POWER CALCULATION (Power Card & Draw Card)

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.

Q. How you balance power in two stroke & 4-stroke engine?

Q. What consideration you take when balancing power?

Liquefied Petroleum Gas (LPG)

LPG is the name originally given by the oil industry to a mixture of petroleum hydrocarbons principally propane and butane and mixtures of the two. LPG is used as a clean fuel for domestic and industrial purposes. These gases may be converted to the liquid form and transported in one of three conditions:
(1) Solely under pressure at ambient temperature.
(2) Fully refrigerated at their boiling point (-30°C to –48°C).
(3) Semi-refrigerated at reduced temperature and elevated pressure.
A number of other gases with similar physical properties such as ammonia, propylene and ethylene are commonly shipped on LPG carriers. These gases are liquefied and transported in the same conditions as LPG except ethylene which boils at a much lower temperature (-104°C) and which is therefore carried in the fully refrigerated or semi-refrigerated condition.

Oil Mist Detector

  ¨   Maintenance & Checking accuracy of OMD operation:

¥      Light scatter Type (QMI)

• The light scatter device detector heads are set – up by the manufacturer. systems are self calibrating against deviation.

• Maintenance consists of cleaning lenses and changing filters.
•  The CPU has a self diagnostic facility which checks for errors.
•  The device can be tested by using aerosol which mimics an oil mist at alarm level (2%) LEL.

¥      Graviner type

• Alarms & slowdown are tested against a known concentration of mist
• The lenses and mirrors should be cleaned periodically.
• In this model the total mist concentration is measured with respect to clean air.
• An alternative models draws samples through both references and measuring tubes which from C/C and atmosphere and compare the level of mist.

  ¨   Take to safeguard against the risk of C/C explosion in the event of OMD become inoperative:

• If the engine is not equipped with bearing temperature monitors linked to slowdown/shutdown, then in the event of the oil mist detector becoming inoperative, then subject to classification society rules, the machinery space cannot operate under UMS conditions and will have to be manned on a 24 hours watch keeping basis.
• Because one of the important safety devices is now inoperative, extra vigilance will be required by the engineers on duty.
• If bearing temperature probes are fitted, then these must be closely monitored.
• The watch keeping engineer must use his senses of smell, hearing, sight and touch to check for any unusual noise, heat or smell which may indicate a potentially dangerous situation. In the event of any doubt, them the engine must be stopped for further investigation.
• It will be even more important than ever to ensure that the engine is not overloaded.
• The crankcase pressure should be closely monitored; a rise in pressure on a trunk piston engine will indicate excessive blow by from one or more units.

The OMD should be investigated to establish the cause of failure, and necessary spares ordered to rectify the problem as soon as possible. 

Midship section

Container Ship

Purpose of Torsion Box in a container ship:

       Problems may arise in these vessels due to the lack of torsional strength caused by the large hatches.

       This problem is overcome to some extent by fitting torsion boxes on each side of the ship.

       These boxes are formed by the upper deck, top part of the longitudinal bulkhead, sheer strake and upper platform, all of which are of thick material.

       The boxes are supported inside by transverses and wash bulkheads in addition to the longitudinal framing.

       These boxes are only effective if they are efficiently tied at their ends.

       At the after end they extend into the engine room and are tied to deep transverse webs.

       Similarly at the fore end, they are carried as far forward as the form of the ship will allow and are welded to transverse webs.

       The longitudinal bulkheads below the box may have to be stepped inboard to suit the shape of the ship, the main longitudinal bulkhead being scarphed into the stepped section.

                                           

Fuel Injector (drawing)