Ignition and Combustion Properties of Marine Heavy Fuels and the Effect on Engine Performance and Condition.
In this paper is briefly presented a new instrument for establishing the ignition and combustion properties for marine heavy fuels. Typical test results are presented and described as well as examples of operational problems observed when operating on marine heavy fuels with bad ignition and combustion properties. Possible methods to overcome bad ignition and combustion properties on an already bunkered heavy fuel through blending of different marine fuels, are described. Finally a comparison is presented and deviations commented between the ignition quality for a variety of marine heavy fuel oils established by this new instrument and the corresponding Calculated Carbon Aromaticity Index - CCAI for these fuels.
Key words: Marine Heavy Fuels, Ignition, Combustion, Engine Performance
In the late 1970's reported operational problems on large marine diesel engines seemed to indicate that changes in the heavy fuel properties were taking place. These changes were reflected in operational disturbances and on occasions in damages in the cylinder units which seemed to be related to changes in the ignition and combustion process. At the same time a rapid changes in the refinery structure took place in order to increase the output of the lighter fuel fractions (jet fuel, gasoline etc.). This had evidently the effect that changes in the composition of heavier refinery fractions being used in the production of marine fuels, resulted in a decline in the ignitability and burnability of these fuels. This was reflected in increased ignition delay, more extensive explosive combustion and increased afterburning.
The instrument used in the investigations described in this paper, is a commercialised version of constant volume combustion rigs developed and utilised for fuel research in Norway in the 1980's. Investigations and experimental test results have been reported on various occasions (ref.  ). A lot of the investigations performed on marine fuels, have also been carried out in Japan using this instrument in a cooperation between research organisations in Norway and Japan.
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2. DESCRIPTION OF THE METHODOLOGY
This instrument is, as mentioned above, based on a constant volume combustion rig methodology. A sample of the fuel to be investigated is injected into a combustion chamber containing pressurised and heated air. The fuel spray self-ignites and burns and its ignition delay is established as a criteria for the fuels ignition property.
Fuel injection is achieved by actuating the plunger piston in a conventional high pressure fuel injection pump. The actual testing procedure incl. instrument control, compilation of test data and presentation of the test results are completely automatic and computer controlled. In Figure 1 is shown the instrument.
During operation of the instrument monitoring and control is done on-line through a computer screen picture (refer to Figure 2). Furthermore, the charge conditions (temperature and pressure) as well as cooling of the fuel nozzle and, in case of testing a heavy fuel, fuel heating are controlled and conditions presented automatically by the computer on the computer screen.
At a predefined combustion chamber pressure and temperature, the fuel is injected through a fuel nozzle with one centrally located injection hole with a diameter typically of 0.35 mm. The ignition delay is defined as the time in milliseconds between start of injection (injector needle lift detected) until, due to the initial combustion phase, a predefined pressure increase (0.2 bars, refer to Figure 3) in the combustion chamber is recorded.
The combustion chamber condition at the time of injection can be selected freely, but a set of reference charge conditions have been suggested and are being used in the fuel testing described below.
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