The first hurdle in the BOXER DIESEL’s development was to shorten bore and elongate stroke. Horizontally opposed gasoline engines gain shorter stroke and larger bore as engine displacements increase. But with regard to the BOXER DIESEL, its combustion chamber must be made compact to optimize combustion. To increase engine displacement, its bore size must be made small, and its stroke longer, yet an elongated stroke results in expanded engine width, ruling out an easy fit in the engine room.
The first BOXER DIESEL trial model was a 1.3-liter displacement engine, which was then expanded to 1.7 liters after numerous improvements, including placing the injector in the cylinder head at an angle. The challenge was to develop a 2-liter model, since Subaru wanted to make inroads into the popular 2-liter diesel engine market in Europe. In fact, 80% of diesel cars there are 2 liter models.
A breakthrough came after reviewing the manufacturing processes of the boxer gasoline engine. Normally the piston is inserted through a service hole in the cylinder block and is connected inside the block. For the assembly of the BOXER DIESEL, however, the piston and the connecting rod are inserted inside the engine together, and then a cap is placed. This assembly eliminates the service hole entirely. The design of a shorter connecting rod inserted at a certain angle is also seen in the 6-cylinder, 3.6-liter EZ engine on the Tribeca model, which allows optimal stroke within the limited width of the engine.
To contain the width of the BOXER DIESEL within limited space, its cylinder head was designed to be 17 mm thinner than that of the gasoline boxer engine, while still ensuring rigidity in the combustion chamber. Intake and exhaust ports are laid out almost horizontally, but they have been shaped to effectively generate a swirl inside them. The injector, which is positioned at almost 90 degrees to the cylinder, is about 40 to 50 mm shorter than the one used in the conventional 2-liter inline diesel engine to perfectly contain it inside the cylinder head. The BOXER DIESEL features bore and stroke dimensions of 86mm x86mm, by optimizing the shape of its combustion chamber and piston rigidity.
The BOXER DIESEL is revolutionary in that the common conception of the heavy and large diesel engine was reversed. The BOXER DIESEL did away with reinforcements for withstanding high combustion pressure, and it requires no balance shafts for vibration reduction. Maeda stated: “Our goal from the start was to take advantage of the boxer engine configuration and make a compact and lightweight BOXER DIESEL. When a heavy engine is mounted, a car gets heavy in front, which affects its drivability. One of our challenges in development was to make the BOXER DIESEL even more compact than the gasoline boxer engine.”
The BOXER DIESEL is almost as wide as an equivalent gasoline boxer engine, despite its elongated stroke, which is 19 mm longer than that in the gasoline boxer. Its length is 61.3 mm less. Its compactness is revolutionary. One can easily observe it inside the engine compartment of the Legacy model.