Quick look
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HORSE and Repsol have developed a 1.2-litre hybrid engine reaching 44.2% thermal efficiency, using an ultra-high 17:1 compression ratio to maximize power from every drop of fuel.
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The engine, to be presented in the near future, is designed to run on 100% renewable petrol, making the powertrain a ‘technology-neutral’ solution.
It seems a coincidence that as the Year of the Horse starts, a company called HORSE Powertrain is also in the news. This is a relatively new company formed to specialize in hybrid electric and combustion engine powertrain development for supply to interested automakers.
Established in 2024 and headquartered in London, it operates as a joint venture with Renault Group and Geely each holding a 45% stake, while Saudi Aramco holds the remaining 10%. “HORSE Powertrain Limited will have the portfolio, scale and capacity to offer the low-emission solutions the automotive industry of tomorrow demands,” said Eric Li, Geely Holding Chairman.
Through its Horse Technologies division, the company has collaborated with energy firm Repsol to unveil a next-generation hybrid powertrain developed specifically to operate on 100% renewable petrol. This development is the HORSE H12 Concept engine, an evolution of the existing 1.2-litre 3-cylinder HR12 architecture.
Exceptional thermal efficiency
The engineering teams from Horse in Valladolid and Repsol’s Tech Lab in Madrid focused on significant enhancements to the combustion system to maximize energy extraction. The result is a stated peak brake thermal efficiency (BTE) of 44.2%. In internal combustion engineering, BTE represents the percentage of fuel energy converted into usable mechanical work; typical modern production gasoline engines generally hover around the 35% to 40% mark.
17:1 compression ratio
To achieve these gains, the H12 Concept utilizes an exceptionally high compression ratio of 17:1. In a standard petrol engine, such a high ratio would typically lead to knocking or pre-ignition, where the fuel-air mixture ignites prematurely due to the heat generated by intense compression. Horse and Repsol have addressed this by optimizing the engine specifically for the properties of Repsol’s 100% renewable fuel, which allows for more stable combustion under high pressure.
Thermodynamically, a higher compression ratio increases the pressure and temperature at the moment of ignition, allowing the engine to extract more work during the expansion stroke. This longer effective expansion cycle ensures that more of the heat energy is converted into mechanical motion rather than being lost as waste heat through the exhaust.
The H12 architecture integrates several critical sub-systems to support this high-compression cycle. A sophisticated next-generation Exhaust Gas Recirculation system is used to re-introduce cooled exhaust gases into the intake. This dilutes the intake charge, which lowers the peak combustion temperature and reduces the likelihood of knocking, while also minimizing pumping losses at partial loads.
High-energy ignition system
To ensure reliable combustion of these diluted or high-compression mixtures, a high-energy ignition system is employed. This provides a more powerful spark to initiate a faster, more consistent flame front, which is essential for maintaining stability when the engine is pushed to its efficiency limits. Furthermore, a new-generation turbocharger has been tuned to work in tandem with the high compression ratio, ensuring the engine maintains its efficiency across a wider range of operating conditions.
Efficiency is gained not only in the combustion chamber but through the reduction of internal losses. Repsol developed specialized low-viscosity lubricants tailored for the H12 to minimize parasitic friction between moving parts. Additionally, the powertrain features an improved hybrid gearbox with updated energy management software, designed to keep the engine operating as often as possible within its sweet spot — the narrow band where that 44.2% peak efficiency is achieved.
According to WLTP testing, this technical package results in a fuel consumption figure below 30.3 kms/litre. When operating on 100% renewable fuel — produced from organic waste and residues — the net CO₂ reduction is significant. The companies calculate that a mid-sized car driving 12,500 kms annually would emit 1.77 tons less CO₂ per year than a comparable vehicle using traditional fossil fuel.
Technology-neutral approach
The project emphasizes a technology-neutral approach, arguing that with more than 97% of Europe’s current vehicle fleet still being combustion-based, highly efficient engines using renewable fuels offer an immediate path to decarbonization.
Repsol has already commenced industrial-scale production of its Nexa 95 renewable petrol, which is compatible with existing vehicle engines without modification. A demonstrator vehicle featuring the H12 powertrain is scheduled for presentation in the near future.
