Tankful Outcome

Marlon Matthäus

Bernhard Huber

Audi e-fuels are a key building block of CO₂-neutral driving. The experts of Audi’s Technical Development are testing these fuels of the future to the nth degree.

What would otherwise be hidden in the cylinder behind metal walls is made visible to the human eye by our glass engine.
Thomas Schladt

Audi Ingolstadt, Technical Development, Building T06. Behind a steel door several centimeters thick, the engine development experts have their gazes fixed firmly on their computer screens. The blinds on the windows are down, the light in the room has been dimmed, a tense silence fills the air. Then measurement engineer Guido Grosse starts the test program in the pressure chamber. There’s a hissing sound as the injector in the steel ball with the round windows made from hardened quartz glass slides forward and clicks into place.

Suddenly, bright flashes dart through the room, throwing long shadows and bizarre patterns onto the walls. Guido Grosse casts a satisfied eye over the first images, which are now appearing at one-second intervals on his monitor. “With every flash, the injector sprays a tiny quantity of fuel into the pressure chamber. We are using a special camera to scan the spray at intervals of 50 microseconds, enabling us to see exactly how the fuel behaves during the injection process,” he says, explaining the so-called light-scattering process. The chamber, which is about the size of a soccer ball, has to withstand quite some forces. Inside it are pressures of up to 15 bar and temperatures of around 350 degrees Celsius. “We’re simulating the same conditions that exist inside a real engine,” adds Grosse.

But this is not just any fuel that’s being tested. What we are dealing with today is the future of CO₂-neutral mobility. On the test stand are the liquid representatives of Audi e-fuels, the synthetic fuels of tomorrow. “For about one year, we have been successfully testing the production of Audi e-ethanol at our demonstration facility in the USA, and hope that e-diesel will follow this year,” explains Reiner Mangold, Head of Sustainable Product Development at Audi. He and Project Manager Sandra Novak brought a few liters of it with them for their co-workers in Technical Development. “We have long proven that we can produce synthetic fuels. Now we are testing them to the nth degree,” is how Mangold explains the purpose of today’s mission.

While measurement engineer Guido Grosse monitors the test in the pressure chamber, Peter Senft sets about analyzing the data. The thermodynamics expert studies the print-outs of diagrams and tables and compares the values collected with existing data. “It’s looking really good,” he says, as he directs his gaze toward the pressure chamber, where flashes are still going off every second. “The e-fuels behave exactly the same on injection as conventional fuels. Clean mixture formation in the chamber is the basis for optimum combustion,” he continues. In eager anticipation, he picks up another diagram. On it is a cross-section of the fuel spray from the pressure chamber. “While the fuel is being sprayed out of the injector in the chamber, we cut through it with a laser, taking a photograph at the same time. The whole thing takes just a few milliseconds,” says Senft, explaining the so-called laser light section process. “From the resulting images, I can see the inner structure of the spray.” After looking at the print-out for a while, he comes to a clear conclusion, “Everything is absolutely fine here, too. The individual droplets are evenly distributed.”

Inside the chamber are pressures of up to 15 bar and temperatures of around 350 degrees Celsius. We’re simulating the same conditions that exist inside a real engine.
Guido Grosse

Test passed! Compared with conventional fuels, Audi e-fuels are often better.

However, the pressure chamber is just the first test bed for the Audi e-fuels. A few rooms further along, Thomas Schladt, Team Coordinator for Measurement Technology, monitors the flow and combustion characteristics of the synthetic fuels in the so-called glass engine. What would otherwise be hidden in the cylinder behind metal walls is made visible to the human eye. A ring of quartz glass shows the onlooker how the fuel behaves in the cylinder. With every one of the maximum of 3,000 revs per minute performed by this research engine, a tiny amount of fuel shoots into the glass cylinder, is compressed, ignited and expelled. “We have mixed a tracer, a kind of chemical colorant, into the e-fuels. We stimulate this with the laser and it begins to glow. The places in the glass cylinder that are particularly bright are where most of the fuel is,” says Schladt, explaining the laser-induced fluorescence process.

Using a high-speed camera, the combustion process is captured with time-lapse photography. “We examine where and how the fuel ignites in the cylinder,” says Peter Senft as he checks the images. “The blue flame is an indicator that the fuel has been cleanly and fully combusted.” But it doesn’t end there. In contrast to fossil fuels, which have varying compositions depending on their geographical source, Audi e-fuels are absolutely pure. Peter Senft explains, “Thanks to their chemical characteristics, they generate fewer pollutants during combustion. They contain no olefins and no aromatic hydrocarbons.” To sum up – better mixture formation, cleaner combustion and fewer emissions. Test passed!

Left: In their cells, the micro-organisms produce fuel molecules that form the basis for Audi e-ethanol and Audi e-diesel. Right: All that the organisms need to produce the synthetic fuels are sunlight, CO₂ and water.

Left: The experts from Technical Development use light scattering to analyze the behavior of the fuel as it is injected into the pressure chamber. Right: They then send a laser beam through the fuel spray to investigate its inner workings.

Left to right: When the intake valves are open, the fuel is sprayed directly into the cylinder, where it distributes evenly. Combustion is triggered by the ignition sparks, from where the flame front propagates. The increasing pressure generated by combustion pushes the piston downward.

Only four elements are required for the production of the Audi e-fuels – water, CO₂, sunlight and tailormade micro-organisms, single-cell organisms just a few thousandths of a millimeter big. Like plants, these organisms use oxygen photosynthesis, i.e. they use sunlight and ambient CO₂ to grow. All they need as a living environment is brine or waste water. “With our American partner Joule, we have been able to modify and optimize the process to make the micro-organisms directly produce either ethanol or long-chain alkanes for diesel,” explains Audi Project Manager Sandra Novak.

At the end of this photosynthesis process, the ethanol or the synthetic diesel fuel is separated from the water and cleaned. The characteristics of Audi e-ethanol are exactly the same as those of regular bio- ethanol and can be used immediately as the basis for E85 fuel (85 percent ethanol, 15 percent gasoline). Audi e-diesel, too, can be mixed without restriction with fossil diesel.

Audi e-ethanol and Audi e-diesel do not need biomass for their production and can be made in regions unsuited to agriculture. “This finally puts paid to discussions about ‘food or fuel’,” says Sandra Novak. “Obviously they, too, produce CO₂ when burned. However, our Audi e- fuels are climate neutral, as the micro-organisms consumed the same amount of CO₂ from the atmosphere. The bottom line is that a car powered by e-fuels has a similarly good carbon footprint to that of a battery-powered car driven by electricity from renewable sources,” says Novak.

We have long proven that we can produce synthetic fuels. Now we are testing them to the nth degree.
Reiner Mangold

The experts from the Sustainable Product Development department are delighted with the results. “We now know that our e-fuels are the same as or even better than conventional fuels,” says Reiner Mangold. The next task is already lined up and waiting – the production process associated with e-ethanol and e-diesel must be further optimized, then these new fuels will be ready to bring to market. “In the near future, we will be in the position to produce several hundred thousand liters of synthetic liquid fuel per day,” says Sandra Novak. This marks a major step toward sustainable mobility.