Wednesday, May 18, 2011

Variable valve opening & diagram

Variable valve opening

With variable valve opening, the intake valve is arranged to open further at higher engine speeds.  This increases the intake airflow and improves the engines volumetric efficiency.  There are different ways of achieving this.
In one system, each cylinder as an extra intake rocker arm operated by a high lift cam on the camshaft.  At lower engine RPM the Cam moves the rocker arm up and down, but the rocker has no effect on the intake valves.
In one system, each cylinder has an extra intake rocker arm operated by a high lift cam on camshaft.  At lower engine RPM, the cam moves the rocker arm up and down, but the rocker has no effect on the intake valves.  At higher RPM, a plunger is move hydraulically to connect the high lift rocker arm to the Normal intake rocker arm.  Both rocker arms then operate together, but the intake valve will now be opened further because of the high-lift cam.
Fig. 2.42
There is another method that is used with a bucket type tappet.  The tappet has a spring loaded plunger that provides lost motion at low engine RPM.  This has the effect of reducing the intake valve opening because the full movement of the tappet is not used.  At higher engine RPM, a wedge is hydraulically inserted between the plunger and the body of the tappet. This eliminates the lost motion and so the intake valve is opened further than at lower engine speeds. Again, the increased valve opening improves the air flow and engine’s efficiency.

Valve timing diagram
Valve timing diagrams shows how the valves are timed during the four strokes of an engine (intake, compression, power and exhaust). The diagrams show how the valves are timed to open and close in relation to top dead center(TDC) and bottom dead center (BDC) of the piston strokes.
To illustrate variable valve timing and intake camshaft advance, the diagram 2.42 has been drawn with two intake strokes.  The diagram shows how advancing and retarding the intake camshaft affects the valve timing:

1.       Intake valve timing, both opening and closing, can be varied 40°.
2.       Exhaust valve timing does not change.
3.       Valve overlap will vary from fired 5°-45°.
The valve timing is adjusted by the ECM to suit driving conditions.  In general, at idle speed and light loads, there are slower piston’s speeds and slower intake movement, so opening the intake valve can be delayed. The opposite conditions apply at higher engine speeds.  Piston’s speeds are higher and the intake air flow is faster, so the intake valve opening can be advanced.
            There are also advantages by varying the valve overlap for different conditions:

1.    At low speed and light loads, less overlap allows more exhaust gas to be expelled from the cylinder before the intake valve opens-idling is improved.
2.   At medium speeds, increasing overlap re-circulates more exhaust gas within the engine (Internal EGR).
This lowers combustion temperatures and reduces the nitrous oxide emissions from the exhaust.
3.   At higher speeds and heavier loads, the timing of the intake valve closing is varied and this helps with the volumetric efficiency of the engine.

Valve timing terminology
Lead, lag and overlap are terms associated with valve timing.  The intake valve opens before TDC and the exhaust valve opens before BDC. This is a referred to as lead.
The intake valve closes after BDC and the exhaust valve closes after TDC.  This is referred to as lag.
The intake and the exhaust valves are both open for a few degrees around TDC of the exhaust stroke and this is referred to as overlap.
·    Advancing our retarding the intake camshaft alters the intake valve lead and also its lag.  It also alters the valve overlap.

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