Subaru Developing Next-Gen Boxer Four-Cylinder Engine, Targeting 50% Thermal Efficiency

On the evening of November 12, 2026, Subaru clarified the development goal for its next-generation internal combustion engine: to raise thermal efficiency to 50%. Subaru is developing a new-generation horizontally opposed four-cylinder engine, and engineers have begun improving combustion efficiency, with a focus on enhancing in-cylinder airflow. Preliminary development results show that the in-cylinder airflow intensity generated by the new engine exceeds that of other engines used as benchmarks. Subaru has introduced three-dimensional combustion analysis technology to predict flame propagation within the cylinder after ignition. Data released by Subaru shows that, compared with the naturally aspirated engine used in current hybrid models, the new engine's high-thermal-efficiency operating range has expanded by about twofold. The new engine under development adopts in-cylinder direct injection and turbocharging technology. Subaru is also studying technologies such as variable-geometry turbochargers, electric motor assistance, and the Miller cycle. Subaru said that these technologies alone cannot achieve 50% thermal efficiency, so it is also considering pre-chamber combustion technology. BMW recently introduced pre-chamber combustion technology on the inline-six engines powering the M2, M3, and M4 sold in the European market.