Introducing... The Hippie Crack Engine

[brian hughes]

*** Can I/how do I embed videos in a post?***

I've made a bit of progress with a long running project, based round discarded Nitrous Oxide bottles. Basically, kids buy full bottles here in the UK for £20-£30, drive to a layby, have a party, and discard a nice chunk of thick walled tube, 70mm diameter by 300mm long by 2.4mm just waiting for my angle grinder and welder. I've partially built an engine round a couple of them, almost successfully just using basic tools, and found/upcycled/hardware store components. I've got to the stage of applying some heat and measuring the pressure fluctuations. At the moment, the vacuum will suck up a column of water about 400mm- this isn't remotely enough to get any read force out of the power diaphragm.

Things will become a lot clearer once I can post a video, but for now, my suspicions are with the displacer. I think there's too much dead space. Am I correct in thinking.... there has to be a compromise between the regenerator and the compression ratio.

[Tom Booth]

*** Can I/how do I embed videos in a post?***

The video would have to exist on the internet someplace, like YouTube or Vimeo. Then just copy/paste the link into your post. It helps to leave a few blank lines between any text and the video URL.

The forum program will automatically embed the video.

Under the YouTube video hit "share" then "copy link" (gray circle on left:

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Then come here and paste the link into the post from the clipboard.

[brian hughes]

https://www.facebook.com/10000927902974 ... l/?app=fbl

[Tom Booth]

...At the moment, the vacuum will suck up a column of water about 400mm- this isn't remotely enough to get any read force out of the power diaphragm.
...

That looks like enough pressure to at least keep the engine turning itself over to keep running. There is also some loss using a long flexible tube.

[brian hughes]

With the first attempt with a diaphragm, and a much shorter coupling tube, it wouldn't keep running- my guess is that the force produced under that configuration wouldn't overcome the frictional losses. I think, though, that I'm onto something with the water manometer. The displacer was pretty crude- pot scouters stuffed into soldered up tin cans. The next displacer will be solid without a regenerator- to ask the question again, am I right in thinking that there's a compromise between regeneration, and compression ratio? That a regenerator, by it's very nature, will introduce dead space, and reduce the compression ratio?

[Tom Booth]

With the first attempt with a diaphragm, and a much shorter coupling tube, it wouldn't keep running- my guess is that the force produced under that configuration wouldn't overcome the frictional losses. I think, though, that I'm onto something with the water manometer. The displacer was pretty crude- pot scouters stuffed into soldered up tin cans. The next displacer will be solid without a regenerator- to ask the question again, am I right in thinking that there's a compromise between regeneration, and compression ratio? That a regenerator, by it's very nature, will introduce dead space, and reduce the compression ratio?

What do you mean by "it wouldn't keep running"? Would it run for any length of time at all?

Your question about the regenerator probably does not have any clear yes or no answer.

It might help if you could post some photos or video. Showing the actual displacer and power piston.

So far, the only resident expert I know of on the workings of an experimental hippie crack engine is brian hughes.

[brian hughes]

On the first test with the heat applied.... it wouldn’t keep running beyond one cycle- I would have thought that if it’s ever going to work, it should just need a quick nudge to get it out of a dead spot, after that, it’s on it’s own. I could tell *something* was happening- it was harder to crank backwards than forwards.

To rephrase the question about the displacer.. do I build a solid- by solid I mean hollow and gas tight displacer piston- or a regenerator piston stuffed with steel wool? The latter, AIUI, will introduce dead space, and reduce the compression ratio, so my understanding is that there’s a trade-off between power to size ratio efficiency, and power to heat ratio efficiency. Because I’ve more heat than I know what to do with, and plan to capture the heat from the cold side, I’m leaning to the former, and abandoning regeneration for now.

Photos of the displacer piston, and power diaphragm to follow.

[Tom Booth]

My personal issue(s) surrounding "compression ratio" are 1. I come from a gasoline engine - small engine repair background where the term compression ratio applies to the power cylinder and ignition chamber.

A Stirling engine has this big displacer compartment added on. I guess maybe that could be viewed as an ignition chamber of sorts

Anyway, some people start out with a tin can displacer. Later adding a regenerator by wrapping the displacer with steel wool. In that instance the "dead air space" is actually being reduced.

Generally the working fluid mostly takes the path of least resistance, so, from what you described: "pretty crude- pot scouters stuffed into soldered up tin cans". The air may be just going around this apparatus rather than through the regenerator, so the regenerator would just be useless weight plus dead air space, especially if the pot scrubbers are tightly packed and resisting air flow.

One possible problem I noticed is that it looks like the heating/displacer chamber is being heated by a big flame in the center part of the can.

Usually heating up the far end, furthest away from the cold water jacket would be more effective. It looks like there is some blue silicone or something there though. Is that to seal the engine? Or what is that blue stuff.

Regardless, that is probably where the heat should be applied if possible, but if that's silicone it would burn up.

[Tom Booth]

Imagining where the displacer/regenerator is positioned in the chamber:

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Supposing it is all the way to the rounded end with the blue paint (if that is just blue paint).

When the displacer is pulled back what looks like maybe 2 or 3 inches, the air in the chamber would only just barely then begin to be fully exposed to the heat where the flame is positioned. By that time, if the timing is the typical 90° advance, the piston is already half way down the cylinder past TDC.

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With the heat applied right at the nose, where the blue paint is, the heat will begin to be introduced a little before Top dead center and heating will continue through TDC.

I assume if you had already tried that the blue paint would have been burned off.

With the flame that far back towards the water jacket, I think that would have the effect of mostly just transferring heat from the regenerator to the cooling water rather than to the air which is being driven out to the end.

The hot regenerator is moving between the flame and the cooling water, just heating up the water, is how it looks to me.

[Tom Booth]

On the first test with the heat applied.... it wouldn’t keep running beyond one cycle- I would have thought that if it’s ever going to work, it should just need a quick nudge to get it out of a dead spot, after that, it’s on it’s own....

This is very often far from true. I'd even say that it almost never happens, especially when first starting up. Possibly when restarting after being stopped. This can be even more true when a regenerator is involved.

To a great extent, a Stirling heat engine works by acting as a heat pump, keeping the heat separated, or isolating the heat, pushing it back where it came from.

The heat pumping action is something that results from the compression and expansion, combined with the movements of the displacer and/or regenerator while the engine is running.

It is quite normal that a heat engine will need to be turned over mechanically by hand several times before this heat pumping action starts to kick in.

Think of a refrigerator before it is plugged in. It will need to run for some time before the ice box begins to get cold, and the longer it runs the colder it gets.

As a DIY heat engine generally does not have any kind of start up motor, it may need to be turned over several times by hand before there is enough heat separation for it to run on its own.

It would also help to apply heat to the engine as far away from the cooling jacket, or cold end as possible to assist start up.

Anyway, it is not at all unusual for these engines to be stubborn about starting up and will almost always run quite slowly, have a number of false starts, and when they finally do get going, build up speed and torque gradually.

Limping along and needing a lot of assistance to get started is more the rule than the exception. Your engine with the.slider track also has considerably more frictional resistance than a typical model, so if anything would likely need more help getting started.

[brian hughes]

Thanks for the replies, although I would be a little bit skeptical about having to spin engines before they start running on their own. I would have my doubts that this is anything to do with the Stirling Engine operating principle, and more to do with thermal lag and bearings warming up. If there's any elasticity in the system from a diaphragm, that could make a difference.

I've now sourced some 63.5mm exhaust tube, which is a perfect fit inside the cylinder, so the experimenting can continue with a solid displacer, and a steel wool stuffed displacer for comparison.

[Tom Booth]

Thanks for the replies, although I would be a little bit skeptical about having to spin engines before they start running on their own. I would have my doubts that this is anything to do with the Stirling Engine operating principle, and more to do with thermal lag and bearings warming up. If there's any elasticity in the system from a diaphragm, that could make a difference.

...

The reasons are debatable, but rarely will even a well manufactured Stirling engine start running the first time it's turned over, and even when it does get going, may run a little haltingly the first few revolutions, or even need to be restarted a number of times if it is not up to full "operating temperature".

I've seen probably hundreds of examples of this, but taking a random example. BTW, I never saw this video before now, but it is far from unique:

https://youtu.be/fdD1JmFm2o8


The point being, to give some encouragement, I would not necessarily write off your engine as a failure just because it doesn't start up immediately the first time it's turned over. These engines typically need some coaxing to get them started.

[brian hughes]

An interesting point that perhaps got missed in the video was the engine running on after the heat source was gone- there’s quite a lot of thermal mass in that little engine that needs to be “charged”- and my educated guess is that’s why you won’t get instant starting... and why Stirling Engines are difficult to throttle and govern like a steam or IC engine.