On Mon, 4 Nov 1996 Simon.MATTHEWS@st.com wrote:
> William Eastman wrote an excellent article on this. However, there are
> a couple of additional points that should be made:
>
> Valve Overlap -- all other things equal, minimal valve overlap would
> be good, however, all other things are not equal and (my understanding
> is that) valve overlap allows a greater charge of Fuel/Air mixture --
> hence more pressure.
I want to go a little more into detail here.
Valve overlap really comes into play at high rpms, and it is necessary
because of the inertia of the fuel/air mixture.
Without overlap, the intake valve opens when the cylinder is at TDC Top
Dead Center). The piston starts travelling downwards, and the F/A mixture
is being sucked through the intake port into the combustion chamber.
At BDC, the valve closes.
Now, at high rpm, the described events happen very fast. After the valve
opens, the AF charge needs some time to accelerate, so that by the time
the AF mixture really gets going, the valve is already closing again. The
cylinder is not filled optimally. Because of the high speed of the
piston, by the time the valve closes, the AF mix has attained an
impressive velocity.
So if one leaves the valve open a little longer past BDC, the
cylinder keeps getting filled even though the piston has started going
back up. This is again due to the inertia (or momentum) of the AF mix.
Clearly, with this design, at lower rpm, when the piston and thus the AF
speed is not as high, the upwards moving piston will reverse the flow and
push AF back out of the cylinder through the open valve!
Hence, for every point in the rpm band, there is an optimal valve timing.
This is one of two aspects of what performance camshafts do (the other is
increased lift). A "hot cam" has large valve overlap, insuring good
cylinder fill at high rpm and thus high max. hp, but they also cause
uninspiring low end performance because intake charge is being pushed back
out of the cylinder at low rpm.
A similar argument can be made for the exhaust valve and in the course of
that discussion, higher fuel consumption can be explained. Completely
purging the combustion chamber of spent gases is important, because
remaining exhaust gases leave less room for fresh (hp generating) charge.
For this reason, increased valve overlap between intake and exhaust valve
is used to flush spent gases out with incoming fresh gases.
Clearly, the portion of fresh charge that exits with the exhaust gases is
wasted and is a waste of fuel resulting in lower mpg.
I have only talked about the timing of the closing of the intake
valve and the closing of the exhaust valve. Much more can be said about
the timing of the other events throughout the 4 strokes, but this is
already long.
Ulix __/__,__
.......................................................... (_o____o_).....
'67 Sprite
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