Spring 2007 Forward to a Friend


I-active valve lift system (AVLS)
by Scott Heidbrink
AVLS AVLS IS ONE OF THE KEYS TO THE POWER AND EFFICIENCY FOUND IN TODAY’S 2.5-LITER NATURALLY ASPIRATED SUBARU BOXER ENGINES.

Contemporary Subaru engines deliver more performance per liter than their predecessors while operating with dramatically reduced emissions. One key to these advancements is the development of computer-controlled systems. Computers provide new levels of management in a wide range of areas – from the amount of fuel injected into the combustion chamber to transmission shift functions.


Each intake valve is operated via rocker arm by its own camshaft lobe – one lobe with a high profile and the other with a lower profile. The resultant staggered valve openings increase the speed of the air entering the combustion chamber, which increases torque.
 
The rocker arms of each cylinder’s two intake valves are locked together, and the high-profile camshaft lobe operates them both. The two wide valve openings reduce resistance to intake air, helping to improve horsepower.

THE ROLE OF CAMSHAFTS
On all four-stroke engines (which power the great majority of the vehicles on the road today), camshafts open and close intake and exhaust valves by the shape of their cam lobes. In the past, modifying an engine to deliver more horsepower entailed physically disassembling the engine and then installing a new camshaft with different cam lobe profiles. The only trouble was that engines “tuned” for maximum horsepower allowed other attributes to suffer. They tended to have rough idles and/or reduced fuel economy.

Today, thanks to the power of the computer and AVLS, modern Subaru engines deliver the best of all worlds – performance, economy, and low emissions. (The i in the name i-Active Valve Lift System stands for intelligent – meaning the system automatically responds to the driving and atmospheric conditions to deliver optimum performance.)

INTELLIGENT ADAPTATION
The camshafts on an AVLS-equipped engine have specially designed lobes for intake valves. They feature two different cam profiles:

1. A low/mid-lift profile
2. A high-lift profile

Which cam profile is utilized is regulated by the Engine Control Module (ECM) computer. The AVLS-equipped Subaru engines use oil pressure generated by the engine to activate the different valve lift settings:

Today, thanks to the power of the computer and AVLS, modern Subaru engines deliver the best of all worlds – performance, economy, and low emissions.
  • At low engine speeds, the low/mid-lift profile increases the speed of the air rushing into the combustion chambers from the intake ports. The net result is an increase in torque, which gives you more pulling power.
  • At high engine speeds, the cam lift is switched to the high-lift profile to reduce resistance to air coming into the engine. At high speeds, this allows the engine to breathe deeply and generate strong horsepower for ease of passing at highway speeds.

AVLS-equipped Subaru engines have two intake valves per cylinder. But AVLS only controls one of them. The other always operates in the high-lift mode. This results in an imbalance in pressure as air enters the cylinder, which creates a swirling pattern. The turbulence provides a better distribution of the air-fuel mixture at low engine rpm, which translates into more even combustion. The ECM automatically selects the different cam lift profile settings based on engine load, driving requirements, and atmospheric conditions.

SUBARU APPLICATION
For the 2007 model year, the i-Active Valve Lift System is standard on all 2.5-liter naturally aspirated Subaru engines. In essence, AVLS provides two engines under your hood – one tuned for optimum fuel economy and one tuned for performance – and both deliver very low levels of emissions.