Fuel Injection and Spray Research

The fuel-injection process is critical to attaining high fuel efficiency and low emissions in modern engines. Accurate control of fuel injection parameters (timing, delivery, flow rate, pressure, spray geometry, etc.) is the most effective means to influence fuel and air mixing and to achieve both clean burning and high efficiency. Unfortunately, the physics of spray atomization and its influence on combustion, pollutant formation, and fuel efficiency are not well understood, and final tuning of the engine is a trial-and-error procedure. A deeper understanding of the injection process and spray atomization is needed to enable new strategies for clean and efficient combustion.

Argonne scientists have developed several novel diagnostic techniques that use x-rays to study the detailed structure of fuel sprays. X-rays are not hindered by multiple scattering processes that 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), Argonne has developed a non-intrusive absorption technique that yields a highly quantitative characterization of the dynamic mass distribution in the spray from both diesel and gasoline engine injectors.

Diesel Sprays

Diesel engines are significantly more fuel efficient than their gasoline counterparts, so wider adoption of diesels in the United States would decrease the nation’s petroleum consumption. However, diesels emit much higher levels of pollutants, especially particulate matter (PM) and nitrogen oxides (NOx). These emissions have prevented manufacturers from introducing diesel passenger cars. Researchers in Argonne’s Engine and Emissions Research group are exploring ways to reduce pollution formation in the engine using clean combustion strategies. A key component to the development of clean combustion is controlling the fuel spray and fuel/air mixing.

Argonne’s studies of diesel sprays have led to several new discoveries. For the first time, quantitative measurements of the mass distribution within fuel sprays have been obtained with very precise time resolution. In addition, the density of the fuel can be calculated at any position and time within the spray, a result that proved for the first time that sprays from modern diesel injectors are atomized only a few millimeters from the nozzle. 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 shock waves in the spray chamber, which we have quantitatively measured for the first time. Sprays from nozzles with different internal structure have also been quantitatively measured under identical experimental conditions; the resulting differences in the mass distributions of the sprays will prove very useful to spray modelers trying to understand the effects of nozzle geometry on the structure of sprays.

Gasoline Sprays

Gasoline Direct Injection (GDI) engines are now beginning to enter the US market. These advanced gasoline engines inject the fuel directly into the engine cylinder rather than into the intake port. These engines can achieve higher fuel efficiency, but they depend on a precise fuel/air mixture at the spark plug to initiate ignition. This leads to more stringent requirements on spray quality and reproducibility. Direct injection also enables new combustion strategies for gasoline engines. Such “lean burn” engines may achieve efficiencies near that of a diesel while producing low emissions. Again, this advanced combustion strategy relies on precise mixing of the fuel and air to achieve clean, efficient power generation.

Argonne's fuel injection and spray researchers are studying the process of gasoline injection to enable these advanced combustion strategies. We have performed the first quantitative, dynamic three-dimensional reconstruction of a fuel spray, which revealed the striking asymmetry of sprays from a prototype gasoline injector. We have worked with several US manufacturers to help them understand the performance of their injectors, and have assisted in the development of a new GDI injection system, from prototype to final production design.

Alternative Fuel Sprays

Non-petroleum fuels are gaining popularity in the United States. Ethanol is being blended with gasoline in varying proportions, and biodiesel can be found today at pumps across the country. These fuels can reduce our nation’s dependence on petroleum, but their effects on engine performance and emissions are still not well understood.

The physical properties of these alternative fuels can vary quite dramatically from those of petroleum fuels. Ethanol has significantly less energy per gallon than gasoline, and biodiesel has a much higher viscosity than diesel. Changes such as these require the engine to adapt to the fuel in ways that have not previously been necessary. These changes also fundamentally alter the operation of fuel injectors and the structure of spray in ways that are not well understood. These uncertainties are preventing the adoption of new clean combustion strategies in both gasoline and diesel engines.

Argonne’s researchers are studying the fuel injection process using fuels such as biodiesel, vegetable oils, pyrolysis oil, ethanol, and butanol, with the goal of understanding how changes in fuel properties affect the spray, combustion, and ultimately, the operation of the engine. Our experiments have discovered structural differences between sprays of conventional fuels and biodiesel, revealing that biodiesel sprays require more time to atomize and produce more compact sprays with higher density.

Dynamic Imaging of Injector Operation

The high penetrating power of x-rays makes it possible to penetrate through the outer steel structure to image the internal components of fuel injectors. The high x-ray flux at Argonne’s Advanced Photon Source makes it possible to image these components while they are in motion. Measurements such as these are critical for the development of computational spray models, since they can precisely measure the time-dependent geometry of the fuel passages inside the injector. These measurements are also useful to injector manufacturers, since they can reveal whether a particular component is functioning as designed. Measurements such as these have revealed “wobbling” of the needle valve inside an operating injector, which can lead to spray asymmetry, decreased fuel efficiency, and increased emissions.

More

• Near Field Characterization of Direct Injection Gasoline Sprays from Multi-Hole Injector Using Ultrafast X-Tomography (980KB)
• X-Ray Measurements of the Mass Distribution in the Dense Primary Break-Up Region of the Spray from a Standard Multi-Hole Common-Rail Diesel Injection System (1.1MB)
• X-Ray Radiography Measurements of Diesel Spray Structure at Engine-Like Ambient Density (672KB)

May 2008