Q. What is power card?
ü
The power generated in the engine cylinder can
be evaluated by indicator diagram.
Q. What you will get from power card?
ü
We can get by power card:
¨ Mean Indicated Pressure
¨ Combustion and compression pressures
¨ Evaluation of the combustion process
¨ Evaluation of scavenging process
¨ Calculation of air temperatures in
the compression chamber.
Q.
How
you will calculate power
o
Determination of the indicated engine output… using
power card and PLAN formula
o
In order to determine
the indicated power, a set of diagrams are taken consisting of one diagram for
each cylinder. The area of the diagrams, and therewith the (MIP) Mean Indicated Pressure is determined by means of a planimeter.
o
MIP = Wnet (Area of the p-V diagram)
Stroke volume
Power (Watts) = Pm x L x A x n
Where Pm = Mean Indicated Pressure (N/m2)
L = Length of piston stroke (m)
A = Piston area (m2)
n = Power strokes / second ….
Note: (for 2 stroke engine n
power stroke/second
for 4 stroke engine n/2
power stroke/second)
Q. What are the things you consider when
calculating power?
o
The following to be considered:
ü
The prop is fully immersed.
ü
The ship is fully laden at an even keel.
ü
The ship is running neither with nor against a
head wind or heavy sea.
ü
The ship skin is not fouled by an underwater
growth.
Q.
What
are the safety measures you are taking?
o
Before taking a set of indicator diagrams the
following precautions should be taken. It is only then the diagram will record
truly the engine power.
a)
To reduce the effect of friction the indicator
piston is removed, the piston and the cylinder cleaned and lubricated with a
few drop of cylinder oil.
b)
The tightness of the indicator piston in the
cylinder should be checked. This can be done by dismantling the piston and
allowing it to drop slowly through the cylinder by its own weight. If the
bottom of the cylinder is closed the piston should be held at its position.
c)
The drum should not hit the stop at end
positions. Play in the pencil mechanism will distort the diagram.
d)
The cock should be free from accumulation of
soot and oil.
e)
The stylus should be adjusted to a light writing
pressure.
Q. How you will take power card without any error?
o
To take the indicator diagram —
— fasten the instrument to the cock so
that the cord is taut.
— Trace the atmospheric line with the
cock shut.
— Open indicator cock and lightly press
the stylus against the paper till the cycle is described.
— Close the cock.
Q. What is Draw Card?
ü
A draw diagram is in effect a pressure-time
diagram which is obtained by moving the drum at constant velocity during the
period covering a few degrees before and after the TDC.
Q. How you will take draw card without any
error?
ü
Indicator diagram 900 out of phase
ü
After taking the indicator diagram
— disconnect the c Open the indicator
cock and pull it by hand. A con1prcs and ignition sequence is recorded. The
pull is to be timed when the pencil is upward i.e. the piston is at the TDC.
Q. You have taken
draw card, you got compression pressure low, what are causes?
ü
Engine conditions which lower the compression
pressure are:
·
Piston ring and liner wear out,
·
Exhaust valve leak,
·
Defective lubrication,
·
Wearing out of bearings,
·
burning out of piston crown
·
Low scavenge pressure, fouling of exhaust
and/or scavenge air system, etc.,
Q. What is light
spring diagram? How you will take?
ü
The variation of pressure in the cylinder during
exhausting and scavenging is magnified in this diagram.
ü
This diagram is taken with a light spring.
Q. How you will asses the light
spring diagram?
1-
Exhaust valve open
2-
Scavenge port open
3-
Scavenge port closed
4-
Exhaust valve closed
Fig:
Light spring diagram
The
pressure during this part of the cycle being low, a light spring is utilised.
For most of the time the cylinder pressure is outside the range of the spring.
As the pressure drops part of the expansion curve is shown in the diagram. The
exhaust opens at 1 and there is an instantaneous drop in pressure (Blow down).
Exhaust gases rush towards the exhaust valve creating a pressure wave leaving a
low pressure region behind. The pressure drops till the scavenge ports are
opened at 2 and the scavenging takes place till scavenge ports close at 3.
The
drop in pressure (Blow down) is a measure of clearness of exhaust passages
(exhaust back pressure, Fouling of turbine and EGE).
This
diagram also shows the pressure of scavenge air inside cylinder when scavenge
port closes at 3. The scavenge port opening timing 2 and closing timing 3 is
same.
Choked scavenge ports
In power card this
defect will show as loss of power, low compression and ignition pressure and
high exhaust temperature. A light spring diagram will show a low pressure at
point 3 and 4 (closing of scavenge port and exhaust valve). The blow down will
be normal indicating normal exhaust back pressure.
High exhaust back
pressure
High exhaust back pressure manifests as loss of power in affected cylinder as well as high exhaust temperature and sooty exhaust. First power card is taken. If the compression pressure is normal and no defect is noticeable in fuel injection, a light spring diagram is taken. High pressure affects scavenging and fuel cannot burn properly.
Blow down is less compared to normal. This increase
in back pressure may be due to fouled turbine or EGE.
Q. How could you confirm indicator drive is
perfect?
Q. 4-stroke engine, power calculation (different
type).
Q. 4-stroke engine power/draw card analysis. What are
the limitations in case of 4-stroke engine?
Q. What is power balancing?
n Power produced in a diesel
engine is related to the quantity of fuel injected and time when injected,
n Balancing of power between
the cylinders is carried out by small adjustments to individual fuel
pumps on a running engine.
