock sensors allow optimum engine
performance and protect the engine from potential
damage caused by detonation.