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Making air compressor that also works as air motor

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  • Making air compressor that also works as air motor

    Hey everyone,

    I recently purchased an older twin cylinder oil lubricated craftsman air compressor. I took it apart, and was amazed how much it is like an engine, only with one way valves instead of cam actuated poppet valves.

    I really want to replace the one way valves with some kind of electronically controlled valves. This would let me either run it as an air compressor, or by changing the timing as a compressed air powered engine.

    The main problem, is I didn't realize most solenoid air valves are made to only work in one direction. However, for this application I would need the valve to allow air to pass through it in both directions.

    Does anyone know of any off the shelf valves that might work in this application (~100PSI, air, works in both directions)? Or have any ideas how I might make one?


  • #2
    I guess one solution would be to use a check valve in parallel with each valve. I am thinking of having two valves. Valve 1 (IN: Air Reservoir, OUT: Piston Cylinder) Valve 2 (IN: Piston Cylinder, OUT: Ambient Air). That way when used in air compressor mode the piston could push air through the check valve around valve 1 and into the air reservoir. Actually, if the valves only work one way that may be fine. The only time the piston cylinder pressure would be higher than the air reservoir would be when running in compressor mode. In this case air leaking backwards through the valve would actually be desirable.


    • #3
      Put in a tee and use a valve oriented in different directions on each outlet?

      Next question: why exactly do you want to do this again?


      • #4
        As for why, mainly just for education and a fun project. I keep thinking about how cool it would be to have this device act like a giant compressed air "spring". You could wind it up using a motor or other energy source, putting air into the compressed air tank, then later switch it over to compressed air motor mode and power things. Not really practical, but really fascinating to me.

        I guess I could see it being practical in certain limited circumstances. It would be interesting to use it like a mechanical battery, charging it up a motor powered with solar panels and then discharging it to power a generator.
        Last edited by HunterL; 09-22-2022, 04:53 PM.


        • #5
          Electronically controlled valves is actually a idea for an internal combustion engine. It would allow it to eliminate a camshaft, all losses associated with it and make the timing infinitely variable. I am not sure if any automaker actually implemented this idea into real cars. Any actuator must be powerful, very fast, cheap and reliable. I am not aware of any practical solutions to these problems to date, but they may exist.

          Same principles would apply to a 2-way air compressor/motor. I think it can be done with a hydraulic pump/motor, but it may not have a reciprocating design.


          • #6
            Kind of reminds me of a fireless locomotive. If you're not familiar with them, HunterL, you might get a kick out of them. Basically just big air tanks on wheels where the compressed air was used to propel the machine in environments where a firebox wasn't practical (e.g. in mines or mills/factories with fire / explosion risks).

            Ha! Here's a cutie... "the whistling pig". Tiny little guy for mines:
            Last edited by Fasttrack; 09-22-2022, 06:02 PM.


            • #7
              That was very cool. Thanks for that Fasttrack.

              The valve timing needed for turning the compressor into a motor would need to be similar to a steam engine. Namely air entry a touch before TDC and close off the pressure part way through the power stroke and permit the air to continue expanding to aid with being frugal with the air volume. And that's all quite different from the purely above or below internal cylinder ambient that you have in an air compressor setup. You'd need a way to monitor the crank and thus piston position and some manner of programming to trip the valves open and closed at the proper times.

              Even the air compressor side would need some care and perhaps a pressure sensor to adjust when the inlet opens. When tank pressure is low the ambient left over in the cylinder would lower to atmospheric early in the down stroke. But when near to full pressure there would be some blowback through the inlet if the opening is not delayed until the residual chamber pressure is equal to atmospheric. The reed valves do this automatically. But your solenoids would not.
              Chilliwack BC, Canada


              • #8
                My friend Jonathan has taken Curtis 1 cylinder compresors
                and made them into engines. He built a motorcycle out of
                one or two of them. Added a cam and valve in the head
                as I recall. I think he made an engine out of a piston
                well pump too.



                • #9
                  How do the air starters work on delivery trucks? I don't know but perhaps there is something to learn there.


                  • #10
                    Air vane motors.



                    • #11
                      Thanks for all the helpful info! As for doing the timing, I was thinking about using a toothed wheel and a crankshaft position sensor. I would connect this to some kind of Arduino like computer that I could program to open the valves with an electrical signal. For example, you could program it to open the valve from the pressure reservoir from 0 degrees to 35 degrees (assuming 0 is top dead center for that piston).

                      I think I underestimated how tricky finding appropriate valves might be. Sounds like I might have to try to make them myself. I like what this guy has done, maybe I can steal some of his ideas? He has rotating valves that look interesting.

                      Even the poppet valves used in a gas engine probably wouldn't work, they are only made to hold pressure in one directions (keep air inside of the cylinder).

                      Those fireless locomotives are pretty cool! If you had a motor like I'm envisioning you could put it in air compressor mode when descending a hill and charge up the tank a little

                      One problem is that if I add a bunch of piping in line with the valve coming out of the cylinder, it is going to reduce the compression of the compressor. This might be ok, because I probably don't want to run all the way at 150psi for a proof of concept.



                      • #12
                        Russian 9 cyl M14P and Chinese radial aircraft engines use compressed air to start them, they have an small air distributor that injects air through small separate poppet valves. So you could do the same with your 'engine.' They store about 800 PSI in their onboard air tank.


                        • #13
                          Piston air motors are extremely inefficient. You'll quickly find that a huge volume of air yields a very short run time, even at reduced power.
                          Southwest Utah


                          • #14
                            Originally posted by chipmaker4130 View Post
                            Piston air motors are extremely inefficient. You'll quickly find that a huge volume of air yields a very short run time, even at reduced power.
                            That's true. Especially when the pressure will only be around 150psi or less. But then it depends on what sort of work it would be doing. If it needs to provide motive force to do some work for more than a short time from the air held in the tank that it compressed then I'd agree that there are better options. But if this is just for the magic of watching something turning that he built/modified? Then it's just fine.

                            I suspect that those compressed air mining engines needed re-charging with each trip up and down the shaft if they were hauling carts of ore or coal.
                            Chilliwack BC, Canada


                            • #15
                              Yeah, I was reading about compressed-air energy storage here: Basically to be efficient it needs to compress the air slowly enough to not heat up significantly, or it needs to store the heat from compression. My machine would do neither of those things.