It's all about Knocking in IC Engines, their limiting factors,issues,how to nullify their effect and how to control this effect and how to over come knocking inside combustion chamber.

1. Knock
Fundamentals
‘limitations and issues’

2. content
• Basic Definitions of knocks
• Types of abnormal combustions
• Origin of knocks
• Comparison between Spark knock and Diesel knock
• Limiting parameters of knock
• Cetane and Octane ratings with anti-knock agents
• Knock damages
• Knock control strategies

3. Definitions
• Knock is metallic sound caused by spontaneous ignition of end gas.
• Occurs during compression stroke mostly in SI engine and
expansion stroke in CI engines
• Results in pressure fluctuations in cylinder and increases
temperature
• Surface ignition is caused due to hot spots in chamber
• Overheated valve, spark plug, glowing combustion chamber
deposits
• Pre-ignition and post-ignition

4. Definitions contd…
• Acceleration knock produces more sound but can not cause damage
due to short duration
• Constant speed knock can cause drastic damage
1. pre-ignition
2. Runway knock

5. Types of combustions
Normal combustion
Abnormal
combustion
Spark knock Surface ignition
Knocking surface
ignition
Wild ping
Run on
Non knocking
surface ignition
Rumble
Runaway surface
ignition

6. Origin of knock
• Auto ignition of end gas, depends strongly on chemical composition of end-
gas mixture.
• If the flame burns all the fresh gas before auto-ignition in the end-gas can
occur then knock is avoided.
• Knock is a potential problem when the burn time is long.
• Theory of auto-ignition
fuel oxidization occurs during compression of A/F mixture
• Theory of detonation
advancing flame velocity to consume end gas is much higher

7. Limiting parameters for knocks
• end-gas temperature
• time available before flame arrival
• Compression ratio
• Spark timing
• Engine speed
• Delay period
• Inlet pressure and temperature, coolant temperature, fuel/air ratio

8. Fuel factor and knock scale for SI engines
• Octane number ON is measure of resistance to knocking.
• 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.

9. Fuel factor and knock scale for CI engines
• Ignition quality of a fuel cetane number CN.
• For high cetane fuels ignition delay is short so very little fuel is injected before
autoignition.
• Cetane (n-hexadecane, C16H34) has a value of 100.
• In the original procedures a-methylnaphtalene (C11H10) with a cetane number
of zero represented the bottom of the scale.
• This has since been replaced by heptamethylnonane, (HMN) has a cetane
number of 15, which is a more stable compound.
• The higher the CN the better the ignition quality, i.e., shorter ignition delay.
• The cetane number is given by:
CN = (% hexadecane) + 0.15 (% HMN)

10. Comparison between spark and
diesel knock
• In SI knock occurs near start
of compression
• Depends on fuel factor and
engine parameters
• Is more significant and limits
compression ratio and hence
efficiency of engine
• In CI engines knock occurs
near start of expansion
• Depends on delay period and
engine parameters
• Less significant and gives
advantage to higher thermal
efficiencies of engines

11. Comparison between spark and
diesel knock

12. Knock measurement [sensors]
• Optical probes and Ionization detectors can
be used
• Piezoelectric pressure transducer is used
mostly
• Generate a voltage when vibration is applied
to them utilizing the piezoelectric effect and it
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.

13. Knock damages
• Piston ring sticking breakage
• Cylinder head gasket
• Cylinder head erosion
• Piston crown erosion
• Piston melting and holing

14. 1. Provide adequate cooling to the engine
2. Use intercooler on turbo-charged engines
3. Use high octane and cetane gasoline and diesel
4. Anti-knock gasoline additives
5. Fuel enrichment under severe condition
6. Use knock sensor to control spark retard so as to
operate close to engine knock limit
7. Fast combustion system
8. Gasoline direct injection
Knock control strategies