Indicator Diagram

What is the indicator diagram? 

The diagrams indicating, simultaneously, the pressures and the relative position of the piston in the engine cylinder are known as indicator diagrams. 

PURPOSE (WHY) OF TAKING INDICATOR DIAGRAMS

a) Determining the indicated engine output

b) Determining combustion (Pmax) and compression pressures (Pcomp) 

c) Evaluation of the combustion process and its peculiarities 

d) Evaluation of the exhausting and scavenging conditions 

HOW TO TAKE INDICATOR DIAGRAM? (SQUENCE)

a) Check whether the spring fitted on the indicator instrument will meet the peak pressure to be expected (max. pressure of Engine). 

b) Stretch diagram paper firmly over the drum. 

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

d) After drawing atmospheric pressure line, hook the cord to indicator drive, open indicator cock, and take power diagram and shut the cock. 

e) Remove hook, turn the drum by hand to a place clear from the power diagram, took compression pressure line with fuel cut-off. 

f) Having taken indicator diagrams from all cylinders, open the indicator instrument and clean all parts especially the piston, thoroughly. After cleaning, apply high temperature grease into the surface of all parts. 

Note: Do not allow the indicator instrument to become overheated by too many firing strokes, as it will affect the instrument accuracy. 

Types of Indicator diagram? 

1. Power card 
2. Draw card
3. Compression card 
4. Light spring card 

What is indicated power? How to calculate power? 

> It is the power developed in the cylinder. 

> Firstly, take the power diagram for each cylinder. 

> The area of the diagram can be measured by means of a planimeter. 

> Measures the length of diagram; check spring scale. 

> Calculate the mean effective pressure. 

M.E.P = (Diagram area/Mean Length of Diagram) x Spring scale 

> Record the engine revolution(RPM) 

> Calculate the power for each unit 

I.P = MEP x Length of stroke x cylinder bore area x rps (Watts) 

 IP = P L A N x No. of Cyls (KW) 

P = MEP (KN/m2) = (Area of diagram / Length of diagram ) x Spring scale

L = Stroke Length ( m ) 

A = Cylinder bore area ( m2) 

N = N for 2/S single acting (rps) 

 = N/2 for 4/S single acting 

 = 2N for 2/S double acting 

Purpose of Each indicator diagram:

Power diagram

•  To calculate indicate power.
•  It also shows peak pressure. 

 In phase with piston movement, with fuel on. 

Out-of-phase diagram

• compression pressure is to be accurately determined. 
• It shows Pmax (more accurately), P compression and ignition delay period (Nature of expansion curve).
•  It also shows combustion process (early or late combustion) 
• To evaluate injection, ignition delay, fuel quality, combustion, loss of compression, expansion process, fuel pup timing, and after-burning. 
• 90° out of phase with piston movement, with fuel on, to determine Pmax.

Compression diagram 

• To know compression pressure of the cylinder and cylinder tightness. 

 In phase and fuel cut-off.

Light spring diagram 

• This diagram are taken for determining the pressure variation in the cylinder during the exhaust and scavenging periods 
• The diagram shows- 

-Choke exhaust ports or valves 

-Loss of scavenge air 

In phase, using light spring, with fuel on, to determine.

When taken? 

• Taken at every month and after every major O/H on the way. 
• By according to the engine condition. 

PRECAUTION NECESSARY TO AVOID INDICATOR MALFUNCTION

•  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 
• 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. 
• The drum should not hit the stop at end positions. Play in the pencil mechanism will distort the diagram
• The cock should be free from accumulation of soot and oil 
• The stylus should be adjusted to a light writing pressure 
• Before taking indicator diagrams, open indicator cock two or three firing stroke to blow out soots and carbon residues in the cock. 
• Check whether the spring fitted on the indicator instrument will meet peak Pr: to be expected.
• Stretch diagram paper firmly over the drum 
• After drawing atmospheric line, hook the cock to the indicator drive, open indicator cock and take power diagram, shut the cock 
• Remove hook, turn the drum by hand to a place clear from power diagram, took compression pressure line with fuel cut-off 
• Do not allow the indicator instrument to become overheated by too many firing strokes, as it will affect the instrument accuracy 
• Having taken indicator diagrams, from all cylinders, open indicator instrument and clear all parts,especially the piston thoroughly. 
• After cleaning, apply high temperature grease into surfaces of all parts 

How to maintain good Performance 

01) Maintain good power output per cylinder

02) Take power Card, to check power output/cylinder 

03) Take Compression Card, to check for cylinder tightness 

04) Check ratio of Absolute compression pressure to absolute scavenging pressure. 

05) If the ratio is same as that during sea trial, piston rings and exhaust valves are sufficiently tight. (With B&W engine, this ratio is about 30) 

06) If absolute pressure ratio is less, check for cylinder tightness, charge air cooler, scavenge air ports, scavenge valves, piston rings, exhaust valves, TC, etc. 

07) Light spring diagram is taken if necessary 

08) Check exhausts temperatures, exhaust smoke, load indicator and engine running parameters. 

09) Check fuel, CLO & LO consumption 

10) Regular maintenance works and repairs. 

Absolute Pressure = Gauge Pressure (of Manometer) + Atmosphere Pressure (15 psi or 30 Mercuy) 

If compression pressure is low: 

• Carry out unit O/H and renew liner, piston and rings. 
• TC checked, clean and overhauled, to have efficient operation 
• Check Scavenge air line, charge air cooler, for insufficient scavenge air condition
• Check Inlet and Exhaust valves may be leaking. 
• Clean Scavenge Ports, Scavenge Valves, if 2/S engine. 

Early Combustion Effects:

1) Very high peak pressure Pmax at about TDC 

2) Lower Exhaust temperature 

3) Increase Power 

4) Increase Thermal efficiency 

5) Heavy shock load to bearings 

6) Engine knocking 

7) Lower expansion line 

8) Ignition occurs before TDC 

9) Less S.F.O.C 

Causes 

1) Overheated piston 

2) cetane number of fuel than normal 

3) Fuel injection pump plunger has been set too high (Incorrect fuel pump timing or wrong V.I.T

setting) 

4) Incorrect adjustment of the fuel cam on the camshaft 

5) Incorrect adjustment of fuel valve spring pressure (low) 

LATE IGNITION

Effects:

1) Lower peak pressure 

2) Higher Exhaust temperature with black smoke 

3) Loss of power 

4) Reduced Thermal efficiency 

5) Reduced Scavenge efficiency 

6) Higher Exhaust gas pressure at blow down period and increasing pulsation in the exhaust 

manifold 

7) Higher expansion line 

8) Ignition occurs after TDC 

9) Increase S.F.O.C 

Causes 

1) Incorrect fuel pump timing and wrong V.I.T setting 

2) Faulty fuel valve (Excessive injector spring setting) 

3) Injection viscosity too low 

4) Lack of scavenge air or cold air 

5) Bad atomization 

6) Slack chain drive 

7) Lower cetane number of fuel 

8) Worn out fuel pump, cam and roller 

9) Low compression 

10) Under cooling of parts within the cylider

11) Leaky fuel pump delivery valve or spill valve 

It may cause:

a) Exhaust valve burning 

b) Turbocharger surging 

c) Fouling of exhaust system 

d) Uptake fires

e) High cylinder temperature resulting in liner lubrication difficult. 

After Burning 

Symptoms & Effects 

1) Rise in expansion line during latter part of the load 

2) Exhaust temperature and Exhaust pressure will be high, with burning fuel and carbon passing to exhaust. 

3) This may burn exhaust valves and foul the exhaust system, with risk of T/C surging or uptake fires.

4) High temperatures within the cylinder cause deterioration in lubrication and possible damage to liner surface & piston rings. This may be the cause of piston crown burning. 

5) Smoky exhaust 

Causes 

1) Incorrect fuel pump timing (Slow or late combustion of fuel) 

2) Faulty fuel injector (leaky fuel valve) 

3) Heavy fuel oil temperature too low and high viscosity 

4) Lack of scavenge air 

5) Poor compression 

6) Poor quality fuel 

EXHAUST VALVE OPENING 

Exhaust valve opening in a slow speed engine can be checked by means of light spring indicator diagram. 

Early opening 

1) Indicates high exhaust temperature with the risk of overheating and contamination 

2) Causes a loss of power since pressure is released too soon. 

Late opening 

1) Reduces the scavenge efficiency by reducing blow down 

2) Affects the supply of energy to the turbocharger. 

CHOKE EXHAUST PORTS OR VALVES

Symptoms 

1) Higher exhaust pressure with sooty smoke 

Earlier Exhaust 

œ Sufficient time for Cyl: 

Press: to fall below 

charge air press . When 

the scav Ports uncovered. 

œ Increased pulse energy 

available from the 

exhaust gas which can be 

used to improve pulse 

T/C performance. 

œ By advancing exht 

Timing, Gain in T/C 

output is greater than loss 

of expansion stroke. 

2) Higher mean temperature of working parts 

3) Higher Exhaust temperature with heavy smoke 

4) Lower Pmax , Lower Pcomp 

5) Loss in power 

6) Surging of Turbocharger 

Causes 

1) Faulty fuel injection system

2) Lack of scavenge air 

3) Excessive cylinder lubrication 

4) Fouling of exhaust system

Remedies 

# Correct fuel timing and fuel injection system 

# Maintain scavenging system in good order 

# Correct cylinder lubrication feed 

# Clean exhaust grid and Turbocharger 

CHOKED FUEL VALVE 

Symptoms

1) Loss in engine power 

2) Hammering in the fuel pipes between fuel pump and injector (This may lead to rupture of fuel pipe) 

3) Reduce exhaust temperature 

a. It can be confirmed by indicator diagram power and draw cards. The diagrams show 

b. irregularities at the peak of the diagram. 

Causes 

4) The contamination in the fuel in which debris may choke the small atomizer holes in the injector 

5) A leaky injector allowing hot gas to blow back into the injector causing carbon to form and choke the injector. 

6) Overheating of injector nozzle may also cause build-up of carbon.

Remedies 

# Change the fuel valve 

# Clean the whole fuel system

# Ensure correct centrifuging and filtering of fuel 

# Maintain correct cooling temperature of fuel valve 

LEAKY FUEL INJECTORS 

Symptoms 

01. Loss in power 

02. High Exhaust temperature with smoke 

03. Knock or pressure wave in fuel injection system 

It can be detected by taking both Power card and Draw card, which shows fluctuation of pressure during the expansion process due to secondary burning of fuel leaking from the valve. 

It may cause: 

a. Afterburning 

b. Hot gas from combustion chamber blow past into the injector and forming carbon trumpets choking the injector 

c. Fouling and loss efficiency of turbocharger due to afterburning 

d. Coking of exhaust ports and grids. 

 

Diagram showing leaking injector

Remedies

# The fuel valve should be renewed and tested for faults and rectified. 

# Fuel purification and separation system should be kept in good order 

# Fuel system temperature and Nozzle cooling temperature must be maintained in correct levels. 

LEAKY PISTON RINGS OR WORN LINER 

Symptoms 

1) Loss in engine power 

2) Lower compression (Pcom) and combustion (Pmax) 

3) High exhaust temperature with smoke 

Causes 

1) Excessive cylinder liner wear 

2) Lack of cylinder lubrication 

3) Worn, broken, stuck or poorly maintained piston rings 

4) Worn piston ring groove landings allowing rings to cant and jam 

5) Carbon jamming rings in grooves 

6) Turbocharger failure due to fouling 

7) Blockages in the exhaust gas system. 

 It can be detected by taking Draw card which shows higher dip in compression line. It may 

cause:

(1) Blow past of combustion gas 

(2) High rate of cylinder liner wear due to poor cylinder lubrication 

(3) Scavenge fire

(4) Piston seizure due to local overheating 

Remedies 

# Renew cylinder liner if necessary 

# Overhaul piston 

# Clean ring grooves and gauge 

# Machine or fit new groove inserts as necessary 

# Renew piston rings with correct clearances 

# Maintain cylinder lubrication and avoid overload 

 

Leaking piston rings

Cetane Number 

@ A measure of ignition quality of fuel 

@ The higher the Cetane Number the shorter the time between fuel injection and rapid combustion 

@ The higher the Cetane No. the better the ignition quality 

@ Considered as poor fuel, if C< 37 Usual range is 30-45 

High Cetane Number Effects 

1. Shorter delay period 

2. Early combustion 

3. Increased power 

4. Knocking 

Low Cetane Number Effects 

1. Longer delay period 

2. Late combustion 

3. Decreased power

4. After burning

5. High exhaust temperature and smoke 

Diesel Knock 

Violent knocks produced by high rate of pressure rise, RPR, during combustion, as delay period is longer then normal 

Causes 

1. Too low working temperature 

2. Cold start 

3. Too early fuel injection 

Calculate the indicted power diagram by Mid ordinate Method 

Calculate mean height of diagram = Number of partsin diagram

Sum of mid ordinates

Calculate MEP = Mean height of diagram x Spring scale