IGNITION QUALITY PARAMETERS OF SLOW SPEED DIESEL ENGINE

ignition quality parameters:

1. Energy
2. Viscosity
3. Maximum firing pressure.
   4. Injection delay
   5. Ignition delay .

1. Energy comparison

• The injection pump is a volumetric pump
• The higher the density the more energy it contains per volume unit
• The density difference between HFO and MDO is larger than the difference in net calorific value

2.  Viscosity comparison

• The viscosity of MDO is lower than the viscosity of HFO (even HFO is heated)
  • Lower viscosity fuels result in more internal leakage in the injection pump from the high pressure side to low pressure side.
  • Internal leakage has to be compensated by giving more fuel rack

3.  Maximum firing pressure

• Dependent on the charge air pressure
  • To achieve proper combustion, the firing pressure has to be high and thus also the charge air pressure
  • Timing of the fuel pump

4. Injection delay

• Injection delay is the time it takes to build up a pressure higher than the opening pressure of the nozzle.
  • Normally 4…5 ° of crank angle
  • The delay is comparable to the condition of injection equipment

5.  Ignition delay

• is the time gap between the commencement of injection and the commencement of ignition.
• Dependent on the fuel quality
• Compression temperature has a big influence on the ignition delay
• Compression temperature is not high enough for the fuel to ignite longer ignition delay

Source: Wartsila

Besides consider the following for better ignition quality of slow speed diesel engine:

Higher peak pressures and higher exhaust gas temperatures:
• Normally 1…2 ° of crank angle.
• Too early timing of injection
• Higher peak pressure

Exhaust gas temperature after cylinder:

• Measured individually after each cylinder
• Each cylinder has two measuring
• The average temperature of all exhaust temperature sensors is approximately 100 °C lower than the exhaust gas temperature before the turbine.
• Maximum allowed difference between valves in one
• Maximum allowed difference of one cylinders average from the average of the all cylinders ,
• In normal operation the average temperature Cylinder temperature deviation can be up to 30 – 40 °
• Engine is (depending of installation) provided with a safety slow down arrangement if the temperature difference is too big
• A 10 °C increase in ambient temperature will result approximately 15 °C increase in exhaust gas temperature

Exhaust gas temperature after turbocharger:

• Indicates the kinetic energy of the exhaust gases that are running the turbine
• Indicates the condition of the turbo, e. scavenging of the engine
• If a de-Nox catalysator is installed the temperature has to exceed certain level to ensure the function of the catalysator.

Pressure difference over the cylinders:

• Measuring the pressure difference between charge air manifold and exhaust gas
• gives an indication of how well the scavenging works.
• Depends on the turbocharger, therefore values below should be considered as allusive
• The bigger the Dp is the better
• Is normal value with 100% load varies depending of installation once the value has been dropped down, it is an indication that something is wrong with the engine -e.g. dirty nozzle ring & turbine

Written from the news letter of Wartsila. 

main engine performance and how to maintain it?

Indicator Diagrams:

Taken at every month and every major O/H.

Power card: In phase with piston movement, with fuel on, to determine:
IP (Indicated Power)
Pmax (Between Atmospheric line and highest point)
Operational faults.

Draw card: 90° out of phase with piston movement, with fuel on, to determine:
Pmax
Pcom (more accurately)
Nature of expansion curve.
To evaluate injection, ignition delay, fuel quality, combustion, loss of
compression, expansion process, fuel pump timing, and after-burning.

Compression card: In phase and fuel cut-off, to determine:
Compression pressure
Cylinder tightness.

Light spring : In phase, using light spring, with fuel on, to determine:
Pressure variation during Exhausting and Scavenging periods.

How to maintain good Performance:

1. Maintain good power output per cylinder.
2. Take Power Card, to check Power Output / Cylinder.
3. Take Compression Card, to check for cylinder tightness.
4. Check ratio of Absolute Compression Pressure to Absolute Scavenging Pressure.
5. 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.)
6. If Absolute Pressure Ratio is less, check for cylinder tightness, charge air cooler,
scavenge air ports, scavenge valves, piston rings, exhaust valves, TC, etc.
7. Light Spring Diagram is taken if necessary.
8. Check Exhaust Temperatures, exhaust smoke, Load Indicator and engine running parameters.
9. Check fuel, CLO & LO consumption.
10. Regular maintenance works and repairs.

Absolute Pressure = Gauge Pressure (of Manometer) + Atmospheric Pressure (15 psi or 30″ Mercury)

If Compression Pressure is low:

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

Early combustion:
Causes:
1. Cetane no: of fuel higher than normal.
2. Fuel pump plunger set too high.
3. Incorrect adjustment of fuel cam on camshaft. Fuel valve ē low-pressure setting.
Effects:
1. High Pmax.
2. Low expansion line.
3. Less S.F.O.C.
4. Low exhaust temperature.
5. Heavy shock load to bearings.
6. Knocking.

Late combustion:
Causes:
1. Cetane no. of fuel lower than normal.
2. Plunger set too low.
3. Incorrect adjustment of fuel cam on camshaft.
4. Leaky fuel valves or high-pressure setting.
Effects:
1. Loss of power.
2. High expansion line.
3. Increased S.F.O.C.
4. High exhaust temperature.
5. Overheating
6. Lubrication difficulty.

Cetane Number:
1. A measure of ignition quality of fuel.
2. The higher the Cetane Number the shorter the time between fuel injection and rapid combustion.
3. The higher the Cetane No. the better the ignition quality.
4. Considered as poor fuel, if C  37 . Usual range is 30 – 45.

High Cetane no: Effects:
1. Shorter delay period
2. Early combustion
3. Increased power
4. Knocking

Low Cetane no: 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 than normal.
Causes:
1. Too low working temperature.
2. Cold start.
3. Too early fuel injection.