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Effect of CN on PM emissions depends on the engine design. For high speed, light duty engines a small reduction in PM emissions with high CN has been observed while low speed, heavy duty engines showed no significant change.  With high cetane fuels the ignition delay is shorter and more fuel burns in diffusion combustion phase, which may increase soot emissions depending upon the engine operating conditions and  design. Current fuel specifications are setting minimum limits of more than 50 units for the CN and  minimum cetane index is also being specified as the natural high cetane fuels generally give overall better engine performance with respect to PM and HC emissions.

Distillation Range

A typical distillation curve for diesel fuel is shown in Fig. 8.6. The temperature for 50 percent distillation temperature or mid-boiling point, 90 percent point and the final boiling point are the important distillation parameters.

Lower the boiling point of the fuel, more readily it vaporizes and mixes with air giving more complete combustion. The mid boiling volatility is also correlated to the other physico-chemical properties like, density, viscosity and ignition quality.  A higher mid boiling point fuel has higher density and viscosity, and usually a lower CN. Low mid boiling point fuels give faster cold starting and hence lower HC emissions. The fuel components boiling above 350º C may not burn completely, forming high soot concentrations and combustion chamber deposits. Fuels with high T90 and final boiling point are seen to result in an increased injector coking leading to poor combustion and higher smoke emission.

For the modern diesel fuels low maximum permissible limits for T₉₀ or T₉₅ around 350 to 360º C are being stipulated due to their negative  long-term effects on emissions

Density

The density of diesel fuel varies generally in the range 810 to 880 kg/m³. The density, volatility, cetane number, viscosity and heat of combustion of petroleum fuels are interrelated. An increase of 10 percent in density increases the volumetric energy density (MJ/ m³) of the fuel approximately by 6 percent. The balance 4 percent is accounted for by the decrease in heat of combustion (MJ/kg) for the 10% heavier (higher molecular weight) fuels is metered volumetrically by injection pumps. The fuel density affects engine calibration and power as the fuel mass injected/stroke varies with fuel density. High-density fuels also have a higher viscosity thus, influence injection characteristics. Increase in the fuel density advances the dynamic injection timing by up to 1 ºCA. Thus, the fuel density affects engine combustion and emissions.
PM emissions generally increase with increase in fuel density.  As the fuel injection system is calibrated for a particular fuel density, the current fuel specifications set narrow limits acceptable fuel specific gravity e.g. from 0.82 to 0.85.