Author Topic: Little Otto  (Read 8468 times)

Offline Alyn Foundry

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Little Otto
« on: May 20, 2022, 01:59:26 PM »
Little Otto.

Back in 1873 Nicolaus August Otto had a patent granted for a Caloric engine. We now call this type of engine a Vacuum or flame gulper.

The working principle is very simple. At piston TDC a port is opened to allow a volume of heated air to be drawn into the cylinder. This heated air is furnished by the addition of either a flammable gas or liquid hydrocarbon. As the piston descends towards BDC the admission port is closed, sealing the cylinder. As the heated air is now enclosed within the water cooled cylinder it rapidly loses its heat and becomes much smaller in volume. The volume decreases so rapidly that a partial vacuum now exists within the cylinder. Now atmospheric pressure provides the force against the underside of the piston and draws it up towards TDC. Once the pressures have equalised and then slightly increased within the cylinder, an exhaust valve opens to purge the remaining spent air. Thus the cycle repeats over and over until the fuel supply is turned off. Because these engines rely on atmospheric pressure as the power source they weren’t very powerful, as the force is directly related to the piston cross sectional area. However they found uses for driving sewing machines, Dentist drills etc and required little to no experience with things mechanical to operate.

The engine in question, rumour has it that Otto used it to pioneer his now well known 4 stroke cycle. The engine was kept at the Deutz museum for a while before it vanished. Recently a group member sent me some photos and a video of a full scale replica that he was building, needless to say I fell in love with the design and a model was planned. I decided that the flywheel from my Robinson hot air engine would be just right for this project and was used to provide the datum for all the other dimensions needed for pattern making.

That’s all from me for now. I am trying to finish the model in time for the Neunen show next month where, I hope, it will join it’s full scale replica’s debut.

Special thanks to Wayne Grenning from who’s research work I have borrowed.

The bottom picture clearly shows the engine in question in the bottom left hand corner.

Offline Alyn Foundry

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Re: Little Otto
« Reply #1 on: May 20, 2022, 02:31:14 PM »
Otto used a couple of interesting features in his design.

The gaseous fuel was only supplied on demand by the use of a valve that was incorporated into the port slide. Just a small pilot flame remained lit.

The Scotch yoke gave a rapid down/induction stroke followed by a much slower return stroke.

Offline Alyn Foundry

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Re: Little Otto
« Reply #2 on: May 20, 2022, 09:06:01 PM »
I approached Jason several weeks ago with the idea of a model, his help previously providing the last remaining patterns for the Brayton Ready-Motor project was invaluable. The CNC process ensures high accuracy and near perfect results.

We used the Robinson hot air engine flywheel as a scale datum and Jason then used the picture to find all the rest of the measurements using his program. I had callipers, rule and calculator to make my patterns….  :lolb:

I will now had over the reins to Jason with an overwhelming debt of gratitude for his help in getting yet another of the great pioneers in the world of IC into model form.

 :cheers: Graham.

Offline Admiral_dk

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Re: Little Otto
« Reply #3 on: May 20, 2022, 09:23:31 PM »
Getting the  :popcorn: and  :cheers: ready and thank you for the introduction on this historic engine  :ThumbsUp:

Per

Offline Jo

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Re: Little Otto
« Reply #4 on: May 21, 2022, 02:18:32 PM »
I will now had over the reins to Jason with an overwhelming debt of gratitude for his help in getting yet another of the great pioneers in the world of IC into model form.

We will have to wait for him to first finish the engine and gets his running before he posts the build  ::)

Jo
Enjoyment is more important than achievement.

Offline Jasonb

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Re: Little Otto
« Reply #5 on: May 21, 2022, 06:27:34 PM »
Oh well in the mean time for those that are interested I'll have to fill in with some background and the design.

It all started with an e-mail from Graham enclosing a copy of one of the patent drawings and suggesting that it would make a good candidate for my 24mm bore series of engines. I went back to him enclosing more images including the second one he shows in his first post which is of the patent model that was posted by Nick Roland (RMC Engines) on Smokstak forum. This model is held by the Smithsonian and this is a shot from the other side of what remains.



 Knowing his liking for using food tins as parts of his models I did suggest that a large Corned beef or Spam tin would make an ideal engine base He seemed keener that I make a pattern for a cast iron one. So discussing carried on and mention was made of using the existing Robinson flywheel which at 7" dia (178mm) set our scale and was soon followed by a printed drawing of Grahams with some sizes calculated from proportioning from the now known flywheel diameter.



I've moved on from the days of drawing a grid over and image and determining dimensions from that. Instead I used Alibre's "trace" function to import the image and then enlarge it to a point where the flywheel measure d178mm so that any further drawing on it would be actual size. had the image been a straight on side view I could have drawn directly over the image but a sit was at an angle I just drew a series of lines from point to point and used what they measured to start producing a drawing for the pattern.



The overall external shape was not too hard to draw out being little more than a taper sided box with a flange at the bottom and a few rounded corners and internal fillets.



Had the base just been a 5 sided box without the cutouts then it could have simply been made hollow as a "self coding" pattern but the need to form the various windows meant that a core and therefor a corebox to mould it in was needed. I started off by drawing up the core, well half the core actually as the other half is a simple mirror image that is only a couple of clicks away. The protruding parts of the core around the sides will touch the edge of the void produced in the sand by the pattern but as it only has them on three sides the top ones were made to protrude more and they locate in the core pockets produced by the core prints on the pattern which are the black coloured bits in the above photo.



we decided on an over scale wall thickness to the base of 6mm to ensure that the iron flowed easily and that bit of extra thickness should help to keep the chill away. This will only really show around the sides as the top and underside of the bottom flange will have 3mm machined off them and 2nn off the bearing supports.

A couple of images of the core being tried out for size within the pattern which was set to be semi transparent to help seeing if things were correct.







Once satisfied that all was as it should be it was a fairly simple job to form the negative of the core into a block using a Boolene Subtract and then just mirroring that to get the other half.



Well that's enough for now I've an engine to finish ;) Next time I'll write a bit about the CAM and the machining.








Offline Alyn Foundry

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Re: Little Otto
« Reply #6 on: May 22, 2022, 11:53:46 AM »
Many thanks for the introduction of the technical drawing side Jason. I personally find this aspect difficult to get my head around. I’d still be sat in front of the PC now, instead we’re nearly 2/3rds towards completion.

I should have mentioned that the original engine was water cooled, those with Eagle eyes might have spotted the spigots on the side of the cylinder? However, our rendition isn’t….

When I was developing the CHUK engine for Chester UK Ltd, I met up with Bob Sier at the Midlands model engineering exhibition. Bob had been one of the first customers and had got his engine running and on display. Whilst chatting he mentioned the fact that he’d bored his cylinder out to the maximum the casting would allow. He explained that the greater the piston cross sectional area was, the greater the power would be.

Based upon that knowledge I decided on an air cooled cylinder with the maximum bore that the scale could allow. We are using a bore diameter of 44 mm. Being just a model, display runs would be short in duration so wouldn’t need the extra complication of cooling tanks etc. From this value, under “ ideal “ conditions we could expect a maximum pressure of 34.65 PSI acting on the underside of the piston. The engine should at least move !

At this moment the cylinder pattern isn’t fitted with water cooling spigots but they can be added at a later date, as could an extra pattern to make a casting to create a water jacket.

The photo shows the base fitted with crankshaft and Scotch yoke assembly.

 :cheers: Graham.


Offline Jasonb

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Re: Little Otto
« Reply #7 on: May 22, 2022, 01:28:36 PM »
It would also be possible to make a simple bobbin shaped liner to give a water space that will seal at either end should it be required. That might even bring it backdown to 24mm bore :)

Offline Jasonb

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Re: Little Otto
« Reply #8 on: June 01, 2022, 07:18:05 PM »
With all the CAD work done it was time to fire up F360 to do the CAM and generate the tool paths to machine the patterns. The actual CAM would have been quite simple just an adaptive path to remove most of the waste, a finishing pass on the mainly near vertical surfaces and then a couple of small horizontal paths to clean up the top of the core prints and the top of the bearing supports.

However as the pattern is close to 105mm tall I would have needed at least that amount of tool sticking out beyond the collet chuck if I was going to avoid crashing that into the work, F360 certainly turned the screen red with masses of collision warnings even when using an extended shank cutter with 30mm projection. The answer was to split the piece into several layers which could correspond to the layers of wood I would be using to build up the pattern from.

You can see down the left hand side I have a number of adaptive and then ramp paths which are numbered in the order they will be cut.



The actual order they are listed is to allow me to run the simulation on a "solid" block starting from the top and fooling F360 by having a very long cutting tool so it doe snot show any collisions. This shows it about to do the last ramp cut where the blue waste will be removed to leave the green finished surface below.



So time to put it into practice. As I don't have quick change tooling I opted to do all the adaptive cuts first with a 6mm 3-flute cutter just screwing each layer onto the one below. Then I took them all apart, changed to a 4-flute long shank 4mm ball nose cutter and worked my way up again, this time screwing and gluing the layers together.

First layer roughed out


Second layer screwed on and adaptive cut just started


Second layer complete


And the third showing the amount of chips each layer produced


All 5 layers roughed out


With them all taken apart and the tool changed it was time to start the finishing cuts


And all done



Offline Jasonb

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Re: Little Otto
« Reply #9 on: June 01, 2022, 07:35:52 PM »
The core box was not so deep and I was able to get away with using extended shank cutters without having to resort to multiple layers. To reduce the amount of cutting I built up the block from one full size base layer and then just added three strips at the ends and top. Similar roughing and finishing paths were again used with the same two tools. The last path was the drilling of two holes for location pegs to keep the two halves of the box lined up, I used an HSS brad point drill for this as they produce a nice clean entry hole



With the second half done which was basically a mirror image of the first a couple of 6mm steel dowels were put in the peg holes and I was pleased that the two halves pushed together nicely.



It was just after this that I got an e-mail from Graham suggesting that some writing should be added to the inside of the base so that was added to the CAD model and then the CAM done for that. As it was 30mm deep into the corebox my engraving cutters were not going to be long enough so I opted to use a 6mm spotting drill which had enough length to still be able to get a good grip of and a reasonably pointed end. The halves were clocked in again on the mill and the writing engraved - right way round on the core box = wrong way round on the sand core which should hopefully equal right way round on the casting.



All that remained was a very light sanding with 180g paper and a coat of paint, we have just been using grey rattle can primer which seems to work OK.








Offline Johnmcc69

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Re: Little Otto
« Reply #10 on: June 02, 2022, 04:03:30 AM »
 :ThumbsUp: :popcorn:
  very nice work!

 John

Offline Zephyrin

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Re: Little Otto
« Reply #11 on: June 02, 2022, 08:30:38 AM »

Quote
The overall external shape was not too hard to draw out being little more than a taper sided box with a flange at the bottom and a few rounded corners and internal fillets.
this little sentence contains a huge amount of skills...

draft angle, internal filets, rounded corners...
it looks so simple that I wanted to try to make a 3D model for the casting of my next engine, and no it wasn't easy...
I'll finally cut the engine in an aluminum block, as usual, if I want to have a finished engine one day...

Offline Jasonb

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Re: Little Otto
« Reply #12 on: June 02, 2022, 06:20:48 PM »
Compared to a typical crankcase kike those of the Midget or Lightweight that I built it is a much simpler part.

If you take a look down the left of the page to the blue horizontal break bar there are just 4 sketches which are extruded these are just rectangles and a circle, Alibre lets me include a draft angle to the extrusion.

There is then one additional draft angle added to the sides of the bearing supports.

The remaining items are just internal fillets which are fairly straightforward to add though a bit of care can be needed to get them in the right order. I probably could have done away with a couple of those if I had included them in the sketches but it's often easier to edit if they are done separately


Offline Alyn Foundry

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Re: Little Otto
« Reply #13 on: June 02, 2022, 06:50:17 PM »
The mechanicals have been completed for several days now. Getting the little blighter to run is proving rather difficult.

 :cheers: Graham.

Offline Jasonb

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Re: Little Otto
« Reply #14 on: June 02, 2022, 07:05:36 PM »
I was wondering if you could try it without the exhaust valve but just use a bit of shim stock held by one of the head fixings to see if that allows it to breath a bit easier, much like the exhaust on Nattie.

Would also be interesting to hold it on it's side in the vice and see if it runs as this would eliminate the weight of the yoke having to be lifted on the power stroke.


 

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