mix rapidly with the air, and burn smoothly to produce an even thrust on the piston during combustion. Diesel Fuel Oil Grades Diesel  fuel  is  graded  and  designated  by  the American Society for Testing and Materials (ASTM), while its specific gravity and high and low heat values are listed by the American Petroleum Institute (API). Each individual oil refiner and supplier attempts to produce diesel fuels that comply as closely as possible with  ASTM  and  API  specifications.  Because  of different crude oil supplies, the diesel fuel may be on either the high or low end of the prescribed heat scale in Btu per pound or per gallon.  Because  of  deterioration  of diesel fuel, there are only two recommended grades of fuel that is considered acceptable for use in high-speed heavy-duty vehicles. These are the No. 1D or No. 2D fuel  oil  classification Grade No. 1D comprises the class of volatile fuel oils from kerosene to the intermediate distillates. Fuels within this classification are applicable for use in high- speed  engines  in  service  involving  frequent  and relatively wide variations in loads and speeds. In cold weather conditions, No. 1D fuel allows the engine to start  easily.  In  summary,  for  heavy-duty  high-speed diesel  vehicles  operating  in  continued  cold-weather conditions, No. 1D fuel provides better operation than the heavier No. 2D. Grade No. 2D includes the class of distillate oils of lower volatility. They are applicable for use in high- speed  engines  in  service  involving  relatively  high  loads and speeds. This fuel is used more by truck fleets, due to its greater heat value per gallon, particularly in warm to moderate climates. Even though No. 1D fuel has better properties for cold weather operations, many still use No.  2D  in  the  winter,  using  fuel  heater/water  separators to provide suitable starting, as well as fuel additive conditioners,  which  are  added  directly  into  the  fuel tank. Selecting the correct diesel fuel is a must if the engine  is  to  perform  to  its  rated  specifications. Generally, the seven factors that must be considered in the selection of a fuel oil are as follows: 1.  Starting  characteristics 2. Fuel handling 3. Wear on injection equipment 4. Wear on pistons 5. Wear on rings, valves, and cylinder liners 6.  Engine  maintenance 7. Fuel cost and availability Other considerations in the selection of a fuel oil are as  follows: Atmospheric   conditions Frequency of load and speed changes Speed  and  load  range Engine size and design Cetane Number Cetane  number  is  a  measure  of  the  fuel  oils volatility; the higher the rating, the easier the engine will start and the combustion process will be smoother within the ratings specified by the engine manufacturer. Current 1D and 2D diesel fuels have a cetane rating between 40 and 45. Cetane rating differs from octane rating that is used in gasoline in that the higher the number of gasoline on the octane scale, the greater the fuel resistance to self- ignition, which is a desirable property in gasoline engines with a high compression ratio. Using a low octane fuel will cause pm-ignition in high compression engines.  However,  the  higher  the  cetane  rating,  the easier the fuel will ignite once injected into the diesel combustion chamber. If the cetane number is too low, you  will  have  difficulty  in  starting.  This  can  be accompanied by engine knock and puffs of white smoke during warm-up in cold weather. High altitudes and low temperatures require the use of diesel fuel with an increased cetane number. Low temperature  starting  is  enhanced  by  high  cetane  fuel  oil in the proportion of 1.5°F—lower starting temperature for each cetane number increase in the fuel. Volatility Fuel volatility requirements depend on the same factors as cetane number. The more volatile fuels are best for engines where rapidly changing loads and speeds are encountered. Low volatile fuels tend to give better fuel economy where their characteristics are needed for complete combustion and will produce less smoke, odor, deposits, crankcase dilution, and engine wear. The  volatility  of  a  fuel  is  established  by  a distillation test where a given volume of fuel is placed into a container that is heated gradually. The readiness 5-3