FACULTY OF SCIENCE AND TECHNOLOGY
Manufacturing (MOD002554) Submission date: 5/1/2015
This assignment covers the following learning outcomes:
The assignment consists of two sections of equal weightings. Section A (50%) deals with the practical manufacture of a product (Sterling Engine) using workshop facilities. Students are encouraged to work in groups of two or three in this hands-on part of the assignment. Section A is assessed based on the physical product made. Section B (50%) consists of producing an individual report on the safe manufacture of the product, evaluation of the product design, suggestions on design improvements, generation of alternative manufacturing processes and recommendations for the manufacture of final products and parts. This final report should not exceed 2500 words.
Section A: Manufacture of a Product
This section is intended to provide you with the opportunity to acquire the skills necessary for the manufacture and assembly of a simple product (Dolly Sterling engine) and to appreciate the safe practice of workshop. The skills include basic work using hand tools, manual machines and CNC machines to manufacture parts, and determining of appropriate machine settings.
You are required to make the product as shown in the drawings given by the tutor. The standard (bought out) parts and the parts to be made by you will be clearly described. The materials to be used and the main processing instructions will also be detailed in the brief.
On successful completion of this section, you should be able to interpret and utilise engineering drawings, use basic manufacturing techniques in a safe and appropriate manner, determine suitable settings for machine tools, assemble components into a complete product, inspect, test and evaluate your assembled product.
Section B: Design, Recommend for Manufacture and Report
This section is to be completed by individual students. You are required:
Overall report Structure and presentation (10 marks)
Compile your work of section B as a final report (maximum 2500 words) that includes introduction, conclusion and references as 10% of the marks is allocated for the structure and presentation of the report.
Please submit your manufactured product (clearly labelled with your SID numbers) with the report.
How to Construct “Dolly 1”
Since the first project, ‘DOLLY’, appeared in the original publication, MODELLING STIRLING AND HOT AIR ENGINES, it has become accepted as a remarkably simple but effective introduction to model hot air engine construction and quite a number of beginners used this model as their first step.
Fig 1 Completed engine.
‘DOLLY’, (Fig 1) is a basic model specially designed for beginners. It is a compact engine and if well finished makes an ideal demonstration or desk‑top model. The engine can easily attain a speed of 1000RPM and will continue to run steadily as long as a flame is applied. Heat from a spirit burner or solid fuel is sufficient. The construction allows for some minor but nevertheless interesting experiments.
Fig 10.2 General engine layout.
GENERAL ENGINE LAYOUT
The engine consists of two cylinders placed parallel to each other and bolted on opposite sides of a cylinder plate; one of the cylinders houses the displacer, while the other is the power cylinder. The displacer rod and power piston con‑rod run parallel to a central drive mechanism consisting of a flywheel on the power side and a second flywheel on the displacer side. The expansion of gas, increased pressure and compression are transmitted from one cylinder to the other through a ‘hidden’ connecting passage drilled through the cylinder plate in such a way that the working gas (air) is well contained between the two cylinders. Heating is by methylated spirit (solid or liquid) while cooling is by the fin method. A larger scale model can be heated by gas and cooled by a water jacket. The design and construction allows for two parameters to be changed and experimented upon: phase angle and the ratio of volumes.
METHOD OF CONSTRUCTION
The CYLINDER (TRANSFER) PLATE is made from 15 mm aluminium alloy, polished, marked and drilled. On the power side the plate is drilled to take the displacer rod guide bush, the power cylinder (flange), and at an angle shown in Fig 2, the power side of the inter‑connecting port hole. On the reverse, the displacer cylinder side, the plate is drilled to take the displacer cylinder flange and again at the angle shown, the displacer port. The cylinder plate is then drilled crosswise from the power side to connect with the two angled ports from the displacer and power cylinders (Fig 2). The hole is tapped to take an M6 bolt and fibre washer as an airtight plug. The displacer rod guide bush hole is drilled and tapped M6, the holes for the two flanges are tapped M5, the interconnecting air passage is drilled 5 mm. The cylinder plate is also drilled and tapped M6 underneath for bolting to the aluminium alloy base.
The POWER CYLINDER is open at both ends and machined from solid. The cylinder must have a good finish to take the power piston.
The POWER PISTON is machined from a suitable piece of cast iron or mild steel. The piston is finished smooth, however one or two oil grooves may aid compression since a smear of light oil retained in these grooves will provide internal lubrication. A precision gas‑tight fit is essential for the engine to run and to run at a good speed; it should be friction free, sliding freely without binding.
The POWER CON ROD is shaped from 3 mm BMS flat bar, shaped and then drilled at one end to take the gudgeon pin and at the other end the crankpin shoulder screw.
The DISPLACER CYLINDER is made from copper tube with the hot end closed with a compression fitting and the cold end soldered to the flange. It is essential that there is approximately 0.8 mm clearance between the head of the piston and the compression fitting that closes the hot end.
The DISPLACER is made from a machinable ceramic that does not conduct heat from the hot end to the cold end.
The most important requirements in the displacer are two: the annular gap between the displacer and the cylinder wall should not exceed 0.4 mm on the radius or 0.8 mm on the diameter.
The DISPLACER ROD GUIDE BUSH is best machined from 10 mm brass rod, reduced at one end and threaded M6. The bush is centre ‑ drilled, drilled and reamed 3 mm to take the displacer rod and it should be air tight.
The DRIVE MECHANISM consists of a central aluminium alloy block, 20 mm thick, x 25 mm wide x 35 mm in high. A 8 mm hole is drilled laterally (see Fig 5) to take two nylon bushes. The block is also drilled underneath to take two M6 screws.
Fig 3 (Left) Side elevation ‑power side.
Fig 4 (Middle) Side elevation ‑displacer side. Fig 5 (Right) Front elevation.
The POWER FLYWHEEL is machined from 50 mm diameter mild steel. The inner flywheel boss is drilled and tapped M5 to take a grub screw. At this stage the crankpin is fitted 7 mm off centre to give the power piston a 14 mm stroke.
The DIPLACER FLYWHEEL is similar to the power flywheel except the crank pin is 11 mm off centre, giving a 22mm stroke.
The crank pin assembly is a brass washer both sides of the con rod which is retained on their respective flywheels by the shoulder screws.
The HEAT SINK is turned from aluminium bar and should be a tight fit onto the displacer cylinder.
A polished ALUMINIUM ALLOY BASE completes the engine frame; once the cylinder plate and the drive mechanism block are measured, marked and fitted in place, the finished engine is mounted on a wooden base with a rubber disc at each corner.
FINAL STAGES OF ASSEMBLY
The cylinder plate is prepared with the interconnecting air passage, with the plug bolt lightly screwed in. The power cylinder is screwed in place ensuring that the air‑vent connection is completely clear. The displacer rod guide bush is finger tightened with a smear of Loctite Screwlock 222, the displacer rod and displacer are inserted from the reverse side, and checked for alignment and straight fitting.
To ensure that the displacer cylinder is correctly positioned so as to avoid friction between it and the displacer, a piece of electrical tape is wound twice round the displacer hot end, and another piece at the front end. The cylinder plate is held face down in a vice and the displacer cylinder slipped on to the displacer; the cylinder should fit just right ‑ if it is too loose, another turn or two is made with the tape. Finally the position of the flange is marked on the cylinder plate, holes drilled and tapped accordingly. The tape is removed, a light gasket placed between the displacer cylinder flange and the cylinder plate, the whole assembly screwed in and the displacer is tried for friction. Any adjustments required are made at this stage. The main problem normally is friction of the displacer against the cylinder wall sometimes as a result of an unaligned fitting of the displacer rod into the displacer; the method used by the author is to place the displacer rod in the chuck of a bench drill (or lathe chuck), revolving it by hand and using a surface gauge or a suitable piece of makeshift equipment, such a long canvas needle fitted to an upright stand, to check whether the displacer is running true.
The next stage involves the assembly of the drive mechanism ‑ the flywheel and disc are fitted onto the crankshaft with washers and spacers as required for the correct alignment of the con‑rods which must not be too far out on the crankpins or too tight against the flywheel/disc. The drive mechanism and the cylinder plate are then bolted to the metal base.
CON‑ROD LENGTH CALCULATIONS: the following method may be used to ensure the correct lengths of the two con‑rods, assuming that these have been drilled at one end only, and the other end is still unmarked.
The power piston is inserted all the way into its cylinder with a scrap of paper at the top to allow for minute expansion. The flywheel is then turned so that the crankpin is in its nearest position to the power cylinder. A scriber is used to mark the position of the crankpin, which incidentally is known as Top Dead Centre, TDC. With the power piston in this position, the disc on the displacer side is turned so that the crankpin is nearest to the base, and therefore at right angles to the first crankpin. The displacer is pushed almost right in (less 0.8 mm), the disc is turned so that its crankpin is now closest to the displacer cylinder (or cylinder plate), and crankpin position is marked on the displacer con‑rod. The con‑rods are then fitted in place; there should be no need for any alterations if the above exercise and the right measurements have been taken. However the displacer assembly may need the odd adjustment: such measures could include a thicker gasket or reducing the stroke very very slightly by placing the crankpin closer to the crankshaft centre. Care is recommended not to disturb too much the ratio of the two volumes.
TIMING THE DRIVE MECHANISM
The displacer is set 90° (a quarter of a turn) in advance of the power piston. Therefore if the engine is designed to turn clockwise, the setting of the drive mechanism should be done in this mariner:
(a) the power flywheel (power piston side) is turned so that the crankpin is at its highest position above the base (Fig 10.6a);
(b) the displacer flywheel (attached to the displacer con‑rod) is turned so that the crankpin is furthest away from the cylinder plate/displacer cylinder (Fig 10.6b); in this position the displacer is in the cold side of the cylinder and the air in the hot space;
(c) if the flywheel is turned from position (a) clockwise for a quarter of a turn (Fig 10.6c), the power piston is at BDC while ……
(d) the displacer crankpin is in its lowest position relative to the base and the displacer is half way into the hot space (Fig 10.6d).
If it is desired that the flywheel turns anti‑clockwise, all that is required is to reverse the position of the displacer by 180° while leaving the power piston crankpin in its highest position relative to the base.
It may happen that an engine works better if the phase angle varies slightly above or below 90°.
PERFORMANCE & MODIFICATIONS
The first prototype made its first full run after about 15 minutes ‑ the flywheel had been flicked forward a number of times. However the engine bounce and increasing flywheel response indicated from the beginning that there was sufficient ‘feel’ in the engine to show promise. The first run lasted for about two minutes but the engine would not run again until it had cooled down completely; at first it was thought that this was an indication of compression loss. Eventually the running time increased as did the revolutions. The displacer cylinder was modified by further thinning down and reducing the cylinder wall considerably at the hot end with the result that engine response to heat was faster and revolutions increased substantially. A further modification was made much later ‑ the hot end was reduced even more and a steel hot cap mounted over the entire hot space area. This resulted in slower heat conduction but more heat could be applied; another result was less conduction of heat along the cylinder.
Fig 10.6 (a) Power Piston. (b) Displacer (c) Power Piston. (d) Displacer.
Speeds of up to 1000 RPM can be obtained from this engine provided the power piston assembly is gas‑tight fit, the displacer rod/guide bush fit is precise and there is minimum friction in the drive mechanism and elsewhere.
The engine can be scaled up to twice the specification size. Heating by gas and cooling by water jacket will obtain a good performance from such a model, especially if a thin‑walled lightweight mild steel displacer is used.
Ref: Rizzo J G ‘The Stirling Engine Manual’ (1995) Camden
Dolly 1 Engine Specification
|Base||125 × 117 × 6 aluminium|
|Bush 2 off||Dia 20 × 10 Brass|
|Centre Block||40 × 20 × 25 aluminium|
|Power con rod||75 × 10 × 3 mild steel|
|Displacer con rod||48 × 10 × 3 mild steel|
|Gudgeon block||Dia 10 × 10 brass|
|Transfer plate||117 x 60 x 15 machinable Aluminium|
|Displacer shaft guide||Dia 10 × 30 brass or PTFE|
|Displacer shaft||Dia 4 × 81 silver steel|
|Displacer flywheel||Dia 50 × 20 mild steel or Cast iron|
|Power flywheel||Dia 50 × 20 mild steel or Cast iron|
|Flywheel shaft||Dia 5 × 67 silver steel|
|Heat sink||Dia 50 × 30.5 aluminium|
|Power cylinder||Dia 44 × 40 mild steel|
|Power piston||Dia 20 × 22 Steel|
|Displacer cylinder||22 O/D steel pipe 94mm long|
|Displacer + Ends||Dia 19.5 x 64 long, Aluminium|
|End cap||22 I/D, 25 O/D x 19 long Copper|
|M4 shoulder screws crank pin – 2 off||P1030.05-05 (Autommotion.co.uk)|
|Brass washers crank pin – 4 off||Dia 5 bore × 1mm thick|
|Roll pin piston pin||Dia 4 × 18 long|
|Roll pin gudgeon pin||Dia 3 × 10 long|
|M5 grub screws 2 off|
|M5 socket head screws|
|M6 cap head screws|
|Dowel||Dia3 x 15 long|
Dolly Stirling Engine
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