It will help to recall the process of oil refining which is a form of distillation. The crude oil is heated and the different “fractions” condense at different levels. At the top the gas is drawn off, next comes gasoline (petrol) then come the lighter grades of oils which include kerosene (paraffin) and jet fuel, gas oil and diesel oil. After these lighter grades come the heavier oils which are grouped under the general title of “residual oils”. Only the smallest ships now use diesel oil for the propulsion machinery; the very smallest use gas oil. The vast majority of motor-ships today use IFO – Intermediate Fuel Oil. Called Intermediate because Heavy Fuel Oil is that which is burnt in oil-fired furnaces and some very large diesel engines. The older designs of motor-ships still require diesel oil for the generators and the main propulsion machinery in some of these vessels use diesel oil when the ship is manoeuvring when several changes of engine setting will be called for. Otherwise modern vessels use IFO for the main engine and the generators although, of course, different ships will need different grades within the IFO group of oils. It is, therefore, vital that the correct grade and quality of oil is purchased. The wrong grade will affect the vessel’s speed/consumption performance and poor quality can create problems for the machinery; some of which can result in permanent damage. IFO falls into the group referred to as “residual oils” which suggests that many of the crude oil’s “residues” may be present and these have to be dealt with or removed. Furthermore consider the principle of a diesel engine being the direct injection of the fuel into the cylinder in the form of a jet so fine that it can pierce human flesh.