Important fuel specifications for diesel

(a) Viscosity

-Viscosity of a fluid indicates its resistance to flow - higher the viscosity, the greater the resistance to flow.

-It may be expressed as absolute viscosity (Poise, P) or kinematic viscosity (stoke, St).

-It varies inversely with temperature, usually given at 20 - 40 OC

-Fuel atomization depends on viscosity 2 - 8 mm2/s (cSt) at 20 OC

-Lower the viscosity, smaller the diameter of the droplets in the spray

(b) Surface tension

-Surface tension is a parameter which effects the formation of fuel droplets in sprays

-Lower the value, smaller the droplet diameter

-Diesel fuel 0.023 – 0.032 N/m

(c) Cetane number

-Cetane number is used to specify the ignition quality of diesel fuel.

-Running on low Cetane number will produce cold start problems. Peak cylinder pressure, combustion noise and HC emissions will increase -more fuel will be injected before ignition, less time for combustion.

-Higher CN results in a sooner ignition - extremely high CN may ignite before adequate Fuel-Air mixing can take place - higher emissions. Power output can be reduced if burning starts too early.

Measurement of cetane number

-Cetane number is measured by comparing the “ignition delay time” of the sample fuel with a mixture of cetane (C16H34) and alphamethyl naptane (C10H7CH3). The Cetane percentage in the mixture gives the CN of the sample fuel.

-CN of the reference fuel cetane is arbitraryly set at 100, and of alphamethyl naptane at 0.

-CFR engine is used to measure the compression ratio at which ignition starts. CR is gradually increased while the engine is driven by an electric motor - a curve of CN vs critical CR is obtained.

-Inlet air temp is 30 oC and cooling water temp is at 100 oC

An easier and practical method to obtain Cetane Number is by calculating the Diesel Index.

Increasing the DI, increases the tendency to ignite.

-AP is obtained by heating equal amounts of annilin and diesel fuel. While cooling down, the temp at which the annilin separates from the mixture is the AP

-Cetane number is in the range of,

50 - 60 for high speed Diesel engines

25 - 45 for low speed Diesel engines

Normal Diesel fuel CN is 40 - 55