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Increase a gas engine's RPM to drive a water pump

  1. Feb 18, 2016 #1


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    Hello all,

    This is my first post, but I'm no stranger to skimming the boards. Always can find helpful stuff, but I'm stumped on this one. The situation is we have a Kohler 17 hp electric start gas engine horizontal shaft that max's out at 3600rpm. We are attaching a self-priming water transfer centrifugal pump to it that can handle the same HP, but it starts at 3600rpms and max's at 6000rpms... I want to get the most out of my pump so I have to increase the ratio on the pully system it will be attached to.. I am looking for a 1:3 minimum, but 1:4 would be solid... I'll stick with the 1:3 for now... My question is if I would attach let's say a 3.5" pulley to the pump and around a 10" on the engine then would the engine have a tough time starting up on it's electric start system with that big of a pulley attached directly to it? Engine shaft is 1 1/4" and pump shaft is 5/8".. Everything double belt or triple? The entire system runs on a 12v battery which is charged by the command pro Kohler engine... The system turns on and off automatically using murphy switches and a control panel.... Any input would be most appreciated... Thanks
    Last edited: Feb 18, 2016
  2. jcsd
  3. Feb 18, 2016 #2


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  4. Feb 18, 2016 #3


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    Staff: Mentor

    I think your odds are good. The nice thing about a pump is they have small, low inertia impellers and start with fairly low torque as long as they aren't speeding up too fast.
  5. Feb 21, 2016 #4
    Can you have an idler between the two pulleys to tension the belts once it has started? Old technology, it's simple, and it should do the job for you.. a torque convertor would definitely work great, it's just more expensive.

    If you wanted to automate the tensioning system you could connect a line from the engine oil pressure to a small cylinder that tightens the belts... as the engine starts it will slowly build pressure and engage the pump... might squeak a little as it engages.
  6. Feb 22, 2016 #5


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    What rpm has to be achieved before engine will start ?
  7. Feb 22, 2016 #6
    I'd say 'idle speed' on a 17hp industrial engine would be around 1200-1500 RPM, below that it probably doesn't make enough power to turn itself over well...
  8. Feb 22, 2016 #7


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    What I am trying to get at is the engine catching speed and how that relates to the coming on load speed for the pump .

    Say 3:1 speed up to a pump which starts to pick up significant load at - rough guess - 2400 rpm .

    2400/3 = 800 rpm at the engine .
  9. Feb 22, 2016 #8
    Perhaps if you could have a bypass on the pump so it can't build any pressure would help keep load off while cranking.
    I'm kinda thinking out loud
  10. Feb 22, 2016 #9


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    Not enough data to do proper sums but I think that limiting the speed up ratio to around 2 would be a good idea .

    Otherwise you may need an unloading device as suggested .
  11. Feb 22, 2016 #10


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    Why would you want a 3 or 4 to 1 ratio? It seems to me that you'd want a ratio such that the pump and engine both hit maximum speed at the same time, which implies a 5:3 ratio. A 2:1 ratio would probably also work fine, so long as the gas engine is producing near full power at 3000RPM, but if you run a 4:1 ratio, you'll be expecting the engine to run at 1500RPM while driving the pump, and the engine likely doesn't make very much power at 1500RPM.
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