Horse H12 Concept and Repsol: 44.2% thermal efficiency and under 3.3 l/100 km

Gareth Wainwright • June 09, 2026 20:51

We have previously covered the potential of this hybrid engine from Horse Powertrain, but there are now hard figures worth paying attention to. The new H12 Concept - created by the Horse Technologies division in collaboration with Repsol - reaches 44.2% maximum brake thermal efficiency (Brake Thermal Efficiency). Until recently, numbers like this were largely the preserve of the most advanced diesel engines.

Another figure stands out even more: Horse is claiming WLTP consumption below 3.3 l/100 km. The company says that equates to a 40% reduction versus the average of new cars registered in Europe in 2023.

44.2% peak efficiency

Brake thermal efficiency indicates how much of the fuel’s energy is actually turned into useful work. In a typical petrol engine, results in the 35% to 38% range are already considered very strong. Pushing beyond 44% puts the H12 Concept - an evolution of the HR12 three-cylinder used across several Renault Group models - into territory rarely seen outside diesel.

Horse H12 Concept: the engineering behind 44.2% peak efficiency

To achieve this, Horse has used a very high 17:1 compression ratio - comparable to diesel - alongside a new exhaust gas recirculation (EGR) set-up, an optimised turbocharger, high-energy ignition, and a hybrid gearbox. On top of that, Horse focused on lowering internal friction, including new lubricants developed in partnership with Repsol.

Repsol renewable fuels and the CO₂ savings claimed

Work between Horse and Repsol also led to the use of renewable fuels from the Spanish company. According to both, an average car fitted with this hybrid system and covering 12,500 km per year will be able to save 1.77 tonnes of CO₂ compared with an equivalent conventional car.

This matters because, as Horse points out, around 97% of Europe’s circulating vehicle fleet is still made up of internal combustion models. The company argues that solutions like this can deliver an immediate reduction in CO₂ emissions without having to wait for the fleet to be replaced by a different technology.

“The Horse H12 Concept is an example of how highly efficient engines and renewable fuels can cut emissions today, without needing to wait for future solutions.”

Patrice Haettel, COO Horse Powertrain and CEO Horse Technologies

When will it arrive?

Horse and Repsol have already built two prototypes to validate the performance of the H12 Concept engine. The next step is to fit it to a demonstration vehicle, which is expected to be unveiled later this year. Only then does the route to industrial production open up.