The high pressure fuel has to reach the inside of the combustion chamber in a manner that is best suited for efficient combustion. The fuel must reach all the air that is inside, sufficiently atomized. Certain observations regarding fuel sprays have been found relevant in CI engines combustion.
As the fuel delivery pressures of nozzle increase, the following changes occur. The flow changes from those of individual drops to a continuous jet. Then the jet stream tend to break up into a spray at a breakup distance that keeps falling as the pressures keep rising. After that the spray becomes thicker developing into a cone whose apex finally coincides with the orifice.
Spray behavior can be expressed in terms of the spray tip velocity and the average individual particle velocity. These indicate the amounts of spray penetration, the maximum and the average. It is found that the rate of penetration of the spray tip increases as the injection pressure is increased, although the maximum penetration is virtually independent of injection pressure. The spray tip from low injection pressures penetrates as well as if the pressures were higher, but a longer time is neccesary to travel the distance but the real penetration for engine operation: "the average penetration of the individual particles" does not necessarily increase with pressure. High injection pressures cause fine atomization and therefore high air resistance to the high pressure spray. The increased air resistance compensates approximately for the higher initial velocity, and the net result is approximately equal average penetrations for either high-or low- pressure sprays.
The high injection pressures afford increased atomization but not increase average penetration of the air within the engine. It was also found that average penetration is best controlled by viscosity of the fuel, while size of the orifice (above 5mm) has only a small effect.