The BOXER engine has a structure that the cylinder block is connected from each side at the center of the engine, which basically has a characteristic of rigidity and less vibration. To develop a BOXER DIESEL, Subaru engineers have ingeniously implemented new technologies to showcase these intrinsic benefits of the BOXER engine.
To further bring out the driving performance, safety, and contribution to the environment that are unique to Subaru, the engine needs to be light weighted and compact. So our engineers decided to use aluminum to make the BOXER DIESEL. However, the combustion pressure in a diesel engine amounts to twice of that in a gasoline engine. To counter this problem, a hybrid structure of aluminum and iron has been adopted.
These methods are also used in inline engine designs but cast irons are often used in bearings on the bottom where high pressure is applied. In the BOXER DIESEL, due to the high pressure applied on both sides of the cylinder block, all crankshaft journals are also made in combination with cast irons. It is not easy to retain rigidity when combining two very different materials: namely, cast iron and aluminum. Even when computer simulations yield a perfect balance between steel and aluminum components, the actual trial models sometimes produce inconsistent results.
Tatsumi Obayashi, a manager in the No. 2 Power Unit Design Department recalls, “We did check the computer simulation results, but also confirmed those result by testing engine after engine for troubleshooting, which was an unrefined way to do. We sometime used all of the test benches at our test facilities to run three or four engines simultaneously for 500 to 1,000 hours. Time was what we needed most then.”
The BOXER DIESEL adopts a semi-closed cylinder block deck to improve the rigidity around the head gaskets, following the precedent of the semi-closed type used in the Subaru EJ20 turbocharged gasoline engine.
To realize the rigid and reliable BOXER DIESEL, improvements to the manufacturing processes have been constantly made. At the assembly line at the Oizumi Plant, quality is thoroughly monitored. For example, engine oil leaks are checked by injecting fluorescent liquid into the engine and applying pressure to the fuel line. In addition, not only torque control was used in tightening bolts, but additional method is also applied: tightening bolts by a certain angle to ensure precision in the tightening process.
The common rail fuel injection system is used in the BOXER DIESEL. Diesel engines in general feature very high injection pressure, around 1800 mini bars, which is 600 times higher than in gasoline engines. This level of pressure is equivalent to a fingertip supporting an elephant that weighs 5 tons. Therefore, not even a small scratch, dent, or any contamination can be introduced into the chamber at any point in its assembly. Utmost attention is paid to the fuel injection system on the assembly line of the BOXER DIESEL.
The BOXER DIESEL features an even compact cylinder block than that of an equivalent gasoline BOXER engine, thanks to rigorous tests, analyses, and numerous improvements in design by Subaru’s engineers. In order to reinforce rigidity, an easy path might have been to fabricate thick walls. But the engineers never compromised and steadfastly pursued their goal of making a lightweight engine to enable the ideal driving performance of a Subaru diesel car.
The Development of a rigid and strengthen BOXER DIESEL exaggerated even more of the characteristic of the BOXER engine like seamless rotation and less vibration. By minimizing its moment of inertia and reducing inner friction, the BOXER DIESEL has achieved smooth acceleration and outstanding fuel economy.
