The longer the delay, the greater and more rapid thepressure rise since more fuel will be present in thecylinder before the rate of burning comes underdirect control of the rate of injection. The aimshould be to reduce the delay as much as possible,both for the sake of smooth running, the avoidanceof knock and also to maintain control over the pressure change. There is, however, a lower limit to thedelay since, without delay, all the droplets wouldburn as they leave the nozzle. This would make italmost impossible to provide enough combustionair within the concentrated spray and the delayperiod also has its use in providing time for theproper distribution of the fuel. The delay periodtherefore depends on:● The pressure and temperature of the air.● The cetane rating of the fuel.● The volatility and latent enthalpy of the fuel.● The droplet size.● Controlled turbulence.The effect of droplet size is important, as the rate ofdroplet burning depends primarily on the rate atwhich oxygen becomes available. It is, however,vital for the droplet to penetrate some distance fromthe nozzle around which burning will later becomeconcentrated. To do this, the size of the dropletsmust be large enough to obtain sufficient momentum at injection. On the other hand, the smaller thedroplet the greater the relative surface area exposedand the shorter the delay period. A compromisebetween these two effects is clearly necessary
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