The BOXER DIESEL engine comes equipped with a turbocharger. Engineers were confident that it's a winning combination for the enhancement of driving pleasure. A diesel engine, because of its more precise control of combustion, is less prone to knocking and detonation than a gasoline engine. This factor makes the turbo system a good fit with the BOXER DIESEL when optimal designs and adjustments are applied to the turbocharger.
During the initial testing of the turbocharged BOXER DIESEL, an obvious issue surfaced: the lack of torque at low speeds. Toshio Masuda, General Manager, Subaru Product & Portfolio Planning Division summarized: “We were not able to demonstrate the best of the BOXER DIESEL. Maybe because we didn’t have extensive experience in diesel engines then, and we didn’t quite grasp the essence of diesel engine performance, which is largely characterized by its powerful generation of torque. Even though, we did realize that the BOXER DIESEL should generates ample torque at low engine rotations of 1000 rpm and it should retain the power to accelerate when we drive slowly or shift down a gear, so we can drive on under any conditions. Once we reached that understanding, our development work picked up speed.”
Subsequently, the Subaru engineers took the BOXER DIESEL to Spain for long-distance road tests. The engine was much improved by then, and the unique revving sounds of the BOXER DIESEL were heard which cannot be heard with the inline diesel engines. While the engine still struggled on mountainous roads, other benefits became apparent: the BOXER DIESEL gave outstanding powerful performance in the high-speed ranges, between 2500 rpm and 4000 rpm, which no other conventional diesel engine had demonstrated. The engineers were excited that their new diesel engine would soon overtake conventional diesel engines, as soon as its amazing driving performance in the high-speed range was recognized by drivers and the low-speed performance issue was fixed.
To remedy the lack of torque in low-speed range, design changes were made in intake and exhaust ports and channels, and specifications for valve timing and injectors were modified. At the same time, tuning of the turbo system was also reviewed. The turbocharger itself is important, but the optimal tuning in relation with the engine is more crucial. “Subaru has a long history of developing a high-power turbocharger for the World Rally Championships, and it’s one of Subaru’s core technologies. So what we focused on most was applying that technology to fit with the new BOXER DIESEL,” Masuda recalled. He stressed that the BOXER DIESEL builds on the accumulated expertise gained with turbocharged gasoline boxer engines. Subaru and its suppliers worked on revision after revision to specifications for the compressor, the turbo’s capacity, and the turbine blades, until they found the best matches for that engine.
Lastly, the turbo system on the Legacy employs a variable nozzle. When the nozzle vane is closed, the flow gets narrower and faster, boosting torque in the low speed range below 1800 rpm. On the other hand, at high speeds with high engine rotations, the vane opens to reduce resistance in the flow, which also helps lower fuel consumption and contributes to fuel economy. While delivering ample torque at low speeds, the Turbo BOXER DIESEL exhibited smooth acceleration at high speeds—the engineers had achieved another breakthrough!
The turbocharger is attached under the front part of the engine, a position closer to the exhaust port. This layout not only serves to make emissions cleaner, but further ensures optimal placement of the center of gravity of the powertrain. To finalize the layout, more tests were conducted to make sure the diesel particulate filter (DPF) was properly installed and that the low-speed and high-speed characteristics were retained.
While taking environmental considerations fully into account, the engineers applied dynamic modifications to the Turbo BOXER DIESEL to set Subaru’s model apart from other diesel engines.
