I'll try to summarize what we wrote here:
Variable valve timing
1. In any engine there is
valve overlap, as Belatti, Riff_Raff and Fridge13 explain.
In the following pictures, the blue arc shows the time the intake is open, the red arc shows the time the exhaust is open. Notice the arcs overlap in a regular car and overlap
a lot in a racing engine.
Standard Camshaft: the overlap is small (red and blue segments of circle on the top)
Performance Camshaft: huge overlap. Both valves are open for a long time.
Why do you
need to have overlapping valve openings? Because
air has inertia. I copy and paste an explanation I already gave here, in line with Fridge13 and Riff Raff:
If you close the intake valve just when the piston reaches the bottom of its movement (what is called "BDC" or "Bottom Dead Center"), you are going to stop the inrush of air right when is entering the cylinder at its highest speed.
The air has inertia, like almost anything in this world. So, you left the valve open a little more time, with the final effect that, even while the cylinder is moving up, on the compression stroke, the air is still entering the cylinder because of that inertia, giving you a little extra mixture inside the cylinder.
The opposite is true on the exhaust stroke: even when the piston has moved away from the TDC (top dead center) and it's starting to move down, (theoretically it has started the intake stroke), the air is exiting at top speed and it "pulls" a little extra air out.
Finally, when you overlap the exit and intake of air, the exiting air helps to "pull" in the intake air.
This effect of the "inertia of the air" is more noticeable at high rpms, simply because the air is moving faster. This is why the overlap of the valves is greater in race engines, that develop ultra-high rpms.
2. I copy another of my posts verbatim:
When you close the throttle,
the engine manifold vacuum increases. I think this is the same thing that happens when you block the pipe of a vacuum cleaner with your hand, for example.
On carburated cars, like NASCAR (ehem...), the 'idle port', (below the throttle plates), is then subjected to high intake manifold vacuum throwing extra fuel into the combustion chamber.
On some cars, injectors are located between the throttle plate and the intake valve. The intake engine vacuum on deceleration pulls fuel from these injectors and send it to the cylinders.
In modern cars the injectors have a tight shut-off on deceleration to avoid that, specially at normal, highway speeds, so flames don't come out of them (I think F1 cars behave in the same way).
3. There are more things: at low rpm the spark is not as energetic as at high rpm
and when you close the throttle there is less air in the mixture.
Actually there are "flamethrowers kits" that
cut the spark and use an ignition coil in the exhaust for you to impress your friends:
4. Finally, many racing engines
use more fuel than needed to cool the cylinders. This is notorious in NASCAR aspirated engines, but it is also widely used in F1.
