Auto giant Stellantis and Saudi state oil company Aramco have confirmed through testing that e-diesel and e-gasoline — made using renewable hydrogen and captured CO2 — are compatible with 24 engine types that are used across 28 million existing vehicles in Europe.

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This would appear to prove that these e-fuels can be utilised as a “drop-in” fuel, despite having some differences in chemical properties compared to their fossil equivalents.

Stellantis’ European head of propulsion systems, Christian Mueller tells Hydrogen Insight that e-fuels “are not completely identical” when it comes to density and inflammation points, although they are within the specification frame of engines.

“Hence, we wanted to verify that indeed no side effects would occur, neither on emission[s], on-board diagnosis, long-term durability or other items, such as startability in extreme conditions,” he says.

The French automaker — which owns brands such as Fiat, Chrysler, Citroen, Peugeot, Vauxhall, Opel, Alfa Romeo and Maserati — is testing a further four engine types, although the 24 tested “make up the largest portion of the Stellantis’ legacy vehicle fleet in Europe”, Mueller explains.

All 28 of these engines are designed to the Euro 6 emissions standard, which measures carbon monoxide, nitrogen oxide (NOx) and particulate matter released by internal combustion engines.

While this may confirm the claim that e-fuels can be used in existing engines — and thereby reduce transport emissions in the short term without requiring new vehicles to be purchased — these fuels have been criticised by environmental groups as not only inefficient in terms of renewable electricity use and extremely expensive, but also continuing to emit net greenhouse gases.

Every 100kWh of renewable electricity used to produce e-fuels supplies only 13kWh on the road, due to energy losses in production and conventional engines, according to European non-profit Transport & Environment (see chart below). This compares to 77kWh of energy on the road in battery-electric vehicles for every 100kWh of renewable electricity.

In other words, almost six times more renewable energy would be needed to run road vehicles on carbon-neutral e-fuels compared to batteries.

Earlier this year, the EU relaxed a proposed 2035 ban on sales of internal combustion engine vehicles, allowing them to continue to be sold as long as they only run on synthetic fuels.

The differing efficiencies of batteries v hydrogen v e-fuel in passenger cars. Photo: Transport & Environment

Stellantis itself aims to sell 100% battery-electric vehicles in Europe by 2030, although it is unclear whether this precludes sales of fuel-cell electric versions of its vehicles.

However, Mueller told a briefing earlier this month that, based on the estimated lifetime of a passenger cars or light-duty vehicles sold by the firm, internal combustion engines would still be in use up to 2050, necessitating carbon-neutral fuels.