Does tapered cam profile change valve timing with valve lift ???

Ferrari had used the tapered cam profile for varying valve timing. But
valve colse at the same time when they have to. So just valve lift is
varible. But in some articles i have refered it is included in variable
valve timing.
Is it right to include this case in Variable Valve Timing ????

Raj wrote:

> Is it right to include this case in Variable Valve Timing ????

Yes. Valves aren't suddenly either "open" or "closed" They spend much
of their time in a partly-open state. The behaviour during this time,
and how fast they move, is important to valve design. This is
particularly important for engines with large valves -- gasflow through
these at low gasglows can be quite poor. A smaller opening for these
over at least part of the stroke can give a useful increase in
low-speed torque on what would otherwise be a very "peaky" engine.

There are two techniques to this. One is an axially sliding camshaft
with conical profiles on the cams. This allows great control of
profile, but it also requires a narrow cam follower face and so
accelerates wear. Another technique is for a twin-camshaft engine to
have identical profiles throughout, but to vary the relative timing of
the inlet and exhaust cams, and thus the crucial value of their overlap
time.

"Andy Dingley >" > wrote in
ups.com:

>
> Raj wrote:
>
>> Is it right to include this case in Variable Valve Timing ????
>
> Yes. Valves aren't suddenly either "open" or "closed" They spend much
> of their time in a partly-open state. The behaviour during this time,
> and how fast they move, is important to valve design. This is
> particularly important for engines with large valves -- gasflow through
> these at low gasglows can be quite poor. A smaller opening for these
> over at least part of the stroke can give a useful increase in
> low-speed torque on what would otherwise be a very "peaky" engine.
>
> There are two techniques to this. One is an axially sliding camshaft
> with conical profiles on the cams. This allows great control of
> profile, but it also requires a narrow cam follower face and so
> accelerates wear. Another technique is for a twin-camshaft engine to
> have identical profiles throughout, but to vary the relative timing of
> the inlet and exhaust cams, and thus the crucial value of their overlap
> time.
>
>


Another approach is Honda's VTEC. Hydraulic solenoids engage the followers
with a third cam lobe on the same camshaft.

http://honda_fans.tripod.com/vtec-info.html

--
Tegger