smuttley
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How to build and run a Stirling engine the first time out...............
PLEASE NOTE: This is not an explanation on how Stirling engines work just some main tips, construction details and running them with fewer hassles.
OK you want to build a Stirling engine and the first time (most builders) you try and run it, it fails to run and you then apply more heat than can power you home for a week and the blasted thing will still not turn over ....................... WHY NOT?
1/ First make sure the flywheel will turn freely on giving it a good flick with your finger; it should run for several minutes without wobbling and stopping. This is done before connecting the push or con rods. Using good quality bearings is a must, or very well machined bronze or brass bushings with precision ground shaft. This also applies to connecting push rods; make sure they are as free from friction as possible and FREE running as you can.
Unlike powerful steam and IC engines, the Stirling engine is low powered and does not like friction to hinder it from running.
2/ Try and build an engine that has distance from the HOT and COLD cylinders and pistons????? What's this?
Well the Stirling engine or hot air engine needs a differential temperature to run, that is the hotter the hot side is and the colder the cold side is will help the engine to run faster, with more power and run longer or if you follow my advice will or can run continuously without stopping and on lower flame temperatures. So the further your cold cylinder is from the hot the better. But not all is lost if you want to build a hot and cold back to back engine as many want to build............. Read on dudes.
3/ Make sure that the hot displacer piston has a very small clearance in both directions, in other words if the hot cylinder has a bore depth of say 50 mm, and you have a 10 mm crank through, then you will need a piston that is 39 mm in length ..............
50 mm 10 mm (the crank through) 1 mm (for gap of .5 mm on both ends) = 39 mm (god I hope I get this right).
You can push it out a little, but if yah want an engine that runs like a dream on a smaller flame, then get with the small distance thing. Same goes for the bore size (although I am working on trying smaller bore diameters) and keep this as small as possible without rubbing against the cylinder sides.
Why so smaller gap size?????
Well you are talking air expansion and contraction (hot and cold air), if the gaps are large, then the air can and will compress and this will act as a damper, we want the gasses to grow on being heated and shrink on cooling (hot air balloons or crashing to the ground), so trust me on this, more time spent on gap sizes will pay dividends on you first or even tenth build.
4/ Do the same for the cold side power piston, it will need less than 1 mm gape between the bottom of the piston and cylinder (.5 or less would be better) to run perfectly.
Very important NOTE on cold side piston and probably the most important thing of all points
Take your time machining the cold cylinder, it has to be smooth and free from burs and also symmetrical. Once you are happy with the cylinder, then go make a piston to fit perfectly, yes perfectly.........................it has to slide under its own weight FREELY within the cylinder, NO jamming or stopping under gravity if held between thumb and forefinger without sealing off the air. Now if it passes the free movement test, then you need to also check to see if it will not leak air past the piston. Hold the cylinder vertically and seal the bottom off with your thumb, then gently place the piston in the top of the cylinder and it should stay there without dropping to the bottom. It is ok if it slowly sinks to the bottom with a very small leak, but if it leaks too much, then it will be very hard to run the engine or need a lot of heat energy to get it working.
Warning: Dont drop the piston testing this out as I did once.
You can use brass cylinder with an aluminium piston, but one of the best cylinder and piston sets you can use is glass cylinder with a graphite piston; we use them for our cold side as friction is so low and will run for ever. We get them specially made by Airpot corp in the US and you can obtain a free sample from them off the shelf at http://www.airpot.com/html/piston_cylinder.html
I recommend spending some time on this site, as it will give you a much better understanding of friction and the magic of glass/graphite combinations.
Can I machine my own graphite piston with a borosilicate glass cylinder? YES YOU CAN ................ obtain the highest standard test tube that is used in very accurate measuring labs of the approx size you want to use. Cut this to length using a diamond cut off wheel (fleBay for them) in a dremel tool and check the inside for roundness. Then obtain some mechanical graphite bar (High-purity graphite rod/bar) and machine to exact fit to cylinder. Jan Ridders has a good site with details of using this method http://ridders.nu/ You may have to translate this website.
5/ the hot displacer piston shaft will need to glide freely and not bind, this will also need to have a very small tolerance on the shaft and bush to stop as much air leakage as possible and keep the working air trapped within the engine and allow the air to expand and then contract without leaks.
6/ ONLY use the smallest amounts of oil on the moving parts, never use grease or thick oil. Sowing machine oil is one of the light oils that I always use. If you havent got a precision oiler, then a good tip is to use a piece of thin wire and dip this into the oil leaving a very small droplet on the end, touch this on the part you want to oil and only that much.
NOTE: If you use glass cylinder with graphite piston, then don't oil them, as the graphite is self lubricating.
To recap major points:
Friction is a Stirling or vacuum engine killer
Keep the pistons gap on the working gas side as small as possible
NO air leaks or as small as possible
Keep the cold working cylinder side as cool as possible (differential temperature)
Try and use cooling fins to assist with heat reduction on cold sides to help with the last tip
Never over oil your engine
Hope that helps and if anyone sees spelling mistake or need to help my grammar, then please chip in, as I am not that good at it.
Please feel free to contribute and join in and I am always happy to help if anyone gets stuck.
Kind regards,
Steve
PLEASE NOTE: This is not an explanation on how Stirling engines work just some main tips, construction details and running them with fewer hassles.
OK you want to build a Stirling engine and the first time (most builders) you try and run it, it fails to run and you then apply more heat than can power you home for a week and the blasted thing will still not turn over ....................... WHY NOT?
1/ First make sure the flywheel will turn freely on giving it a good flick with your finger; it should run for several minutes without wobbling and stopping. This is done before connecting the push or con rods. Using good quality bearings is a must, or very well machined bronze or brass bushings with precision ground shaft. This also applies to connecting push rods; make sure they are as free from friction as possible and FREE running as you can.
Unlike powerful steam and IC engines, the Stirling engine is low powered and does not like friction to hinder it from running.
2/ Try and build an engine that has distance from the HOT and COLD cylinders and pistons????? What's this?
Well the Stirling engine or hot air engine needs a differential temperature to run, that is the hotter the hot side is and the colder the cold side is will help the engine to run faster, with more power and run longer or if you follow my advice will or can run continuously without stopping and on lower flame temperatures. So the further your cold cylinder is from the hot the better. But not all is lost if you want to build a hot and cold back to back engine as many want to build............. Read on dudes.
3/ Make sure that the hot displacer piston has a very small clearance in both directions, in other words if the hot cylinder has a bore depth of say 50 mm, and you have a 10 mm crank through, then you will need a piston that is 39 mm in length ..............
50 mm 10 mm (the crank through) 1 mm (for gap of .5 mm on both ends) = 39 mm (god I hope I get this right).
You can push it out a little, but if yah want an engine that runs like a dream on a smaller flame, then get with the small distance thing. Same goes for the bore size (although I am working on trying smaller bore diameters) and keep this as small as possible without rubbing against the cylinder sides.
Why so smaller gap size?????
Well you are talking air expansion and contraction (hot and cold air), if the gaps are large, then the air can and will compress and this will act as a damper, we want the gasses to grow on being heated and shrink on cooling (hot air balloons or crashing to the ground), so trust me on this, more time spent on gap sizes will pay dividends on you first or even tenth build.
4/ Do the same for the cold side power piston, it will need less than 1 mm gape between the bottom of the piston and cylinder (.5 or less would be better) to run perfectly.
Very important NOTE on cold side piston and probably the most important thing of all points
Take your time machining the cold cylinder, it has to be smooth and free from burs and also symmetrical. Once you are happy with the cylinder, then go make a piston to fit perfectly, yes perfectly.........................it has to slide under its own weight FREELY within the cylinder, NO jamming or stopping under gravity if held between thumb and forefinger without sealing off the air. Now if it passes the free movement test, then you need to also check to see if it will not leak air past the piston. Hold the cylinder vertically and seal the bottom off with your thumb, then gently place the piston in the top of the cylinder and it should stay there without dropping to the bottom. It is ok if it slowly sinks to the bottom with a very small leak, but if it leaks too much, then it will be very hard to run the engine or need a lot of heat energy to get it working.
Warning: Dont drop the piston testing this out as I did once.
You can use brass cylinder with an aluminium piston, but one of the best cylinder and piston sets you can use is glass cylinder with a graphite piston; we use them for our cold side as friction is so low and will run for ever. We get them specially made by Airpot corp in the US and you can obtain a free sample from them off the shelf at http://www.airpot.com/html/piston_cylinder.html
I recommend spending some time on this site, as it will give you a much better understanding of friction and the magic of glass/graphite combinations.
Can I machine my own graphite piston with a borosilicate glass cylinder? YES YOU CAN ................ obtain the highest standard test tube that is used in very accurate measuring labs of the approx size you want to use. Cut this to length using a diamond cut off wheel (fleBay for them) in a dremel tool and check the inside for roundness. Then obtain some mechanical graphite bar (High-purity graphite rod/bar) and machine to exact fit to cylinder. Jan Ridders has a good site with details of using this method http://ridders.nu/ You may have to translate this website.
5/ the hot displacer piston shaft will need to glide freely and not bind, this will also need to have a very small tolerance on the shaft and bush to stop as much air leakage as possible and keep the working air trapped within the engine and allow the air to expand and then contract without leaks.
6/ ONLY use the smallest amounts of oil on the moving parts, never use grease or thick oil. Sowing machine oil is one of the light oils that I always use. If you havent got a precision oiler, then a good tip is to use a piece of thin wire and dip this into the oil leaving a very small droplet on the end, touch this on the part you want to oil and only that much.
NOTE: If you use glass cylinder with graphite piston, then don't oil them, as the graphite is self lubricating.
To recap major points:
Friction is a Stirling or vacuum engine killer
Keep the pistons gap on the working gas side as small as possible
NO air leaks or as small as possible
Keep the cold working cylinder side as cool as possible (differential temperature)
Try and use cooling fins to assist with heat reduction on cold sides to help with the last tip
Never over oil your engine
Hope that helps and if anyone sees spelling mistake or need to help my grammar, then please chip in, as I am not that good at it.
Please feel free to contribute and join in and I am always happy to help if anyone gets stuck.
Kind regards,
Steve