Experimental investigation of the non-uniformity of jets’ velocities exiting a multi-hole injector

Coratella, Carlo (2021). Experimental investigation of the non-uniformity of jets’ velocities exiting a multi-hole injector. University of Birmingham. Ph.D.

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Abstract

Over the last few decades, the global demand for vehicular mobility has seen an unstoppable growth on a worldwide scale, setting the stage for a faster depletion of fossil energy. Besides some implications of a geopolitical character, this scenario has also led to an increase in pollution levels. As a consequence, progressively stricter criteria of pollutants‟ levels from IC engines are being demanded by the emissions‟ regulations. The automotive field has thereby endeavoured to pursue the minimisation of the environmental impact of IC mobility, directing research efforts towards the development of cutting-edge solutions.

With regard to the fast exhaustion of oil supplies, this issue is deemed to significantly determine radical changes within the global automotive scenario in the next few years. This criticality has urged the automotive field to turn its attention both to investigation of fuel spray and to the development of biofuels, until the worldwide spread of electric vehicles
becomes a viable pathway for green mobility.

The work in the present thesis thereby aimed to provide a further perspective when approaching these critical subjects. More specifically, the doctoral research developed an experimental framework to assess the variability of jets‟ velocities at early stages of injection, employing diesel and methylfuran-based biofuels. The scope was to provide a physical explanation of the combined impact of injection settings with the fuel properties and injector geometry on the uniformity of the jets‟ development.

The first stage of this study addressed the design of a sensor for the detection of needle lift in a diesel injector. This part was prompted by the key role of needle lift in the characterization of the injection process. Diesel injections were performed, demonstrating that the sensor is able to follow needle displacement over the injection process. Besides the accurate detection of needle motion, peculiarities of the sensor include a largely shorter realization time and a higher cost effectiveness. With regard to the findings which emerged from the tests, it was observed also that along with injection pressure, backpressure plays a non-negligible impact on the time history of needle displacement over the injection process. More specifically, it emerged that, at unvaried injection pressure, higher backpressures boost needle lift as well as injector closure.

The analysis of the impact of injection settings on needle lift prompted the second stage of the present study. This stage aimed to assess the non-uniformity of diesel jets‟ velocities. From the tests, it emerged that besides the greater injection pressure, an increase in backpressure minimises the orifice-to-orifice variability of diesel jets‟ velocities at the early stages of the injection process. In this regard, keeping in mind that needle wobbles contribute to the imbalance of the injected flow rate, the combined impact of injection pressure with backpressure on needle motions was discussed.

The outcomes emerging from the second stage led the author to shift the focus to the combined impact of injection setting with the fuel properties on the variability of jets‟ velocities. For this purpose, injection tests were performed, employing diesel and 2-methylfuran/diesel blends (here referred to as MF30 and MF50, given the 30% and 50% volumetric content of 2-methylfuran in diesel, respectively). It was observed that MF30 jets‟ velocities are spread out over a wider range than diesel in most cases; whilst MF50 exhibited the lowest orifice-to-orifice variability. Besides the jets‟ velocities, the injection delays were also acquired. Afterwards, the combined action of the injection settings with the fuel properties both on the needle off-axis motion and on the injection delays was fully discussed.

The ascertained impact of the injection settings on the jet development prompted the author to assess the injection-induced pressure fluctuations taking place in the rail. Therefore, keeping in mind that injection pressure plays a pivotal role on the uniformity of jets‟ development, and as the injection pressure is subjected to variations stemming from the precedent shot, it was deemed necessary to assess the injection-induced pressure fluctuations occurring in the rail.
Injection tests were performed employing neat diesel and the aforementioned 2- methylfuran/diesel blends at various injection configurations. Besides the injection settings, the fuel properties of viscosity, surface tension, density, and bulk modulus were found to
meaningfully determine rail pressure fluctuations. The injection tests showed that the furanbased blends, provided with greater bulk modulus, are subjected to lower pressure fluctuations, as well as the expected decrease in the injection-induced pressure drop from greater fuel viscosity. Moreover, higher backpressure was found to mitigate the diesel pressure drop within the rail. By contrast, MF30 and MF50 pressure drops tend to increase in response to increasing backpressure. It was observed also that, at unvaried injection pressure and greater backpressures, decreasing viscosity (but still higher than diesel) alleviates the increase in rail pressure drop in most cases.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):