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Could a team of researchers in Valencia turn the combustion engine’s fortunes around with a system that promises to produce zero NOx and capture the entirety of its CO2 production?

It’s easy to see why the internal combustion engine has dominated the automotive market for so long. With energy-dense liquid fuels, widely available raw materials and a mature global infrastructure in place, it’s hard to imagine the industry moving away from this technology if the crucial issue of tailpipe emissions could be solved. And a team of researchers from Spain claims to have done just that.

Headed up by the CMT-Motores Térmicos (CMT) group at the Polytechnic University of Valencia and the Spanish Institute of Chemical Technology (ITQ), the project combines CO2 capture and oxy-fuel combustion to produce an engine that emits no CO₂, no NOX and very low levels of engine-out particulates.

The concept is based around a technology developed at ITQ that uses ceramic structures known as mixed ion-electron conducting membranes (MIEC) to separate gases. This enables the intake air to be filtered into almost pure oxygen, removing the potential to form NOX. Furthermore, this frees powertrain engineers from the NOX/particulates trade-off that comes with conventional combustion. The small amount of particulate matter that remains is captured along with the CO₂ rather than released into the atmosphere.

“This technology works for both spark ignition [SI] and compression ignition [CI] engines,” explains Dr José Serrano, a professor at CMT. “We have been able to reduce particulates virtually to zero and CO far lower than a state-of-the-art engine in both cases. The exhaust gas is basically CO₂ and water. There are some hydrocarbons too, but we oxidize those in a simple catalyst.”

Another benefit of this process is that the resulting CO₂ is around 95% pure, which simplifies the task of capturing it. This is done by compressing the exhaust gas to the critical point where it liquefies. A condenser is used to remove the water vapor beforehand, and a pump maintains flow through the exhaust.

There are a variety of potential approaches to compressing the CO₂ itself, but the idea put forward by Serrano and his colleagues is to adapt one of the engine’s cylinders to act as a compressor. “We think it would be possible to retrofit this system to an existing engine using that principle,” the professor notes.

This would result in an engine that’s carbon neutral when run on fossil fuels. But perhaps the most exciting prospect is that of combining the engine with synthetic fuels.

“If you run the engine on synthetic fuels you can actually operate with negative emissions, so the vehicle would actively remove CO₂ from the atmosphere,” notes Serrano. “We can see a monetary value to that, with manufacturers being awarded CO₂ credits, plus the CO₂ itself has a value once it’s captured and compressed.”

Gas recirculation

As anyone who’s ever used an oxy-acetylene torch can attest, burning fuel in pure oxygen can result in extremely high temperatures. To prevent this, the engine feeds a significant portion of the CO₂ and water vapor – 60-70% – back into the cylinder as a form of EGR.

“The in-cylinder conditions are similar to a current state-of-the-art production engine,” comments Serrano. “The maximum temperature is around 1,000°C at full load, and maximum pressure is around 180 bar. We can also use a lot of conventional technology. For instance, the fuel injection hardware is completely unchanged; the combustion chamber geometry can be optimized, but it’s not radically different. The only thing that’s totally new is the MIEC, but the technology is quite similar to a DPF, so it’s something OEMs are already familiar with.”

Serrano says there’s the potential to establish a circular economy around CO₂, with the liquefied gas exchanged at filling stations when the vehicle refuels. The relatively high purity of the CO₂ produced by this process means it can be put directly back into a variety of industrial processes, including the production of synthetic fuels.

These are ambitious targets, but Serrano points out that oxy-fuel combustion and CO₂ recovery are both technologies that are already used in industry. The team’s original project successfully demonstrated the combustion side of the process in two working prototypes – a single-cylinder SI engine and a four-cylinder CI engine – leading to an international patent. A second project backed by the Valencian Innovation Agency is expected to begin shortly, looking at the CO₂ capture aspect.

“For me, the most significant finding to come from this work is that we have demonstrated zero NOXemissions, both theoretically and experimentally, and still maintained the BSFC [brake-specific fuel consumption] of a conventional engine,” comments Serrano. “And the high purity of the CO₂ shows the potential for zero tank-to-wheel emissions.”

With careful management, the oxy-fuel combustion process is said to significantly extend the knock limit in an SI engine. It also reduces the trapped mass, which enables the compression ratio to be increased without breaching the maximum cylinder pressure limits in a CI engine. Consequently, the team was able to raise the compression ratios from 11:1 to 22:1 on the SI engine, and from 16.2:1 to 28:1 on the CI engine.

This enabled an increase in peak torque from 410Nm at 2,000rpm to 560Nm at 1,600rpm for the CI engine. Peak power fell slightly from 172ps to 160ps, but BSFC at 2,500rpm increased from 211g/kWh to 219g/kWh.

Chemical processing

Carrying around your own chemical processing plant isn’t without its challenges, although there is a parallel here with existing aftertreatment systems, which the automotive industry has become very adept at producing and managing. At present, the additional hardware for the oxy-fuel system takes up around the same space as the engine itself.

The planned CO₂ capture system is not said to dramatically increase the overall footprint if it uses one of the existing cylinders, and its power consumption is projected to be a manageable 10% of the brake output. Storage still has to be taken into account, with every tank of fuel releasing roughly three times the volume of CO₂, but Serrano is confident that the system can be downsized for real-world applications.

“Initially, we see this technology being used in long-haul trucks, ships and locomotives because it’s quite a bulky system,” he says. “The packaging is obviously easier in a big ship, and when they return to port they have the infrastructure right there to discharge the CO₂. A lot of refineries where synthetic fuels are produced are already based in harbors due to the logistics of carrying crude oil.”

Serrano also believes the potential is there to use the system for automotive applications: “With the exception of a basic oxidation catalyst, you don’t need any conventional aftertreatment systems, so that will free up space and offset the cost. Ultimately, we can see this being used in passenger cars – maybe not as the only engine but as part of a hybrid system or a range extender with zero tailpipe emissions.”

Discussions have already taken place with a well-known truck manufacturer. Work has begun on the carbon capture system for the next phase of the project, and the team is targeting an aggressive timescale.

“We hope to have a fully tested TRL 8 technology covering both the oxy-combustion and the CO₂ capture by the end of next year,” says Serrano. “It will then be a question of costs, opportunities and regulations. Being optimistic, I think we could see a commercial vehicle with this technology in three years.”

There are still challenges to overcome, but this project raises the tantalizing prospect of zero-emissions propulsion from conventional liquid fuels. It’s another reason to question whether the demise of the internal combustion engine is as imminent as some would have us believe.

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