Knocking in Engine- CI & SI

Factor tending to reduce knocking in SI engine:-

♦Table of content♦

  1. THE COMBUSTION IN CI ENGINE IS CONSIDERED.
  2. IGNITION DELAY PERIOD
  3. PHENOMENON OF DIESEL KNOCK
  4. FACTORS AFFECTING DIESEL KNOCK
  5. METHODS OF CONTROLLING DIESEL KNOCK

knocking

The combustion in CI engine is considered to be
taking place in four phases:
1) Ignition Delay period /Pre-flame combustion
2) Uncontrolled combustion
3) Controlled combustion
4) After burning

IGNITION DELAY PERIOD

Ignition delay (0.7-3ms) period is counted from the
start of injection to the point where the pressure
time curve separates from the compression curve
indicated as start of combustion.
The ignition delay period can be divided into two
parts,
I. Physical delay
II. Chemical delay.

The physical delay depends on
The type of fuel,
◦ For light fuel the physical delay is small
◦ For heavy viscous fuels, the physical delay is high
injection Pressure
◦ The physical delay is greatly reduced by using high injection pressures
combustion chamber temperatures and Turbulence to facilitate
◦ break up of the jet and
◦ Improving evaporation.

Chemical delay depends
◦ On the temperature of the surroundings
◦ At high temperatures, the chemical reactions are faster
◦ In most CI engines the ignition lag is shorter than the duration of
injection.
◦ Generally, the chemical delay is larger than the physical delay.

PHENOMENON OF DIESEL KNOCK
Knocking is violent gas vibration and the audible sound produced by extreme pressure
differentials leading to the very rapid rise during the early part of uncontrolled
the second phase of combustion.
In C.I. engines the injection process takes place over a definite interval of time.
Consequently, as the first few droplets injected are passing through the ignition
lag period, additional droplets are being injected into the chamber.
If the ignition delay is longer, the actual burning of the first few droplets is
delayed and a greater quantity of fuel droplets gets accumulated in the chamber.
When the actual burning commences, the additional fuel can cause the too rapid
a rate of pressure rise.

FACTORS AFFECTING DIESEL KNOCK
A low combustion pressure due to worn out piston, rings, and bad valves
Low cetane number of fuel
Poorly atomized fuel spray preventing early combustion
Coarse droplet formation due to malfunctioning of injector parts like spring
Low intake temperature and pressure of air.

METHODS OF CONTROLLING DIESEL KNOCK
Using a better fuel:
1) Higher CN fuel has a lower delay period and reduces knocking tendency.
2) Controlling the Rate of Fuel Supply:
3) By injecting less fuel in the beginning and then more fuel amount in the combustion chamber
detonation can be controlled to a certain extent.
4) Cam shape of a suitable profile can be designed for this purpose.

The self-ignition temperature of the fuel High
Compression ratio Low
inlet temperature and pressure Low
Super Charging No
Spark advance No
Flame travel distance Small
Engine size Small
Turbulence High
Engine Speed High
Octane rating High

Factor tending to reduce knocking in CI engine:-

Compression ratio High
Self-ignition temperature Low
Dealy period Low
Inlet temperature and pressure High
Combustion chamber tempreure High
Rotation per minute(R.P.M) Low
Engine size Small
Cetane rating High
Supercharging Yes
Injection angle advance No
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