Subaru aims to raise boxer engine efficiency to 50%, but it may never go on sale in Europe

Subaru looks to the combustion process for greater efficiency

According to a presentation of Subaru’s development work reported by Japanese publication Motor-Fan, engineers want to retain the boxer engine’s low centre of gravity while significantly improving its thermal efficiency.

The boxer engine is distinguished by its low, wide layout, with pistons moving in opposite directions. This helps keep the car’s centre of gravity low, but makes it more difficult to use a long piston stroke.

A longer stroke often allows improvements in combustion chamber geometry and efficiency. Subaru therefore has to seek alternative solutions.

The new experimental engine has straighter, narrower intake ports than before to increase airflow speed and improve fuel-air mixing.

The engineers also use three-dimensional combustion simulation to track flame-front propagation, assess the risk of knock and optimise ignition timing.

Initial tests indicate that the turbocharged engine’s high-efficiency operating range is approximately twice as wide as that of the current naturally aspirated reference engine.

Variable-geometry turbocharger and pre-chamber

Subaru is considering several technologies to achieve 50% efficiency.

One is a variable-geometry turbocharger, which can adjust the flow of exhaust gases to the turbine to suit the engine’s operating conditions. This could improve response at low engine speeds and maintain higher efficiency across a wider load range.

Electric boosting is also under development as an option to help reduce turbo lag.

Another solution is the Miller cycle, in which the engine adjusts intake valve closing timing to achieve a more favourable relationship between the effective compression stroke and the expansion stroke.

However, Subaru sees the greatest potential in a pre-chamber combustion system. In this arrangement, the fuel-air mixture first ignites in a small chamber, then jets of burning gas ignite the mixture in the main chamber at multiple points.

This approach could accelerate combustion and allow the use of a leaner fuel-air mixture.

Subaru acknowledges that combining these technologies does not yet guarantee the desired result. 50% is a development target, not an efficiency figure already achieved.

Toyota and Mazda pursue the same goal

Subaru’s development work is part of an initiative announced jointly with Toyota and Mazda in 2024, under which the Japanese manufacturers decided to continue developing more efficient internal combustion engines.

Each company nevertheless chose a different technical approach.

Toyota is developing compact 1.5- and 2.0-litre engines suitable for hybrid powertrains. Mazda is focusing on Skyactiv-Z combustion technology, while Subaru remains committed to the boxer engine.

Subaru’s approach is technically more complex than further developing a conventional inline engine, but helps preserve the engine layout characteristic of the brand.

The new engine’s real value would be most apparent in a hybrid powertrain, where an electric motor allows the petrol engine to operate more frequently in its most efficient load range.

Subaru has already confirmed that it is developing new internal combustion engines specifically to meet the needs of electrified powertrains.

European buyers may miss out on the new engine

The biggest contradiction in Subaru’s strategy concerns Europe.

The company is preparing to end sales of models with internal combustion engines, including hybrids, in Europe by the end of 2027. From 2028, its European model range is expected to focus on the all-electric Solterra, Uncharted and E-Outback.

This means the new boxer engine is being developed primarily for North America, Japan and other markets where petrol and hybrid vehicles retain greater importance.

Subaru has not confirmed plans to introduce the new engine in the European market.

The real value of Subaru’s plan lies in improving the boxer engine’s efficiency, rather than merely preserving its historic layout. If the company achieves 50% thermal efficiency, the new engine could help reduce hybrid cars’ fuel consumption without a larger battery.

However, Subaru must demonstrate that the more complex combustion technology will not make the engine too expensive or compromise reliability. The start of series production, engine displacement and final performance figures have not yet been disclosed.