**7. Fluidity**

Fluidity encompasses all aspects of the ability of the automotive fuel to flow. This encompasses the fuel's characteristics when it is moved when in liquid state and what happens when it is cooled. These properties are measured by viscosity (ASTM D445) and cold flow properties where viscosity is a measure of the fuel's resistance to flow and cold flow properties (cloud point, CFPP and pour point) describe the fuel's reaction on cooling.

Viscosity is determined by the intermolecular structure. Thus, the same considerations for density apply where viscosity is dependent on composition and temperature at which it is measured. N-paraffins have the lowest viscosity (as intermolecular interactions are the weakest Van der Walls forces) while the aromatics have the highest viscosity (as they are polarizable). Also, the longer the chains the greater the intermolecular forces, and thus the higher the viscosity. As gas oil fuel typically is in the C10 to C25 range compared to gasoline fuel, which is C5 to C12, gas oil fuels have a higher viscosity. In fact, gasolines viscosity is so low that it is not normally requested in specifications.

Both a minimum and maximum are normally specified for viscosity at 40˙C for gas oil fuel. This is done as the pump which carries the fuel from the fuel tank to the combustion chamber are either mechanical pumps (such as a diaphragm pump) or electrical pumps which deliver a constant pressure on the fuel. Thus is the viscosity is too low excessive fuel may be delivered resulting in incomplete combustion. This would increase the fuel consumption and result in harmful exhaust emissions. On the other hand, if the viscosity is too high flow rates can reduce to an extent where insufficient fuel is being delivered to the engine resulting in difficulty in start-up.

Cold flow properties measure the characteristics when the sample is cooled. In addition to intermolecular forces, these are also greatly determined by molecular symmetry. Since molecules in the liquid state are crystallizing into a solid, ability to pack well into a crystalline lattice is of fundamental importance. In fact, n-paraffins tend to be the main contributors to cold flow properties as they are symmetrical when compared to iso-paraffins or cycloparaffins and aromatics with side chains.

Once again the short molecular structures of gasoline fuel precludes cold flow issues while there are three cold flow properties which can be performed for gas oil fuel. These are:


• Pour point by ASTM D97, D5949, D6749 or D7346 where the wax crystals forms an interlocked 3D network lattice big enough to trap the remaining liquid. This prevents the fuel from moving and thus no fuel can be transported.

Cold flow improvers can be added to improve low temperature performance of the gas oil fuels. These consist of different types of polymeric materials of hydrocarbon chains and polar segments. The polymers' hydrocarbon chains provide interactions with the gas oil's paraffin segment while the polymers' polar groups are responsible for modifying the wax crystals. This results in a sort of scaffolding where wax crystals are not allowed to agglomerate resulting in crystal morphological changes. There is no universal additive for all gas oils and the choice of additive would depend on the interaction of the additive with the particular fuel.
