Nissan is developing an engine for road cars that could reach 50% thermal efficiency, meaning the potential for much more fuel-efficient cars.

An engine's thermal efficiency is a measure of how much of the heat energy generated by combustion is actually converted into useful work. A good rating for a road car is about 40% whereas 50% is a rating normally associated with the best Formula One engines.

Nissan said Friday that an engine designed for road cars could achieve 50% thermal efficiency, but there's a catch: the engine would need to be running at a constant rpm, for example when used as a generator in an extended-range electric vehicle.

In a conventional car, an engine experiences a wide variety of rpm and loads, which prevents it from running in its most optimal state due to the rich air-fuel mix required at certain rpm levels. This isn't the case for an engine acting as a generator, which can constantly run at a level that allows a lean air-fuel mix.

But Nissan plans to further improve the efficiency of an engine running at a constant state by optimizing in-cylinder gas flow (the flow of the air-fuel mixture that is pulled into the cylinder) and ignition, and as a result enable the engine to reliably burn an even leaner air-fuel mix at a higher compression ratio. By also adding heat energy recovery systems, Nissan managed to achieve a 50% thermal efficiency, the automaker said, without providing further detail.

Such an engine could be used in one of Nissan's E-Power models. Nissan's E-Power models are essentially electric vehicles with batteries that are charged on the go via a combination of energy regeneration and a small engine serving purely as a generator. Electricity generated by the engine can also be sent directly to the vehicle's electric motor(s) during high-load situations.

The first E-Power model was the Nissan Note E-Power subcompact hatchback launched in Japan in 2016. More are coming, including from Nissan premium brand Infiniti. Nissan hasn't said when we'll see one with 50% thermal efficiency, however.