Toyota, Subaru, and Mazda on Tuesday announced a joint effort to improve efficiency and reduce emissions from internal-combustion engines.

The development effort will take a number of different approaches, including smaller engines, according to a press release from the automakers. These engines will also allow for lower hoods, opening up new design possibilities as well as allowing for aerodynamic styling that can further improve efficiency, the automakers noted.

The automakers also said they would “aim to optimize integration with motors, batteries, and other electric drive units” for hybrid powertrains. Driving home this point, Subaru brought a camouflaged prototype of a next-generation Crosstrek Hybrid to the presentation of the joint development effort.

A third main area of focus is alternatives to conventional gasoline and diesel. New engines will be able to run on what the automakers describe as carbon-neutral fuels, including synthetic fuels (also known as e-fuels), biofuels, and liquid hydrogen.

Mazda rotary range extender

Toyota has already been developing hydrogen combustion engines for racing, and it’s said that it sees a hydrogen-combustion Corolla Crossas an EV alternative. In both cases, hydrogen is burned in an engine in place of conventional fossil fuels. This does produce some emissions—including NOx, which was at the core of the VW diesel scandal.

Synthetic fuels have received some support from other automakers, and also benefit from a loophole in European Union emissions regulations that otherwise call for a ban on new internal-combustion vehicle sales by 2035. But for now, the math simply doesn’t work out well in favor of synthetic fuels—they cost more and result in more CO2 emissions than conventional fuels.