Make your Stirling Engine yourself

Our Stirling Engine competition atracted a few students from abroad. So, in this column you will find some basic information in English. And perhaps, in May we will have an opportunity to share our expierences with Stirling Engine building with boys from a foreign country.

From now on, you can contact me on my email address jiritom@seznam.cz

Thank you for your interrest.

Jiri Toman

So, who is the competition for ?

– It is for high school students (from 15 to 19 or 20) not for university students. If it is just you and if you want to make your Stirling engine yourself, write me and you can get a set of pre- made components, bearings included. In this case, you have to eather take part in the competition or return the set back. (Of course, you can make any of the component yourself)

What is the main goal?

– The main goal is to build a functional model of Stirling Engine and take part in the Stirling contest

in May 2015, 16 etc.

What are the basic conditions?

– Your engine must be built from a tin 100mm in diameter and 65mm high = an average tin (or you can make this shape yourself). For construction of the engine you can use drawings and pictures in this blog, but you can do everything your way. The engine is powered with a gass flame for 2 min. And the revolutions are displayed on the computer. The highest are noted. Everybody has two runs.

What will you compete in?

– The main discipline is – REVOLUTIONS. How fast is your engine? You”ll reach approx. 400 rpm with an engine made by an ordinary way. This year record was 1804 rpm It means that you must try to make things very carefully and think on the basic principle of the hot air engines.

– But not only the revolutions. Your engine is checked by a “commetee” (which is me and my friend Milos) and especially your inventions and inovations are appreciated.

– And for the third – the design of your engine is checked by experts of the National Technical Museum where the competition takes place.

What are the main principles?

– Try to get as much heat into the inside air or Helium (which is better) and try to cool the top as well as possible. So, try to make the biggest differrence between the hot and cold side.

– The displacer and the piston are oriented at 90* by each other (not 180*)

How can you influence the turnings of your engine?

– By better heat transfer, by some dimensiones like the piston stroke, by the displacer stroke, by the mass of moving details, by diminishing of friction, by choosing good materials, by precise work…

……

I wish you good ideas and a lot of patience

177 years ago, explosive combustion engines did not exist. Heavy machines used mostly steam engines, smaller powers were represented by hot-air engines, invented by Stirling in 1816.

Principle:

A closed volume of air (gas) is heated on one side of the engine and cooled on the other side. Displacer piston moves the air from one side to another. A working piston is influenced by the overpressure and underpressure that arises inside the engine. The piston drives the crankshaft and crankshaft drives the displacer. The greater is the difference of temperatures of hot and cold sides, the greater is the engine output. Theoretical cycle of the Stirling engine comprises two isotherms, two isochors and its efficiency is relatively high. But use of the heat on the input is very low and so the output of an engine is small.

Participants make their own Stirling engine from the components received (price approx 500 CZK). However, some parts needs to be completed. Any of the components can be replaced by another – your own one. Only two parameters must retain: The diameter of the chamber – D = 100 +/- 3 mm and the height of the chamber H = 65 +/- 3 mm. This is guaranteed by a normal tin can. The principal criterion for the evaluation of the engines is their speed. A digital tachometer will be used to measure the engines speed (at constant gas flame). At home, the engine can be tested at the smallest gas ring set on the smallest flame (flame dial corresponds to less than the security fence burner). During the competition, after the warm-up and start-up (approx 2 minutes), the speed will be measured for 60 seconds. The greatest speed will be recorded. Design of the engines as well as technical improvements will be evaluated. For more info see the website of the competition.

– Use two-component epoxy adhesive to make joints (for instance Bison epoxy).

– It is handy to make two spots against each other on a shaft and also make a little “key” from a metal sheet # 5.5 mm.

– Because of an angle of 90°, the thread in a crank handle is made by a second stage of a screw tap M5./ M5. If rigidity is not sufficient, it is good to insert a pad between position 12 and 10.

– Slots for connecting rods in positions 5 and 16 should be cut through using a saw. Fix the pivots any way you like.

– The piston moves in a glass cylinder with a small clearence that should be filled up with an oil.

– Grind the chamber (tin) using emery/sand paper on a flat plate.

– Displacer piston – the clearance between the displacer and the chamber is about 1-2 mm. Fronts can be cut using larger scissors. Drill the holes of both fronts together, you will avoid inaccuracy.

-The hole in the possition 18 is made very preciously 4H8. The piston rod must move with minimum friction. The semifinished product of the rod must be finished by very soft sandpaper. It will take you at least half an hour.

-Lubrication of the piston and piston rod makes the engine tight.

-The flywheel must be well centered and the crank mechanisms can be ballanced for example by a piece of a magnet.

-Fix the bearings with a drop of glue at the hub and the shaft. (not a smallest drop of glue should get into the bearing!)

-Keep the perpendicularity during the manufacturing.

–Radius of handles -Coefficients of friction – oil quality -Diameter and height of displacer (the clearance between the displacer and chambre wall) -Vibration – precision of manufacturing, balancing -Cooling the baseplate – think about heat transfer and using a ribbed cooler -Mass of moving parts

I wish you all,who has turned to building his or her own engine, a lot of patience and creative work.

Feel free to ask me for an advice on jiritom@seznam.cz

Jiri Toman

Process of making a Stirling (basic version)

Dear friends

Here you can see the process of making a simple Stirling described in several steps.

Take the aluminium sheet, find its centre and make a centre punch.

Make circles of diameters 100mm (your tin) and 112mm with hexagon marks.

Make 6 centre punches on diameter 112mm and drill holes 3,5mm for screws.

Draw a line between the centre and the centre of two holes. There, on this line, there you make a centre punches approx. 40mm far from the centre (for the glass cylinder hole – d-10mm) and approx. 20mm (for the stand hole – d- 6,5mm).

Make three holes step by step a) d3mm – all, b) d6,5mm – all, c) d8mm for

pos. 18- Guide and the glass cylinder, e) d) 10mm for the glass cylinder.

Take a tin – d100 x 65mm, properly opened, without a paper label. Put a sheet of a soft sand paper on the smooth table. Grind the fringe of the tin by circular movement

on the sand paper – for about 3min – so that a flat area will appear on the fringe of the tin. Then the flat area will seal the air inside.

6 Screws will be made from steel sticks. At first file the ends not to be sharp. Then bend one side to make hooks (about 10mm). And finally fix the hook into a clamp and make M3 thread about 15 mm long.

Now, the bottom base is finished, you can try to put the pieces togethe

2) Poz. 24(Piston rod) + poz. 18(Guide)

The hole in the Guide is made very preciously. The aluminium pipe d4 is a little bit thicker. Take a piece of a very soft sand paper and grind the pipe so that it could move in the hole very smoothly – without resistance, BUT without a gap.

This is very important, the air must not leak and the movement must not be hindered by friction. (not all the rod must be grinded, just the part moving in the Guide)

3) Displacer – is made from four 0,5mm aluminium parts poz. 23 + 24. A roller box should be made of a diameter 3-4 mm smaller than the tin is

A) Roll the rectangle piece to have the right diameter of the roller, make a mark, make a 2mm hole, take 2mm rivet (cut it shorter) and rivet it (on a piece of pipe or a roller steel fixed in the clamp). Make sure it is round and messure the diameter. (The rectangle is longer, a small piece can be cut off)

B) Make a centre punch and draw the circle on the square and draw some 6 “tongues” about 4 x 4mm (see picture in galery) for mechanical connection of the displacer. Cut the piece off by using an ordinary (larger) scissors. Draw the second piece according the first one. Put them both exactly one on the other and drill 4mm hole. Now, you can make a round box by bending the tongues.

C) Make two small cuts on the not grinded end of the rod (about 4mm deep by a

thin saw or a knife (don’t cut yourself) and bend the four parts by 90*. Insert the

rod into the 4mm holes and press a little bit the “Heads” against each other and

make a mark on the rod. Then put the rod off, make a 1mm hole on the mark.

Take an ordinary paper clip (of 1mm in diameter) and cut some 15 mm off. Bend

one end (5mm), insert the rod back, press the”Heads” again and insert your little

pivot into the rod. Bend the outcomming part and your displacer is finished. It

should be tough and outcomming part of the rod should be in right angle with the

heads. See pictures in the gallery. Sticking the parts is not recommended. The

temperature in the bottom can be up to 350*C.

4) TOP – poz 14b(stand) + 9(hub) + 11(bearings) + 12(crank) + 13shaft + 10flywheel + 8(crank pin)

Take the stand and make M6 thread on the longer end. Take the hub and drill a 6mm hole in the middle of its length. (On a stand drilling machine to keep right angle).

Put the end of the stand without thread into the hole. It shouldn’t overreach the hole in the hub. File it if neccessary. Stick the stand and the hub with an (5min) epoxide glue.

Put both bearings on the shaft and put both bearings into the hub. (The axial possible move will be fixed in the end).

Flywheel: Take the steel sheet, find the centre and make the centrepunch. Take the compasses and make a circle of approx. 13cm. Cut the round shape off. (A bench compasses is reccomended not to make the flywheel wavy) File in the end to make the edge smooth. Drill the hole 4 or 5mm according to the shaft.

Take the cranks and drill 2,5mm holes. The radiuses are halves of the strokes of both crank mechanisms that you are making. The stroke of working piston is approx. twice as long as the stroke of the displacer.

Take the screw tap M3 and make threads into both cranks (nb.1 = one stripe, nb.2 = two stripes, nb.3 = without stripes, be very careful not to break the tap).

Put the brass screws M3 (= crank pins) to the cranks, the outcomming piece of thread must be cut off. Put the flywheel and washers on the shaft (they are not on the drawing), put the cranks on the shaft (washers where necessary) and make them tight (use two pliers) There must be exactly right angle between the two crank pins.

Now, the top is finished, the flywheel spins smoothly, and there is 90* between the crank pins

Poz. 5 piston noose + 6 connecting rod + 3

piston

Take the semi-finished product, make the thread M5, make a hole 3mm, put it into a clamp and make a slot with a saw – perpendicular to the hole and approx. 9mm deep. Then make it widen by a needle file so that the future connecting rod could easily move in the slot. Take the future connecting rod, file the sharp edges, file one end round and make a 3mm hole approx. 4mm from the end. Take the 3mm rivet and join the noose with the connecting rod. Make the rivet shorter and make it secure not to get out. You have just made a joint that must move very smoothly.

Screw the noose into the piston tightly.

Poz. 16 displacer noose + connecting rod 15

Make the joint the same way as the previous one.

The piston rod can be connected with the noose by an epoxide glue. But if it is made this way you can not to put it into pieces. The better way is a connection by a little peg made from a paper clip. Put the piston rod into the noose and drill 1mm hole and prepare a peg from a clip. (1 mm hole might be a problem, so 1,5 is easier, but you need a thicker piece of wire)

5) Montage:

Your Stirling engine is finished and you can try it on the flame. (the smallest on a gass cooker). But before, oil a little bit all the moving places – piston in the glass cylinder and the piston rod in the guide, bearings , joints and crank pins.

If your engine doesn’t work there may be two possible mistakes:

How to find it easily? – Put your engine to the fridge, prepare warm water to a

pot, put the engine to the water ( up to the half of of cylinder) and you will see

bubles.

2. There is too much friction somewhere, find it and get rid of it.

I wish you the great feeling, when your own engine starts working😄

Remember the direction of spinning : Piston tries to catch up the displacer!

And also have on mind: YOU CAN ALWAYS DO ANYTHING IN A DIFFERENT WAY

Only the dimensions of the tin has to be kept – d100x65 mm

And also: If your engine works you are a great mechanik, if it works and you are thinking why and if you try to improve it, you are a future engineer.

I wish you very much patience and good ideas

Jiri Toman