Pneumatic MotoGP

[erik]

nope....would be interested to know where you heard that presumption??? rotary valves have a huge ability to flow air when compared to poppet valves...the timing/duration can have an effect on that...but you wouldn't design an engine to be low efficency for its design now would you??

I can't remember where I heard it, I've tried searching the net and looking in a couple books i've got with no luck.

The reasoning was that if a rotary valve is turning at a constant speed, it's going to open gradually and close gradually and will only be fully open for an instant. But for a poppet valve controlled by a cam, pneumatics or electronics, it can be opened more quickly to the maximum opening, then be held at the maximum opening and then closed quickly.

I really can't remember where I heard it or if I'm getting it confused with something else...

[Mental Trousers]

The time it takes to fully open a rotary valve is always related to the engine speed as they are turning at 1/2 crank shaft speed, so therefore the time to fully open or fully close at any given rpm cannot be varied. However, if a poppet valve were to use something like solenoids to open them, in theory you can dial them in to open at any speed you want at any rpm.
So in theory a poppet valve that uses solenoids or some other method to open them (not a rotating cam) could fully open the valves faster, hold them open longer, then fully close them faster so that you get more mixture into the chamber.
The reality of it is that it's not the speed of the valve opening that's the problem but the piston. If the piston is at TDC and starts to move down the bore the valve has to follow the piston and therefore cannot open any faster. Camshafts are able to do exactly that now. So opening and closing the valves faster isn't actually going to help anything as they're governed by the speed the piston moves.

[Sanx]

The time it takes to fully open a rotary valve is always related to the engine speed as they are turning at 1/2 crank shaft speed, so therefore the time to fully open or fully close at any given rpm cannot be varied. However, if a poppet valve were to use something like solenoids to open them, in theory you can dial them in to open at any speed you want at any rpm.
So in theory a poppet valve that uses solenoids or some other method to open them (not a rotating cam) could fully open the valves faster, hold them open longer, then fully close them faster so that you get more mixture into the chamber.
The reality of it is that it's not the speed of the valve opening that's the problem but the piston. If the piston is at TDC and starts to move down the bore the valve has to follow the piston and therefore cannot open any faster. Camshafts are able to do exactly that now. So opening and closing the valves faster isn't actually going to help anything as they're governed by the speed the piston moves.

All of which begs the question why don't they use something else other than the crank to turn the rotary valves? I'm not sure a stepper motor could move fast enough, but there's no reason why you couldn't have pneumatic or electro-magnetic actuation of a rotary-type valve. Or at least a reciprocating valve that doesn't intrude on the cylinder at all. It's an appallingly crude diagram, but maybe something like the one attached.

[sAsLEX]

I can't remember where I heard it, I've tried searching the net and looking in a couple books i've got with no luck.

The reasoning was that if a rotary valve is turning at a constant speed, it's going to open gradually and close gradually and will only be fully open for an instant. But for a poppet valve controlled by a cam, pneumatics or electronics, it can be opened more quickly to the maximum opening, then be held at the maximum opening and then closed quickly.

I really can't remember where I heard it or if I'm getting it confused with something else...

The time it takes to fully open a rotary valve is always related to the engine speed as they are turning at 1/2 crank shaft speed, so therefore the time to fully open or fully close at any given rpm cannot be varied.

This issue could be solved with elliptical gears. You can have a fast opening time, a long pause on fully open and a fast shut.

[Mental Trousers]

All of which begs the question why don't they use something else other than the crank to turn the rotary valves? I'm not sure a stepper motor could move fast enough, but there's no reason why you couldn't have pneumatic or electro-magnetic actuation of a rotary-type valve. Or at least a reciprocating valve that doesn't intrude on the cylinder at all. It's an appallingly crude diagram, but maybe something like the one attached.

This issue could be solved with elliptical gears. You can have a fast opening time, a long pause on fully open and a fast shut.

Yup, both right. Using cam chains/belts/gears to drive the rotary valves off the crankshaft is relatively crude and difficult to vary (some of the VVT systems currently being produced can be bloody complex). In theory using VVT type systems on rotary valves would have a greater effect on them because poppet valves are limited by how fast and when they can open cos of the piston and clashing with the other valves.

[sAsLEX]

To help explain what I meant a picture


Right hand side is rotating at a constant speed, ie crankshaft rate, whilst the other side is rotating at a varying pace!

[cowpoos]

The reasoning was that if a rotary valve is turning at a constant speed, it's going to open gradually and close gradually and will only be fully open for an instant

nope...depends on the size/shape of the porting and duration.

The time it takes to fully open a rotary valve is always related to the engine speed as they are turning at 1/2 crank shaft speed, so therefore the time to fully open or fully close at any given rpm cannot be varied..

thats correct...but...it depends how the are ported...how big the hole is?? how far around the duration its open for,etc a poppet valve engine's duration is specified by the cam shaft profile...and rotary valve engines duration is done with port size/shape/duration...same concept really...just a different way of doing it.

All of which begs the question why don't they use something else other than the crank to turn the rotary valves?

because they don't need too.

This issue could be solved with elliptical gears. You can have a fast opening time, a long pause on fully open and a fast shut.

good concept from a engineering point of veiw...but...it will have to be one mean arsed tensioning system to handle the forces while spinning at 5000+ rpm...ad maybe a lil unbalenced?

[Mental Trousers]

Yep. But the duration doesn't change how long the rotary valve takes to be 100% open/uncovered, just how long it's open for.
The time it takes for the leading edge of the rotary valve to be uncovered until that same edge gets to the otherside of the port is governed by how fast the rotary valve is rotating. Port shape can add time to how long it takes to achieve 100% wide open, but it can't reduce the time it takes for that leading edge to move from one side of the port to the other.

[cowpoos]

Yep. But the duration doesn't change how long the rotary valve takes to be 100% open/uncovered, just how long it's open for.
The time it takes for the leading edge of the rotary valve to be uncovered until that same edge gets to the otherside of the port is governed by how fast the rotary valve is rotating. Port shape can add time to how long it takes to achieve 100% wide open, but it can't reduce the time it takes for that leading edge to move from one side of the port to the other.

as in the same situation with a poppet valve re: opening and closing?? even with a massivly sprung poppet valve...

Don't think about it in the extremes of engine development as I thinkn you are....think of it as a everyday highperformance motor...like a gsxr,etc...

[Mental Trousers]

I'm just thinking theoretical rather than practical, ie the application is irrelevant.

[rwh]

I can see the elliptical (or even odder shape) gears (or sprockets) working on a single, or maybe even a twin, but I imagine it would be hard to get it right for more than that - unless you have more than one shaft.

Hmm - perhaps you could have a single central shaft running at constant speed, with elliptical gears driving each individual valve? More friction though.

Richard