VISUALISATION RESEARCH INTO FUEL SPRAY PROPAGATION

Numerous papers indicate that the present classical systems of fuel injection directly into the combustion chamber, in the most economical directly injected Diesel engines, have reached the limit of development from an ecological point of view. In order to keep the emission of toxic components of exhaust gases within the ranges defined by both the EURO III standard and by the projected EURO IV standard, various modifications to the combustion system become necessary. Initially the fuel injection phase and the distribution of injected material within the combustion chamber should be considered [1, 2, 3, 6, 10]. It is known that injection through a conventional multi-hole nozzle (classical injector), in combination with induced swirl in the air in the chamber fails to ensure that particulates and oxides of nitrogen are not formed. These toxic components are among the most difficult to be subsequently removed from exhaust gases. For a significant advance in this area the mechanism by which the combustible mixture is formed must be radically altered. The overall process involves the following stages: fuel injection/spray formation, evaporation of fuel droplets, admixture with air, reactions immediately before combustion, the combustion process itself.


Introduction
Numerous papers indicate that the present classical systems of fuel injection directly into the combustion chamber, in the most economical directly injected Diesel engines, have reached the limit of development from an ecological point of view. In order to keep the emission of toxic components of exhaust gases within the ranges defined by both the EURO III standard and by the projected EURO IV standard, various modifications to the combustion system become necessary. Initially the fuel injection phase and the distribution of injected material within the combustion chamber should be considered [1,2,3,6,10]. It is known that injection through a conventional multi-hole nozzle (classical injector), in combination with induced swirl in the air in the chamber fails to ensure that particulates and oxides of nitrogen are not formed. These toxic components are among the most difficult to be subsequently removed from exhaust gases. For a significant advance in this area the mechanism by which the combustible mixture is formed must be radically altered. The overall process involves the following stages: fuel injection/spray formation, evaporation of fuel droplets, admixture with air, reactions immediately before combustion, the combustion process itself.
A fuel spray with a different micro and macro structure might have an important effect on these processes. In order to achieve this it is necessary to use a different type of injector -a new type of construction and modus operation [9]. A design for such a device forms the subject of the paper [4,5,7,8]. The special feature of the spray-nozzle of this injector is the variability of the fuel-spraying holes during injection. The variability of the cross-sections of these holes is achieved by a rotary/swinging movement of a needle (RSN injector). The results of investigations described below show that a spray generated by this design of injector has macro-structural parameters that differ from those of a classical/traditional injector.

Results and discussion
The parameters of macrostructure of the stream of sprayed fuel were determined on the basis of measurements carried out using specially constructed equipment, which enabled both a direct observation of the development of the spray during the fuel injection to a chamber of fixed volume [4,7,8] and the measurement of the fuel distribution within the spray of droplets. The visual studies enabled the following to be analyzed: the range of the spray front -L c , the top angle of the sprays and the area of the spray projection in a plane perpendicular to the injector axis -A s . The last criterion of the spray macrostructure estimation was introduced because of the irregular shape of the spray generated by the RSN injector. The method of determination of the values of the analyzed parameters of the spray of injected fuel is depicted in Fig. 1.
A classical injector with a D1LMK 140/M2 pattern sprayer and the new RSN injector denoted "B" were compared. Both sprayers had three outlet holes, the diameter of the holes in the In Fig. 2. the example pictures of the fuel sprays propagation achieved in visualization research were shown, which created a basis for their quantitative analysis. The figures illustrate injection of diesel fuel (DF), rape oil (RO) and mixture of these fuels to visualization chambers in identical conditions. From the figures it can be clearly seen that fuel spray generated by a spray nozzle with rotary-swinging needle movement is formed in a different way than fuel spray formed by a classical spray nozzle, which causes differences in the values of an estimation parameters of the macrostructure of fuel spray. In particular it shows that fuel spray formed by a spray nozzle with rotary-swinging needle movement has an irregular form and its surface (in the view on perpendicular surface to the axis of spray nozzle), cone angle and tip penetration in comparison to the classical spray nozzle are mostly evidently larger. The range of the spray front for diesel fuel, formed by the RSN sprayer under various values of the background pressure in the observation chamber is presented in Fig. 3. It can be seen that an increase of nitrogen pressure in the observation chamber caused -as was expected -a reduction in the range of the spray front. This phenomenon is characteristic of classical sprayers, and may be ascribed to the effect of the aerodynamic resistance on droplets of variable size. An increase in the background pressure (gas density) causes an increase in aerodynamic resistance, and a reduced dyna-mic pressure of the gas into which the injection is made, creating adverse conditions for the disintegration of secondary droplets. Therefore, larger droplets with greater penetrative capability are formed (obviously a larger droplet has greater kinetic energy and will, therefore, travel further).

The range of the spray front
The greatest range of the front of the diesel fuel spray formed by both the classical injector (Fig. 4)  As it could be expected, the use of fuels of considerably greater viscosity affected both types of injectors by considerably increas- . An additional reason for the increase in range of the spray front when using higher viscosity fuels, observed for both types of injectors, was probably the increase in droplet size, due to the conditions conducive to their disintegration being worse.
From a comparison of Figs. 5 and 6 it may be seen that, as in the case of diesel fuel, the spray-range of other fuels was greater for the RSN injector over the whole time of spray development.

The apex angle and surface area of the spray
In Fig. 7 it may be seen that in the case of the RSN sprayer a change in background pressure did not significantly affect the values of the apex angles of the spray over the whole time of its development. However, the spray surface area varied, the greatest area being observed for p b ϭ 15 [bar], i.e. at the background pressure at which the range of the spray was greatest.
Conversely, in the case of the classical injector the effect of pb on the apex angle s was more visible -cp. Fig. 8. As it could be expected, the largest apex angles occurred at maximum background pressure. The values of the apex angles of the spray diminished during its development, i.e. the penetration of the spray in a direction perpendicular to its axis was reduced. For mixing this is a negative effect. It may be only partly compensated by the fact that the spray surface area increases with its development. The smallest surface area of the spray was recorded at the intermediate background pressure, p b ϭ 20 [bar], i.e. for a value corresponding to the shortest range of the spray front.
From a comparison of Figs. 7 and 8 it will be seen that the values As achieved by the RSN injector were greater than for the classical injector. It may additionally indicate the better properties of the spray from the RSN injector, due to better air/fuel mixing processes.

Conclusions
The results of the investigations show that the spray of fuel formed using an RSN-type in a different way than that generated by a classical injector. In particular, the parameters analyzed, i.e. the range of the spray-front, the apex angle of the spray and its surface area, reach greater values for a spray formed by the new RSN type of sprayer. It may positively affect the ecological impact and the performance of engines fitted with injectors of this type.
The variation of the conditions of injection (change of the pressure of the gaseous medium into which fuel is injected, change due to use of fuels of differing viscosity) affects the macrostructure of sprays, generated by each type of injector differently. The best example may be the variance in the apex angle of the spray during spraying RO.
In the case of the classical injector it was found that this angle diminished as the spray developed, while in the case of the RSN injector the opposite tendency was observed.