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What's Inside: Active Valve Control System (AVCS)




Pulling the connecting rod, the crankshaft pulls the piston toward the center of the engine, drawing in air and fuel from the intake system. The air and fuel enters the cylinder through the intake ports opened by the intake valves. This is like a person taking a breath – inhaling. AVCS adjusts exactly when intake valves begin to open.



With both valves closed, the turning crankshaft forces up the piston, compressing the air/fuel mixture.



The spark plug ignites this compressed mixture, causing an explosion that forces down the piston and connecting rod, which, in turn, rotates the crankshaft. These explosions within the cylinders provide the engine’s power.



The crankshaft forces the piston to the top of the cylinder again, this time pushing leftover gases out of the combustion chamber past the opened exhaust valves and into the exhaust system. This is similar to a person exhaling.

 

 

The 4-Stroke Cycle

 

Most of today’s automotive gasoline engines function via a 4-stroke cycle. Engine components continuously cycle through four strokes, named for their functions of intake, compression, power and exhaust.

 

AVCS affects the roles of the camshafts in this process. Actuation is mechanical, by direct contact or through a combination of lifters, tappets and/or pushrods, depending on engine design. How the camshafts are designed essentially gives engines their personalities.

 

Camshafts in Subaru engines are belt-driven (4-cylinder) or chain-driven (6-cylinder) by the crankshaft. Intake valves open to let the air into the combustion chamber, and exhaust valves open to let out the exhaust gases. AVCS operation affects the intake valve timing or at exactly what point each valve opens and closes.

 

Overall, intake- and exhaust-valve operation during the 4-stroke cycle follows this pattern:

 

However, there are nuances in operation, and that’s where AVCS plays a part.

 

In the 4-stroke sequence, the exhaust cycle immediately precedes the intake cycle. Overlapping the timing of the closing of the exhaust valves and the opening of the intake valves can help the engine perform better under heavy loads, but not under light loads. AVCS continuously varies this overlap through an infinite number of positions. Overlap ranges between a slight overlap (“retard” position) through as much as 35 degrees of the crankshaft rotation (“advance” position).

  

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