multivalve engines

Hi all,

I am a car laic trying to understand reasons for some design decisions.
So far the best explanation I found for multiple valves is based on
the assumption that valves must be round; then, to cover more area of a
round cylinder head with circles, it's better to use more smaller
circles.

My question to the group is, why do valves need to be round? With
"D"-shaped valves there could be one intake and exhaust valve that has
maximum possible flow.

Thanks!

foobarian wrote:
> Hi all,
>
> I am a car laic trying to understand reasons for some design decisions.
> So far the best explanation I found for multiple valves is based on
> the assumption that valves must be round; then, to cover more area of a
> round cylinder head with circles, it's better to use more smaller
> circles.
>
> My question to the group is, why do valves need to be round? With
> "D"-shaped valves there could be one intake and exhaust valve that has
> maximum possible flow.
>
> Thanks!
>

If the valves are not perfectly round, then you have the added
complexity and likely friction of a mechanism to keep the valves from
rotating. Regular round valves do rotate, and indeed it is desirable
for them to do so, to keep deposits from building up and causing the
valves to burn and/or beat imperfections into their seats (which will
eventually cause them to burn...) in fact many heavy-duty versions of
various engines have exhaust valve rotators built into the retainers.

It's a good question, but unfortunately not a good idea.

nate

--
replace "fly" with "com" to reply.
http://home.comcast.net/~njnagel

They are usually round so that they can rotate and prolong the sealing
properties against the seat.
A D shaped valve might work for a while, but would soon lose its ability to
seal efficiently.

There are other types of valves which have been used commercially and
experimentally.

To get the most power from an engine, you want to get the maximum gas
mixture into the cylinder and the exhaust out, efficiently. Multiple valves
help attain this.

foobarian > wrote in message
ups.com...
> Hi all,
>
> I am a car laic trying to understand reasons for some design decisions.
> So far the best explanation I found for multiple valves is based on
> the assumption that valves must be round; then, to cover more area of a
> round cylinder head with circles, it's better to use more smaller
> circles.
>
> My question to the group is, why do valves need to be round? With
> "D"-shaped valves there could be one intake and exhaust valve that has
> maximum possible flow.
>
> Thanks!

How would you design a machine that could cut D shaped valve seats that
perfectly matched D shaped valves to such fine tolerances that they sealed
properly? Clearly you can't rotate them to lap them in and you also have to
build in a mechanism that stops them rotating in use. If you work it out
then patent it quick because no one else can manage it.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)

foobarian wrote:

> Hi all,
>
> I am a car laic trying to understand reasons for some design decisions.
> So far the best explanation I found for multiple valves is based on
> the assumption that valves must be round; then, to cover more area of a
> round cylinder head with circles, it's better to use more smaller
> circles.
>
> My question to the group is, why do valves need to be round? With
> "D"-shaped valves there could be one intake and exhaust valve that has
> maximum possible flow.
>
> Thanks!
>

Valves need to rotate in order to maintain a good sealing surface. You
can't rotate non-round valves.

"Steve" > wrote in message
...
> foobarian wrote:
>
> > Hi all,
> >
> > I am a car laic trying to understand reasons for some design decisions.
> > So far the best explanation I found for multiple valves is based on
> > the assumption that valves must be round; then, to cover more area of a
> > round cylinder head with circles, it's better to use more smaller
> > circles.
> >
> > My question to the group is, why do valves need to be round? With
> > "D"-shaped valves there could be one intake and exhaust valve that has
> > maximum possible flow.
> >
> > Thanks!
> >
>
> Valves need to rotate in order to maintain a good sealing surface. You
> can't rotate non-round valves.
>

Valves seat just fine without rotating.

Steve > wrote in message
...
> foobarian wrote:
>
> > Hi all,
> >
> > I am a car laic trying to understand reasons for some design decisions.
> > So far the best explanation I found for multiple valves is based on
> > the assumption that valves must be round; then, to cover more area of a
> > round cylinder head with circles, it's better to use more smaller
> > circles.
> >
> > My question to the group is, why do valves need to be round? With
> > "D"-shaped valves there could be one intake and exhaust valve that has
> > maximum possible flow.
> >
> > Thanks!
> >
>
> Valves need to rotate in order to maintain a good sealing surface. You
> can't rotate non-round valves.

Some diesel engines have valves with a shroud on one side to direct the
airflow and are pinned to stop them rotating. There's no absolute necessity
for rotation to maintain a good sealing surface.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)

Dave Baker wrote:
>>
>>Valves need to rotate in order to maintain a good sealing surface. You
>>can't rotate non-round valves.
>
>
> Some diesel engines have valves with a shroud on one side to direct the
> airflow and are pinned to stop them rotating. There's no absolute necessity
> for rotation to maintain a good sealing surface.
> --
> Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
>
>

One advantage to round valves is doing a valve job. Valve and seat
grinding, lapping, to get a good seal when doing a valve job requires a
round seat. It would be very hard to do this, but not impossible,
without a round valve. One would almost have to replace the valves, and
have a thread-in seat, for non circular valves. Not impossible, but
complicated. All in all, a round valve is nice.

Of course, there have been sleeve valve engines, and other methods of
porting, but these have gradually become dead ends.

Although I respect the opinions of a previous poster, round valve rotators
are designed
to help lengthen the time that the valve will function well. The design is
intended to help
prevent warpage, lessen deposits, extend the period of positive seal.

Odd shaped valves can certainly seal efficiently at first, and possibly with
advanced design
can last a long time. Design and metallurgy of this degree of refinement
are seldom a part
of production automotive engines.

If you are willing to spend the money to do it right, you can get by with a
lot of things.