There’s a lot that is not understood about these systems, and thus a lot to be learned. Our work is focused on identifying the time and location of cavitation events—to study the injector with the ability to see cavitation in action.

—Todd Toops

While there are a number of neutron imaging studies of cavitation in diesel fuel injectors in the literature (e.g., Takenaka et al. below), there is little on the more recent SGDI injectors.

In August, they conducted their research at HFIR’s CG-1D beam line, which is used for neutron radiography and computed tomography, to study non-destructively the internal structure of the fuel injector.

In order to create an experiment that closely mimics natural conditions of an engine running, Nafziger, Splitter and Toops developed a closed-loop fuel injection system designed to operate with commercial and prototype injectors and deliver fuel to the injectors at pressures up to 120 atmospheres.

With 48 hours of observations for a given operating condition, they compiled approximately 1 million injection events to capture a 7-millisecond composite injection sequence, with 1 millisecond before injection, 1 millisecond of injection, and 5 milliseconds after injection. This compilation was accomplished with a 0.02 millisecond time resolution.

In the initial analysis of the composite neutron images, it is possible to see both internal injector motion and the spray exiting the nozzle. Just inside the nozzle area, a marked difference in fluid density is also observed during the injection event, indicating vaporization of the fluid and possible cavitation.

—Eric Nafziger

The team is working on more detailed analysis of the data, and will collaborate with the ORNL high performance computing team for fluid dynamics modeling as part of the second year of their project.

HFIR and neutron imaging. Neutrons (along with protons) are fundamental particles that constitute the nucleus of most atoms (hydrogen being the exception). Neutrons do not possess any electric charge; they are stable inside the nucleus, and outside of it, they decay with a lifetime of about 15 minutes (886 seconds)—a long time for a fundamental particle.

They can be released from the nucleus by fission as in a nuclear reactor (HFIR) or by firing a high-energy beam of protons into a target of a neutron-rich element (a process called spallation).

Operating at 85 MW, HFIR is the highest flux reactor-based source of neutrons for research in the US, and it provides one of the highest steady-state neutron fluxes of any research reactor in the world. HFIR currently offers 13 beamlines (instruments) for users. This ORNL team is using beamline CG-1D: Neutron Imaging Prototype Facility.

Other complementary research on fuel injectors has been done with lasers, X-rays and even with fuel injectors made partially with acrylic to make them transparent. However, those experiments had temperature and pressure limitations.

And although neutron imaging has some similarities with x-ray imaging, neutrons are much more penetrating than x-rays. While x-ray scattering occurs close to the surface, neutrons can penetrate more deeply. Another major difference is that the intensity of x-ray scattering increases with the electron density of a material; light elements such as hydrogen and lithium thus make very little contribution to scattering.

Neutrons, on the other hand, can obtain an appreciable scattering signal from light elements. As a practical result of these differences, most metals typically used for manufacturing purposes are readily penetrated by neutrons (surpassing even the maximum depth of high energy x-rays) while hydrogen atoms have a high probability of scattering neutrons out of the incident beam. This makes neutrons quite suited for the detection of hydrogenous material within engineering materials and structures, noted Oak Ridge researchers in a 1999 paper.

This neutron technique, said Toops, is the first to have the potential to see what’s happening inside the injector at normal operating conditions.

We can measure the spray of a fuel injector using X-rays, but imaging the internal structure in operation is very challenging. Neutrons are ideally suited for this study due to their high sensitivity to hydrogen atoms in the fuel and low interactions with the metal part of the injector.

—Hassina Bilheux, HFIR instrument scientist for CG-1D

Resources

• N. Takenaka, T. Kadowaki, Y. Kawabata, I.C. Lim, C.M. Sim (2005) “Visualization of cavitation phenomena in a Diesel engine fuel injection nozzle by neutron radiography,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 542, Issues 1–3, Pages 129-133, doi: 10.1016/j.nima.2005.01.089