I-Active Valve Lift System

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i-Active Valve Lift System (AVLS) is an automobile variable valve timing technology used by Subaru. It varies the duration, timing and lift of the intake valves by using hydraulic pressure to lock two sets of independent camshaft lobes together. This system is similar to Honda's VTEC. Cam locking is done by moving a pin with hydraulic pressure. As with other variable valve timing systems, it allows the engine to have a quiet low-speed operation with a smooth idle, while also using a camshaft lobe profile similar to "race cam" at high engine RPMs.

Wide spread use of AVLS began with non-turbocharged Subaru 2.5 liter four-cylinder engines in Model year 2006. AVLS was first used in the 2005 six-cylinder 3.0 liter Subaru Outback. Turbocharged engines and Subaru's six-cylinder engines continue to use Subaru's AVCS (variable cam phasing) system instead of AVLS. Reports generally agree the new engine feels like a brand new engine with flatter torque curve compared to previous non-AVLS, non-AVCS 2.5 liter engines. Initial applications raised maximum power from 165 hp (123 kW) to 175 hp (130 kW), but more significantly raised the power at most RPM levels. The goal is better fuel economy, improved driveability and increased power.


Operation

At low engine speeds, each of the two intake valves for a given cylinder run on a different camshaft profile (the engines are 4-valve per cylinder type). One valve is operated by a low-lift, low-duration lobe, while the other is actuated by a high-lift, high-duration lobe. This increases air speed causing a smooth idle, while at the same time promoting a swirl factor, increasing efficiency.

At high speeds, the ECU (Engine Control Module) computer signals a solenoid to let oil pressure move a pin which locks the two lobes together. This means at high speed both intake valves are effectively driven by the high-speed camshaft lobe. This increases high-RPM power due to increased valve lift, duration and overall timing.

The switching point varies depending on engine RPM, engine load (vacuum sensor), atmospheric pressure (barometer sensor) and other factors. It is controlled by the ECU.

Usage

Model years:

  • 2005: 3.0 liter naturally aspirated H6 Subaru engines (used along with AVCS)
    • 250 hp (186 kW), 219 lb·ft (297 N·m) torque in 2005 3.0-liter Subaru Outback
  • 2006: all 2.5 liter naturally aspirated Subaru engines
  • 2007: all 2.5-liter naturally aspirated Subaru engines
  • 2008: all 2.5-liter naturally aspirated Subaru engines
  • 2009: all 2.5-liter naturally aspirated Subaru engines

References