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.