Abnormal
Combustion
Delay period:
Delay period is
the time interval (measured in milliseconds)
between the commencement of fuel injection and the beginning of
ignition and combustion. The start of combustion
is indicated
by the deviation of point of the pressure curve above the normal
compression pressure. In practice, this actual time is as low as
0.006 seconds.
The delay period consists of the following:
- Physical delay
period.
- Chemical delay
period
Factors
influencing delay period:
1. Characteristics
of fuel (self-ignition temperature,
volatility and viscosity).
2. Temperature and
pressure of compressed air.
3. Degree of
atomization of injected fuel.
4. Air motion /
turbulence present in the combustion
space.
5. Engine speed.
6. Injection
timing.
7. Characteristics
of the combustion chamber.
Factors
affecting knock
Knocking
Knocking is violet gas vibration and audible sound produced by extreme pressure differentials leading to the very rapid rise
during the early part of uncontrolled second phase of combustion.Delay period is directly related to Knocking in CI engine. An extensive delay period can be due to following
factors:
• A low
compression ratio permitting only a
marginal self -ignition temperature to be
reached.
• 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 and
Coarse
droplet formation due to malfunctioning of injector parts like spring
• Low intake
temperature and pressure of
air
METHODS
OF CONTROLING DIESEL KNOCK
We have discussed the factors which are
responsible for
the detonation in the previous
sections. If these factors are controlled, then
the
detonation can be avoided.
Using a better fuel. Higher CN fuel has lower delay
period and reduces knocking tendency.
Controlling the Rate of Fuel Supply. By injecting
less
fuel in the beginning andthen more fuel amount
in
the combustion chamber detonation can be
controlled to a certain extent. Cam
shape of suitable
profile can be designed for this purpose.
Knock reducing fuel injector:
This type
of injector avoid the sudden increase in
pressure inside the combustion chamber
because
of accumulated fuel. This can be done by
arranging the injector so that
only small amount
of fuel is injected first. This can be achieved by
using two
or more injectors arranging in out of
phase.
By
using Ignition accelerators :
C N number can be increased by adding chemical
called dopes. The two chemical dopes
are used
are ethyl-nitrate andamyle –nitrate in concentration of
8.8 gm/Litre and 7.7 gm/Litre. But these two
increase the NOx emissions.
Increasing Swirl : Knocking can be
greatly
reduced by increasing swirl ( or reducing
turbulence). Swirl helps in
knock free
combustion.
Injection
systems :
Direct – Injection
systems:
In largest size engines, where mixing rate
requirement are lease stringent, quiescent direct –
injection systems of the
type are used. The momentum
and energy of the injected fuel jets are sufficient
to
achieve adequate fuel distribution and rates of mixing
with the air.
Additional organized air motion is not
required. The combustion chamber shape
is usually a
shallow bowl in the crown of the
piston, and a
central multi hole injector is used.
