Variable valve timing-cam phasing-VVT- how does it work

Variable valve timing-cam phasing-VVT- how does it work ?

Valve overlap.

valve-overlap
valve-overlap

Valve overlap is the amount of time the intake and exhaust valves are both open at the same time. Less overlap produces a smooth idle and more slow speed torque. But poor high speed performance because there is not enough time for complete scavenging to occur.

More valve overlap allows better engine breathing at high speeds. But poor performance at low speeds, rough idling, and higher exhaust emissions.

Also with hydraulically-actuated cam phasers, the VVT system is usually not active until the engine reaches normal operating temperature. As engine speed and/or load change, the PCM looks at its various sensor inputs and commands the oil flow control valve solenoids to open.

There are two types of variable valve timing, or VVT – cam phasing and cam changing.

Cam phasing VVT varies valve timing by shifting the phase angle of the camshaft.

Consequently this will increase the amount of valve overlap. This is controlled by the engine management system, and actuated by hydraulic valve gears.

Continuous or fixed phasing change – What is the difference.

  • Continuous systems normally vary the phasing angle between 0 and 40 or more degrees according to engine load and speed requirements.
  • Fixed phasing systems alter phasing by a specific angular value at a specific speed and load condition.

Single overhead camshaft engines can use cam phasing. However double overhead camshaft engines can receive greater benefits from phasing change VVT. Separately controlling the intake and exhaust camshafts is a  definite advantage.

Some manufacturers choose to alter phasing on both intake and exhaust camshafts. Other might have the inlet camshaft phase controlled with the exhaust camshaft being fixed.

Cam changing VVT uses different cam profiles to lift the valves depending on engine load and speed.

honda-vtec-system
honda-vtec-system

One common system uses two rocker arms for normal operation on its two intake valves, with a third, higher profile, rocker arm between the other two arms. At engine speeds above 5000 to 6000 rpm, the engine ECU activates an oil pressure controlled pin that locks the three rocker arms together. The center rocker arm follows a larger and more aggressive profile, transferring its movement to the intake valves which now open further and for longer.

How does it work ?

  • When engine speeds fall below the threshold speed, oil pressure is removed from the pin and a spring deactivates the pin.
  • As a result the rocker arms are no longer locked together and the valves are controlled by the less aggressive outer lobes.

Cam changing VVT can also be used in a similar way to, deactivate a second intake valve at low engine speeds. This will increase the velocity and swirl of the air/fuel mixture as it enters the combustion chamber.

Other Benefits of Variable Valve Timing.

  • Internal exhaust gas recirculation. By allowing for more direction for internal gases, the variable valve timing system can cut down on emissions, which is critical for auto makers working to get their cars and trucks in compliance with federal or state emissions controls.
  • Increased torque.Variable valve timing systems can provide better torque for an engine.
  • Better fuel economy. With more precise handling of engine valves, some automakers have shown that VVT can produce better fuel economy for vehicles.

Consequently never reuse a bad cam phaser. It is no different than reusing a worn oil pump. It will cause problems and end up costing you money and possibly a new engine.

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