 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Solar : Solar Steam
  Page 3 of 7   |
|||||
| Author | Message | ||||
| VK4AYQ Guru  Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Don Just a word of caution on using the valves as an inlet. The amount of time the valve is open is to long to harness the expansive nature of compressed air and if you get to steam it is much more pronounced. also if you use high pressure it will blow the valve open and waste air. To get any efficiency in this type of engine using the gas expansion the inlet valve open time to control revs/power allowing the pressure to decay to zero psi at bottom dead center or as close as you can is the aim. All the best Bob Foolin Around |
||||
| lwright Newbie  Joined: 14/12/2009 Location: AustraliaPosts: 8 |
Hi Don, Yes the first one I set so that the receiver sat level with the top of the parabola in terms of depth. I used a great program called "Parabola Calculator" (freeware). Yes the LED's are used as photo sensors. I was heating water with the energy. See the improvised tank underneath. It did heat up 20 litres no problem. I noticed that the pictures didn't upload at very high res so I've included in a zip file. Not to dampen your enthusiasm but why I stopped persuing the trough idea was that I found that an evacuated tube solar heater does the same job for reasonable cost $500, doesn't need tracking & the header is well insulated. One problem I found with the trough was significant heat loss from the receiver. The wind would blow & your steam would just die. Lee. 2010-01-12_203739_IM001769.zip |
||||
MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
[Quote=Iwright]I used a great program called "Parabola Calculator" (freeware). That being said, if you're building a parabola out of plywood, to act as the sides of a trough, I have a method that would have to be called "The Fred Flintstone Parabola Calculator". It works like a champ and here's how: Select the piece of plywood you wish to cut the parabolic curve from. Since it'll be about a 99% chance this thing will be used outdoors, maybe 'marine' plywood would be the ticket, eh? Using a carpenter's square, accurately mark out an "X" and a "Y" axis (one up, one down). Make the "X" axis run all the way across the board and make the "Y" axis at the center of the "X" axis, (which should be drawn close to the bottom of the board, by the way) and extend it upwards to the top of the board. When you're done, it should look like a giant "T" upside-down along the bottom of your plywood. Grab a 16-penny nail and hammer it into the "Y" axis at the place you want the "focal point" of the soon-to-be-parabolic-mirror. Make sure the point you choose is no greater distance away from the "X-Y" junction than the shorter side of your contractor's square. Now, the fun part. On each side of the "Y" axis (this will make more sense after you've actually done it on the plywood a little), use the carpenter's square by placing the short side against the nail and then moving the square so the 90-degree tip (where the metal makes a square end) is on the "X" axis -- draw a line from that point on the junction of the "X" axis all the way out to the end of the square's longest side. Continue doing this: Keep the short side of the square on the nail, but move the tip of the square's apex outbound along the "X" axis just a little bit and draw another line. Do this until the short leg isn't long enough to remain on the nail. Now switch-hit and do the other side. When you're done, the points at which all the lines you've drawn from the "X" axis outbound along the square, will approximate (actually, it's very accurate!) a parabolic curve. When you're done, use a band saw with the blade set at 90-degrees to the table and carefully cut out the curve. What I did was to cut the first one, then use it as a template to draw the second one. When both ends were cut out, I clamped them together and hand sanded each together, so they matched. These two ends now become the form on which you will lay a piece of sheet metal and the shape of the entire thing will be a fairly-accurate parabolic curve. Once the metal is on it, you can make ribs to support things by turning the contraption upside down (curve facing the ground) and laying wood pieces up next to the metal and secured at the ends, through the plywood pieces. It's a little Fred Flintstone, but it works great and you don't need anything but a hammer, nail, pencil, carpenter's square and plywood. Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
||||
| lwright Newbie Joined: 14/12/2009 Location: AustraliaPosts: 8 |
Thanks MacGyver, Learn something new every day. Just another thought based on my experience. If your trough is going to be larger than say 2sqm, then I'd probably avoid a timber frame. It needs to have quite good precision which is hard to get with timber. A steel frame might be better. Here is a good design in timber. http://www.iedu.com/DeSoto/Projects/Stirling/Heat.html Lee |
||||
| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Iwright If you're intending to make the troughs really long, think about building a wood template, then having someone spot-weld an aluminum framework, which will pivot on bearings slipped over your focal axis. You can make the trough(s) to hang from a frame, which holds a piece of sheet metal from each side and hangs from the focal axis like a stirrup. There's a place out in the desert near Ridgecrest, California that makes electricity using miles and miles of these troughs. Their working fluid is ammonia (no thanks!) and they use a closed system, re-constituting the spent gas into liquid again somehow. These mirrors face east-west and follow the sun through the sky. When the sun is low in the sky, it skews the reflection, but apparently it's easier doing it the way they do, rather than tilting the troughs as the wind in the area runs 60+ miles per hour sometimes for weeks on end. They have a 12-foot fence around the place, which is interwoven with a soft screen-like material to absorbs blowing sand before it impacts the mirrors surfaces. Razor wire strewn across the tops of the fence absorbs lookie-lous!  Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
||||
Don B Senior Member  Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi all, Many thanks for all of the comments to date on both steam engines and parabolic reflectors. I have now resolved the interference problem in the B & S engine between the exhaust camshaft lobe and the big end cap by grinding away a small portion of the lobe. At last, I now have the exhaust valves opening just after BTDC, and closing just before TDC. With regard to the inlet valving, I have located a steam service solenoid with a 3mm orifice that I hope will be satisfactory. It certainly has the correct temperature and pressure ratings. Incidentally, it seems that you need PTFE seals in the solenoid for this duty, as the more standard Viton seals are not really suitable. I am not sure though about how well it will operate at my target engine speed of about 600 rpm. This means that it would have to operate at up to 10 times per second, which is a pulse each 100 milliseconds. As each pulse would only be of between say 5 to 50 ms long, the inertia of the solenoid mechanism might be too great for this duty. This is why I was initially attracted to using fuel injection solenoids, as they are specially designed for high speed operation, and would be perfect if it was not for their flow limitations at the modest pressures that I was looking at. Anyway, I will give the steam solenoid a try and see how it performs. There is also a concern about the amount of power that it will take to operate this solenoid. The fuel injector solenoids have a resistance of around 18 Ohms, meaning that they draw about 670mA, or 8 Watts when on. I don't have coil data for the steam solenoid as yet, but I expect that it will have a heavier draw. Even though each cylinder solenoid will only be on for a maximum of 50% of the time, with a 3 cylinder engine, this could equate to a steady electrical power requirement of around 20 Watts or so. This was the total output of one of the engines featured in one of the links, although I would hope that my 3 cylinder engine would output considerably more power than that. Concerning the possibility of using mechanical inlet valving instead, the problem is that you need to be able to readily adjust the number of rotational degrees after TDC through which steam is admitted. Steam train engines (and probably most other commercial steam engines) had this facility, but at the cost of a cumbersome and slow control, and considerable mechanical complications. I came across this article yesterday relating to possible future steam cars, and its content would no doubt be of interest to those following this thread http://autospeed.com/cms/title_Alternative-Cars-Part-5-Steam /A_109206/article.html. Regards Don B |
||||
| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Don Have a look at the SUNSTRAND hydrolic variable displacement pump they use a soliniod controlled servo valve controlling a ballanced shuttle valve, while it is a bit complex it may be adapled to your requirements of full electronic control. I did play with several electronic valves with little success in the control side of things as the inertia of the mechanical components when large enough to letenough steam through and the inductance of the coils where a problem I went back to a mechanical controlled rotary swasch plate system and it worked up to 1500rpm OK Steam is a bit difficult to valve as it carries bits of pipe scale and other odds and ends it picks up in the system so high precision valving tends to fail by contamination. All the best Bob Foolin Around |
||||
| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
[Quote=VK4AYQ]Steam is a bit difficult to valve as it carries bits of pipe scale and other odds and ends it picks up in the system so high precision valving tends to fail by contamination. You're partially correct about steam being hard to valve, but the hard part is that it is rapidly expanding, so you have to "time" events a bit more closely than with air There's a thing called a "steam bell". It looks like an inverted round-bottom pot and is welded to the top of the boiler. It collects steam and if you'll draw your steam from this thing, you'll find there is very little particle contamination. Not sure how it works, but it does. Maybe it's magic! . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
||||
| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Mac I am familier with the steam collector you mention but I think Don is planning to use flash steam without a steam collector as that would require a steam ticket issued by the government "big drama", I believe it applies to over 20 psi containers and all instalations must be certified and installed by licenced people, in the past we got by this using a 16 psi radiator cap on the boiler and condenser to pick up another 7 psi energy, model engineers and model train peop;e could give the latest boiler requirements. earlier days the flash steam tube boiler wasnt a problem as there is no container of any sort only a small high pressure pump injecting a specific amount of water into the tube. All the best Bob Foolin Around |
||||
| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi all, Today my single cylinder Briggs and Stratton steam engine had its first run on compressed air. This was with a steam solenoid as the inlet valving, feeding in through the spark plug hole. As it happens, the spark plug thread seems to be compatible with the 1/4 NPT thread supplied with the solenoid, so not having to make a fancy adaptor was a bonus. The exhaust valving is via the engine's original inlet and exhaust valves, running on a modified crankshaft, as previously described in this thread. The engine has a slight clunk when running, so maybe I didn't grind enough off the camshaft lobe to completely clear the interference with the big end cap, but otherwise it seems to chuff away quite happily. It will run down to 20 psi, but is happier with 40 to 60 psi air. Being single cylinder, it needs a flick to start it, but it then picks up and runs steadily with no problems. The inlet valve solenoid timing is via two reed switches operated by the flywheel magneto magnets. In theory, one opens the inlet solenoid at TDC, and the other closes it again near BDC Because there are two magnets in the flywheel for the magneto, some electronics were needed to ignore the second pulses from the open and close reed switches. The electronics also incorporate an adjustable pulse time limiter to close the solenoid before the BDC reed switch operates, and a hit and miss governor to inhibit the solenoid operation above a set speed. Both of these circuits need work, but look as though they will be useful for this experimental engine module until I get a shaft encoder working. I am now encouraged to proceed with modifying two other B & S engines, and coupling them up so that I have a 3 cylinder self starting steam engine. I have also realized that the simplest load for the engine, once it is working properly, is a standard squirrel cage motor used as an induction generator. All that is needed is some gearing to spin it at above its synchronous speed, a speed sensor to only connect it to the grid when it is running above synchronous speed, and I have a very suitable grid connect device that will only generate while ever the grid is operating. Then I need some steam, and a system for controlling and condensing it. Still working on this, though I have recently acquired an air compressor from the tip shop that looks as though it will supply me with a condensate tank, vacuum pump, safety valve, and pressure switch. Thinking about Lee's comment on how wind tended to kill his solar collector, I believe that the answer is to have a glass sheet in front of the parabolic trough and make it totally enclosed, apart from maybe some controlled ventilation. Something else to think about. Regards Don B |
||||
| lwright Newbie Joined: 14/12/2009 Location: AustraliaPosts: 8 |
Hi Don, Sounds like you're making progress. Just by way of encouragement, you might be interested in this article. It's about the Whitecliffs solar power station. It seems that the government modified a Lister diesel (I have one of these in pieces) as a steam engine. Seemed to work pretty well. They got an efficiency of 21% - not bad at all. They were running at 415c. I don't think you'll quite get that from a trough but still food for thought. http://www.rossen.ch/solar/wcengine.html Lee. |
||||
| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Don Well done good to hear you had positive results. Just a little suggestion, try opening the inlet a few degrees before TDC as it cushions the piston inertial loads at TDC and allows a build up of pressure and close the valve at 90 or so degrees to allow for expansion otherwise you waste air and it dosnt produce much torque after 140 deg. I think after a while you will get a fully mapped inlet going and use it as an expansion governer, as when you get to steam it is much more expansive than air. On the consenser side I found the best was to use a plunger pump on an exentric to provide boiler feed and vacume condenser with a ratio of displacement of four to one with a sealed condenser it was possible to get -7 psi without to much trouble, automotive MP guage reading. If you are using a flash steam tube boiler make a variable displacement link betweern the pump and the ecentric, you could use a stepper motor to set the pump output in line with loading of the engine if you can get a smart helper to do the control circuit, I just used a centrifical governer and it was a bit inclined to surge at light loads. Keep up the good work. All the best Bob Foolin Around |
||||
| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi all, Just an update for those thinking about using a small 4 stroke motor like the Briggs and Stratton 3.5 HP engine as the basis for a steam engine. I have been crunching some numbers, looking to get about 4 kW output from a 3 cylinder engine (3 B&S engines coupled at 120 degrees). My target engine speed was 600 RPM, which the steam solenoid that I was using for inlet valving could easily handle. I was also looking to use steam at around 5 bar (about 70 psi in imperial measure), which did not involve excessive pressures or temperatures. Unfortunately, it seems that the torque that can be obtained fron 5 bar steam in an engine with a bore and stroke of 65.1mm and 44.4mm respectively (2.56 in by 1.75 in) is way too low to output 4 kW at 600 rpm. What is needed for 4 kW is a torque of about about 63 Nm at 600 rpm, and what is available from 5 bar steam is somewhere below 10 Nm. To get somewhere into the ball park using 5 bar steam and this engine, I would have to increase the engine speed to around 3,600 rpm. Unfortunately, while operation at this speed is well within the capability of the fuel injector solenoids that I was originally playing with, they can't handle the flow required. It also seems that the steam solenoid that I am using works well to about 600 rpm, but could not operate 6 times faster. As my old inspiration, Wylie Coyote (of Road Runner cartoon fame) would say, oh well, back to the old drawing board. Regards (PS Wylie Coyote's schemes are much more ingenious than mine, but we do seem to enjoy about the same success rate) Don B |
||||
| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Don Good to hear that you are progressing through your development phase and be heartened by the fact that you have done it not just dreamed about it. On to your development, I feel that the next stage is to go for a double acting larger ddiameter piston mounted on top of your existing engine. The option to run it faster is counter productive as it takes time for the steam cycle to extract the full potential energy from steam, The fastest piston steam engine I have seen was a 1000 rpm 500 hp engine driving a generator set in a sugar mill, it had some very hi tech mechanical valving to run at those revs. I feel you are better to fabricate a engine bed and crank shaft using standarrd pillow bock bearings and ball bearings to keep the cost down the only draw back is the cylinder and valve castings, go and see some of the steam train modelers in your area as they often have standard castings available and you may find a retired machinist prepared to help you if you can raise some interest in your projest. The one I built myself was based on a design for a steam launch engine two cylinder 90 degree vee engine so as to be self starting with a common crank pin to simplify construction, it could be built as a simplex engine or a compound engine if you want to go to condensing steam. It produced 10 HP at 550 rpm so was in the ball park of what you need. You could make it a uniflow engine to use your valving solinoides and simplify the valving no end. The uniflow isnt as efficient as a full expansion engine but it is a lot easier to build also dosnt work as well as a compound design and the cylinder needs to be a bit longer as a hint for the cylinder you could use a GM two stroke diesel sleve from a 71 series or a 92 series these could be available as scrap in an engine repair shop. If you cant get any I still have some here and could send you two if you pay the freight cost and could machine to length for you if you wish. As said before the uniflow engine can only use a bit over half of the energy in the steam so it is more of an experiment than a final solution to the project. Work on a pressure graph of 4" bore 5" stroke to do your power calculations. One advantage of the uniflow engine is that it will work a bit faster so you could do your assumptions at 600 rpm the limit of your valving solinoide. Hope this is of some help. All the best Bob Foolin Around |
||||
| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi again, To those interested in the progress of my Briggs and Stratton steam engine conversion, I have looked again at the numbers, and been encouraged to progress it further. The first change is that the target speed of the engine will be increased to around 1,600 rpm. This will mean that I can get more power out, and can also direct drive the (4 pole 50 Hz) induction generator, which saves a messy and lossy speed step up arrangement. The second change is that the operating pressure will be increased to 8 bar (about 100 psi). This means that the steam temperature will be at least 170 degrees C (and hopefully a little more), but neither of these values are extreme. Unfortunately, the speed change in particular means that I need to find a solenoid that can operate much faster than the one that I have been using. At 1,600 rpm, assuming that the steam admission angle will be about 90 rotational degrees, the solenoid needs to be able to be on for 9.25 milliseconds, with a pulse repetition frequency of 37 milliseconds. Parker Hanifin can supply a "pulse valve" solenoid that can easily operate at that speed. It is essentially a scaled up axial fuel injector solenoid. Unfortunately, it does not have the temperature rating for steam, and has a flanged rather than a 1/4 inch NPT output, but otherwise looks very promising. I am still waiting for a quote, and hope that I won't need to change the batteries in my pacemaker before I read it. I have also obtained four old Briggs and Stratton 4 stroke mowers with the hope that I could get 3 usable motors suitable for conversion. The motor that I have already converted turned out to be quite worn, so I wanted to start out with ones with good compression. One of the motors was a "Series 260" deluxe with cast iron bore. On partial dismantling, it proved to have an internal mechanical governor rather than the usual vane type operated off the flywheel/fan, so I did not progress further and have reassembled it for some future project needing a good 4 stroke motor. I also dismantled a "200 series" engine, only to find that it had a plastic camshaft and gear wheel that I did not think would be suitable to machine, cut, re-orient, and re-splice. I still have two other engines with good compression, and old fashioned cast iron camshafts that I will proceed to modify. I will also try to get another older motor with good compression. It seems that the oldies are really the goodies for this purpose. I also have a friend who is the full bottle on steam, and who is reviewing the numbers for me and, as soon as I hear back from him, I will post an update on this project. Thanks also for your kind offer Bob. I am keeping it in mind if the B&S effort won't stretch as far as I am hoping. Regards Don B |
||||
| dextercath96 Newbie Joined: 07/01/2010 Location: PhilippinesPosts: 6 |
Yes you should use sunlight. Because it transforms mechanical energy to chemical energy. Sunpowerport Solar Generator |
||||
| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
DonB "Ix-nay" on the solenoid valves. Use mechanical spool or "D" (slide) valves. That's the way it's been done for 300 years and it works well; why change horses mid race? You'll need a lathe and if you're swimming in cash, pop for a mill as well. If you can line-bore a 1" hole in flat stock (4-jaw required), do that and use a rotary valve in the head. I've made several this way and linked the valve with a chain and sprocket. Just keep your fingers out of things and it works like a champ. If you're determined to use live steam, lubrication will soon become a nightmare. Use sintered bronze where possible as it holds oil. It's sort of like open-pore cast iron only much softer; very easy to machine. When possible, use HDPE or Teflon for bearing surfaces. I have sleeved several rotary head valves in Teflon and it works great. If you've never used live steam, you're in for a real learning experience; hope you have lots of hair, as you'll be tugging on it a lot! Oh, one last hint: The difference between a good-running steam engine and a great-running one is determined by how fast you can exhaust the used steam. I know you're probably scratching your head, but it's true. Getting the stuff in through a small hole under high pressure is child's play. Getting it all out of the cylinder after it's expanded 1,000 times its input volume in the same time it took to get it inside is the real trick. A hollow, rotary valve was born for just this. Best wishes. Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
||||
| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi MacGyver, Thanks for the comments, however, I will persevere with a solenoid inlet valve till I satisfy myself that it is hopelessly unworkable. One of the reasons for continuing down the solenoid inlet valve path is that it means that the actual steam injection point can readily be varied either side of TDC. Additionally, the rotational angle during which steam is admitted can also be readily varied. This should let the engine be optimized to operate at maximum efficiency, and simplify the governing. The problems with mechanical inlet valving arrangements, particularly slide or rotary valves, are their friction losses, and the fact that the admission and cut off point can't be readily varied - especially while running. No doubt I have yet to find out that solenoids have their own special problems. With regard to exhausting the steam, I am looking to use a condensor in the exhaust system, and to operate it at a vacuum. The exhaust valve will open shortly before BDC, and will be open to just before TDC. This is in contrast to the inlet solenoid, which will be open for probably only about 90 degrees, at most. If the initial trials of the engine are encouraging, I will look at ways to double the cam lobes to open both exhaust valves on each upstroke, rather than on alternate upstrokes, as happens in the present contrapted engine arrangement. The lobes are also probably not of optimised shape for a steam engine exhaust, but I first need to get it operating in the simplest possible way. Hopefully then, sucking the steam out of the exhaust with a vacuum from the condensor will remove the necessary steam volume, and will not be a severely limiting factor for engine operation. I appreciate the fact that that steam engines have used mechanical inlet valving only to this point in time, but I believe that, as with the present day petrol and diesel engines, electronically controlled steam inlet valving is the way of the future for performance steam engines. I would also stress that I am still trying to progress up the learning curve, and understand well that proven methods should never be lightly dismissed, as they represent the sum total of generations of experience. Thanks once again for the comments. Regards Don B |
||||
| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Not wishing to be the bearer of bad news, I reluctantly must advise you that everything you've said here is basically wrong; sorry. By using a cam to operate a crescent-shaped lever with a slot in it, both the direction (forward or reverse) and the timing can be very accurately accomplished. As far as friction losses, it's a moot point. In the big picture, unless your steam engine is the size of a marble, friction losses are minimal. Re-using your exhausted steam by condensing it is a great idea. Trouble is, it's way more work than you're seeing at this stage. Remember the oil that's mixed with the inlet steam to lubricate the working parts? That oil has to be separated from the exhaust steam. If it is left in the condensed steam (water), it emulsifies and your problems more than double! Steam capture also requires mountains of energy. If you're worried about friction losses in your valving, steam recapture is light years from becoming a reality. Still, I know it's fun to run with an idea, so I wish you well. Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
||||
| Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Speeding Up Solenoid Valves Hi all, For those who have been following my posts on this thread re converting a Briggs and Stratton 4 stroke engine into a one stroke 3 cyinder steam engine, here is an update. One of the problems with using a solenoid as the inlet valve for a steam engine, particularly one intended to operate at 1,600 rpm, is that you need the solenoid to operate 26 times a second (ie once every 38 milliseconds), and open for maybe about 10 ms or so. Even reasonably fast axial solenoid specs quote times of 10 ms to open, and up to twice as long to close. As I see it, one of the problems with the opening time of a solenoid (apart from inertia of the element) is the time taken for the current in the coil to increase to give the minimum operating flux to attract the operating element. Once the element has been attracted, it is well recognized that the smaller air gap means that the solenoid can be held open with significantly reduced voltage on the coil. The other problem on closing if full voltage is applied to that point is the time taken for the field to decay to a value where the operating element in the solenoid is no longer held open against the closing spring. I am presently playing with a 12 V steam solenoid that has a resistance of 11.6 Ohms, and an inductance of 65.8 mH. This gives it a time constant (the time for the current to rise to 63% of its final value, = L/R) of 5.6 ms, which is certainly a factor in the total time needed to open. The minimum voltage to open at no pressure has been measured at 5.25V, and the minimum voltage to hold open is 1.25V. With 80 psi on the solenoid, the voltage to open increases to about 7.25V. I did not measure the voltage at 100 psi, but I guess that there is probably a linear relationship between pressure and minimum operating voltage. Flow past the element seems to have no effect on the minimum voltage to hold open, which seems to be constant at about 1.25V. I am therefore experimenting with a (Picaxe 08M) microcontroller based circuit that does the following things: Prior to the point where opening is required, the coil is pre-energised using pulse width modulation to an effective voltage just below the minimum operating value. When the solenoid is required to open, the coil gets a short pulse of full voltage. The theory for this action is that, when the coil has been pre-energised, it should take much less time for the field to build the remaining amount to attract the solenoid element open when required. Once open, the coil voltage is reduced, again by pwm, to a value just above its minimum operating value. The theory is that, when required to close by removing this voltage, this should hasten the closing time, as there is substantially less magnetic field to decay. It is also worth noting that the common practice of fitting a reverse polarity diode across the coil to absorb the back EMF only serves to extend the opening time of the solenoid, as it maintains the current flow in the coil as the coil field decays. I have only got the electronics functioning today, and my oscilloscope is on the blink, so that I am unable to tell just how much, if any, I have sped up the solenoid, but this might provide food for thought for other experimenters. One other thing that I have noted is that there needs to be a facility to adjust the opening angle before TDC as the input pressure changes. At present, the admission point (using compressed air) is fixed at a few degrees before TDC. This is fine until the operating pressure increases to above about 80 psi, when I start to note something akin to pre-ignition in a petrol engine. No doubt, engine speed is also a factor, as, the faster the engine is running, the earlier can the air (or steam) be admitted without causing it to ping. Subjectively, with the new electronic solenoid control operating, the single cylinder B & S engine that I am experimenting with seems to run faster, and more sweetly (on compressed air). I am looking forward to being able to take some measurements to see what, if any, actual improvements have been made. I drafted this post as a word document, and did attempt to include a photo of the single cylinder engine operating on compressed air, however, there is obviously something that I don't understand about how to achieve this part. Regards Don B |
||||
| Page 3 of 7 |
|||||
 Print this page |
