Supercritical fuel jets - resolving controversy

New high efficiency engine combustion modes hold the promise to significantly reduce the contribution of transport vehicles to climate change. All of these modes rely upon direct injection of fuel into the combustion chamber, and so fuel/air mixture preparation is a controlling process in terms of engine efficiency (i.e. reduced CO2 production) and pollutant emissions (including soot, another contributor to climate change). As these new combustion modes push to higher pressure and temperature, it would appear that the fuel jets probably undergo a thermodynamic change, becoming supercritical at the edges of the jet. If that were true, it would completely change our current understanding of fuel/air mixture preparation, and that would have a significant effect on engine performance and design. At this time several theory groups are in disagreement on whether or not this change happens, and if it does how best to understand it. This project will resolve those disagreements, and it will lead to the understanding required to adapt both fuel injectors and engines to this potential new reality.

Aside from the motor industry, supercritical mixing is important to the pharmaceutical, food processing, catalyst production, and other nanomaterials industries, and our goal is to team with researchers in these areas as well.

This project has four main parts. A specialized, optically-accessible cell will be designed and built based on a successful design under operation at the Technical University of Darmstadt. A laminar liquid jet (with clear access to the fluid/gas interface) will flow down through this chamber, which can be set to various pressures and temperatures below and above the liquid critical point.

A line-Raman scattering instrument will be developed in order to characterize the chemical composition of the flowfield as a function of time and position.

Next, laser induced thermal acoustics (LITA) will be developed. LITA will be used to measure the sound speed as a function of position and time as conditions are varied. The sound speed reaches a minimum at the critical point, and increases very steeply as pressure and temperature go above the critical point, so it can be a significant marker for thermodynamic states.

Finally, Förster resonance energy transfer (FRET) will be evaluated as a way to observe changes in the density (mean free path) as the jet approaches a supercritical state. Such a change is considered to be a distinctive marker for this state as well.

This program has been designed specifically for our theory partners (City University London, Sandia National Labs, Stanford University, and University of Wisconsin), who have taken part in planning discussions for this proposal. We will thus use the experiments to provide quantitative information never before available to academia or to industry. The information will provide unambiguous, quantitative results with two aspects: 1) Industry can use them to re-think their mixture preparation strategies, while 2) theoreticians can use the results to inform and validate models. Ultimately those models will be delivered to industry.

As mentioned, there are many other subject areas interested in similar problems. Once this system is operating well we will approach UK researchers working in related areas and offer this facility for collaborative research.

The US Air Force Office of Scientific Research has committed to support partially this effort, with $360,000.

The urgent need to reduce greenhouse gas emissions is a principal driver for this project. The climate problem must be attacked from many directions, and this is one of them. It has to do with informing and improving the way in which transportation powertrains are developed, via much better understanding for direct injection of fuel into the combustion chamber, a controlling parameter.

The Automotive Council UK publishes roadmaps for future vehicle development, and topping the list under "Powertrain (ICE) technology" is "Fuel injection system optimisation" within the "Thermal Efficiency" section. It would be impossible to optimise a fuel injection system if we don't even know how the fuel jet behaves. Researchers in the motor industry thus require better understanding of how fuel/air mixtures are prepared in their engines. In this case, a significant leap towards higher efficiency has suddenly introduced a complex question about changes in thermodynamic states and thus changes in how the engine operates. One could reasonably ask if we have found an optimum under these circumstances. Fuel injector manufacturers are also affected. Motor manufacturers will be requesting a change in the flowfield produced by the jet, and they may well request temperature stabilization of the injector. These changes would have a significant impact on cost as well. Guidance from a program like this one would make a significant contribution.

Theoreticians who are developing models for changes in the thermodynamic states of fuel jets at high pressure and temperature in high efficiency engines are currently in disagreement; they require new, quantitative experimental findings to work out their differences. Professor Linne collaborates with all of these groups, and this proposal is based upon conversations with them regarding their most pressing needs. This proposal has thus been designed in collaboration with theorists, with the goal to provide necessary, quantitative information to develop insight into how the fuel jet actually behaves.

Finally, many other emerging processes in, for example, the pharmaceutical, food processing, and catalyst industries can benefit from the techniques and the new knowledge generated in this project. This project is thus just a first step, with the goal to provide a users' facility for supercritical mixing experiments to support any industry that relies upon the process.