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. 

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