Understanding Power Card / Indicator Diagrams and Fault Analysis

Power Cards:

A power card is a graph of cylinder pressure against time.

Purpose Of Taking Power Cards:

·         To calculate indicated power of the engine

·         To determine peak pressures and compression pressures

·         To evaluate the process of combustion inside the engine

·         To evaluate scavenging and exhausting conditions

How to Take:

It is originally drawn using a mechanically driven pen onto graph paper mounted on a drum. The drum was rotated by string, via a cam on the camshaft and pushrod. As the drum rotated the pen mounted on the linkages was pressed up to the paper. For clarity the pen is released once a single cycle has passed otherwise slight fluctuations in power demand could lead to several cycles being superimposed on one another blurring the image.

The indicator is a sensitive piece of equipment which can malfunction and so it must be treated with care. It can only be used effectively on an engine operating below 200 rpm due to the difficulty involved in getting only a single line on the card. In addition the inertia in the drum can lead to delays distorting the shape. For higher speed diesels either peak pressure indicators are used, or sophisticated electronic monitoring equipment is required with oscilloscope type displays. The time base for these is off transducers mounted on the flywheel.

It is important that the indicator is kept well lubricated with a light high quality oil. Prior to mounting the indicator the indicator cock is blown through to ensure it is clear. Compression cards are then first taken to check for errors caused by wear or friction/stiction in the instrument.

§  Check whether the spring fitted on the indicated instrument will meet the peak pressure (maximum combustion pressure) to be expected

§  Stretch diagram paper firmly over the drum.

§  Before taking diagram, open indicator cock, allow two or three firing strokes, to blow out soot and combustion residues in the cock.

§  After drawing atmospheric pressure line, hook the cord to indicator drive, open indicator cock, take power card and shut off the cock.

§  Make sure that the instrument is not exposed to high temperature for long time. This may affect its accuracy.

§  Remove the hook, turn the drum by hand to a place clear from the power diagram, take compression pressure line with the fuel cut off.

§  After taking indicator cards from all the cylinders, open the instrument and clean all the parts, and lubricate the same.

Precautions While Taking Indicator Cards

• Always lubricate indicator instrument parts to prevent seizure at high temperatures.
• Tightness of the indicator piston inside the cylinder to be checked. It should be a free sliding fit.
• The cock to be free from accumulated carbon particles.
• Do not apply much pressure on the stylus while taking the diagram.
• Allow sufficient cooling time for the instrument after taking diagrams from each units

Types of indicator diagrams:

• Power card / Power indicator diagram
• Compression diagram
• Draw card / Out of phase diagram
• Light spring diagram

Purpose of Indicator Diagrams:

§  The indicator diagram is very important to know the combustion in the cylinder and also to adjust the engine.

§  The diagram is taken periodically from the indicator valve equipped on he cylinder head and combustion condition is to be confirmed.

§  The compression pressure and maximum pressure in the cylinder can be presumed from the indicator diagram.

§  Engine indicator is the device used to take the indicator diagram, which can be considered as a ‘stethoscope’ for diesel engines.

§  Indicator diagrams give efficiency of combustion in the cylinder, condition of the running gear, irregularities in fuel pumping and injection and a lot of things.

Pcom – Compression Pressure

Pmax – Maximum Pressure

Power Card

§  Power card is taken with the indicator drum rotating in phase with the piston movement

§  The area within this diagram represents the work done during one complete cycle to scale

§  Mean Indicated Pressure (MIP) is obtained from this diagram to calculate power produced in the cylinder

Power card of 2 stroke Engine:

1.  scavenge port closed
2.  exhaust port shut-commence of compression
3. fuel injection
4. top dead center
5.  7 post combustion expansion
6. exhaust port opens     

  Four stroke cycle

   

   

•          3-4-5 fuel injection and combustion
•              5-6 expansion
•             6-7-8-Exhaust valve open
•       8-9-10 overlap, exhaust remains open whilst air enters
•       10-1 aspiration and exhaust valve closes 
• Power calculation
  The area swept out by the power stroke will give the power developed by the engine. It should be noted on a four stroke most of the non-power stroke occurs below atmospheric on a naturally aspirated engine and so gives a net loss of power.
  Power = p.A.L.n
   p - Mean average pressure in the cylinder
  A-area of piston [m3]
  L-stroke [m]
  n-revolutions per second
  From a power card this is altered to
  Power = area of diagram/length of diagram x Indicator spring constant
  By use of an instrument called a Planimeter the area scribed out by the pen could be measured giving the power generated by the cylinder. In addition, through experience, certain problems could be diagnosed by looking at the shape drawn. 

Compression Diagram

• Compression diagram is taken in similar manner to the power card but the fuel shut off in the cylinder
• The height of this curve shows maximum compression pressure
• If the compression and expansion line coincide, it indicates that indicator is correctly synchronized with the engine
• Reduction in height of this diagram shows low compression which may be due to worn cylinder liner, faulty piston rings, insufficient scavenge air or leaky exhaust valve

Draw Card / Out of Phase Diagram

• Draw card is taken in a similar manner to power card with fuel pump engaged but with the indicator drum 90 degree out of phase with the piston stroke
• This diagram illustrates more clearly the pressure changes during fuel combustion. Fuel timings or injector faults may be detected from its shape

Light Spring Diagram

• Light spring diagram is taken similar to the power card and in phase with the engine and with a light compression spring fitted to the indicator
• This diagram shows pressure changes during exhaust and scavenge to an enlarged scale
• It can be used to detect faults in these operations

Irregularities in Indicator Diagram

Early Ignition

Indications and Effects

• Abnormally high peak pressure of the unit is recorded at the top of the piston stroke.
• Knocking sound comes out of the engine due to heavy loads passed to bearings via running gear.
• Early ignition causes increased thermal efficiency of the engine. Also exhaust temperature reduces since combustion starts long before it is supposed to. But the shock loads and vibrations results in damage of the engine.

Causes

• Incorrectly adjusted or accidentally changed fuel pump timing
• Damaged or incorrectly set fuel valve or fuel injector
• Undesired fuel quality
• Parts inside the cylinder are overheated.

Late Ignition 

Indications and Effects

• Low peak pressure is indicated for the unit after top dead center.
• Combustion continue during expansion stroke, give rise to incomplete combustion of fuel, loss of energy, elevated exhaust gas temperature for the unit, and black smoke at the engine exhaust.
• Reduced power of the engine due to incomplete combustion of the fuel and energy lost in the exhaust.

Causes

• Faulty fuel injector or injector spring tension tighten beyond setting.
• Poor fuel quality
• Wrongly timed or leaking fuel pump
• Engine parts inside cylinder are under cooled
• Incorrect atomization
• Compression pressure low
• Combustion air supply is low

After Burning:

Indication and Effects

• A rise in expansions line is recorded in the later part of piston stroke
• Since burning of fuel continues, exhaust gas temperature and pressure increases, causing black smoke at engine exhaust.
• Unburnt carbon deposits fouls exhaust system, cause damage to exhaust valve and seat, turbocharger surges and there are chances of uptake fires in exhaust gas economizer.
• Elevated temperatures inside cylinders cause breaking up of lubrication and increased wear of liners, piston rings, burning of piston crown, etc.

Causes

• Slow fuel combustion
• Quality of fuel is less
• Low temperature of fuel (Means high viscosity)

Leaky Fuel Injector

Indications and Effects

• Reduced power in the affected unit, high exhaust temperature and presence of black smoke in exhaust.
• Possible knocking sounds or pressure waves in fuel injection system.
• Sudden up and downs in indicated diagram in the fuel injection and expansion side.
• After burning due to incomplete combustion of fuel.

Causes

• Leaking fuel injector.
• Chocking of fuel injector spray holes, which leads to improper atomization and dripping of fuel.

Partly Choked Fuel Valve

Indications and Effects

• Low exhaust gas temperature of the unit
• Power card and draw card indications
• Loss of engine power

Causes

• Fuel oil contamination and improper purification
• Carbon formation at injector tip
• Carbon deposits on fuel valve due to over heating

Low Compression

Indications and Effects

• Low pressure in the indicator card
• Reduced power of engine

Causes

• Improper combustion
• Insufficient air for combustion
• Leakage of air in between piston rings and liner while compression stroke due to worn out liner or piston rings.

Exhaust Valve Opening

Light spring diagram gives indications on faulty exhaust valve and intake port operations.

Early Opening

• Elevated exhaust gas temperature and fouling of exhaust system
• Loss of engine power

Late Opening

• Reduced blow down effect and hence reduced scavenging efficiency
• Low quality of exhaust gas delivered to the turbocharger inversely affect its operation

Choked Exhaust

Indications and Effects

• Power loss in the unit
• Increased exhaust temperature
• Turbocharger surging
• adversely affect scavenging efficiency

Causes

• Improper combustion
• Increased cylinder lube oil

Watch This Video for Better Understanding:

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References: 

marineengineeringonline.com

www.marineinsight.com

www.youtube.com

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