# Experiments with a solenoid engine...



## John Hill (Oct 11, 2011)

...I have been messing around looking for ways to improve on the typical "solenoid engine".


One avenue I have been researching is the best way to add permanent magnets to the 'piston' with the object of gaining a longer power stroke.


My best configuration so far is to make a soft iron/steel piston about twice the length of the solenoid coil, I put a permanent magnet each end, mounted in opposition so that, for example, there is a north pole at each end and two south poles induced in opposition at the mid point of the piston.

The coil I am using is 500 Ohm, 50mm long, 45mm diameter and 20mm bore diameter.

The armature, i.e. "piston", is 18mm diameter mild steel 110mm long.

There is a neo. magnet on each end, they are cylindrical 20mm diameter and 12mm long.

When DC power is applied the piston slides to the end of available travel which is limited by the magnets being unable to pass into the bore. Reversing the polarity slides it back the other way, almost 60mm.

I used electronic kitchen scales to measure the thrust at different points of the travel and found, somewhat to my suprise, that the force is almost uniform for the entire length of the stroke!

With 20V on the coil the force at the beginning of the stroke was measured at 136 grams, midway 127 grams and 122 grams near the end of the stroke.


I therefore have no hesitation in recommending this form of 'piston' to anyone seeking to build themselves a model "solenoid" engine!


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## Herbiev (Oct 11, 2011)

Sounds good. Any pictures ?


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## John Hill (Oct 11, 2011)

No pictures, yet.


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## Coilmotorworks (Oct 12, 2011)

I don't understand how this design gets movement with the magnets in the repelling position on the slug? Can you post a picture or diagram? A solenoid using a voice coil type arrangement will offer long travel with little mass moving. I think what you have made is an inverted speaker voice coil where the coil is stationary and the iron core (magnet) movers.


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## John Hill (Oct 13, 2011)

The coil does not move. The moving armature is a simple steel rod which is twice as long (more or less) as the coil. There is a magnet on each end which induce two opposed poles near the middle of the rod. 

These induced poles are repelled by one end of the coil and attracted by the other.

This is quite unlike a speaker coil in that it is a high mass device.


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## John Hill (Oct 26, 2011)

I made a little engine to prove my ideas regarding solenoid engines... 

http://flic.kr/p/az9wqs



... I dont know why the audio is so loud as the engine is really not quite that noisy..

As you can see this is a two "cylinder" machine and each cylinder is double acting. That may seem an odd way to arrange things but the double magnet and pole piece that makes up each armature is rather heavy and having two like this tends to balance things somewhat.

The engine can run quite slowly, just a little slower than in the video and it will even run without the flywheel. I think this demonstrates the long power stroke achievable with this arrangement of the armatures.

It is running on about 24V in the video and it goes quite a bit faster if I wind up the voltage to the maximum of my power supply which is 36V. The solenoid coils are quite happy and show no sign of heating at 100V which I suspect would be beyond the capability of the very light weight crankshaft I have made in this engine.


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## Blogwitch (Oct 26, 2011)

John,

That is one of the smoothest running solenoid engines I have ever seen, so it looks like your theory has worked out just fine.

I don't really understand the magnet thing. Are they there to impart a magnetic field along the length of the iron core, so you get a smoother action as it is going through the coil?

John


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## John Hill (Oct 26, 2011)

Thanks John..

The 'piston' is a steel bar which is twice the length of the coil. I put a magnet on one end which induces a pole on the other end of polarity opposite to that on the free end of the magnet, then I took another magnet and stuck that to the end of the steel but I flipped it over so that it was repelled by the induced pole. However, because the bar is quite long the second magnet completely overwhelmes the induced pole at the end of the bar so this, the second magnet, sticks to the steel too.

Now there is a steel bar with a N pole at each end and two S poles induced right in the middle of the bar.

Having the induced poles right in the middle of the coil puts them in the area of strongest flux density from the solenoid coil so that when the coil is energised these induced poles are very effectively repelled by one end and attracted by the other. I did some tests with some electronic kitchen scales and found the thrust was very even throughout the travel of the steel bar.

I hope that is clear..

John

PS, since the video I have been playing around with lubrication and alignment of the bits and got it to run a lot slower..now if I can find another solenoid coil I will be able to make a three 'cylinder' engine which would not have the dead centres top and bottom and should be able to run even slower.... ;D

PPS, I need a better scheme for fabricating the crankshafts, this style is far too flimsy but if I turn one out of solid how would I get the ball races into place for the piston rods?


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## Blogwitch (Oct 26, 2011)

One answer to that John, unless you leave one end of each crankpin unsoldered, you can't.

That is where split big ends come into the equation.

If you really want to put ball races in, then the unfixed ends would need to be able to come apart easily, either with grub screws (not recommended) or taper pins to lock everything together for running. Neither method is a really rigid fix. In fact for a three crank version, I personally don't think it could be got accurate enough to run smoothly without having each crankpin permanently fixed unless you put crank supports in.


John


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## John Hill (Oct 26, 2011)

John, that crank is just glued together (Loc Tite), but I think the flexibility issue is from the long unsupported piece between the cranks which is unavoidable with the big diameter of the solenoid coils. 

But never to fear, I do have a few options to consider!

BTW, we used to have an engine on the farm that had a ball bearing big end. It was something a local engineer salvaged from a war surplus aero engine, quite large internal diameter, so large that it could the threaded along the crank then locked in place with a couple of interlocking pieces that fitted around the crank pin.


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## Blogwitch (Oct 26, 2011)

John,

Early single cylinder BSA m/cycles had a bearing big end, but the crank disks were press fitted on around them.

A real PITA job when you had a knocking big end, or in my case, a disintegrated one on a small C15.

Big hydraulic presses became involved.


John


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## Admiral_dk (Oct 26, 2011)

Since this type of engine can have a power-stroke both ways by reversing the polarity of the electric power to the coil, you only need a 90 degree crank to have an engine with no dead spots ...!... just like a double acting steam engine.

This engine will be a perfect candidate for a MCU to control the coils. It can be done with a cam and four switches too, but the MCU will allow you to control speed and direction.


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## John Hill (Oct 26, 2011)

Admiral_dk  said:
			
		

> Since this type of engine can have a power-stroke both ways by reversing the polarity of the electric power to the coil, you only need a 90 degree crank to have an engine with no dead spots ...!... just like a double acting steam engine.



Yes, I am using reversing polarity via a relay that is under the cover on the top of the engine but I did not use a 90 degree crank as the 'pistons' are very heavy and the 180 degree crank allows one piston to balance the other, but it does have the dead spots. Perhaps someone might make a three cylinder, 120 degree, engine.


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## KirkMcLoren (Oct 16, 2013)

A voice coil transducer normally has one pair of poles. I actually like to make the whole plunger out of neodymium. There are thick washer sized magnets with a countersink for a screw. Probably used on shelving. These make a simple way of attaching non ferrous materials to a plunger. They are strong as well. I can barely get them apart.


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## BronxFigs (Oct 17, 2013)

Mr. Hill:

Love the design, and magnetic concepts.

Can you give us the source of your supplies. i.e. solenoids, neo magnets, etc?  Thanks.


Frank


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