The radical change in the piston of the Subaru engine began four years ago with the second generation Legacy, when the Subaru piston underwent a change in its shape and style. Its rebirth was heralded as a "piston revolution."
Toward the twenty-first century, an engine with lower harmful emissions and reduced energy consumption has become highly sought after. Even Subaru's Horizontally- Opposed Engine, which has undergone continuous improvement over the last forty years required thorough re-examination of the smallest details.
The engine can be called the heart of a car and the piston is the most important part of an engine.
The piston's job is tough and complex. It plays a role in compression, combustion, exhaust, and air intake through its up-down motion. For instance, a piston which executes 6000 revolutions in one minute and in the same time achieves speeds of about 90 km/h (approximately 50 mph) . This means, in a fraction of a second, the piston accelerates from zero to 90 km/h and then returns again to zero and this extremely vigorous motion reoccurs 6000 times a minute. A further example of the piston's brutal motion is a force of nearly 7 tons at the instant of combustion for it to generate 200 hp. The piston works tirelessly under an unrelenting motion. A mass-produced piston needs to be capable of undiminished durability and reliability even after 500, 000 kilometers and 30 years of use. Pistons were not parts that underwent revolutionary changes and mass produced pistons were designed based on conservative empirical engineering principles.
Tatsumi Obayashi, in charge of Subaru Powerunit Research and Experiment Dept., Subaru Engineering Division at the time of this groundbreaking revolution in pistons, reflects;
"The ideal piston is light and moves smoothly. This ideal is something we resolutely strive for."
Development of the piston took one and a half years. Experimentation was repeated many times to reach the conclusion that "a piston that moves up and down in a smooth rolling-like motion is ideal." The piston can be thought of as sliding up and down smoothly inside the cylinder. However, in reality the motion is frenetic. The piston hits the inside of the cylinder wall randomly in a number of places and this secures its up-down movement. Contact between the piston and the cylinder wall cannot be avoided. In view of this, can the violent motion of the piston be a smooth rolling-like motion?
Through experimentation, the engineers learned that when the piston is slightly barrel-shaped, the piston makes a smooth, rolling-like up and down motion, while friction, and vibration and noise are reduced.
On the other hand, in order to reduce weight, we focused on improving the piston skirt. In former technology, a long thick skirt was considered necessary to provide adequate rigidity for the piston and ensure a precise up and down motion. However, the engineers challenged this with the opposite concept of a shorter, thinner and softer skirt, which can change shape freely like a spring.
"We requested a short, thin, soft prototype skirt from the piston manufacturer. They wondered if we should even attempt such a thing. The concept departed from conventional wisdom to that extent," laughs Mr. Obayashi.
In a break with the conventional wisdom, the world's shortest, thinnest and softest skirt is the most appropriate for "a piston that moves up and down with a smooth, rolling-like motion" When it came to accumulating the fundamental technology for this new piston, a range of improvements and refinements for details were made possible.
The compressed gas that builds up in the small spaces between the piston head above the compression rings and the inner wall of the cylinder is difficult to combust. This unburned gas is contaminated with exhausted gas and reduces fuel economy. To prevent this problem, the gap between the piston head and the inner wall of the cylinder was made smaller. For this reason, it was necessary to reduce the amount by which the piston pin is offset. To reduce the offset it was necessary to align the weight with an piston to adjust for the center of gravity.
The finishing touch was the coating process. To ensure that the grooves of the piston rings do not wear, they were treated with alumite. Since it is difficult to apply a coating only to the grooves of the piston ring, the entire piston head was treated with alumite. The skirt was coated with molybdenum to reduce frictional resistance. This results in a two-toned piston. The former piston was a mono-tone aluminum-gray.
The newly developed piston enhances fuel economy remarkably. Running fuel economy is improved 0.5 percent and idling fuel economy is improved 2.3 percent. Since fuel combusts well, fuel economy is improved, and at the same time, the exhaust gases are cleaner. This piston has been named the low fuel (LF) piston.
Currently, the LF piston has been adopted on the Legacy, Impreza and the minicar, Pleo (Japanese domestic model). Recently, the engines of European manufacturers and Formula 1 cars have adopted pistons with similar development concepts to the LF piston. These are not copies of the Subaru piston but the technology is similar due to the similar direction and aims in development. Subaru developed its piston technology a little faster than other companies.
The LF piston has a pleasing shape and a stylish two-tone color much like a precious metal. In the search for a technical rational for one part, a beautiful shape and color is certain to result. The Subaru piston provides evidence for the aesthetic nature of the technology.