Though the successful development of the Diesel engine depends upon its ability to digest the widest possible range of fuels between kerosine and the heavier residual type of fuel such as bunker C fuel (1, 2, 3), the need for some general specification, particularly for the higher speed engine, is generally admitted. As a result of a multiplicity of conflicting specifications drawn up by various engine manufacturers (4, 5, 6), Committee D-2 of the American Society for Testing Materials first presented in June, 1934, a classification of Diesel fuel oils, dividing them into five grades. This classification has been revised from time to time as a result of suggestions and experimental work carried out by several oil and engine manufacturers' research laboratories, and a further modification has recently been made in respect to the determination of ignition quality. Broadly speaking, in any fuel specifications there are two basic problems to consider: one is the problem of handling the fuel and dispatching that fuel from the refiner's supply tanks to the final fuel injection valve which admits the fuel at the proper time and in the proper form of spray to the combustion chamber; the second problem is to supply a fuel at this valve which will burn completely in the combustion chamber without objectionable exhaust smell or without leaving objectionable deposits which will eventually result in destroying the “tune” or mechanical adjustment of the engine, thus impairing its efficiency. Under the first of these problems we have to consider such properties of the fuel as its flash point, viscosity, pour point, cloud point, and last, but by no means least, its cleanliness (water, sediment, and ash contents). Under the second problem we have to consider its ignition quality, volatility, as expressed by carbon residue or distillation, and sulfur content. The following discussion concerns the various standard and tentative tests used to control these items and the significance of such tests in relation to engine performance.