International Journal of Engine Research

The effect of fuel properties and fuel temperature on the behaviour of the internal nozzle flow, atomization and cyclic spray fluctuations is examined for a three-hole direct injection spark ignition injector by combining numerical simulation of the nozzle flow with macroscopic and microscopic spray visualization techniques. A dominant influence of the liquid fuel viscosity on the highly unsteady, cavitating nozzle flow and spray formation was observed. A reduced viscosity (or larger Reynolds number) increases the flow velocity, turbulence and cavitation in the nozzle and leads to a slim spray with a reduced width but increased spray penetration. Furthermore, the spray cone angle is larger for lower Reynolds numbers due to the changed internal nozzle flow profile as predicted by the numerical calculation. The shot-to-shot fluctuations of the sprays were found to have their origin in the highly unsteady, cavitating nozzle flow. Larger cyclic spray fluctuations were observed at low Reynolds numbers although the predicted vapour formation in the nozzle is weaker. This can be explained by flow instabilities at low Reynolds numbers leading to large fluctuations in the nozzle flow.

High-pressure multi-hole injectors for direct-injection spark-ignition engines have shown enhanced fuel atomisation and flexibility in fuel targeting by selection of the number and angle of the nozzle holes. The nozzle internal flow is known to influence the characteristics of spray formation; hence, understanding its mechanisms is essential for improving mixture preparation. However, currently, no data exist for fuel temperatures representative of real engine operation, especially at low-load high-temperature conditions with early injection strategies that can lead to phase change due to fuel flash-boiling upon injection. This challenge is further complicated by the predicted fuel stocks, which may include new (e.g. bio-derived) components. The physical/chemical properties of such components can differ markedly from gasoline, and it is important to have the capability to study their effects on in-nozzle flow and spray formation, taking under consideration their different chemical compatibilities with optical materials as well. The current article presents the design and development of a real-size quartz optical nozzle, 200 µm in diameter, suitable for high-temperature applications and also compatible with new fuels such as alcohols. First, the internal geometry of a typical real multi-hole injector was analysed by electron microscopy. Mass flow was measured, and relevant fluid mechanics dimensionless parameters were derived. Laser and mechanical drilling of the quartz nozzle holes were compared. Abrasive flow machining of the optical nozzles was also performed and analysed by microscopy in comparison to the real injector. Initial validation results with a high-speed camera showed successful imaging of microscopic in-nozzle flow and cavitation phenomena, coupled to downstream spray formation, under a variety of conditions including high fuel temperature flash-boiling effects. The current work used gasoline and iso-octane to provide proof-of-concept images of the optical nozzle, and future work will include testing of a range of fuels, some of which will also be bio-derived.

Cavitation plays an important role in the formation of sprays in fuel injection systems. With the increasing use of gasoline–ethanol blends, there is a need to understand how changes in fluid properties due to the use of these fuels can alter cavitation behavior. Gasoline–ethanol blends are azeotropic mixtures whose properties are difficult to model. We have tabulated the thermodynamic properties of gasoline–ethanol blends using a method developed for flash-boiling simulations. The properties of neat gasoline and ethanol were obtained from National Institute of Standards and Technology REFPROP data, and blends from 0% to 85% ethanol by mass have been tabulated. We have undertaken high-resolution three-dimensional numerical simulations of cavitating flow in a 500-µm-diameter submerged nozzle using the in-house HRMFoam homogeneous relaxation model constructed from the OpenFOAM toolkit. The simulations are conducted at 1 MPa inlet pressure and atmospheric outlet pressure, corresponding to a cavitation number range of 1.066–1.084 and a Reynolds number range of 15,000–40,000. For the pure gasoline case, the numerical simulations are compared with synchrotron X-ray radiography measurements. Despite significant variation in the fluid properties, the distribution of cavitation vapor in the nozzle is relatively unaffected by the gasoline–ethanol ratio. The vapor remains attached to the nozzle wall, resulting in an unstable annular two-phase jet in the outlet. Including turbulence at the conditions studied does not significantly change mixing behavior, because the thermal nonequilibrium at the vapor–liquid interfaces acts to low-pass filter the turbulent fluctuations in both the nozzle boundary layer and jet mixing layer.

Reducing the sac volume size of medium-/heavy-duty diesel engine injector nozzles can minimise the fuel dripping into the combustion chamber at the end of injection events, which has been linked to reduced engine-out emissions. This study demonstrates the effect of reduction in the sac volume of diesel fuel injectors utilised in medium-/heavy-duty applications on the internal nozzle flow. This is realised by comparison of two heavy-duty diesel nozzles that feature a large difference in sac volume size of almost three times. For visualisation purposes, the nozzles have been enlarged by six times, and replicas were manufactured from a transparent material. High-speed digital imaging was used to capture the instantaneous spatial and temporal characteristics of geometric as well as dynamic vortex cavitation structures. The investigation was conducted in a steady-state flow test rig for three different needle valve lifts. For all tested conditions, the flow behaviour was analysed at three distinct areas of the nozzle, these being the needle seat, the sac volume and the injection hole. Interpretation of experimental observations was supported by parallel computational fluid dynamics simulations of the exact conditions measured during the experiments. Post-processing of the captured images has revealed the ensemble – average cavitation location, its standard deviation and the cavitation structures life – time inside the sac volume. Results showed a significant dependency of the internal nozzle flow on the sac volume size and identified clear differences in the structure of the cavitation pockets inside the sac volume under certain operating conditions.

Samples of commercial diesel were subjected to 40 h of intense cavitation flow across a diesel injector in a specially designed high-pressure recirculation flow rig. Changes to the optical absorption and scattering properties of the diesel over time were identified by the continuous measurement of spectral extinction coefficients at 405 nm by means of a simple optical arrangement. Identical diesel samples were maintained at 70 °C for 40 h in a heated water bath, in order to distinguish the effects of hydrodynamic cavitation and temperature on the cavitated diesel samples. The commercial diesel samples subjected to high-pressure cavitation flow and water bath heating revealed a response to the flow and temperature history that was identified by an increase in the optical extinction coefficients of the cavitated and heated samples. The contribution of cavitation flow and temperature to the variation in spectral extinction coefficient was identified. It was concluded that the increases observed in the spectral extinction coefficients of the cavitated commercial diesels were caused by the cavitation affecting the aromatics in the commercial diesel samples.

This study undertakes an optical comparison of the cavitation phenomena present in a true-scale optical diesel injector at fuel injection pressures up to 2000 bar. The cavitating nozzle-flow characteristics of rapeseed methyl ester B20 and B100 bio-diesel blends are compared to those of standard ultra-low sulphur petroleum diesel as a means to establish any potential differences in fuel injector performance when bio-fuel blends are introduced to the fuel injection system. A difference in the propensity for cavitation to form is shown, with cavitation inception observed earlier in the 2 ms fuel injection event tested for the bio-diesel blends, with B20 cavitating more readily than B100. Once cavitation inception has occurred, the characteristics of the flow appear to be no different to those of petroleum diesel for fuel injection pressures up to 2000 bar.

The characteristics of ambient gas motion induced by a single diesel spray were measured quantitatively by using a laser-induced fluorescence–particle image velocimetry technique under non-evaporating quiescent conditions. The effects of fuel injection pressure, ambient gas density and nozzle hole diameter on the ambient gas mass flow rate into the spray through the whole spray periphery (spray side periphery and tip periphery) were investigated quantitatively according to the gas flow velocity measurements. The results show that the captured gas mass flow rate through the spray tip periphery is prominent in the whole periphery and the proportion of the gas entrainment through the spray side periphery increases with spray development. The higher injection pressure significantly enhances the total gas mass flow rate through the whole periphery; however, the increase in the ratio of ambient gas and fuel mass flow rate becomes moderate gradually with the increase in the injection pressure. The higher ambient gas density results in a slight increase in ambient gas flow velocity along the spray side periphery and the tip periphery and a reduction of the spray volume; however, the ambient gas mass flow rate was apparently enhanced. The smaller nozzle hole diameter results in a significant decrease in the ambient gas mass flow rate and an increase in the ratio of the gas and fuel mass flow rate. Numerical simulation results provide more understanding of the spray-induced gas flow field and validate the measurement accuracy of the laser-induced fluorescence–particle image velocimetry results.

A common rail injection system for diesel engines has a potential to realize low emission and low fuel consumption diesel combustion. The core technology of this system is accumulated on spray characteristics such as mean droplet diameter, spray cone angle and tip penetration. And these characteristics are greatly influenced by internal flow in a diesel nozzle. In order to clarify the internal flow phenomenon such as cavitation, a real-size transparent nozzle had been developed, which can realize simultaneous cavitation visualization during a single-shot injection under high-density and room temperature condition using a high-speed camera. In this article, relationship between internal flow in the nozzle and spray combustion had been investigated. A high pressure and temperature environment in which the spray ignites were achieved in a rapid compression machine, and internal flow in the nozzle and the spray combustion were visualized. Regarding the evaluation of the spray combustion, the formation of the luminous flame triggered by the spray ignition was observed. Two cases of inlet radius of the nozzle hole (R = 0.016 and 0.033 mm) were prepared for this analysis. Under the case of larger inlet radius hole (R = 0.033 mm), probability of the string-type cavitation was greater, spray cone angle was larger and ignition delay was shorter.