it runs on the same principals as this [ame]http://www.youtube.com/watch?v=tqZyyonyRRI[/ame]
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would this engine work
- Thread starter zig 2007
- Start date
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B
Bogstandard
Guest
Zig,
As is, it would not run, the vital part you are missing is the poppet valve in the head.
This used to be a standard conversion of two stroke engines when running them on steam.
Usually there was a striker pin sticking up on top of the piston that hit a small spring loaded ball valve in the head at tdc and so allowed steam in until the the ball valve closed again. Or the other way around, a pin sticking down from the valve that was hit by the rising piston.
Timing was the most crucial bit, because if the valve opened just a fraction too early there was no momentum to get it over the top of the stroke and the crankshaft would just oscillate backwards and forwards very fast. That is why on the vid they have fitted a basic flywheel, to get the momentum going.
Hope this helps
John
As is, it would not run, the vital part you are missing is the poppet valve in the head.
This used to be a standard conversion of two stroke engines when running them on steam.
Usually there was a striker pin sticking up on top of the piston that hit a small spring loaded ball valve in the head at tdc and so allowed steam in until the the ball valve closed again. Or the other way around, a pin sticking down from the valve that was hit by the rising piston.
Timing was the most crucial bit, because if the valve opened just a fraction too early there was no momentum to get it over the top of the stroke and the crankshaft would just oscillate backwards and forwards very fast. That is why on the vid they have fitted a basic flywheel, to get the momentum going.
Hope this helps
John
B
Bogstandard
Guest
Just in case I confused you, I have now woken up and done a little sketch of the type of valve that is required in the head for you engine to work.
In fact the exhaust port is in the wrong place as well, because as the piston is rising it will be attempting to compress the air that is already in there and might not reach the valve to fire it. If you look at my post I am doing the same sort of thing, but firing the poppet valve externally. Maybe Chuck can help on that one.
http://www.homemodelenginemachinist.com/index.php?topic=612.0
You have to vent the cylinder on the up stroke, by using another little valve, that closes when the poppet fires.
On the u-tube vid I suspect they are most probably using the original inlet and exhaust ports (modified) to allow the engine to breathe between firing.
I have seen somewhere on the net about converting glow plug engines to run on steam or air, but because the mind is getting a bit frail, I can't remember where. Maybe one of our other members can help on this one.
It is not such an easy solution as you think. But don't be put off by this, you will get there in the end.
John
In fact the exhaust port is in the wrong place as well, because as the piston is rising it will be attempting to compress the air that is already in there and might not reach the valve to fire it. If you look at my post I am doing the same sort of thing, but firing the poppet valve externally. Maybe Chuck can help on that one.
http://www.homemodelenginemachinist.com/index.php?topic=612.0
You have to vent the cylinder on the up stroke, by using another little valve, that closes when the poppet fires.
On the u-tube vid I suspect they are most probably using the original inlet and exhaust ports (modified) to allow the engine to breathe between firing.
I have seen somewhere on the net about converting glow plug engines to run on steam or air, but because the mind is getting a bit frail, I can't remember where. Maybe one of our other members can help on this one.
It is not such an easy solution as you think. But don't be put off by this, you will get there in the end.
John
B
Bogstandard
Guest
Zig,
I wish everything was a simple as that, but it is never the case.
The basics are - When the piston is at the top you then fire a charge into the cylinder, stopping the fire before the piston hits the bottom of the stroke, this is the power stroke. Then the piston starts on its way up, on the way up you have to open above the piston to atmosphere to prevent pressure building up on top of the piston before it reaches the top, this is the exhaust stroke. Then the cycle repeats itself.
The longer the stroke of the engine, usually the slower it runs. A fairly fast engine will be a 'square' engine where the piston diameter is equal to the stroke length.
The poppet valve system is the way to go if you want a 'fast' engine.
John
I wish everything was a simple as that, but it is never the case.
The basics are - When the piston is at the top you then fire a charge into the cylinder, stopping the fire before the piston hits the bottom of the stroke, this is the power stroke. Then the piston starts on its way up, on the way up you have to open above the piston to atmosphere to prevent pressure building up on top of the piston before it reaches the top, this is the exhaust stroke. Then the cycle repeats itself.
The longer the stroke of the engine, usually the slower it runs. A fairly fast engine will be a 'square' engine where the piston diameter is equal to the stroke length.
The poppet valve system is the way to go if you want a 'fast' engine.
John
C
compound driver
Guest
Hi
Using a poppet valve on a steam engine like that is going to admit steam 50% before and 50% after TDC. The 50% before TDC is the time you want the exhaust to be open and free to atmosphere. I doubt the poppet setup would come even close to a useable running solution.
try downloading a valve simulation for steam and put in teh 50/50 event times and see how the engine runs.
A rotary valve is a much more useable solution and was featured in a ME build some time back.
Cheers Kevin
Using a poppet valve on a steam engine like that is going to admit steam 50% before and 50% after TDC. The 50% before TDC is the time you want the exhaust to be open and free to atmosphere. I doubt the poppet setup would come even close to a useable running solution.
try downloading a valve simulation for steam and put in teh 50/50 event times and see how the engine runs.
A rotary valve is a much more useable solution and was featured in a ME build some time back.
Cheers Kevin
B
Bogstandard
Guest
Kevin,
I think Zig is looking for an engine that runs fast on air not steam, and as you know they have totally different characteristics. The poppet valve is for use on inlet only, not exhaust.
John
I think Zig is looking for an engine that runs fast on air not steam, and as you know they have totally different characteristics. The poppet valve is for use on inlet only, not exhaust.
John
C
compound driver
Guest
Hi John
You still have the same problem of cut off events. Steam and air are very different in an engine
but the valve events still play the same roll in the running of the engine. A 50/50 before and after either wastes a lot of energy either from steam or air, or acts to damp out the expansion part of the cycle.
Cheers kevin
You still have the same problem of cut off events. Steam and air are very different in an engine
but the valve events still play the same roll in the running of the engine. A 50/50 before and after either wastes a lot of energy either from steam or air, or acts to damp out the expansion part of the cycle.
Cheers kevin
B
Bogstandard
Guest
Kevin,
Can you please tell me where I have said that the valve should admit air (or steam) 50% before or after TDC.
John
Can you please tell me where I have said that the valve should admit air (or steam) 50% before or after TDC.
John
C
compound driver
Guest
John
Look at the drawings you put up! the poppet valve is opened by the piston and allowed to close when the piston drops back. What other valve events can you get from that. Unles you have a magice shrinking valve stem you can only get a 50 50 event.
Cheers kevin
Look at the drawings you put up! the poppet valve is opened by the piston and allowed to close when the piston drops back. What other valve events can you get from that. Unles you have a magice shrinking valve stem you can only get a 50 50 event.
Cheers kevin
