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Kinda OT- Engines and compressors

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  • Kinda OT- Engines and compressors

    I was thinking about using a couple pushmower engines and a couple weedeater engines along with an air compressor to make a pressure booster for my paintball gun's tank. It needs to get to at least 3000 PSI, better if it could get to about 4500. I'm thinking also 2-3 stages in which the larger bored pushmower engine would power the smaller bored weedeater ones. My question is- How much pressure can those things take without just completely fudging out on me? I have no clue how high pressures get inside an engine. Anybody know?
    You never learn anything by doing it right.

  • #2
    I believe the pressure in an engine cylinder reaches around 750psi (+/-). It's lower than you might think because the piston begins to move away from the flame front and the exhaust valve begins to open before BDC, in most cases.

    The power of an IC engine is gained from how fast you cause fuel to burn exherting force across the surface of the piston. This explosion force drives the piston down or away from the force.

    Besides that, I wouldn't trust a thin aluminum piston, cylinder and head to hold up past 1000psi at the top end. I've seen "blown" aluminum heads on racing engines and it ain't pretty.

    You will need specialized equipment to reach pressures that high with any volume. Most hydraulic cylinders are rated for only 2500-3000 psi. If you look at a manual automotive grease gun, you'll see that they reach up around 2kpsi but the piston is only about 1/4" in diameter with a stroke of about 3/8".

    The rating on you tank would be in question too. You are talking about some serious and dangerous pressures!

    [This message has been edited by CCWKen (edited 06-13-2005).]


    • #3
      I don't think engines get to 750psi.....Diesel's usually run around 350psi from memory...Petrols a lot lower because otherwise the petrol would self ignite at the high pressures...I think <----(disclaimer )
      Precision takes time.


      • #4
        well compression tests i have done (petrol) usually give 120-180 psi on a good engine and under 90 on a bad one .
        hope this is of some help.
        all the best.mark


        • #5
          Diesels (Navistar) are around 400psi.17.5 comp.ratio.


          • #6
            I was talking about the cylinder pressure created during ignition not compression. The reference was to show a comparison for Scatter's thought of an engine used as a compressor and it's capabilities.

            One thing for sure, he won't get 4,000psi out of it and still be in one piece.


            • #7
              I have considerable experience with 3000 and 4500 PSI air (submarine storage and emergency air) and I'm here to tell you that air unter that pressure is nothing to fool with.

              First of all the pressure boundaries of those gas engines are inadequate for the pressure you're working towards. They're deisgned for 1600 max PSI or so. Trying to reach 3000 PSI or more by compounding exhaust into intake of engines used as compressors is asking for a nasty high presssure air situation of not an actual explosion.

              If you want 3000 PSI air for your paintball gun I sure hope you reduce it so some safe pressure. 3000 PSI will push low sectional density projective to supersonic vleocity even in short barrels - velocities high enough to be fatal at close range if it strickes flesh where it's vulnerable.

              If your intention is a simple paintball gun air re-charge station I suggest you use SCUBA (new ones go to 3000 PSI I think) bottles and suitable valves. Have the SCUBA bottle re-charged at a dive shop. Buy a SCUBA "trickle charger" air compressor. Do anything but plumb together engines in series to use them as separately driven compressors because as soon as the final stage gets up enough pressure it will fail - possibly with shrapnel.

              There's no safe, low cost way to make 3000+ PSI air unless you go with a home brew intensifier and these are horribly wasteful of energy.

              [This message has been edited by Forrest Addy (edited 06-14-2005).]


              • #8
                Aside from all the other specifics it simply won't work because the IC engine uses
                poppet valves. To develop the kind of pressure you want you need reed type valves.
                Also be very careful playing with that kind of psi. I have seen it kill people.
                The firing pressure (psi inside the cylinder @ ignition) of a ALCO 251C is 1350 psi.
                There is also the fact that you would burn
                up the unit before you ever got close to the psi you desire.
                Non, je ne regrette rien.


                • #9
                  The SCUBA tank was my first thought too. The aluminum 80cf tanks are rated at 3000psi, tho they're normally only filled to 2800.

                  But I think you'll find that dive shops will require you to present proof of dive certification before they'll fill your tank.
                  Lynn (Huntsville, AL)


                  • #10
                    I guess I should explain myself a little more. The paintball tanks used for high pressure air are rated for that high pressure, and are regulated down to 800 or so, maybe 1000 sometimes to go into the gun before it gets out of the tank. Usually there is another regulator on the gun somewhere, sometimes two, to reduce operating pressures to somewhere like 100-400 psi to actually fire the paintball. This isn't for a homebrew paintball gun, it's for a normal one. Scuba tanks are regularly used for paintball to refill the gun-carried tanks. I'm not making my own tank here, just filling the one I alredy own.

                    And thanks for the advice. I was thinking about this project but wanted to make sure all of the components I was loking for would be rated for high enough pressure. If I had to guess the pressures in an engine, I was actually thinking around the 7-800 psi range, to be on the safe side, but I actually had no real idea. So I wanted to check before I blew myself up. The boosters for normal pressures that are usually used to fill tanks are a couple thousand bucks, and if I could make one for less than 200 with stuff I already had I would save some money .

                    About burning up the unit, if you were talking about burning up the engines by using them as engines- I was thinking of connecting the driveshafts and using shop air to drive the large bores to compress more air at the small side. I dunno if you were talking about just me breaking the engines from heat generated.

                    In any case, I won't be doing this (at least with engines) anytime soon. Thanks for the heads up. Man this board is great !
                    You never learn anything by doing it right.


                    • #11
                      My friend that is into paintball went to a scuba shop and pursuaded them to sell him a big scuba tank and he uses it as a refill station for his paintball gun tank. A lot of money tied up in something that lobs little pellets of paint.


                      • #12
                        Yeah, unfortunately paintball (can) cost a lot of money. Many people with $1000+ guns, 400 dollar tanks, 200 dollar barrels, go through 130 dollars worth of paint per DAY, it can add up. Unfortunately paintball and tools are 2 hobbies that work well together, as you can make some of your own stuff (as well as sell it, there's a pretty high demand for custom stuff) but they're both nice and expensive. Honestly though I have paintball to thank for getting me into machining (along with a little help from battlebots )
                        You never learn anything by doing it right.


                        • #13
                          Stupid question?
                          You don't have a welding supply nearby?(3,000# 180 cf bottles of day air or even co2 aren't that expensive,and 180 cf at 3,000 is a LOT of air)

                          Safty,at 2500+ psi plus ANY oil present in your AIR system will cause it to deisil and explode while it is being filled.Thats why they use nitrogen in shock absorbers and hydraulic accumilators.
                          Forrest is right,high pressure anything is dangerous.

                          But if you insist on building your own pressure amp,a differential air cylinder is whats needed.
                          Assuming you feed the amp with 150psi air you will need a 20:1 ratio to achieve 3,000psi.
                          That would mean a 2" bore cylinder,with a chromed ID(no oil allowed in the amp cylinder).500" wall thickness with 20" of stroke for the amp cylinder.And a 10" bore air cylinder with a 20" stroke for the driver as a bare minimum.With frictional loss it would yield 2900-3100psi depending on cooling.

                          Even if you machined all your own parts and bought only the seals and honed id tubing you would still have over $600 in materials and seals invested,before buying a compressor to supply the 150psi inlet pressure.Cylinders from the welding supply are looking better all the time IMHO.
                          I just need one more tool,just one!


                          • #14
                            You know I never even checked that out- I didn't know they (3k tanks) were that readily availible. Now when you say "day air" do you mean just plain old air? You mentioned CO2- can't use that, it's a liquid above around 800 psi, also freezes the valves in the gun. The big tanks sound pretty good though, definately won't be building my own comp anytime soon.

                            [This message has been edited by Scatterplot (edited 06-16-2005).]
                            You never learn anything by doing it right.


                            • #15
                              The local welding supply here has breathing air tanks that lease for $60 a year and cost $20-25 per fill up.
                              I just need one more tool,just one!