SPRING 2003 — Engine and Emissions
Fuel Spray Studies Being Conducted at Elevated Pressures
CTR researchers are using high-brilliance x-rays from the Advanced Photon Source (APS) to obtain detailed quantitative data on the structure and behavior of sprays from diesel fuel injectors. Fuel injector design is considered key to reducing particulate and nitrogen oxide (NOx) emissions from diesel engines. The x-ray techniques the researchers are using improve on laser-based techniques because x-rays can penetrate fuel sprays even in the crucial near-nozzle region, which the other techniques cannot do.
The researchers are currently testing Bosch fuel injectors in a pressurized chamber fitted with special windows that permit x-ray passage. Data from recent experiments are still being analyzed, but, generally speaking, the researchers have found that increasing chamber pressure decreases the speed of the spray, as expected. This results in an increase in fuel density at higher pressure because spray particles move away from the nozzle at lower speeds. The x-ray technique is unique because it is the only method that can precisely measure this change in fuel density. The experiments were conducted at ambient temperature at pressures of 1 bar and 5 bar. Experiments at 10 bar are the next step, with 20- 30-bar tests coming next year. One objective of the test series is to map out how increasing the pressure affects spray behavior, so that a comprehensive model of the structure of fuel sprays can be developed that remains valid all the way to diesel engine cylinder pressures of about 120 bar.
Reaching the pressure and temperature regime of a modern diesel engine in the x-ray laboratory will require a novel approach. The researchers are designing a rapid compression machine (RCM), which will permit x-ray experiments under diesel engine conditions. It may even prove possible to simulate the swirling gas motions found inside real engines with the RCM. A primary issue with the development of the RCM concerns designing x-ray windows that can withstand harsh engine conditions. The researchers are testing prototype windows on the current pressure chamber that they hope will work on the RCM.
A long-range goal of the research program is to see whether it is possible to gain new information about fuel spray combustion using x-rays. Should x-ray research prove useful, it could open the door to the development of an x-ray engine research center based at the APS, where fuel oxidation and NOx and particulate formation could be studied under realistic engine conditions.
X-ray images of diesel fuel sprays encountering (top) atmospheric pressure and (bottom) two atmospheres of pressure. The measurements were taken 115 microseconds after the start of injection. Measured fuel density is indicated by color. The images show greater penetration at lower pressure and higher density in the higher-pressure case.
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U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, FreedomCAR and Vehicle Technologies Program
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