5.6 Result of piston overhaul
5.6.1 Piston head 1]
In the visual inspection when the cylinder cover was overhauled, it seemed that the fouling condition by the deposition of the soot on the cylinder cover and the piston head part was less different from that in the A-oil mono-combustion mode (Fig.10); however, the white substance was deposited on the piston head part of every cylinder along the fuel injection traces (see Fig.11). The emulsified fuel sprayed from the fuel valve is longer in the combustion time (poor in burning) than that in the A-oil mono-combustion mode, the flame becomes longer and the unburnt part is blown against the piston head part, and thus, the piston head part is exposed to the high temperature. This deposition looks like a glossy and hard glass, and Si and Na are detected from the piston head part according to the component analysis of the combustion product. It is thus estimated that this combustion product was generated through the alternate high temperature and cooling for a long time on the piston head part such that the element (Si, etc.) in the emulsified fuel was exposed to the high temperature in the expansion (explosion) process of the combustion stroke, and then, subjected to the rapid cooling in the suction process through the exhaust. Such a phenomenon has never been experienced in the A-oil mono-combustion mode, and since the deposition was very hard and melted, it was very difficult to remove it.
No glassy combustion product was visually identified on the exhaust pipe or the blades of the turbocharger forming the passage of the gas emitted from the cylinder, and little combustion product was found in the analysis of the combustion product, and it seems that the combustion product passed but was not deposited.
In the emulsified fuel used in this test, tap water was added, and it seems that more mixed components such as Si were contained than in the A-oil mono-combustion mode; however, the environment repeatedly subjected to the high temperature and the low temperature is considered to be very influential.
5.6.2 Piston head 2]
After the above-described hard deposited substance was removed, the regular wrinkle-like pattern was found on the metal surface of the piston head part (formed of aluminum alloy). Fig.12 shows the sketch thereof. These wrinkles were possibly caused by the high-temperature corrosion such as vanadium attack in which the flame of the sprayed emulsified fuel was extended, and touched the piston head part directly, and the surface temperature was locally high. In addition, a part of the piston head of the cylinder No.5 was found eroded.
