The initial development work for the world’s first BOXER DIESEL for passenger cars began in 1999 with a team of only four engineers. By that time diesel engine performance had been dramatically improved through technological advances in the control of combustion, and the diesel engine had proven to be a promising power unit with even less carbon dioxide emission. Automobile manufacturers around the world were then in the midst of an increasingly keen competition in developing high-performance diesel engines. When Subaru started its development project for the BOXER DIESEL, we soon realized that we were in an unprecedented, unchartered area in diesel engine development and were undertaking a technological challenge for which no benchmarking comparisons existed.
Compared to those in a gasoline engine, the dimensions of bore and stroke differ in a diesel engine. The difference between the two stems from dissimilar combustion mechanisms: the combustion chamber in the diesel engine is usually more compact to facilitate compression ignition by injecting fuel directly into the combustion chamber.
The diameter of the piston is made longer to increase gasoline engine displacement. In the diesel engine, however, enlarging the bore leads to a bigger chamber, which doesn’t support efficient combustion. To address this issue, the bore is made shorter and the stroke longer in a conventional inline diesel engine to increase engine displacement, yet, in the layout of the BOXER DIESEL, which features horizontally opposed cylinders, an elongated stroke would widen the engine, making it impossible to fit it under the hood.
Consequently, at the beginning of our project, when Subaru discussed the development of the BOXER DIESEL with allied automakers or a research institute specialized in diesel engines, they concluded that the boxer configuration is not structurally suitable for diesel engines. Subaru then also considered the possibilities of developing a new inline diesel engine or purchasing a diesel engine from another manufacturer. In light of this situation Subaru engineers reflected on our fundamental philosophy of developing Subaru brand identity. What sets Subaru apart from others is the BOXER engine in combination with AWD technology, which provides outstanding driving performance coupled with safety and environmental features. The engineers were convinced that this unique brand identity must be incorporated in the new diesel engine.
Satoshi Maeda, a general manager of the Engine Design Department who led the development team, remembers that conviction. “It made no sense that we would just create a new 2-liter diesel engine without thinking about the Subaru brand identity. Diesel engines were already considered more economical and durable than gasoline engines, and the need for developing diesel engines was great. But we believed that an innovative diesel engine with our brand identity would generate high demand–and that the SUBARU BOXER DIESEL would distinguish itself from diesel engines made by Mercedes or one by Audi. We were sure that ours must be in the boxer configuration.”
Along those lines, advantages of the boxer engine were further discussed. Diesel engines generally provide more torque, but they produce more noise and vibration than gasoline engines, because combustion pressure created inside a diesel engine is twice as powerful as that of a gasoline engine. Almost all inline diesel engines come with balance shafts to reduce noise and vibration. In addition, diesel engines are generally larger and heavier than gasoline engines, since they must withstand strong combustion pressure. Placing a heavier engine in a car affects the weight distribution of the vehicle in motion and may impinge on agility. The BOXER engine, however, cancels out inertial forces that cause vibration and noise, thanks to their horizontally opposed pistons. The compact and simply designed BOXER requires no balance shafts. The BOXER engine configuration is also structurally rigid. Unlike inline diesel engines, the BOXER DIESEL features low vibration, a low center of gravity, and high rigidity. The advantages of the BOXER DIESEL became obvious to the engineers, which further encouraged them in their endeavors.
The first BOXER DIESEL prototype was completed in 2004. In November of that year, a trial model was placed on the test bench and started. The engineers observed stable idling, low vibration, and powerful boost. They were convinced of the success of their BOXER DIESEL.