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A long stroke two cycle engine functioning as a direct air compressor for ultralight applications. Imagine a cylinder closed at both ends. On one end there are all the components of a two stroke valveless engine. On the other end we have compressor valves directing air to an air tank. The piston is
a double headed free floating design with two sets of rings supporting it. To prevent the piston from bottoming out on the power stroke we place the output to the tank high enough that an airspring remains in the bottom of the compressor. This springs the piston back up the cylinder to the airspring formed in the combustion chamber. Ignition is provided by an inductive magnet built into the cylinder wall energized by a section of ferrous metal in the piston as it passes by. Starting of the engine is allowed using a shunt valve that momentarily vents pressurized air into the bottom of the cylinder powering the first complete cycle. Inefficient but simple, robust, light and maintenance free. You could even build one that didn't need a tank, simply shake hard up and down a few times to start. Portable air tools, bicycle power booster, emergency starter for vehicles with air starters, boat inflater, anything where portability and high output/weight are a must.
Similar
http://www.realspac...com/waterpiston.htm
Lacks polish and elegance. [WcW, Feb 24 2008]
Double-ended concept
http://www.imagebuc...&img=Compressor.JPG
Always powered, always pumping [neutrinos_shadow, Feb 25 2008]
[link]
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You seem to be missing the forced air part of the 2-stroke, though I guess you could just bleed air back from the air tank. I love the magneto built into the piston. I don't think I've seen that before. I also like the image of pulling out something the size of a spray can, banging it on a table to start it and then using it to fill all the tires in my car. (+) |
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Im not positive but I think you might have a conservation of energy problem with this one. Im too tired to think it through tonite though. Two strokes need big flywheels for a reason. |
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The combustion invests a lot of energy (velocity) in the piston. We take part of that to compress air. Some we save in the form of an airspring which we use to power the next compression stroke. Induction would be tricky but I think the equivalent of a chainsaw carb and intake would be near right. As for the flywheel the piston would need to be heavy to match piston velocity to the burn rate of gasoline. The speed of the oscilation would be low compaired with a conventional two stroke. The whole thing would have to be designed so airspring, induction, exhaust and compression all worked in concert. |
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Even a low level detonation would be alright if we keep everything small enough. |
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//shake hard a few times to start// but make sure you don't drop it afterward. |
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It's going to shake like crazy... guess that cuts down on the portability unless you can mount it. hmmm. |
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i think that your air spring will provide insuffcient forcew to offset the velocity of the piston. The sw3ept volume of the cylinders will be equivilent except that on one end you will have an explosive force driving the piston out and on the other nothing at ll but air compression, I suspect that you would need all the energy available in the compressed air to achieve turnaround for the piston, With a flywheel the rotary motion brings the piston back around using its own force. |
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I really dont see any limit on the abilities of the airspring. remeber that the piston has already done some work compressing air on the way down. Further nothing indictates that we must use a full atmospheric charge on the power side. |
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To get around the airspring problem, you could make it a double-ended design (quick sketch linked - I'm not much of an artist). The whole thing will shake itself wildly, so it would need a mount/handle that could cope with the motion. |
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Thats a good idea but i'm not sure how it improves the basic concept. Now we need additional seals, valves and carbs. The balancing mass issue seems the most crucial to me. |
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Weird. An airspring instead of flywheel, that's ... boy, that's different. |
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One of the reasons behind the sealed crankshaft chamber in a typical two-stroke - and also the reason why oil must be mixed in the fuel - is the bottom side of the piston is pre-compressing the next intake cycle's charge of air. At the bottom of the cylinder's stroke, then, you're suddenly pulling some of the pressure out of your airspring. |
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If you don't mind using nasty exhaust as your inflation gas, you could simply plumb the exhaust to your inflator hose and go that route. Off-roaders used to do it all the time, with a cheap little doohickey called a "chuffer." |
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Of course, most of them now just carry a big bottle of dry ice. |
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dry ice?
I understand the diversity of two stroke design, not all are crankshaft scavenged, and since this is a constant speed design I'm sure you could design a resonanat manifold. I was actually thinking that we vent the exhaust but capture the heat to add to the air volume output. |
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...Who carries dry ice while offroading? I mean it's an awesome idea, but I'm not aware of anyone doing it... |
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Ive seen those little CO2 inflators (liquid) but never dry ice (solid). |
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Nice idea...but, probably will not work for most of the reasons stated above. I think a more practical solution would be a small alcohol fuled engine, perhaps along the lines of a model airplane engine, set up to power a scaled down air compressor...pull the string, start the engine and begin the compressing of air. These engines are no longer the pains in the rear they once were...they start reliably, even in cold weather and run very long times as long as you keep the induction of air clean. Engines that weigh only ten ounces can produce up to one horse power. I imagine expense of precision engineered fuel powered small compressors would not be practical next to the cheap, $14.99 at Wal-Mart electrics you just plug into your cigarette lighter. |
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How about an electric one that fits into a cigarette lighter instead? Problem with a gas powered 2 stroke piston type compressor is 1) stall once the backpressure becomes too great and 2) oil and gas from the exhaust would inevidebly get into your compressor no matter how good the gasket is between the explosion chamber and the pump chamber because of the nature of the 2 stroke design. |
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Obviosly i don't rely on an untested concept to inflate my tires. I was actualy thinking that this design might work as a suplimentary power source for bicicles but the vibration issues seem to doom the simplicity of the idea. That being said I disagree about the stall pressure issue. The unit would stop at a certain pressure but that seems to be a desireable failsafe. Further the addition of some oil to air equipment is usually seen as desireable, include a filter and you are all set. |
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The airspring will not work. When fully extended the pressure will be less than atmospheric. It starts at 1atm, gets compressed, some air gets taken out, then it extends, thus less than it started. That will not be enough to compress the charge. Why not just use a metal spring? Otherwise you could use a sealed air cylinder but that is an added component that defeats the aim of small and simple. |
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Actually, I'm having second thoughts but I'll leave the above thought anyway. You could have a hole (constantly open air intake valve) half way along the cylinder and a long cylinder head. |
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Marklar consider again. Only part of the air is removed the rest can be compressed to a godawful psi. As the piston bounces back fresh air is drawn in thus the pressure on the spring side is never sub atm. Further the energy required for compresion is entirely a matter of design in chamber "displacement" and induction. |
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I'm liking the model engine idea. I've seen some .28ci models - I guess that's around 4cc - that wind up to electric motor speeds and develop around 2hp. That would turn your compressor over, for sure. |
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The downside is having to keep extra/different fuel to run it. They don't go on plain gas. |
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Big narsty bone to whoever even mentioned Wal*fart. A pox upon the Wal*fart. |
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That was why I had second thoughts. It would work but the pressure could not be more than 1atm and would probably be less as you wouldn't be able to allow sufficient time for the chamber to fill completely. |
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As you mentioned, it would be very inefficient. If you are only worried about size, why not make a turbine with a rotary compressor? You could use an R/C jet engine for that. |
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speaking of model airplane engines, what about one of those turbine engines for model planes that are about the size of a soda can? (google "wren gas turbine"). |
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You can tap into the engine just behind the compressor blades for a good source of compressed air. commercial airliners use this "bleed air" method to get compressed air for cabin pressure, etc. |
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The plus side is that technically such a device would only have a single moving part (assuming the compressor and turbine disks are both mounted to the same shaft). It would be prohibitively expensive though, but very very light, and very halfbaked. |
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To get around the vibration problem, consider the following redesign: |
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The first part is a small free piston engine, whose piston doesn't do any direct work, except to vibrate the heck out of the engine body. |
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Connect the vibrating engine body to a second piston in a cylinder. This second piston does the work of compressing, and the cylinder around it is connected to the casing, which the user holds in his hand. |
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yeah, its a problem, but you wouldn't be able to "shake start" a double piston engine opposed engine. Vibration doesn't defeat the action of the compressor it just wastes some of the energy and I suppose it doesn't matter if we conceive of the piston moving or the cylinder moving since both are functional. |
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Why not just burn the liquid fuel to produce gaseous, high pressure products, and then inflate the tyre with them ? |
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So, not about an ultralight aircraft that can be used to inflate a bounce house? |
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8/7, the problem with that, is that once the hot, gaseous products cool, their pressure will drop. |
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This might not be a problem with air tools, and other devices which derive power from the compressed air (since it won't have time to cool between generation and use), but it would be horrible to try to use for filling an inflatable boat, or a car tire. |
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The results might be ok for a few minutes, or maybe even a half hour, but as the temp goes down, so will the pressure and volume. |
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The combustion of gaseous fuel can produce a net reduction
in gas volume:
Consider that the pure combustion of methane takes 4 mol.
of gas and converts it into 1 mol. of CO2 and 2 mol of H2O a
reduction in overall volume by any measure.
Ethane takes 6g mol and converts it into 2g and 3l. The trend
is to make less and less total volume of end product gas from
longer and longer HC chains. This is also how the oxidation of
HC in a non combustion situation can collapse containers and
cause leaks. |
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//airspring// exceedingly cool |
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If you make the shuttle-piston hollow then you have a fuel tank: just unscrew the spring end and drop the piston out to refuel; put a solid piston inside that for momentum-driven power for a low-pressure fuel injector. |
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So... only 4-5 distinct parts so far: how are you going to scavenge it ? |
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1) the fuel is added to a resovoir inside the piston through the exhaust port |
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hmm... you'd be increasing the CR when inflating a high-pressure item since your airspring would compress air at the target's pressure. |
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