1-1. Specifications of bunker fuel oil for marine engines
Engine builders have been trying to develop reliable engines, and in the case of SULZER RTA-engines, IS08217:1996 class F, RMK55 is "Bunker limit" which indicates minimum quality of heavy fuel oil to feed. When we look into the statistics of DNV fuel oil analysis, however, there are so many varieties of RMH35 and corresponding fuel oils available in the market "Recommended fuel quality" which specifies the quality at engine inlet, and commonly use fuel oil can clear this recommendation by a continuous and careful use of the pre-treatment systems on board.
In the above standard/limit, however, physical characteristics such as specific gravity, kinetic viscosity and water content, metallic components such as S, Al, Si, Na etc. are specified quantitatively. On the other hands, it is said that various kinds and sources of fuel oils are mixed at shore, sometimes chemical waste such as used lub. oils are mixed, and supplied in the market as residual fuel oils for marine engines. It is well known that automobiles and airplanes, they are using distillated fuel oils.
1-2. Combustion phenomenon in diesel engines using residual fuel oils in the market
Ship operators and engine builders know empirically, that the different combustion characteristics used to observe in the field by using different residual fuel oils of similar specification. And, according to my knowledge in school days, there were two phases in diffusion combustion like diesel engines. One was chemical reaction and another was physical phenomenon.
1) Fuel spray/fuel droplets formation, entrainment of air into fuel spray, evaporation of fuel droplet and mixing with air are controlled by physically and rather slow compared with chemical reaction.
2) Fuel gas and air reacts chemically and forms a flame. This reaction is rather fast. Due to these combined physical phenomenon and chemical reaction, the mixing of different kinds of fuel oil and sometimes chemical waists make the combustion characteristics more and more complicated one.
Nevertheless, operators used to operate their main engines using residual fuel oils, not knowing about the results of chemical analysis of fuel oil or a special instruction how to handle it.
In this meaning, it is useful to report the chemical compositions and physical property of each fuel oil precisely and before use. And it is important to know the to overall characteristics of combustion process using a diffusion combustion simulator such as a combustion bomb.