Factor tending to reduce knocking in SI engine:-
|♦Table of content♦
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
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
◦ 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
◦ 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|
|inlet temperature and pressure||Low|
|Flame travel distance||Small|
Factor tending to reduce knocking in CI engine:-
|Inlet temperature and pressure||High|
|Combustion chamber tempreure||High|
|Rotation per minute(R.P.M)||Low|
|Injection angle advance||No|