with which a liquid changes to a vapor is known as the volatility of the liquid The 90 percent distillation temperature measures volatility of diesel fuel. This is the temperature at which 90 percent of a sample of the fuel has been distilled off. The lower the distillation temperature, the higher the volatility of the fuel. In small diesel engines higher fuel volatility is needed than in  larger  engines  in  order  to  obtain  low  fuel consumption, low exhaust temperature, and minimum exhaust smoke. Viscosity The viscosity is a measure of the resistance to flow of  the  fuel,  and  it  will  decrease  as  the  fuel  oil temperature increases. What this means is that a fluid with a high viscosity is heavier than a fluid with low viscosity.  A  high  viscosity  fuel  may  cause  extreme pressures  in  the  injection  systems  and  will  cause reduced atomization and vaporization of the fuel spray. The viscosity of diesel fuel must be low enough to flow freely at its lowest operational temperature, yet high enough to provide lubrication to the moving parts of the finely machined injectors. The fuel must also be sufficiently  viscous  so  that  leakage  at  the  pump plungers and dribbling at the injectors will not occur. Viscosity also will determine the size of the fuel droplets, which, in turn, govern the atomization and penetration qualities of the fuel injector spray. Recommended  fuel  oil  viscosity  for  high-speed diesel engines is generally in the region of 39 SSU (Seconds  Saybolt  Universal)  which  is  derived  from using a Saybolt Viscosimeter to measure the time it takes for a quantity of fuel to flow through a restricted hole in a tube. A viscosity rating of 39 SSU provides good  penetration  into  the  combustion  chamber, atomization of fuel, and suitable lubrication. Sulfur Content Sulfur has a definite effect on the wear of the internal components of the engine, such as piston ring, pistons, valves, and cylinder liners. In addition a high sulfur content fuel requires that the engine oil and filter be changed more often. This is because the corrosive effects of hydrogen sulfide in the fuel and the sulfur dioxide  or  sulfur  triioxide  that  is  formed  during  the combustion  process  combines  with  water  vapor  to  form acids. High additive lubricating oils are desired when high  sulfur  fuels  are  used.  Refer  to  the  engine manufacturer’s specifications for the correct lube oil when using high sulfur fuel. Sulfur content can only be established by chemical analysis of the fuel. Fuel sulfur content above 0.4% is considered as medium or high and anything below 0.4% is low. No. 2D contains between 0.2 and 0.5% sulfur, whereas No. 1D contains less than 0.1%. Sulfur content has a direct bearing on the life expectancy of the engine and its components. Active sulfur in diesel fuel will attack and corrode injection system components in addition to contributing to combustion chamber and injection system deposits. Cloud and Pour Point Cloud  point  is  the  temperature  at  which  wax crystals in the fuel (paraffin base) begin to settle out with the result that the fuel filter becomes clogged. This condition   exists   when   cold   temperatures   are encountered and is the reason that a thermostatically controlled fuel heater is required on vehicles operating in cold weather environments. Failure to use a fuel heater will prevent fuel from flowing through the filter and the engine will not run. Cloud point generally occurs 9-14°F above the pour point. Pour  point  of  a  fuel  determines  the  lowest temperature at which the fuel can be pumped through the fuel system. The pour point is 5°F above the level at which oil becomes a solid or refuses to flow. Cleanliness and Stability Cleanliness is an important characteristic of diesel fuel. Fuel should not contain more than a trace of foreign substances; otherwise, fuel pump and injectors difficulties  will  develop  leading  to  poor  performance  or seizure. Because it is heavier and more viscous, diesel fuel will hold dirt particles in suspension for a longer period than gasoline.    Moisture in the fuel can also damage  or  cause  seizure  of  injector  parts  when corrosion occurs. Fuel stability is its capacity to resist chemical change caused by oxidation and heat. Good oxidation stability means that the fuel can be stored for extended periods of time without the formation of gum or sludge. Good thermal stability prevents the formation or carbon in hot parts, such as fuel injectors or turbine nozzles. Carbon deposits disrupt the spray patterns and cause inefficient combustion. COMBUSTION  CHAMBER  DESIGN The fuel injected into the combustion chamber must be mixed thoroughly with the compressed air and 5-4