greggors84
|
| posted on 4/11/05 at 10:13 PM |
|
|
Why are valves round?
Just a quick Q.
Why are valves round? Surely they could be another shape to maximise surface area in the combustion chamber, Im talking race engines here, so
manufacturing isnt much of a worry.
Chris
The Magnificent 7!
|
|
|
|
|
Triton
|
| posted on 4/11/05 at 10:19 PM |
|
|
I might be barking up the wrong tree but round is better....as no nasty edges to upset gas flow
My Daughter has taken over production of the damn fine Triton race seats and her contact email is emmatrs@live.co.uk.
www.tritonraceseats.com
www.hairyhedgehog.com
|
|
|
JoelP
|
| posted on 4/11/05 at 10:19 PM |
|
|
a circle or sphere has the maximum circumference to area or surface area to volume ratio, so these shapes are the most economical for many
applications. Hence why pistons are round too! 
|
|
|
novarallycar
|
| posted on 4/11/05 at 10:29 PM |
|
|
as far as i know valves are round because they rotate in the guide and still need to seal when shut.
|
|
|
cymtriks
|
| posted on 4/11/05 at 10:48 PM |
|
|
Other shapes have been tried, ake a look at rotary valved engines such as the Cross or Aspin. The valve and cam shaft are combined in the Cross
engine!
Square, recangular and eliptical pistons have been tried. A Japanese race bike engine from the late eighties had eliptical pistons with eight valves
per cylinder, two rows of four.
All sorts of weird designs have been tried over the last century or so. All that we are left with is the four stroke with round valves, two stroke
with ports and the Wankel rotary. There are dozens of other designs that worked but never caught on.
|
|
|
Volvorsport
|
| posted on 4/11/05 at 10:54 PM |
|
|
rotax motor ? desmodromic ?
the main point is how do you grind the seats ?
www.dbsmotorsport.co.uk
getting dirty under a bus
|
|
|
Hellfire
|
| posted on 4/11/05 at 11:59 PM |
|
|
The bike manufactured using oval pistons was the Honda NR750
(-=CLICK ME=-
Other reasons for producing round pistons is to try to avoid the onset of locallised heating causing premature failing of the valve in the smallest
radius area (minimum metal condition). I doubt valves are ground in anymore as the finish and accuracy that they are produced now supercedes the
previously essential manual process
|
|
|
madman280
|
| posted on 5/11/05 at 01:27 AM |
|
|
From what i recall from trade school the benefit of round valves is that they rotate and wear more evenly.
Rotary engines and some two stroke engines have ports cut right into the cylinder wall. The piston or rotor seal uncovers the port. Called piston
port engines. They can also have reed valves and rotating inlets.
Desmodromic has to do with valve control in the engine. With desmodromic control, no return springs are used and the engine relies on compression
and linkage to seat the valves. This is a design used previously by Mercedes and others but became an integral part of the Ducati motorcycle engine.
It allowed for much higher rpm and horsepower, but is quite complicated.
Rotax is a two stroke engine manufacturer. There used in snowmobiles, motor cycles, boats and PWC's. They have used piston port, reed valve
and rotary valve systems over the years. Light, powerfull and finicky as hell at times. They tend to have very narrow powerbands, and CV
dirvelines.
[Edited on 5/11/05 by madman280]
|
|
|
NS Dev
|
| posted on 5/11/05 at 02:15 AM |
|
|
To touch on a point put by cymtriks, the Aspin engine uses round valves of a sort, not that you would recognise them, and is a GENIUS design of
engine.
Seriously, if materials technology had been seriously applied to this engine, "normal" piston internal combustion engines would no longer
exist.
The Aspin engine achieved outputs of over 100hp per litre (similar to the cosworth DFV race v8's of the time) running on CREOSOTE as
fuel!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I wont go into the detail here, suffice to say the aspin engine tackled most of the problems of the internal combustion piston engine and solved them,
with one major issue, which was never truly solved.
|
|
|
madman280
|
| posted on 5/11/05 at 02:48 AM |
|
|
The Aspin rotary valve engine, the cross rotary valve, Froede rotary valve, Mellors rotary valve, the Norton rotary, and the Coates spherical rotary
valve engine, none of which has ever been mass produced.
The basic problem, which still remains essentially unsolved, is that the pressures in the cylinder of an internal combustion engine are high, due to
both the compression stroke and the explosion of the fuel-air mixture. This produces large forces on the valve system, the beauty of the poppet valve
is that such forces simply push it harder against its seat, and have no effect on the valve-actuating mechanism.
However, the geometry of rotary valve systems is inherently different; the vertical valve cone is pushed up axially against its bearing, this can
cause excessive friction and seizure. The solutions usually involve things that increase oil consumption and emmisions.
|
|
|
gazza285
|
| posted on 5/11/05 at 07:55 AM |
|
|
quote:
Originally posted by NS Dev
The Aspin engine achieved outputs of over 100hp per litre (similar to the cosworth DFV race v8's of the time)
The DFV had a little more than that, the first one they ever ran on the test bed had 408bhp, which is a little more impressive 136hp/L.
As for the Aspin engine, it was a clever idea, but the friction issue meant it was fatally flawed. We did some experimenting in the late '80s
and early '90s with a rotary valve engine for the Shell Ecomarathon. We made a few prototypes with varying cone angles to try and stop the
excessive frictional losses and ended up with a valve with no taper at all, being essentially a piece of round bar with the combustion chamber in it
(and a hole obviously). This ran fine, but suffered from blow by when cold until the valve had expanded to seal the chamber properly, burnt oil, as
the valve lubricant was total loss (we ran with stroker oil in the sump), and, unless engine temperature was religiously monitored, even slight
overheating would lead to the valve dragging in the chamber and seizing. After much fun and frustration we went back to the good old, reliable,
efficient, and easy to make poppet valve engine.
DO NOT PUT ON KNOB OR BOLLOCKS!
|
|
|
zzrpowerd-locost
|
| posted on 5/11/05 at 07:55 AM |
|
|
quote:
Originally posted by greggors84
Just a quick Q.
Why are valves round? Surely they could be another shape to maximise surface area in the combustion chamber, Im talking race engines here, so
manufacturing isnt much of a worry.
because they are! 
|
|
|
NS Dev
|
| posted on 5/11/05 at 09:07 AM |
|
|
quote:
Originally posted by gazza285
quote:
Originally posted by NS Dev
The Aspin engine achieved outputs of over 100hp per litre (similar to the cosworth DFV race v8's of the time)
The DFV had a little more than that, the first one they ever ran on the test bed had 408bhp, which is a little more impressive 136hp/L.
As for the Aspin engine, it was a clever idea, but the friction issue meant it was fatally flawed. We did some experimenting in the late '80s
and early '90s with a rotary valve engine for the Shell Ecomarathon. We made a few prototypes with varying cone angles to try and stop the
excessive frictional losses and ended up with a valve with no taper at all, being essentially a piece of round bar with the combustion chamber in it
(and a hole obviously). This ran fine, but suffered from blow by when cold until the valve had expanded to seal the chamber properly, burnt oil, as
the valve lubricant was total loss (we ran with stroker oil in the sump), and, unless engine temperature was religiously monitored, even slight
overheating would lead to the valve dragging in the chamber and seizing. After much fun and frustration we went back to the good old, reliable,
efficient, and easy to make poppet valve engine.
I did say "over 100hp per litre" not 100hp per litre!!!!!!!!!!!!!!!!
I was not more specific because I don't have the aspin engine book to hand, it is at work! I was also rather drunk when I posted so I am
surptised that the post made any sense at all!!
I think (iirc) the Aspin made similar specific output to the Cosworth engine, but with dramatically reduced ignition advance, and running on extremely
poor fuel.
Yes though, it was never a finished practical design.
However, do you really think the poppet valve engine was perfected instantly. If engineers ran away from ideas every time they hit problems we would
never have progress!!!
|
|
|
gazza285
|
| posted on 5/11/05 at 04:36 PM |
|
|
Only trouble with the claims for the Aspen engine are that non of them were ever tested independantly. Not to suggest that a dead man might have been
fibbing, but some of his claims were a little on the unbelievable side.
Pretty good site about Frank Aspin and his engines here.
http://www.lortim.demon.co.uk/aspin/
DO NOT PUT ON KNOB OR BOLLOCKS!
|
|
|
britishtrident
|
| posted on 5/11/05 at 05:36 PM |
|
|
One other valve technology is known to work in 4 stroke engines sleeve valves --- almost silent, good gas flow but after a few thousand miles the
oil consumption becomes a real problem -- I think the last applications to use them were some british piston aero engines (Napier Sabre ? Bristol
Centaur ??)
|
|
|
britishtrident
|
| posted on 5/11/05 at 05:49 PM |
|
|
quote:
Originally posted by gazza285
The DFV had a little more than that, the first one they ever ran on the test bed had 408bhp, which is a little more impressive 136hp/L.
DFV was severely rev limited in its initial version because of camshaft torsional vibration problems. Duckworth change the stiffness of the camshaft
and instantly liberated an extra 2,000 rpm.
|
|
|
gazza285
|
| posted on 5/11/05 at 06:09 PM |
|
|
quote:
Originally posted by britishtrident
quote:
Originally posted by gazza285
The DFV had a little more than that, the first one they ever ran on the test bed had 408bhp, which is a little more impressive 136hp/L.
DFV was severely rev limited in its initial version because of camshaft torsional vibration problems. Duckworth change the stiffness of the camshaft
and instantly liberated an extra 2,000 rpm.
Didn't really affect revs or power though, it won 34 races before the problem was solved, four years after the DFV was introduced. Different
cams and a shock load absorbing cam drive finally overcame the intermitant cam gear breakages.
DO NOT PUT ON KNOB OR BOLLOCKS!
|
|
|
NS Dev
|
| posted on 6/11/05 at 01:37 PM |
|
|
quote:
Originally posted by gazza285
Only trouble with the claims for the Aspen engine are that non of them were ever tested independantly. Not to suggest that a dead man might have been
fibbing, but some of his claims were a little on the unbelievable side.
Pretty good site about Frank Aspin and his engines here.
http://www.lortim.demon.co.uk/aspin/
very fair point!
|
|
|
jon_haggerty
|
| posted on 7/11/05 at 08:48 AM |
|
|
Actually most pistons are designed slightly oval and with a taper from top to bottom. This is so they become round when hot.
|
|
|
