Subaru Wants To Save The Gas Engine With A More Efficient Boxer

Subaru is developing a next-generation flat-four engine with an ambitious 50% thermal-efficiency target.

Next-generation Subaru hybrid boxer engine

Adrian Padeanu

By: Adrian Padeanu

Nov 12, at 5:36am ET

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The internal combustion engine isn't going down without a fight. Toyota, Mazda, and Subaru made that abundantly clear when they announced a joint effort to develop new combustion engines in 2024. Rather than sharing the same powertrain, each automaker would pursue its own technology: Toyota would work on a new family of smaller, more efficient four-cylinder engines, Mazda would continue developing its rotary and a new inline-four Skyactiv-Z, and Subaru would stick with its beloved horizontally opposed boxer.

Since then, Toyota and Mazda have offered more details about their respective plans. Subaru, however, has been relatively quiet about what comes next for its signature engine layout. That's finally changing, with the Japanese automaker providing a closer look at its next-generation boxer and revealing an ambitious target: 50 percent thermal efficiency.

The goal isn't simply to keep the flat-four alive for nostalgia's sake. Subaru wants to make it significantly more efficient without sacrificing the driving characteristics that have defined its cars for decades. Engineers are looking for ways to improve combustion without fundamentally changing the layout. They've been working on creating stronger airflow inside the cylinders, helping air and fuel mix more thoroughly before ignition.

Next-generation Subaru boxer engine

Next-generation Subaru boxer engine

Subaru Doubles The High-Efficiency Operating Range

Japanese magazine Motor-Fan attended a Subaru briefing about its next-generation engine, where the company revealed promising early results. Despite retaining a relatively short stroke, the development boxer already generates stronger airflow inside the cylinders than competing benchmark engines. Subaru has also introduced three-dimensional combustion analysis to its development process. This allows engineers to predict better how flames spread inside the cylinders after ignition, making it easier to anticipate knocking and optimize combustion.

The results are encouraging. According to data presented by Subaru, the development engine has roughly twice the operating range with high thermal efficiency compared with the naturally aspirated engine currently used in its hybrid models.

That's important because an engine shouldn't deliver impressive fuel economy only under specific driving conditions. Expanding its efficient operating range should help improve real-world consumption across a wider variety of situations.

The prototype is a direct-injection turbocharged engine, whereas the reference powertrain is naturally aspirated. Forced induction also gives Subaru greater control over the amount of air entering the cylinders, helping deliver stronger torque across a broader operating range.

Next-generation Subaru boxer engine

Next-generation Subaru boxer engine

Subaru's Target: 50 Percent Thermal Efficiency

Subaru isn't stopping there. Engineers are exploring several additional technologies that could help push thermal efficiency closer to the 50 percent target.

One possibility is a variable-geometry turbocharger (VGT), which adjusts the turbine's flow passages depending on exhaust gas volume. This could improve responsiveness, although Subaru acknowledges that electric motor assistance could also help compensate for turbo lag.

The company is also considering the Miller cycle, which changes intake valve timing to achieve a more favorable compression-to-expansion relationship. Besides improving efficiency, this approach lowers exhaust temperatures, potentially making a variable-geometry turbocharger easier and less expensive to implement.

However, Subaru admits that these technologies alone probably won't be enough to reach 50 percent thermal efficiency. That's where pre-chamber combustion comes in, as BMW recently introduced it on the inline-six powering the M2, M3, and M4 sold in Europe to meet stricter emissions regulations.

Next-generation Subaru boxer engine

Next-generation Subaru boxer engine

The New Boxer Engine Is Coming... Eventually

That said, there's no word on when the next-generation boxer will reach production or which models will get it first. Subaru is still working through the technical challenges, and the 50 percent thermal-efficiency figure remains an ambitious development target rather than a confirmed specification.

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Still, it's encouraging to see Subaru investing in an engine layout that has become synonymous with the brand. The company could have taken the easier route and moved away from its horizontally opposed engines, but it clearly wants to preserve that identity while making its powertrains more efficient.

There's an interesting contradiction in Subaru's strategy. While the company is investing in a more efficient boxer engine for the future, it's moving in the opposite direction in Europe. Subaru has announced plans to transition to an electric lineup on the continent by 2028, effectively ending sales of new gasoline-powered models. The boxer might remain central to the brand's identity in other markets, but its days in European showrooms are numbered.