An additional investigation carried out for the same shipping company on a mixture of one part of above very bad heavy fuel quality with two parts of another type of marine heavy fuel (a new bunker) with normal ignition properties, showed a significant quality improvement as this mixture has normal ignition and combustion properties for a marine heavy fuel (refer to Figure 7).
Another vessel observed severe operational problems expected to be related to the ignition and combustion properties of the heavy fuel bunkered. Due to extensive engine damages the vessel had to divert its route for immediate repairs on cylinder units.
A sample of the heavy fuel used was tested for ignition and combustion properties. Test results are presented in Figure 8. The investigation showed that the heavy fuel oil had very bad ignition properties. This is reflected in a FIA CN below 18.7. An initial low level heat release extends for a very long period before the actual main combustion starts. This is an indication of a low intensity initial combustion probably due to the contents of a lighter fuel fraction ("dumbell fuel"). Furthermore, we observe a significant delay in the start of the main combustion phase.
However, the actual main combustion proceeds fairly rapidly and efficiently with a high maximum ROHR level and correspondingly a normal duration.
The shipping company, having app. 1.000 metric tons of above heavy fuel onboard the vessel, would like to establish if the ignition quality of this heavy fuel could be improved sufficiently by adding certain amounts of an available marine diesel oil to the heavy fuel oil.
Thus, an investigation was carried to establish the ignition and combustion properties of the pure MDO as well as of various mixtures of the heavy fuel and MDO.
An investigation on the pure MDO showed that the ignition and combustion quality of this distillate fuel was very good, reflected in a FIA CN of 54.4.
By adding 20% MDO to the HFO the ignition quality of the heavy fuel oil is improved significantly ie. from a FIA CN ＜18.7 to a normal level for heavy fuel - FIA CN = 26.4. Also the actual combustion process is improved reflected in an increased level for the maximum ROHR
Figure 9 shows the test results for a mixture of 20% of this MDO into the marine heavy fuel with very bad ignition properties.
Additional tests with 25% and 30% MDO added to this HFO resulted in a FIA CN = 27.1 (25% MDO) and a FIA CN = 29.4 (30% MDO).
Thus, the test results show fairly small but still significant, improvements in the ignition and combustion properties when increasing the contents of MDO from 20% to 25% and 30% in the blend. However, the major improvement is achieved when adding 20% MDO to the HFO.
As all the three MDO/HFO mixtures show normal ignition and combustion process neither of these are expected to give rise to operational problems related to the ignition and combustion of the fuel in a large marine diesel engine.
CCAI related to ignition delay.
In relation to the investigations presented in Figure 4, the CCAI for the heavy fuels included have been established.
A correlation between the recorded ignition delay and the corresponding CCAI level, are presented in Figure 10.
As clearly seen in this figure, there exists only a very weak correlation (R = 0.41557) between these two parameters.
Thus CCAI can be used as a very rough indicator of the ignition quality of heavy fuels, but one should have in mind that CCAI below app. 860 is no insurance against heavy fuels with bad ignition properties.
4. CONCLUSION.
Above investigations confirm that marine heavy fuel oils even when these comply with the ISO marine fuel specifications and show a generally accepted level for the CCAI, can show unacceptable ignition and combustion properties.
This may result in extensive and time consuming operational problems and even severe and costly engine damages.
In order to overcome this problem it will be necessary to introduce additional parameters in the marine heavy fuel specifications to make certain that the fuel have an acceptable ignition and combustion quality.
REFERENCES
[1] SHIODE. K, KIJIMA T, FISKAA G. "Ignition properties for Marine Fuels established on the Fuel Ignition Analyser FIA 100/2" 22nd CIMAC int. Congress on Combustion Engines Copenhagen, May 1998
Table 1 - Suggested limits for FIA CN for marine heavy fuels.
