Argonne National Laboratory announced that Cummins, Convergent Science Inc. (CSI), and Argonne’s Virtual Engine Research Institute and Fuels Initiative (VERIFI) entered into a Cooperative Research and Development Agreement (CRADA.) The purpose of the agreement was to understand the complex fluid dynamics of fuel injectors in engines. The result of the work is now available in CSI’s CONVERGE software, according to Argonne.
Fuel injectors are designed to atomize fuel into droplets for efficient combustion. Without an effective way to simulate the inner workings of the injector, manufacturers use prototype hardware and physical testing to design efficient, durable injectors. An accurate simulation tool would shorten engine development time by allowing designers to better visualize the spray in the crucial microseconds of injection and its effect on the hardware.
Injector anomalies affect combustion
Why did Cummins want to join the CSI and Argonne team? “One of the key shortcomings in modeling in-cylinder combustion is our inability to accurately and quantitatively model what goes on inside the injector and relate that behavior to the resultant in-cylinder sprays,” explained Dr. John Deur, Director of Combustion Research at Cummins in an interview with SAE Magazines. Up to now, most engine combustion simulations start at the orifice face of the injector, as he describes it, glossing over “critical details of phenomena upstream of that which aren’t adequately known.”
However, there are a number of factors inside the injector that affect combustion. Cavitation can damage the injector through erosion as well as impacting the character of the spray injected into the cylinder. There is also needle motion and float, where the injector needle position minutely changes from event to event, introducing unwanted variability.
“There are waves inside the whole injection system that affect combustion behavior,” Deur said. Better designs rooted in fundamental understanding of such unwanted behavior can lead to better, more durable injectors. “In our effort to attack all uncertainties in [engine development] we would like to attack our uncertainties in injectors,” he stated, explaining why Cummins joined the work through a CRADA. Cummins participated in a non-exclusive arrangement under the CRADA.
“The true complexity of how fuel injectors function is not yet well simulated,” agreed Sibendu Som, also speaking to SAE Magazines. He is the principal investigator and principal mechanical engineer at Argonne’s Center for Transportation Research and a founding member of the VERIFI program. For example, using existing approaches, researchers could only model the output of one injector opening at a time. Modern fuel injectors often have up to nine holes in each injector and inject multiple times during a combustion cycle.
Confidence through uniquely validated simulated models
According to Som, the new software the CRADA team developed and validated dynamically couples the flow inside the nozzle with the ensuing spray.” Due to the high performance computing (HPC) capabilities at Argonne, some simulations could be performed that have never been performed before,” said Som. He noted that injection transients are fast processes governed by the opening and closing of the needle—within 20 micro seconds. “The spatial and temporal resolutions necessary to resolve such fast processes are demanding. In collaboration with CSI and Cummins, Argonne has developed, tested, and validated these coupled models,” he explained.
Initially they were run with Reynolds-Averaged Navier–Stokes (RANS) turbulence models. They graduated from these time-averaged equations to higher fidelity simulations using large-eddy simulations (LES). The validated models now use LES, implemented within a higher fidelity Eulerian spray model, according to Som. According to data presented by Som at a workshop held at Argonne on Nov. 12, simplified RANS modeling cannot replicate actual vapor penetration data as well as high-fidelity LES turbulence models. He attributes this to the fact that the LES resolves more flow structures and therefore predicts air-fuel mixing better.
The software was then validated through a variety of published data in literature, including the use of X-ray radiography data from the Advanced Photon Source (APS), another DOE Office of Science user facility located at Argonne. The APS produces the brightest X-ray beams in the western hemisphere, according to Argonne. A dedicated portion of the APS enables VERIFI researchers to collect experimental data on fuel injection and spray phenomena (think cavitation and near-nozzle jet interactions) to simulation tool development. Argonne can visualize and quantify needle float (also known as wobble), using the APS. VERIFI’s work is funded by the Vehicles Technology Office of the DOE’s Office of Energy Efficiency and Renewable Energy.
The validated models are now available in the CONVERGE standard release, according to CSI.
HPC remains a necessary tool for simulations such as these. An injection simulation modeled with LES can consume more than 256 processors for three to four weeks to produce a result, according to Som.