1.6 Dimensionless criteria of the atomization process

To achieve a fuller understanding of the atomisation process, a number of nondimensional numbers have been derived as follows:

• Weber number (We): expresses the ratio of dynamic forces of an ambient gas to the surface tension. It defines the effect of external factors on the drop development: where: ?g is the gas density, d the droplet diameter, ud the droplet relative velocity and s l the liquid surface tension.

• Reynolds number (Re): denotes the ratio of inertial force to viscous force: where: L – nozzle length.

• Ohnesorge number (Oh): defines the ratio of internal viscous forces to surface tension forces: .

These dimensionless numbers will be used in the structure of Diesel spray section to describe the break-up and the penetration length.

2. Experimental instrumentation

2.1 The common-rail fuel injection equipment

The purpose of the fuel injection equipment (FIE) is to provide the engine with the adequate quantity of fuel needed in proportion to the power required and timed with a good accuracy, so that the engine will deliver that power within the limit prescribed for fuel consumption, exhaust smoke and noise. The FIE ensures that the engine not only runs smoothly but also economically. The fuel is injected through one or several holes, located at the tip of the injector, at pressures high enough to cause the required degree of atomisation in the combustion chamber and ensure that it mixes properly with sufficient air to complete the combustion.

The fuel injection equipment includes a fuel pump, a common high pressure rail, an injector, an electric motor and an electronic control unit. The injector is mainly constructed of a triggering element (solenoid valve), a valve spring, a valve control plunger, a valve ball, a bleed orifice, a feed orifice and a needle:

Figure 2.1.1: Bosch common rail injector

A voltage applied to the solenoid as a shaped pulse activates the coil, because the force exerted by the triggered solenoid is now higher than that of the valve spring, which then opens the bleed orifice. The lower pressure in the valvecontrol chamber causing a reduction in the force exerted on the control plunger will open the needle. The fuel is then injected as the pressure difference between ambient and fuel line pressure.

The Diesel fuel injection system has the most dominant effect on flow processes in the combustion of compression ignition engines; because it controls the process of fuel atomisation, and the subsequent fuel/air mixing, mixture ignition, combustion and pollutant formation (Figure 2.1.2).

The characteristics of Diesel spray usually include spray tip penetration, spray angle, droplet velocities, droplet sizes and distributions, and global spray structure. These characteristics in time and space form are used to describe the performance of the fuel injection system. A good understanding of these characteristics is essential for raising the combustion efficiency and reducing environmental pollution. For instance, size distribution of droplets in a spray has a fundamental effect on compression-ignition engines. Smaller fuel droplets vaporise much more quickly than large ones, but their penetration is shorter and therefore the size distribution must be optimised. Factors that may influence the droplet size distribution include the injection and cylinder pressures, geometry of the injector nozzle and speed operation of the needle. To study the droplet formation from an injector effectively, very finely resolved time measurements are essential.

The different studies previously reported attempt to explore the results of such strategies upon droplet distributions and behaviour with the emphasis on data from the core of the spray. Understanding the droplet behaviour will help explain the benefits of these injection strategies and aid in improving the mixture formation process in Diesel engines.

Figure 2.1.2: Diagram of the Diesel combustion system