For testing of marine heavy fuels expected to show fairly long ignition delays, the reference temperature is 480℃ at a pressure of 45 bars, while when testing marine distillate fuels the temperature is maintained at 480℃ with a charge pressure reduced to 20 bars.
Marine heavy fuels show when tested with this instrument, a fairly wide range of ignition delay (typically from below 5 msec up to well above 10 msec at the selected charge conditions). In order to convert the recorded information on ignition and start of main combustion into a cetane number, a calibration of each instrument is performed and a reference diagram established. The reference data are based on recorded information on ignition and start of main combustion for fuels composed from secondary diesel reference fuels T20 mixed with U13 (Phillips Petroleum) giving a cetane number range from CN =18.7 to CN = 72.7.
Based on experience limits are suggested (depends also on engine type, engine load level and profile and engine condition) for the FIA CN for in particular marine heavy fuels. These suggested limits are presented in Table 1.
3. TEST RESULTS AND OBSERVATIONS
Marine Heavy Fuels - Ignition and combustion quality and operational problems.
According to statistics from the Japanese Class NK, a total of 90% of serious damages causing ships to be inoperable, occurred in the ships main engine. Furthermore, Japanese investigations presented in ClassNK annual report, indicate a significant increase in number of serious failures on large 2-stroke engines assumed to be caused by poor combustion.
In order to investigate the present situation world-wide of marine heavy fuel ignition quality and combustibility, the Japanese Shipowners Association - JSA together with the Ship Research Institute, American Petro. Chemical (Japan) Ltd and Kanematsu Chemicals Co. Ltd. conducted a joint study.
Up until January 1998, Japanese ships belonging to JSA collected more than 300 samples of heavy fuels. Bunkering places were divided in three geographical areas - Asia, Europe and America. The collected samples were sent to a laboratory in Japan and tested for their ignition properties.
The ignition delays as observed for these fuel samples, related to the three geographical areas are presented in Figure 4. Frequency is established as the percentage of all samples from one region showing an ignition delay within the various time segments, e.g. between 5.5 and 6.0 msec.
As can be seen from this diagram, there exist significant differences in the ignition quality, defined as an ignition delay in msec., between these geographical areas. The best qualities are being delivered in Asia while heavy fuels hunkered in America in general show the longest ignition delays. It is worthwhile to notice in relation to these observations, that Japanese engine builders recommend that the bunkered heavy fuel shall have a Calculated Carbon Aromaticity Index - CCAI of less than 850. Thus, the heavy fuels delivered in Asia in general show CCAI levels of 850 or lower while the bulk of heavy fuel deliveries in Europe and America show CCAI in the range between 850 and 860.
There are large variations between marine diesel engines in their dependency of the ignition property for the fuel being used. Thus variations will also be present in the probability for the occurrence of operational problems and damages when operating on fuels with bad ignition properties. Of importance in this respect are engine type (2-stroke or 4-stroke, slow, medium and high speed), engine design and specifications as well as the engine condition, the operating load and operating conditions in general.
However, on the basis of a number of heavy fuel tests and observations of operational problems, limits for a FIA CN have been suggested, as presented above in Table 1. We observe that when operating on heavy fuel, very bad ignition quality is reflected in a FIA CN below 20. In the investigations presented, only one heavy fuel sample (from Asia) falls into this category. Furthermore, bad ignition qualities are reflected in a FIA CN in the range from 20 to 25. In the investigation referred to above, app. 13% of the European fuel samples and 8% of the fuel samples from America falls into this category. Over a number of years ignition tests have been performed on heavy fuels where the operator suspected that operational problems experienced, might be attributed to the fuels ignition and combustion quality. Some examples will be mentioned in the following.
A vessel reported operational problems reflected in high maximum cylinder pressures and occasionally blowing off of relief valves, increased exhaust temperatures and cracked liners.
Ignition investigations showed an ignition quality (45 bars and 480℃) equivalent to a FIA CN of app. 23. An additional ignition test carried out at the reduced pressure of 20 bar, showed that the fuel at this charge condition only just ignited and burned extremely slowly.
Another vessel reported "hunting" and unstable operation on both main and auxiliary engines operating on the same heavy fuel. Ignition investigations showed a FIA CN in the magnitude of 18.
The introduction of the rate of heat release (ROHR) as additional information on the combustion properties of a heavy marine fuel, has extended the capabilities for this test method. Test results indicate that the combined use of a pressure trace and the corresponding ROHR diagram will give information related both to the ignition property and on the actual combustion quality for the fuel being investigated.
In Figures 5 and 6 below are shown typical traces of a heavy fuel oil with good and very bad ignition and combustion properties respectively.
A good marine heavy fuel shows a short ignition delay with a rapid start of the main combustion. The main combustion period is short, furthermore the level of maximum ROHR is high and reached after a short time (refer to Figure 5).
A heavy fuel oil (refer to Figure 6) with all properties within the ISO specification, was giving severe operational problems on a large bore 2-stroke main engine. Problems included sticking exhaust valves, heavy knocking and start failure.
The ignition and combustion test of this fuel show a very long ignition delay and correspondingly a very delayed start of the main combustion. Furthermore, the heat release is slow with a low maximum ROHR level reached after a very long period. The complete combustion period (positive heat release) is very long, indicating increased possibility for slow combustion and afterburning in an engine.
