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Need guidance on pneumatic motorcycle fuel system

  1. Oct 29, 2015 #1

    I'm a physics student at Michigan State and I started an applied physics club. One of our first major projects is converting a 4-stroke combustion engine into a pneumatic engine to drive a motorcycle. We completed the prototype using a 4.5 hp tecumseh 139cc motor, a 5lb CO2 tank, some regulators, and a custom one-way valve opened by the piston at its peak. We're now working on a 1984 Yamaha Virago 1000 cc motor (2 cylinder, 8.3:1 compression ratio) and need some guidance!

    Basic Plan:
    We plan to inject pressurized CO2 twice per cycle, once at a low pressure (15-100 psi) during the intake phase and again at a high pressure (~800 psi) in place of the ignition phase. This way we don't have to modify the cam shaft to convert to a 2-stroke motor; the motor will require very little modification. We estimate the maximum chamber pressure of the combustion motor during ignition to be around 1000 psi so we'll try to emulate that with CO2 and expect to sit around a maximum of 800 psi when the piston is at the peak just before the engine would usually ignite the fuel. We will put together an optical tachometer and observe the flywheel to time the injections. We hope to regulate pressure and/or flow in response to the throttle being turned.

    1) Flow or pressure regulation: We expect the 1000 cc motor at 2000 RPMs to require a very large flow rate. The up side is we won't require regulators with especially quick response times or high precision. We really just need a regulator that has a range between 0-900 psi that can be controlled electronically.
    • From the flow estimates we can decide on which type of regulator would make the most sense in our case
    • We need to source the regulator once we decide on the requirements
    2) Gas phase transitions: This is mostly just something to consider but as it turns out that at around room temperature gaseous CO2 has a phase transition to liquid around 880 psi. While we won't be operating at 880 psi we expect the cooling of the gas as it comes out of the tank (stored as liquid) to lower the transition temperature and result in a gas/liquid mixture in our lines. The higher density may help reduce the required diameter of our fuel lines given a required mass flow rate but this is hard to model. I'm open to suggestions on how to think about this and what problems/benefits we might expect. My intuition tells me that if we inject liquid CO2 it will expand into a gas as the piston lowers causing rapid cooling; is this a problem?

    3) High frequency on/off valve: If we operate at 2000 RPM we need to inject high-pressure gas 33 times/second which may be challenging. Are there high frequency, electronically controlled on/off valves that allow flows listed below?

    We're mostly concerned with filling the volume of the chamber when the piston is at its peak with 800 psi CO2 at around 2000 rpm. We expect that we'll have to settle for lower performance but that is the ideal case.

    Using bernoulli's equation I can estimate the flow velocity given a pressure differential of 785 psi and plugging in different densities. This calculator tells me the density of CO2 considering the pressure and temperature so I plug in different temperatures to get different velocities. Once I have the velocity I use this calculator to estimate the required pipe diameter. At 2000 RPM we need to fill 1/8.3 liters 33 times per second or a flow rate of ~4 L/s.

    At 70 F -> 264 m/s -> 0.17" diameter (mostly gas)
    At 60 F -> 115 m/s -> 0.26" diameter (mostly liquid)

    Lower temperatures don't change much given that the density won't change much (same diameter for 40 F, for example)

    We're open to any suggestions or questions you have. Thanks for reading!
  2. jcsd
  3. Oct 29, 2015 #2


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    What's a pneumatic engine? Using a large source of compressed gas to run the engine instead of combustion? How are you going to carry such a large source of compressed gas on a motorcycle?

    And why are you using CO2 instead of just compressed air? You must have a humongous source of CO2 gas!
  4. Oct 29, 2015 #3
    Good questions. Yes, a pneumatic motor is a motor driven by compressed air but we're using CO2 because it's easy to store in a liquid form at room temperature at relatively low pressures which is not the case with air. Liquid CO2 has a much higher density than air (a factor of ~1200 at 800 psi). The bike is rather large as you can imagine from the engine specs. We may have to lay a large compressed CO2 tank where the seat would be long ways on the bike to get an appreciable run time. It is also cheap to fill tanks with CO2 even though in the long run it would be cheaper to buy a compressor.

    However, the point is that the motor will run if we hook it up to a liquid CO2 tank, compressed air, or even steam! It's an applied physics project; not a 'we're going to change the world by selling these' project.
  5. Oct 30, 2015 #4


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    I've used small co2 motors to power model aircraft. I don't understand how you can inject CO2 twice? If you inject it during the intake cycle whats going to happen on the compression cycle?

    The main issue I had with small model motors was getting enough heat into the motor, the power would drop off in cold weather.
  6. Oct 30, 2015 #5
    We are leaning towards not injecting on the intake stroke simply to save fuel. But if we did inject it would be low pressure then on the compression cycle it would be, well, compressed. My intuition suggested it would act similar to a spring and we would lose very little especially considering the intake stroke typically sucks gas through a jet nozzle creating a vacuum. Although, the lower pressure differential between the high pressure line and the now compressed gas in the cylinder would definitely reduce the flow at the peak of the combustion stroke.

    That's a great point and we definitely will have restricted flow at lower temperatures as the above calculations show. Does this manifest itself in other forms as well?
    Last edited: Oct 30, 2015
  7. Oct 31, 2015 #6


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    To turn from liquid to a gas, a fluid requires energy input. Have you attempted to determine at the flow you think you will need for the engine to run at 2000 rpm, how much heat has to be added to the co2 liquid to become gaseous. With a lessor heat flow into the tank to keep it at room temperature, the co2(l) temperature will drop and so will its vapour pressure. Liquid entering into the engine and turning into a gaseous state needs heat flow also.

    That is to which CWatters is referring.

    You need to study up on the thermodynamics.
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