A Timing question

Discussion on Stirling or "hot air" engines (all types)
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William42
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Joined: Thu Jun 05, 2014 6:05 am

A Timing question

Post by William42 » Thu Jun 05, 2014 7:22 am

I've read that the Piston should trail the Displacer by 90 degrees. Why is this so?

Is the 90 degree offset true for all operating conditions or is it just a good average to use that works under all operating conditions? I mean, if there was a way to advance or retard the timing of the piston because of certain conditions such as load or temperature or whatever, is there even a need? Is the 90 degree offset of the piston in relationship to the Displacer absolute or might there be a need for adjustment of the two when certain conditions are met?

Something I would like to know for determining my design.

Ian S C
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Re: A Timing question

Post by Ian S C » Fri Jun 06, 2014 3:41 am

Bill, 90* is a good starting place, if you can adjust one of the cranks a small improvement can be made. Part of the fun of making these motors is experimenting with them to see if you can get a little more power out of it. Ian S C

William42
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Re: A Timing question

Post by William42 » Sun Jun 08, 2014 9:19 am

In an effort to try to understand the principles involved in the Stirling engine I opened my CAD and started to draw. Please tell me if I'm on the right track with my thinking. To simplify the drawing I just used a 1" wide cylinder and a 1" wide Piston with a 2 1/2" stroke for both the piston and the displacer and went from there.

I put the Piston at the top of it's stroke (2 1/2" inside the cylinder) at which point the air inside the cylinder would reach maximum compression. At this point I assumed that the hot air needs to be released from that end of the cylinder so that it would act on the piston forcing it toward the crank. I then drew the Displacer 1/8" away from the piston and made it's length 5", for lack of a better number.

Next, I drew another piston next to the first one 1.25" lower than the first to account for 90* of rotation. Then another piston 1.25" lower than the second one. I drew four pistons to show it's relationship inside the piston during it's 360* travel. One at 0*, one at 90*, on at 180*, and one at 270*.

Because I know that the compressed, heated air needs to be released from the hot end of the cylinder when the piston is at 180*, I left an air-gap of 1/8" between the displacer and the piston and drew the displacer 5" away from the piston. Because the piston is at 180*, that would put the displacer at 270* and traveling away from the heated end.

I drew the displacer above each piston 1.25" either farther away or closer to the piston depending on the degree of rotation it was at. When the displacer was at it's highest point in the cylinder (at the hot end), I left an 1/8" air gap and made that the length of the cylinder. The cyininder created became 10 1/2" long.

My results are these: When the piston was at 0* (no compression taking place), the displacer was at 90* and had displaced right around 50% of the hot air to the cool side. This seems like an ok statistic to me. The piston is compressing both hot and cool air but the farther it travels the less hot air it has to compress because the displacer is also displacing the hot air to the cooler side.

Piston- 90*, displacer- 180*. 100% heat displaced to the cool side. This seems ok to me too because now all the piston is doing is compressing only cooler air 50% into it's stroke.

Piston- 180*, displacer- 270. (Both are now traveling 1/8" apart in the same direction) 50% heated air expanded. There is no cool air anywhere in the cylinder. At this point the piston has compressed the air as much as it's going to but since the displacer has already let the air start to expand, I'm wondering how much of an affect it's going to have on the piston just prior to this point. The air started to expand before the piston reached 180*. It will surely slow the piston somewhat but enough to seriously affect performance, or not let the engine work at all? Hmmm. This part of the sequence bothers me some.

Piston- 270*, displacer- 360*. 100% heated air expanded. Still no cool air in the cylinder. I'm hoping that the piston had enough enertia to overcome the initial expansion just before it reached 180* and will now except the expansion and go into it's power stroke.

Piston- 360*, displacer- 90*. 50% heat displaced to the cool end. The cycle repeats.

Again, the size of the cylinder and other parts is only for reference. I just needed to try to understand the concept. The only real problem that I see, for the limited knowledge that I have, is with the "Piston @ 180*, Displacer @ 270*" as noted above, and the fact that there is no air in the hot end of the cylinder for a period of time. It has to be made instantaniously as the cool air is displaced. I don't know if that is a problem though or maybe I got lucky and that's the way it's suppose to be.

I guess what I really need to know is if I made an engine with it timed the way I stated, will it work efficiently or do I need to make serious revisions because I have it all wrong?
Thanks again,
Bill

Ian S C
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Re: A Timing question

Post by Ian S C » Mon Jun 09, 2014 4:29 am

If you are going to try different angles, start at 90*, then adjust by 2*+ and run, then another 2*, up to the stage where the performance drops. Go back to 90*, then adjust by 2*--, and on till performance drops, you will need a brake to measure torque, and a tacho to measure the revs, that way you can work out the power increase/ decrease. The revs may go up, and the torque go down, or other way, you may find that you get the same power over quite a range. It's tedious, time consuming, and quite interesting. Ian S C

bertyquaye
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Re: A Timing question

Post by bertyquaye » Tue Dec 13, 2016 8:37 am

My thoughts on displaced piston timing is not to alter the timing but to alter the speed that the displaced arrives at this timing and how long it may stay at this position before that magical 90' is reached. This will require either a very complicated , adjustable linkage or an electronic mechanism. I don't know if these are feasible considering the power of the engine and that needed to drive the "new" linkage / electronics.
This only thoughts at present but one day-----------

Ian S C
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Location: New Zealand

Re: A Timing question

Post by Ian S C » Wed Dec 14, 2016 2:12 am

All of my BETA and GAMMA motors are set at 90*,the ALPHA motor is a Ross Yoke type, and the angle is somewhat different, 120* I think I saw written somewhere. My other motors are either Ringbom, or free piston, and they set their own angles to suit the operating conditions.
with BETA and GAMMA motors it is worth having one crank that can be adjusted. If you have a Prony Brake, and a Tachometer, you can do a series of tests moving the crank angle first + (say) 5* at a time, then back to 90* then - 5* at a time until you find the angle that gives best torque and revs, the greatest power out put, and on a small motor this will be in fractions of a Watt.
Ian S C
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cbstirling2
Posts: 156
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Re: A Timing question

Post by cbstirling2 » Sun Jan 01, 2017 7:14 am

Ian S C wrote:All of my BETA and GAMMA motors are set at 90*...
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From the sounds of it, you did that experimentation and found that 90 degrees is best anyhow?

From your own experience, do you ever think that a variable timing would be useful?

Also one more question, from what I can figure it out, it seems that an alpha engine is the most powerful
, then a beta configuration, then a gamma configuration.
Do you concur?
CBStirling2

Aviator168
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Re: A Timing question

Post by Aviator168 » Sun Jan 01, 2017 2:56 pm

the ALPHA motor is a Ross Yoke type, and the angle is somewhat different, 120*
Yeah. 90* offset is not optimum for alpha engines.

Ian S C
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Location: New Zealand

Re: A Timing question

Post by Ian S C » Mon Jan 02, 2017 2:00 am

I think you will find very little difference in the power of BETA and GAMMA motors, the GAMMA motor will have a little more dead space, the BETA motor can have a little more friction in the moving gland for the displacer rod through the piston.
The Ross Yoke motor can have the angle changed by altering the dimensions of the yoke. In one of the Stirling Engine books (can't remember the author) there are plans for an adjustable yoke.
Ian S C

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