X-Ray Measurements of Fuel-Injection Sprays

Mass distributions of sprays from two different injectors at different injection pressures. The measurements show the effect of nozzle hydrogrinding on the fuel distribution.

The fuel-injection process in modern engines is a critical step in attaining high thermal efficiency and emissions control. Accurate control of the injection performance parameters (timing, delivery, instantaneous flow rate, pressure, spray configuration, etc.) offers one of the most effective means of influencing mixture preparation and combustion kinetics to achieve both clean burning and high thermodynamic efficiency.

Modern fuel-injection equipment provides better control of timing and delivery at much higher injection pressures, which have mproved diesel engine performance and fuel economy. Unfortunately, direct evaluation of the actual fuel-injection spray is still difficult, and the final tuning of the injection system is a trial-and-error procedure. Researchers need new and more powerful means to study and evaluate the characteristics of sprays from fuel injectors so that they can further improve mixing and combustion processes.

Argonne has developed a novel radiographic diagnostic technique that uses time-resolved synchrotron X-rays to study the dynamic characteristics of fuel sprays. X-rays are highly penetrative in materials with low atomic numbers; therefore, they do not encounter the multiple scattering problems typical of diagnostic methods that use visible light. By using highly time-resolved monochromatic X-rays generated at the Advanced Photon Source (APS), researchers have developed a nonintrusive absorption technique that yields a highly quantitative characterization of the dynamic mass distribution in the spray from both diesel and gasoline engine injectors.

To measure the spray's mass distribution, researchers use either a single-point or a two-dimensional X-ray detector. Each detector has different strengths and is chosen on the basis of the desired experimental results. The single-point detector can make measurements with very fine position resolution, but to build a complete image of the spray, the measurement must be repeated at many different positions within the spray plume — a laborious and time-consuming process. The two-dimensional detector can produce a complete image of the spray in a shorter time, but with lower position resolution.

Results

A supersonic diesel spray and the shockwave that it generates.

Argonne researchers have conducted experiments to study the sprays from a modern common-rail diesel system and direct-injection gasoline injectors from several different manufacturers. Several first-of-a-kind measurements have been obtained:

• Argonne calculations proved, for the first time, that sprays from modern diesel injectors are atomized only a few millimeters from the nozzle. This area near the nozzle is normally impenetrable to visible-light-based diagnostics.
• Also for the first time, researchers have obtained quantitative measurements of the mass distribution within fuel sprays with very precise time resolution. In addition, the density of the fuel can be calculated at any position and time within the spray.
• The speed of the spray core can also be measured, not only at the leading edge of the spray, but also at the trailing edge and within the body of the spray itself. Such measurements have shown that the fuel travels at supersonic speeds under certain experimental conditions. These supersonic sprays generate shockwaves in the spray chamber, and Argonne researchers quantitatively measured the shockwaves for the first time.
• Argonne researchers performed the first quantitative three-dimensional reconstruction of a fuel spray, which revealed the striking asymmetry of sprays from a prototype injector.

Sprays from nozzles with different internal structure have also been quantitatively measured under identical conditions. Researchers modeling fuel spray will find the resulting differences in the mass distributions of the sprays very useful as they try to understand the effects of nozzle geometry on the structure of sprays.

One of the key benefits of these measurements will be data that can be used to validate and improve computational spray models. Several groups are developing models to predict spray structure under various conditions. However, quantitative data against which these models can be tested are very limited. The X-ray measurements have the unique ability to provide quantitative data in the near-nozzle region, which is crucial for improving these models. As these models progress and gain predictive power, it will become easier and more cost-effective for engine manufacturers to design engines with improved efficiency and reduced emissions.

Argonne's current research efforts are focused primarily on two different aspects of fuel sprays: the effects of nozzle geometry and the effects of ambient pressure. These two parameters are particularly important to the development of accurate computational models of sprays. Argonne researchers have recently completed the first measurements on valve-covered-orifice (VCO) diesel injectors, which are standard equipment on most modern diesel engines and are expected to produce sprays significantly different from those of the mini-sac nozzles previously studied. Researchers have also extended the range of ambient pressures over which measurements can be made: recent experiments have studied spray structure at 20 bar ambient pressure.

Future Plans

Future experiments will continue to focus on the effects of nozzle geometry and ambient pressure. Recent modeling work has shown that to resolve the predicted differences in spray structure from nozzles of different geometry, position resolution must be less than 20 microns. Argonne researchers are currently designing experimental techniques that will enable them to see such fine features in the structure of sprays. They are also developing new X-ray windows that will allow them to move to even higher ambient pressures while expanding their field of view.

Three-dimensional reconstruction of a hollow-cone spray

A complete understanding of spray atomization also depends on knowledge of the effects of ambient pressure. To explore the effects of temperature on spray atomization, Argonne researchers have constructed a rapid compression machine (RCM) compatible with X-ray measurements. This device will allow researchers to study sprays under high temperature and pressure — conditions similar to those in an operating diesel engine. The RCM is designed to generate an ambient environment of 600 psi and 600°F while minimizing vibration with its dual-opposed piston design. Final tests of the RCM are under way, and the design and construction of a high-precision positioning table for the device will follow. Researchers expect to use the device for taking x-ray measurements of sprays within two years.

Researchers are also exploring the use of X-rays as a tool to solve other problems in engine and emissions research. Techniques to image the internal operation of the injector are currently being developed. When developed, these techniques will allow researchers see the motion of the internal components of the injector while it is operating in real time, which has long been a goal of the major injector manufacturers.

September 13, 2004