Value & Value springs
The valve and valve spring are parts embedded in the engine cylinder head. They are part of the so-called valve train system, which is a small but extremely important part, with many contrivance. Listening to the story about parts like these is a pleasure for a car enthusiast.
"In relation to its layout, the Horizontally-Opposed engine places constraints on the design of the bore-stroke, compared to the in-line engine. So the function of the valve train system is extremely important in the Horizontally-Opposed engine. However, through the high technology of valve train system development, Subaru has succeeded in creating the world's most compact cylinder head for the DOHC engine," says Yasunori Saito of the Subaru Engineering Division, who was in charge of the development of the valve train system.
"The engine's valve train system, made up of the valve and valve spring and the camshaft, is a mechanism that controls the amount of intake air and fuel combustion, which determines engine performance. However, it also produces a great deal of friction. When friction is high, engine output loss increases, and the car becomes less energy-efficient. In addition, friction causes vibrations and noise. Therefore, even though it's a very small part, during its development we must always keep its importance in mind."
The valve spring is a coiled spring that presses the valve against the camshaft cam profile through the tappet. As the cam profile rotates 360 degrees, the valve is pushed and lifts up, and the valve spring extends and contracts. When the engine is running at 8,000 rpm, the valve spring expands and contracts 4,000 times a minute.
If the valve spring is too tight, friction is produced and sometimes the points of contact between the cam profile, tappet and valve become worn. On the other hand, if the valve spring is too loose, the valve won't operate correctly. As a result, setting the optimal tension in the valve spring is the most difficult thing. The rate of valve spring expansion and contraction is set based on detailed calculations and accumulated knowledge from past development.
The valve spring for the Horizontally-Opposed 4-cylinder 16-valve DOHC engine is a small spring, approximately 4cm long and 2cm in diameter. This is longer than the usual valve spring in the in-line engine. A larger valve can be used for the Horizontally-Opposed engine because the bore is larger. In addition, the valve lifts higher for increased fuel efficiency. As a result, because the valve spring receives heavy loads, the spring is made longer to enhance expandability and rigidity.
"As the valve spring is an extremely important part because it pushes the valve firmly against the cam profile, and so it's manufactured from expensive materials using advanced technology. The material is the same metal used in piano wire, and we incorporate additives and carefully eliminate any impurities in order to improve strength."
The spring has two different expansion-contraction rates, achieved by changing the width of a particular coil wind. This will absorb and eliminate any surging of the spring, which can occur when the engine speed increases suddenly.
The spring looks pretty tight, but when I pushed it, it contracted a little. It was flexible, but felt firm and steady at the core. By touching the spring with my finger, I realized that while it holds the valve loosely to the cam profile, it pushes against the valve firmly to prevent any wiggling and unevenness.
I also had a chance to see the valve, which operates supported by the valve spring and is used in the Horizontally-Opposed 4-cylinder 16-valve DOHC engine.
The valve controls the flow of mixed gas and combustion gas in the engine combustion chamber. There are two valves: the intake valve and the exhaust valve. The intake valve opens and allows the air-fuel gas mixture to flow into the combustion chamber, and the exhaust valve opens to discharge combustion gas.
The intake valve measures 36mm in diameter, and the exhaust valve is slightly smaller, at 32mm in diameter. The reason the exhaust valve is smaller is that exhaust gas is expelled easily because of the high pressure in the cylinder after combustion.
The rod-like part of the valve, called the valve stem, is 10cm long. Because this engine has a direct-type valve train system without a rocker arm, the stem tends to be longer. In addition, if an intake valve is lifted higher, tumble flow will occur in the combustion chamber. This tumble flow increases fuel efficiency, which also necessitates a longer valve stem.
The valve face dents slightly for weight reduction. The valve moves back and forth 4,000 times a minute when the engine is running at 8,000 rpm. Therefore, the lighter the valve is, the better its performance will be.
The valve is made of heat-resistant stainless metal, and forged for sturdiness. It weighs only 40g. The black part has been further strengthened through a tufftride heat treatment process.
Fundamentals of developing the valves are "creating a light, sturdy and heat-resistant valve, and preserving sealing performance by preventing wear of the valve face," explains Saito.
"In addition, valves used in high-performance turbo engines must be strong even at high temperatures," Saito adds.
A turbo engine has a high combustion temperature in order to maintain a high level of output and fuel efficiency. Exhaust gas from a turbo engine can reach 900C (1,652F). Because the valve is made of metal, when it heats up it loses some of its strength. Consequently, valves used in turbo engines are filled with sodium, which has greater heat radiation.
Saito thinks that one future direction of valve development will be to make the valve lighter. Specifically, Subaru has plans to develop a lightweight mass-produced valve made of titanium. A lighter valve means less friction, less wasted power and higher fuel efficiency.
The valve and valve spring have existed as basic engine components since the gasoline engine was invented more than a hundred year ago. As a result, there haven't been dramatic changes in their construction. That's why Subaru's engineers are pursuing the kingroad of technological development, such as decreased weight and increased strength, rather than resorting to cheap contrivances. Over the years, accumulation of these efforts has contributed to the distinctive presence of the Horizontally-Opposed Engine.