Newshot Subaru's New-Generation Horizontally Opposed Four-Cylinder Engine Is in Development, Targeting 50% Thermal Efficiency
IT之家 November 12 — According to a report from foreign media outlet motor1 this evening (the 12th), Subaru has set clear goals for its new-generation internal combustion engine: raising thermal efficiency to 50%.
Subaru's insistence on developing horizontally opposed four-cylinder engines is not simply about carrying on nostalgia, but about substantially improving engine efficiency while preserving the distinctive driving characteristics its models have had for decades.
Rather than radically changing the engine layout, engineers set out to improve combustion efficiency, with one focus being to strengthen in-cylinder airflow so that air and fuel mix more thoroughly before ignition.
Initial development results show that although the piston stroke remains short, the new engine generates stronger in-cylinder airflow than the other engines used as benchmarks.
Subaru also introduced three-dimensional combustion analysis technology to help engineers more accurately predict how the flame propagates inside the cylinder after ignition, so they can assess knock risk in advance and optimize the combustion process.
Initial tests produced good results. 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 is roughly doubled.
The new engine under development uses direct injection and turbocharging, while the current engine used for comparison is naturally aspirated. Turbocharging allows engineers to control the intake air volume more precisely and also helps the engine deliver strong torque across a wider range of operating conditions.
In addition, Subaru is studying other technologies in hopes of moving further toward the 50% thermal efficiency target.
One direction is the variable-geometry turbocharger. It can adjust the turbine's gas passages according to exhaust flow to improve response. But this is not the only solution — Subaru is also considering electric motor assistance to offset turbo lag.
The Miller cycle is also under consideration; it adjusts intake valve timing to improve the engine's compression and expansion relationship, raising combustion efficiency while lowering exhaust temperature. At the same time, the Miller cycle can make variable-geometry turbochargers easier to apply and lower in cost.
However, Subaru acknowledges that these technologies alone may not be enough to achieve 50% thermal efficiency, so it is also considering pre-chamber combustion technology. IT之家 learned from the report that BMW recently introduced this technology on the inline-six engines in the M2, M3 and M4 sold in the European market to meet stricter emissions regulations.
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