Stirling engine startup

[walnotr]

Hello all, this is my first post and hope it is not a question discussed ad nauseum. What is the best or proper way to start a stirling engine? The one I have built seems to take an amazing amount of fiddling around to get running. All comments are welcome.

Thanks,

Steve

 

[hitnmiss]

Heat it up and spin it ;D

Mine wouldn't hardly run then I took it all apart and concentrated on making it leak free and as friction free as possible.

Works great now.

 

[black85vette]

Welcome to the board.

What type of engine do you have? There are several configurations and each has its own issues.

In addition to the two suggestions above I would add: Balance. Make sure all parts are well balanced so the engine does not have to overcome gravity.

 

[walnotr]

Well it is relatively leak free. I have dunked it in a bucket of water and looked for bubbles while turning it over. I guess it may just be the nature of the beast to heat the begeezus out of it and rapidly spin the flywheel to get it going. This is my first engine build and had never even seen one of these engines run except on YouTube. Most are already running or have been stopped then restarted as the video begins. Does it matter where the displacer or power piston is while the engine is being heated or do you typically apply the heat and spin like crazy for several minutes until it gets going?

 

[black85vette]

Should not take more than a gentle push on the flywheel to get it going. Does need to get up to temperature before it will run. Can't see the far side of your engine. Timing should be 90 degrees offset from the displacer to the power piston. That also sets the direction the engine will run.

How smooth does it turn? It should turn over really easy and smooth. Mine will go 8 revolutions without heat and a moderate flick of the flywheel.

 

[vlmarshall]

This is my first engine build and had never even seen one of these engines run except on YouTube. Most are already running or have been stopped then restarted as the video begins. Does it matter where the displacer or power piston is while the engine is being heated or do you typically apply the heat and spin like crazy for several minutes until it gets going?

Check your timing. I built mine without the flats on the crankshaft called for in the plans, so I could experiment with timing... and founf that a tiny adjustment can make a big difference.
Sure, the Stirlings on YouTube (like mine) have been preheated off-camera, do you really WANT to watch an extra 30-45 seconds of burner operation? ;D

http://www.homemodelenginemachinist.com/index.php?topic=5329.0

My Stirling takes about a minute of torch heat before it starts with a quick flip. Starting position doesn't matter, displacer up or down, but if the working piston is neat TDC, it'll probably creep downwards as the air in the engine expands.

Mine won't flip over more than half a revolution when cool, compression makes it kick back.

 

[vlmarshall]

My displacer leads the working piston by 90 degrees, so looking at your picture, flip the top of the flywheel toward the cylinders.

 

[mklotz]

A gamma Stirling like you show should be almost self-starting.

On my gamma, I rotate it until the power piston is at TDC then apply heat to the displacer hot end. Within a minute or so, the power piston will begin to move outward because of the expansion of the gas. At that point, a simple flip of the flywheel will set it running.

Are you using a proven design or have you designed this yourself? Stirlings can be unforgiving if the proper mathematical relationships between power and displacer aren't followed. Also, what material is the hot cap? Looks like brass in the picture but it's hard to tell.

 

[walnotr]

Thanks for the comments. I have been lurking through this forum for a couple of weeks now and have really appreciated the talent and helpfulness you all exhibit. The design is a somewhat scaled down and redesigned version of the engine published in The Home shop Machinist back in Nov 2005. I tried to maintain the same displacer/power piston volume ratio but am not quite sure if it would benefit from a smaller diameter power piston. The hot cap is indeed made from brass. The center section has been turned down to have about a .015" wall thickness to try and keep the heat on the hot end as much as possible.

I just ran it again starting with the "Marv position" and paid attention to how the compression feels as it is going through a revolution. As it heats up, for lack of a better term, the "compression point" rotates to about 20 degrees before BDC then takes a little push to get past the hump and start to kick back up to TDC. It will continue like that until the energy in from my hand is overcome by the power generated by the engine. It still takes many smacks on the flywheel to get it to that point though. *knuppel2*

The displacer has a 1" stroke running in a 1" bore. The can is .950" with about .020" clearance at each end of the stroke. The power piston is .750" on a 1" stroke. Do you think a smaller diameter piston/stroke is in order?

Regards,
Steve

 

[zeeprogrammer]

Welcome to the forum Steve.
Very nice looking engine.
Just how wide are those fins? Just curious.

 

[mklotz]

Some guidelines for Stirling design...


1. The length of the displacer chamber = 3 times its diameter.

2. The length of the heated chamber = 2/3 of the length of the displacer chamber (cylinder).

3. The length of the cooling chamber = 1/3 of the length of the displacer cylinder.

4. Swept volume of the displacer = 1-1/2 times the swept volume of the power piston.

5. Length of the displacer = 2/3 of the length of the displacer cylinder.

6. Stroke of the displacer = 1/3 of the length of the displacer cylinder.

Brass is not the best choice for a hot cap. Plain steel or stainless is much better because of their relatively poorer thermal conductivity.

 

[David Morrow]

The late Jerry Howell has some very good info on his web site including a section on "Why my stirling engine won't run."

http://www.jerry-howell.com/Menu-4.html

 

[Deanofid]

Hello Steve; (nice to see you here!)

I don't know a lot about this type of engine, so I just have a comment, more than a suggestion. I see some kind of bearings on the crankshaft. If they are sealed bearings, it could be that they make a bit too much friction for this engine to run until it gets quite hot. Plain, or shielded bearings might be better.

Dean

 

[jpaul]

Good looking build!

Here is a link on the build of this (your) engine. http://bbs.homeshopmachinist.net/showthread.php?t=18894 If this address doesn't work search the site http:/bbs.homeshopmachinist. net using "stirling + Cross" I think you will find quite a few suggestions.

 

[walnotr]

Thanks Marv, I went back and checked the dimensions and this is how they came out. I built the first version of this engine over a year ago and decided to tinker with it more to see if it could be made to run cooler and better. After I got this hot chamber made I started thinking about a glass section to go between the brass cap and the cooling chamber. I might try that if I can find a piece of 1" ID to fit into the design and can figure out a good way to put it all together.

1. The length of the displacer chamber = 3 times its diameter. Actual length 3.094

2. The length of the heated chamber = 2/3 of the length of the displacer chamber (cylinder). Little short 1.806

3. The length of the cooling chamber = 1/3 of the length of the displacer cylinder. A little long 1.288

4. Swept volume of the displacer = 1-1/2 times the swept volume of the power piston. actual 1.604 so a tad on the high side.

5. Length of the displacer = 2/3 of the length of the displacer cylinder. Pretty close 2.025"

6. Stroke of the displacer = 1/3 of the length of the displacer cylinder. Pretty close again 1"

It looks like the two areas I'm off is length of the heated/cooling cylinder and the ratio of the Dv to Pv. When I was doing the latest rework, I calculated the length of the heated chamber so that the cooling fins on the cooling chamber would extend to the aft most point of the extended displacer. I think I said that right. The original design had them about equal. I know the little beast runs but do you think these variations would make it that much harder to start?

Other thoughts for changes are to replace the heavy 12L14 power piston with graphite (thanks to my brother), a heavier flywheel, and some counter weighted crankshafts. I'll post results when those things are done.

Zee: Thanks for the complement. The fins are .040" on .080" centers

Hi Dean: The bearings are abec 7 and are very free running. Shielded not sealed. (Sounds a little James Bondish)

Regards,
Steve

 

[NickG]

Steve,

The ratio of swept volumes Marv mentions is about the minimum. The higher the ratio of swept volumes displacer to power piston, the less heat you will need to run it.

You don't need to stick rigidly to those design parameters, you might have a better chance if you did, but as long as the following is ok it should run.

very free running (low friction) - With the hot cap off .. the whole assembly should spin over by hand 10 or more revolutions with a flick of the flywheel.

as leak free as possible - test in water like you did but connect a tube to it and blow into it somehow. Mine was easier as it had totally separae cylinders connected by a tube. Obviously no leaks are allowed around the hot cap / disp cyl interface. (this was what stopped mine running at first!) Only small leaks allowed around displacer piston rod. The power piston should be a nice sliding fit in its cylinder.

A decent ratio of swept volumes. I originally designed mine with 1:1 ... it was never going to run, so I made my displacer a bigger bore and increased the length ... this just helps prevent heat transfer from one end to the other.

Have you definitely got the cranks at 90 degrees to each other?

By the sounds of what you said, with the compression point shifting around but not far enough, this suggests the ratio of swept volumes is too small to me. The easiest way to change this would be to make a new power piston and cylinder with a smaller bore ...say 1/2". Could you sleeve the existing one and turn the existing piston down?

Nick

 

[walnotr]

I really should pay more attention to my post. Thanks for the added info NickG. I have contemplated making a new power piston/cylinder and may even get one done in the not too distant future. I'll have to look over the drawings to make sure it can work. I believe the port is in the center of the bore. There is a plug on the side of the engine that could be removed to inject air. I will give that a try. There is only a few more weeks before everything is shut down for the winter so there may not be a lot of progress in the near term.

Changing the design of the hot/cold end did make an apparent difference in the temperature the engine will run at. The hot end cap does not need to be red hot to run anymore. It is still hardly discolored and can run off the heat from a gas stove.

Regards,

Steve C.

 

[Longboy]

...........You want to heat the air in the engine, not cook the engine! Your hot cap made from brass is the wrong material. Brass and copper used as a hot cap will surely cook your engine. Mild steel, stainless steel or Pyrex should be used here. You say you heated it red hot and the cold side doen't have enough temp differential to run cuzz the heat migrated in a hurry with the brass hot cap! If you cant keep your finger on the hot cap mounting block for a few seconds its too hot! Invest in a $15 infared temp sensor. The radiator should show a 50 degree drop from the front to back. Remove the set screw (plug) from the side of the engine and use a blow off nozzle with rubber tip against hole with your air compressor turned down to just 7-10PSI to check for leakage. Soap suds around the cyl. to mounting block visualises leakage. Displacer push rod leakage is allowed but no where else. Think about "clearances" instead of "ratios". The displacer can't rub the inside of hot tube and the engine stroke should bring the displacer close to each end of the hot tube. Your dimentions are fine. Step check each cylinder. Block off air transfer passage and pull cold piston back in its bore and release, it should "suck" back in. Put your ear on the displacer cyl. and move the pushrod for the displacer in and out and rotate. You do get some wobbleing between the gland and pushrod. You will hear any rubbing. Displacer pushrods are notorious friction points thru the gland bushing, 3 drops WD-40 here. Graphite for power pistons the best idea since the flush toilet!......................Two reasons a Stirling doen't run if phazed right, friction and leakage. Longboy.