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http://www.subaru.co.nz/Technical_Lab/News/index.php?article=684 wrote: Thursday, May 1st, 2003


WRX and AVCS: all about smooth power
Smoother power delivery and more torque or pulling power - 300 Nm - from the latest WRX did not happen by chance. It is all a product of AVCS.
Active Valve Control System or variable valve timing, as it is more commonly known, allows the engine to produce better performance yet also means it operates more economically and produces lower levels of exhaust emissions.

It's a great example of having your cake and eating it too!

The system is operated by a combination of computer controls and oil pressure to advance or retard the intake camshaft timing by up to 35 crankshaft degrees. In the old days the only way to advance or retard the engine's operation was by manually adjusting the ignition timing by altering the distributor's position and then you were stuck with whatever new setting you made.

With Subaru's AVCS, the adjustment occurs continually for the best operation of the motor at all times. By controlling the intake valve timing to suit the engine load and revs, AVCS optimises the engine's combustion process and efficiency.

This is achieved by advancing or retarding the intake camshaft timing to different positions relative to the crankshaft. As a result the intake valves, which are opened and closed by the camshaft, do their work letting the fuel and air mixture into the combustion chamber at slightly different times to produce the optimum performance.

The variation in the camshaft operating position is controlled by the ECM or engine management computer based on input signals from the airflow sensor, engine coolant temperature sensor, throttle position sensor and camshaft position sensor which send "electrical messages" to the ECM.

The ECM then sends a signal, based on the information it has received, to an oil control valve positioned at each of the intake camshaft sprockets. These have an actuator chamber inside them, that when filled or drained with oil pressure to advance or retard the camshaft timing over a range of 35 crankshaft degrees. This happens instantaneously and is continuously variable depending on the constantly changing conditions the engine is operating under. The oil control valve uses engine oil supplied at the same pressure as in the rest of the motor via two passages.

A computer map in the ECM is the "brains" of the operation and has been set to provide for optimum valve timing for stable idling, improved fuel consumption and low emissions in the medium speed range and maximum power at high revs and high engine loads.

When the engine is running at medium revs - 2000 to 3500 rpm - and the engine load is small with a light throttle, say when cruising on the open road, the intake valve timing is advanced. This reduces intake air blow- back thereby improving fuel consumption.
Increasing the overlap of the intake and exhaust valves is also significant, because when they are open at the same time exhaust gas recirculation is promoted. This reduces NOx exhaust gas emissions and much of what does go into the exhaust system is recirculated through the turbocharger to be burned again in the combustion chamber on subsequent occasions.

When the engine load increases at medium revs, as it does when you accelerate, the intake closing time is advanced to take advantage of the intake air inertia - the speed at which it enters the combustion chamber - to create a supercharge effect on the incoming air and thus increase its velocity.

At high revs and under heavy load - when the accelerator is pushed to the floor - the AVCS creates an intake and exhaust valve and a scavenging effect so the exiting exhaust gases effectively suck more air and fuel mixture in through the intake valves. As the intake valve is closed at the end of the intake stroke of the piston, air intake efficiency is improved and engine power output is increased.

At idling speeds, say when stuck in traffic snarl-ups, the AVCS alters the intake valve timing to improve idling stability and reduce fuel consumption and exhaust emissions.
But the biggest improvement is in the WRX's mid-range performance. Power delivery is now smoother and the torque below 3000 rpm is much stronger. "Turbo lag" is a thing of the past as the stronger performance below 4000 rpm masks the effect of when the turbo really kicks in. The 2003 Impreza WRX now has peak power of 168 kW.