Fig. 2 shows the distributions of the distillation temperatures of the two fuels tested by ASTM method. Comparing the two fuels, BFO-A contains more low temperature distillates (200〜300℃) and less high temperature distillates (300〜500 ℃) than BFO-S. The shape of the distribution graph of BFO-A is near to that of so-called Dumbbell fuel. The black bar in Fig. 2 means the residual portion that is the vacuum residue from the refinery. Though it is more than 50% for both the fuels, its properties cannot be verified.
Fig. 3 shows the data from mass chromatography of the two fuels. According to it, some typical feature of 'cracked distillate ', for example, high peaks of naphthalenes can be seen in the range of lighter distillates (left side) in BFO-A, whereas BFO-S shows the feature of 'straight run stocks', i.e. the peaks of normal paraffins are mainly detected.
Table 2 shows the composition of both the fuels analyzed by the silica-gel chromatography in detail. According to this table, BFO-A contains only 24% saturated hydrocarbons (5% paraffins and 19% naphthenes). This rate is lower than the rate of BFO-S, which contains 31% saturated hydrocarbons. The rate of aromatic hydrocarbons is much higher in BFO-A. When comparing the number of aromatic rings of both the fuels, it must be taken into account that the percentage of aromatics composed of 1 ring and 2 rings, for example, groups of naphthenobenzenes and naphthalenes are much higher in BFO-A. This may be a characteristic of the cutter stocks included in BFO-A.
At first, the authors have supposed that BFO-A is a typical Dumbbell fuel that contains heavier residue than normal base oil [3]. But for the moment, even after so many kinds of analyses as in Table 1, 2 and Fig. 2, 3, the difference in the residual portion (vacuum residue) between the two fuels is not so clear.
Reviewing the properties and the composition of BFO-A, it is possible that the cracked aromatic hydrocarbons, for example, LCO (light cycle oil) or CLO (clarified oil) from FCC (fluid catalytic cracking) process are mixed in large quantities. However, Al+Si content, the catalyst fines for the FCC process, is low enough in both the fuels. (Recently, the electrostatic separators to catch the catalyst fines are being introduced in some oil plants. So, low Al+Si content is not always the evidence that CLO is not mixed.)
V (Vanadium) content and Ni content are as follows. BFO-S has 105 ppm V and 40 ppm Ni. BFO-A has 80 ppm V and 90 ppm Ni.
The two fuels are heated before the injection in order to obtain the same viscosity (20 mm2/s).
3. EXPERIMENTAL RESULT AND DISCUSSION
3.1 Ignition Characteristics Examined with FIA
The ignition delays of the bunker fuel oils, BFO-S and BFO-A are examined using the FIA. In this study, the ignition timing is defined as the timing that the air pressure rise of 3 bar is detected in the chamber. The results are shown in Fig. 4. This figure shows ignition delays of 9〜12 ms for BFO-S and 16〜20 ms for BFO-A. The ignition delay of BFO-A is surprisingly longer and the measured values spread more widely compared with BFO-S.
3.2 Combustion Characteristics Examined with Visual Engine
Fig. 5 and 6 show the photos of the combustion flames of BFO-S and BFO-A in the visual engine. The air conditions at the moment of fuel injection are 6.8 MPa and 500 ℃. Photos are taken from the underside of the transparent piston by a high-speed camera with 5000 fps.
For this test, a 'side-injection system ' with two injection nozzles installed at the side of the combustion chamber, similar to the low-speed marine engines, is used. The injection starts at 4° BTDC and ends at 19° ATDC (for both the fuels). The maximum injection pressure is 100 MPa.
Fig. 5 shows the comparison of combustion process between the two fuels under the condition that air swirl with suitable intensity for combustion is applied. According to it, some differences between the two fuels can be seen. For example, at the final stage of the combustion (22.5° and 27.5° ATDC), the BFO-A flame is still burning brightly and forming black soot-clouds in the center of the visual field, whereas the BFO-S flame is burning up completely.
Fig. 6 shows the flames under the condition without air swirl. In these photos, a clear difference can be seen between the two fuels. Only the BFO-A flame looks burning apart from the injection nozzle, though the fuel keeps on coming from the injection nozzle. That may be because the burning speed is lower than the spray speed and the flame cannot go up to the injection nozzle overcoming the spray speed.
However, as the size of this visual engine is much smaller than the low-speed marine engines that suffered the trouble using BFO-A, above-mentioned data are not sufficient to verify the cause of the heavy wear of the cylinder liners. For that reason, the visual combustion chamber is prepared to observe the full length of the flame.
3.3 Flame Characteristics Examined with Visual Combustion Chamber
Examination using the visual combustion chamber is carried out to investigate especially the spatial flame-1ength and the combustion duration. In this investigation, one more fuel, so-called ' Long Residue' is included as a sample of good fuel. Its properties are shown in Table 3. Reviewing the production process of bunker fuel oil (BFO) shown in Appendix Figure, it is understood that 'Long Residue' contains much better components than 'Vacuum (Short) Residue' normally used as the base oil for BFO. The flames of the three fuels are compared in Fig. 7 under the following two conditions.
(a) Air pressure and temperature: (2.5 MPa. 600 ℃ )
Fuel injection pressure: 100 MPa (80 MPa at the end of injection)
(b) Air pressure and temperature : ( 2.5 MPa, 670 ℃ )
Fuel injection pressure: 66 MPa (50 MPa at the end of injection)
Photos are taken from the window fitted on lower part at the side of the chamber as shown in Fig. 7. In this figure, the flames during the latter half of injection duration and after the end of injection are shown.
In Fig. 7 (a), some differences between BFO-S and BFO-A can be seen. The BFO-A flame is burning apart from the injection nozzle and coming near to the bottom of the combustion chamber. In this chamber, air pressure is only 2.5 MPa and air density is only 1/4 of highly turbo-charged engines. Therefore the spray speed is much higher than in the real engines. For that reason, it is hard for the BFO-A flame with lower burning speed to go up in the direction to the injection nozzle overcoming the spray speed. Moreover it takes more time for the BFO-A flame to burn up completely, i.e. small flame remains near the bottom for a long time after the end of injection, whereas the BFO-S flame burns up much more quickly than BFO-A. As expected, 'Long Residue' bums most brightly and burns up most quickly.
The BFO-A flame in Fig. 7 (b) can be seen at higher position in the window than in Fig. 7 (a). It is because the spray speed under condition (b) is smaller with lower injection pressure than under condition (a). However, the difference of the flame between BFO-S and BFO-A is also clear in Fig. 7 (b). Paying attention to the spatial flame length, the BFO-A flame is much longer than the BFO-S flame not only during the injection duration but also after the end of injection.
