The combustion vessel had previously been designed and built for diesel spray investigations. Modifications were carried out to enable gasoline spray characterisation with additives. This included a new fuel delivery and control systems.
3.1.1 Injection System
The fuel injection system was built based on a gasoline direct injection (GDI) system.
Although modern GDI systems make use of spray guided multi-hole injection systems,
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a single-hole wall-guided DI injector was reasoned more suitable for the current study.
This was due to factors described in Section 2.2 on interactions between spray plumes but also with combustion experiments in mind. Due to the very large volume in the combustion chamber with respect to engine conditions at the end of compression stroke and a single source of ignition, the wall-guided injection system was employed as a spray-guided injection system. The selected injector was a Bosch single-hole GDI injector rated at 110 bar injection pressure. A custom water-cooled injector mount was designed and manufactured for the chosen injector. The mounting assembly can be seen Figure 3.2.
Figure 3.2: Injector mount assembly. Components as named in the figure
It was anticipated that some if not all of the additives used in the testing might carry a memory effect. This would potentially distort results and as a result a full fuel system clean up using an ultrasonic bath was used between different additives.
Consequently, the fuel was pressurised using an in-house designed fuel pressure accumulator [217]. A larger piston is pressurised directly from a nitrogen supply which then amplifies the pressure as the force is transferred to a smaller piston. This system allowed for nitrogen supply pressure to be stepped-up by a factor 7 while all components in contact with fuels could explicitly be cleaned. Cleaned components
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included fuel injector, injector cap, high pressure fuel line, pressure transducer mount and fuel accumulator parts that were in contact with the fuel. This is presented in Figure 3.3.
Figure 3.3: Fuel system components cleaned in the ultrasonic bath. Components include: a) high pressure fuel tank lid, b) common rail fuel pressure transducer, c) pressure transducer mount, d) high pressure fuel line, e) high pressure fuel tank main body, f) fuel side small
piston, g) injector cap, h) DI injector
3.1.2 Injection System Characteristics
Injector characteristics were quantified in order to be able to distinguish between hardware variability and additive effects. Additionally, this data could be used to determine experimental conditions. Figure 3.4 represents the shot-to-shot
Figure 3.4: Injection signal to injector driver output signal delay 6.5
6.7 6.9 7.1 7.3 7.5 7.7 7.9
0 5 10 15 20
Delay, us
Injection Event, no
a
c
d
e
g
h b
f
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variability of start of injection signal to start of injector signal. An average delay was found to be 7.2 µs with a standard deviation of 0.79 %. This, in comparison to the injection signal duration (in ms range), was considered not significant.
Figure 3.5 displays the increase in fuel velocity with increasing injection pressures. The data was obtained from shadowgraph images at each injection pressure and was measured to reflect time taken to reach 60 mm from the injector nozzle. Each point was measured from a 20 image average that was found large enough sample to eliminate the effect of spray-to-spray variability. The measured spray tip velocity varied nearly linearly between 38.5 km/h and 51.9 km/h. Shadowgraph images of typical injections for 50 bar and 110 bar injection pressures can be seen in Appendix A.
Figure 3.5: Spray tip velocity at increasing fuel injection pressure as measured to 60 mm from the nozzle
3.1.3 Droplet Sizing
As was discussed in Section 2.3.2, droplet size is the primary microscopic characteristic of spray atomisation quality. Droplet size analysis characterisation was carried out using a Malvern Instruments Spraytec laser diffraction system. The system uses a 660nm wavelength 10 mm diameter laser beam directed through the working section and sampled on a 32 ring receiver. Maximum sampling rate of the system is 2,500 Hz. Initial testing showed great difficulty in aligning the laser through the 88 mm thick quartz windows due to diffraction of light at different laser to rig alignment angles. If normal positioning between the two was not achieved, small deflections in
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the laser beam caused the inner most rings of the receiver to misread the input and give false readings in the form of large droplets. As a result, two of the windows were removed and the testing carried out under ambient pressure and temperature conditions.
3.1.4 Viscosity and Surface Tension Measurement
In order to better understand drop size behaviour between different additives and fuels, measurements were carried out on viscosity and surface tension. As explained in Section 2.3.2 these are the two prominent physical properties of liquids that affect the droplet size and can change in a non-linear manner to their constituent composition. Viscosity measurements were taken with a Brookfield DV – III Ultra programmable shear rheometer combined with a heated bath for temperature control.
The principle of operation is using a calibrated spring to drive a spindle [218]. The liquid is placed within a cylinder and a rotating spindle is lowered into it. As the spindle rotates, the liquid exerts a measurable torque on it which is converted into shear rate and viscosity of the liquid. The rheometer gave accuracies to within two decimal places.
Surface tension of the fuels was measured with a Kimble & Chase Surface Tension Analyzer. The analyser works on the principle of capillary action, whereby a liquid fuel is forced vertically up a tube and then let fall down due to gravitational forces [219]. In this process wetting of the capillary walls occurs and the surface tension induced tensile stress tends to pull the liquid free surface towards the solid surface. This encourages formation of a curved meniscus and, if the capillary diameter is small, creates a capillary rise. The surface tension could be calculated using an equation proposed by the manufacturer and followed the form:
𝜎 =1
2ρ𝑔ℎ𝑟 3.1
where: σ – surface tension ρ – density of sample, g – acceleration due to gravity, h – distance between menisci and r – radius of the capillary. It was claimed that accuracies to within 20 % of true values were possible with the analyser.
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3.1.5 Spray Apparatus Overview
The overall schematics of the experimental facility, used for droplet size analysis, can be seen in Figure 3.6. The equipment includes a constant volume combustion vessel (CVCV), Malvern Spraytec laser diffraction system, GDI high pressure swirl-type injector and a fuel pressure accumulator.
Figure 3.6: Schematics of experimental facility for spray analysis
The injector was powered by a LifeRacing GDI driver and injections controlled through a LabView control program. The software was additionally used to send a signal to the Malvern system for triggered data acquisition. The same program was also used to monitor fuel pressure and ambient conditions. Shadowgraph images were taken with a Photron APX-RS high speed camera coupled with a Nikon 50 mm f/1.8 lens.