Reed Valves in Two-strokes
 

Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?

Inquiring minds want to know.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

try rec.motorcycles or a motor group
"Tim Wescott" skrev i melding
...
Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?

Inquiring minds want to know.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Håken" wrote in message
...
try rec.motorcycles or a motor group
"Tim Wescott" skrev i melding
...
Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?

Inquiring minds want to know.

Two-stroke design has advanced a great deal since I was interested in it,
but the general idea of reed valves is that they allow a broader torque
range, with fairly even crank-filling over a wide range of rpm.

Disk-type or other crank-timed or piston-timed inlets produce a uniform
number of rotation-degrees of inlet opening, but that becomes a shorter
*time* of inlet opening as rpm increase. Thus, they lead to sharper torque
peaks, even though they may allow slightly greater peak horsepower at the
rpm for which they're tuned. But good reed-valve design hardly steals any
peak horsepower.

--
Ed Huntress

Tim,
In very high performance two strokes both reed valves and rotary valves are
used, like in some go-kart engines. In that way, when the reeds go into
float, the rotary valve provides the extreme high rpm horse power and at low
rpm, the reed valve prevents reversion and low speed torque is retained.
Steve

"Tim Wescott" wrote in message
...
Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed material
in such small sizes?

Inquiring minds want to know.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?

I think model airplain engines are very cost sensitive. Another poster
said reed valves provide low end torque but I don't think that model
airplane engines need low end torque. So my guess is they are not needed.

On Sun, 23 Jan 2005 21:00:53 -0800, Tim Wescott wrote:
Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?

Nope, not that reason. Reeds give the greatest benefit in low-RPM torque,
in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.

Inquiring minds want to know.

There ya go.

Dave Hinz

Dave Hinz wrote:

On Sun, 23 Jan 2005 21:00:53 -0800, Tim Wescott wrote:

Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?


Nope, not that reason. Reeds give the greatest benefit in low-RPM torque,
in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.


Inquiring minds want to know.


There ya go.

Dave Hinz

Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.

Makes sense -- thanks.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Mon, 24 Jan 2005 13:26:44 -0800, Tim Wescott wrote:
Dave Hinz wrote:

Nope, not that reason. Reeds give the greatest benefit in low-RPM torque,
in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.

Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.

Good points.

Makes sense -- thanks.

It's really fascinating how a 2-stroke works. I've seen some computer
modeling of the flow characteristics of a few different engines at
varying RPM, which showed all of this in red/blue flow, with the
intensity of color showing the density of that location. Of course,
I can't google it up right now.

Dave Hinz wrote:

On Mon, 24 Jan 2005 13:26:44 -0800, Tim Wescott wrote:

Dave Hinz wrote:

Nope, not that reason. Reeds give the greatest benefit in low-RPM torque,
in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.



Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.


Good points.


Makes sense -- thanks.


It's really fascinating how a 2-stroke works. I've seen some computer
modeling of the flow characteristics of a few different engines at
varying RPM, which showed all of this in red/blue flow, with the
intensity of color showing the density of that location. Of course,
I can't google it up right now.

Yes, the Day cycle (did you know it was called that? Invented by a guy
named Day, of course) is pretty fascinating. They're doing some amazing
things with direct fuel injection and two-part gaseous mixtures to make
it clean, too -- basically they keep the same cycle, but the fuel (or
fuel-laden air) gets put into the cylinder after the cylinder is
scavenged with fresh air.

Not practical for model two-strokes, of course.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
Dave Hinz wrote:

On Mon, 24 Jan 2005 13:26:44 -0800, Tim Wescott
wrote:

Dave Hinz wrote:

Nope, not that reason. Reeds give the greatest benefit in low-RPM
torque,
in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.



Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.


Good points.


Makes sense -- thanks.


It's really fascinating how a 2-stroke works. I've seen some computer
modeling of the flow characteristics of a few different engines at
varying RPM, which showed all of this in red/blue flow, with the
intensity of color showing the density of that location. Of course,
I can't google it up right now.

Yes, the Day cycle (did you know it was called that? Invented by a guy
named Day, of course) is pretty fascinating. They're doing some amazing
things with direct fuel injection and two-part gaseous mixtures to make
it clean, too -- basically they keep the same cycle, but the fuel (or
fuel-laden air) gets put into the cylinder after the cylinder is
scavenged with fresh air.

Not practical for model two-strokes, of course.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
There's another reason; bike engines have timed ignition systems which
prevent the engine running backwards; most glow plug engines rely on the
induction timing to prevent reversal. The small reed induction engines (e.g
Cox 049) use a starter spring to ensure that they only start in the correct
direction (you hook it around the prop, wind it backwards a few turns and
let go), you couldn't easily do this with a larger engine. I know from
experience that if you try and start one of these reed induction engines
with your finger, they bounce off compression and start backwards; that's
led to a few very short flight on occasions ;-)
Martin
--
martindot herewhybrowat herentlworlddot herecom

On Mon, 24 Jan 2005 13:26:44 -0800, Tim Wescott
wrote:

Dave Hinz wrote:

On Sun, 23 Jan 2005 21:00:53 -0800, Tim Wescott wrote:

Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?


Nope, not that reason. Reeds give the greatest benefit in low-RPM torque,
in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.


Inquiring minds want to know.


There ya go.

Dave Hinz

Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.

Makes sense -- thanks.

There may be another factor. Model engines commonly use the crank as
the inlet valve, with a hollow crank for the air-fuel mix passage into
the crankcase. The counterweight and crank throw are cantilevered.
The big end of the rod slides over the crank throw when the sleeve is
removed.

This is very simple and works well for low mass (small) engines. I
suspect by the time you're up to motorcycle size engines of high
output, the cantilevered design would not be favored compared to reed
valves and crank bearings on both sides.

Pete Keillor

Martin Whybrow wrote:

"Tim Wescott" wrote in message
...

Dave Hinz wrote:


On Mon, 24 Jan 2005 13:26:44 -0800, Tim Wescott

wrote:

Dave Hinz wrote:


Nope, not that reason. Reeds give the greatest benefit in low-RPM

torque,

in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.



Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.


Good points.



Makes sense -- thanks.


It's really fascinating how a 2-stroke works. I've seen some computer
modeling of the flow characteristics of a few different engines at
varying RPM, which showed all of this in red/blue flow, with the
intensity of color showing the density of that location. Of course,
I can't google it up right now.


Yes, the Day cycle (did you know it was called that? Invented by a guy
named Day, of course) is pretty fascinating. They're doing some amazing
things with direct fuel injection and two-part gaseous mixtures to make
it clean, too -- basically they keep the same cycle, but the fuel (or
fuel-laden air) gets put into the cylinder after the cylinder is
scavenged with fresh air.

Not practical for model two-strokes, of course.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

There's another reason; bike engines have timed ignition systems which
prevent the engine running backwards; most glow plug engines rely on the
induction timing to prevent reversal. The small reed induction engines (e.g
Cox 049) use a starter spring to ensure that they only start in the correct
direction (you hook it around the prop, wind it backwards a few turns and
let go), you couldn't easily do this with a larger engine. I know from
experience that if you try and start one of these reed induction engines
with your finger, they bounce off compression and start backwards; that's
led to a few very short flight on occasions ;-)
Martin

I have flown the Cox engines for years, and yes it's an issue (always
check by feeling for airflow). You can learn to flip the thing in such
a way that you neither start it backward or chop off your fingers by
exerting force on the front of the blade about 2" out from center and
following through (and using a finger guard).

Those dang starters rob power and wear out -- I don't use 'em.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Tue, 25 Jan 2005 00:12:32 GMT, "Martin Whybrow"
wrote:

There's another reason; bike engines have timed ignition systems which
prevent the engine running backwards; most glow plug engines rely on the
induction timing to prevent reversal. The small reed induction engines (e.g
Cox 049) use a starter spring to ensure that they only start in the correct
direction (you hook it around the prop, wind it backwards a few turns and
let go), you couldn't easily do this with a larger engine. I know from
experience that if you try and start one of these reed induction engines
with your finger, they bounce off compression and start backwards; that's
led to a few very short flight on occasions ;-)
Martin

Two stroke bike engines will run backwards. Just ask someone who has
stalled an engine on a hill and rolled back a few feet and let the
clutch out to stop rolling.

Andy Asberry wrote:

On Tue, 25 Jan 2005 00:12:32 GMT, "Martin Whybrow"
wrote:


There's another reason; bike engines have timed ignition systems which
prevent the engine running backwards; most glow plug engines rely on the
induction timing to prevent reversal. The small reed induction engines (e.g
Cox 049) use a starter spring to ensure that they only start in the correct
direction (you hook it around the prop, wind it backwards a few turns and
let go), you couldn't easily do this with a larger engine. I know from
experience that if you try and start one of these reed induction engines
with your finger, they bounce off compression and start backwards; that's
led to a few very short flight on occasions ;-)
Martin


Two stroke bike engines will run backwards. Just ask someone who has
stalled an engine on a hill and rolled back a few feet and let the
clutch out to stop rolling.

Whee!

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott wrote:
Andy Asberry wrote:

On Tue, 25 Jan 2005 00:12:32 GMT, "Martin Whybrow"
wrote:


There's another reason; bike engines have timed ignition systems which
prevent the engine running backwards; most glow plug engines rely on the
induction timing to prevent reversal. The small reed induction
engines (e.g
Cox 049) use a starter spring to ensure that they only start in the
correct
direction (you hook it around the prop, wind it backwards a few turns
and
let go), you couldn't easily do this with a larger engine. I know from
experience that if you try and start one of these reed induction engines
with your finger, they bounce off compression and start backwards;
that's
led to a few very short flight on occasions ;-)
Martin



Two stroke bike engines will run backwards. Just ask someone who has
stalled an engine on a hill and rolled back a few feet and let the
clutch out to stop rolling.


Whee!

whee is right... when I was a young man, I worked at a suzuki dealer
unpacking and assembling motorcycles after school... I put together a
trails bike and didn't check the timing, trying to start it, it
back-fired and started up... I put it in gear and took off... boy, was I
suprised when it took off backwards throwing me over bars... timing was
to close to top dead center

Jack

"Andy Asberry" wrote in message
...

Two stroke bike engines will run backwards. Just ask someone who has
stalled an engine on a hill and rolled back a few feet and let the
clutch out to stop rolling.

So will a military 5 ton. The look on the guy's face when the only forward
gear was reverse!

Shawn

On Mon, 24 Jan 2005 15:03:32 -0800, Tim Wescott wrote:
Dave Hinz wrote:

It's really fascinating how a 2-stroke works. I've seen some computer
modeling of the flow characteristics of a few different engines at
varying RPM, which showed all of this in red/blue flow, with the
intensity of color showing the density of that location. Of course,
I can't google it up right now.

Yes, the Day cycle (did you know it was called that? Invented by a guy
named Day, of course)

I've never seen "Schnurle" spelled that way before... ?

is pretty fascinating. They're doing some amazing
things with direct fuel injection and two-part gaseous mixtures to make
it clean, too -- basically they keep the same cycle, but the fuel (or
fuel-laden air) gets put into the cylinder after the cylinder is
scavenged with fresh air.

Have you seen the Orbital 2-stroke engines? Pretty impressive. The
power:weight ratio is amazing.

Not practical for model two-strokes, of course.

Well, what with weight being an issue and all, I suppose not. It's not
like you can't buy a bigger engine if you need a bit more power.