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Air and CO2 Engine Design

  1. Oct 15, 2013 #1
    To give a little background I have machine tool training but not much other than what I have taught myself in general physics. Its a VERY interesting subject for me and I have thought about going back to college just to take a course or two relating to it. I come up with various ideas I would like to test out but cant always get my head around good ways to plan a build calculation wise. One of my current projects is a CO2 or Air powered engine. I am familiar with measuring and caution involved with high pressure but could use some help in sizing calculations for components.

    I would like to try to make a small engine that uses pressurized air (or CO2) as the working fluid but am having some issues getting started with designs. I have been reading a good bit on the bore and stroke's effect on performance but many of the calculations I could find deal with IC engines. How do I calculate for a certain pressure input how bores and strokes will react power and RPM wise? I know the shorter stroke will tend to have a faster RPM but lower torque but how do you come up with an optimal number to start testing at? For example lets say I have 1500 PSI (gauge or absolute whatever is easier to calculate) as an input and want the engine to comfortably put out 60 HP @ 2200 rpms in one cylinder with (60 x 5252)/2200= ~143 ft/lbs torque.

    Another big issue is the flow rate in order to fill that combination of bore and stroke in the right amount of time. So 2200/60=~36.6667 rps and 1s divided by 36.6667 is ~.0273. If I did that right one revolution happens about every .0273 seconds. This means that I have half that time (now 13.65ms) to get the pressure in the cylinder for the power stroke. I need to know how to figure the bore and stroke combination in order to finish this calculation though.

    All of this is just an example to get some formulas started but was the best place I knew to start.

    Thanks for any help.
  2. jcsd
  3. Oct 15, 2013 #2


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    You are considering pneumatic motors.

    Expanding air, CO2 or steam are all the same concept.

    Better efficiency is obtained by compounding the cylinders.

    Have you considered turbines rather than piston engines. Also with pneumatics you can apply pressure alternately to both sides of the piston. By running two cylinders in quadrature you have a smoother torque with a smaller flywheel, equivalent to a 4 cylinder IC engine. If your piston rod is a large diameter you can make a two stage compound engine since piston area is significantly different between the two sides.
  4. Oct 15, 2013 #3


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    What does 'running two cylinders in quadrature' mean? 'Quadrature' is the process of finding or calculating an area.
  5. Oct 15, 2013 #4


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    Quadrature also means 90°. http://en.wikipedia.org/wiki/Quadrature_phase
    The sine and cosine functions are in quadrature.

    With one piston and crank there are two dead spots, at TDC and BDC, where the motor cannot start without assistance. By having a second crank at 90° and double sided pistons you have no dead spots and smoother running.
  6. Oct 15, 2013 #5


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    It also means "90 degrees out of phase", as in the "(in) phase" and "quadrature" components of a
    complex electrical impedance, etc.

    So with two double-ended cylinders "in quadrature", you can get four power strokes per revolution of the crank.
  7. Oct 17, 2013 #6
    Thanks for the replies. I have considered turbines but I don't know enough about them to build them and transfer the power to something else. Turbines seem a little less forgiving and have to be balanced better than i think I can do right now.

    I have thought about the pneumatic cylinders and how they work but wasn't sure how well I could seal the shaft for one and how well it would hold up with a constant high speed movement in and out. I guess you could just reverse the way a set of piston rings works and mount the rings in a fixed position and have the shaft move against that. I did a quick look around and to get a high pressure cylinder it looks like I ma have to look into more hydraulic cylinders but that brings me back to the speed issue and how much drag thy have. Depending on their seals since they are made to work with oil there could be a lubrication issue as well. That was the nice thing about pneumatic cylinders. I do really like the idea of using the cylinder and giving it pressure in both directions. Thats pretty much how an old train steam engine worked right? I wonder how hard that would be to turn into a rotational force in a short distance of less than a foot?

    Anyone have any ideas on how to start sizing the bore and stroke?
  8. Oct 17, 2013 #7


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    To start you must know the gas pressure available and the torque required.
    The gas pressure will act on the area of the piston and result in an axial force.
    The stroke is equal to twice the crankshaft offset which converts the linear force into torque.

    You could make either the stroke or bore a controlled variable and solve for the other.
    There is a range of many solutions.
    If you know the ratio of bore to stroke then you can solve for both bore and stroke.
    Play with the equations and juggle the numbers.

    With compound cylinders you have more degrees of freedom.
    The efficiency of the engine will be a function of the intermediate pressure(s).
    You will need to juggle the equations and numbers again to find an optimum solution.
    Last edited: Oct 17, 2013
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