Electric to Petrol

  1. Spanner88 New Member

    Messages:
    10
    Location:
    UK
    Hi all

    I have a 100 litre 3hp air compressor that's single phase 230v has the one 14cfm belt driven pump on it.

    I've got my hands on a Honda gx240 8hp petrol engine and another secondhand 14cfm pump

    The goal is to make a frame up and along side the 100 litre tank, mount both pumps and drive them with the one 8hp engine.

    So the first thing I'm trying to get my head around is the RPM of the pumps and the rpm of the engine and what gearing to use, then I'll move onto making a twin unloading setup and throttle back for the engine when the tank reaches pressure.

    I assume the pumps need to turn at a certain rpm? Turn them too fast and theyll burn out?

    There is a rpm on the 3hp single phase motor plate that's on the compressor at the moment, but is that atuchally any use at all or is that just telling me the nonload rpm of the motor

    Should I start by setting up a cheap tachometer and measuring the rpm the pump turns at, thats on it at the moment?

    I don't intend to change the pulleys on the pumps, but rather stick two pulleys on the engine output shaft.

    If I measured the rpm of the pump and found that

    Would it be stupid to assume the other secondhand pump turns the same rpm because it's pulley is the same?

    What rpm does the engine need to run at, any? As long as it doesn't stall with the pumps running at rpm?

    So I've found out the rpm of the other pump, which is apprently 965rpm

    I've found this about the Honda gx240 aswel

    Net Power Output* 7.9 HP (5.9 kW) @ 3,600
    Net Torque 13.5 lb-ft (18.3 Nm) @ 2,500 rpm

    Does any of that help? Would the engine want to run flat out to produce the horsepower or at 2.5krpm so the most torque

    Bit lost with this
     
    Last edited: Oct 3, 2019
  2. 8ob

    8ob Forum Supporter

    Messages:
    4,422
    Location:
    moscow on thames
    The engines max torque @ 2500rpm is where the engine is most efficient so work the pulleys back from that.

    Bob
     
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  3. Spanner88 New Member

    Messages:
    10
    Location:
    UK
    Legend ok that gives me something to work with, nice one
     
  4. brightspark

    brightspark Member

    Messages:
    27,029
    Location:
    yarm
    the motor tells you the rpm on the shaft work the ratio out of the pulleys that will give you the rpm of the pump . then you will have to find the rpm of the petrol engine and then work out the ratio to reduce speed to the final drive on pump
     
  5. premmington

    premmington Member

    Messages:
    1,287
    Location:
    Norfolk
    Don't forget a petrol compressor will need an "unloader valve"...
     
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  6. Spanner88 New Member

    Messages:
    10
    Location:
    UK
    Aye it makes sense now using a pulley calculator online, what rpm the engine wanted to be was the missing peice of the puzzle. The calculator now tells me what size pulley to put on the engine, now I can tell it 2.5krevs and the rpm/diameter on the pump.

    I think I've sussed an unloading system using the original pressure switch and two normally open high pressure 1/2" solenoids.

    So when the motor pressure switch turns on (low pressure) what used to run the motor, now powers the normally open solenoids on each pump outlet and they shut meaning air cantc vent so it goes into the tank. Should work and it's only cost £30. The price of unloading valves were making my eyes water. It's not purely mechanical, but I should manage running that off a small inverter when working mobile.

    Been struggling for ages trying to get a decent compressor running mobile in a van, electric just needing way too much power and it being a faff plugging into customers houses and it tripping all the time and it still not being big enough for the job. Carting about stupid long extension leads and messing around getting power at workplaces is a pain in the neck.
     
  7. eil Member

    Messages:
    1,752
    N Wales
    It used to be that you needed a 5hp engine to replace a 3hp electric motor on a compressor, if you intend to spin two pumps you might be underpowered
     
  8. Spanner88 New Member

    Messages:
    10
    Location:
    UK
    Oh man don't tell me that! Really?
     
  9. brightspark

    brightspark Member

    Messages:
    27,029
    Location:
    yarm
    yep 14cfm on electric need a 3hp motor a 5hp needed if petrol :)
     
  10. Spanner88 New Member

    Messages:
    10
    Location:
    UK
    **Language** sticks! Ok that's thrown a spanner right into it, I wonder if I can get more power from the engine with a bigger carb
     
  11. ........... and bigger ports and bigger valves ........!
     
  12. brightspark

    brightspark Member

    Messages:
    27,029
    Location:
    yarm
    try and get a 10 hp honda engine :)what tools are you using on the comp /?
     
  13. Turbo Member

    Messages:
    3,251
    Location:
    Fermanagh, Northern Ireland
    I looked into this before as I was considering converting a compressor to petrol power & they recommend a 5.5hp engine to replace a 3hp motor.

    The way around to use a smaller engine is to gear down the drive to give the engine less work to do. ;) Downside of this is it will take longer to fill the tank as you are effectively reducing the cfm of your pump by running it slower! :(
     
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  14. Spanner88 New Member

    Messages:
    10
    Location:
    UK
    What's likely to happen with the engine under power, get to around 100/120psi ish and just stall?
     
  15. brightspark

    brightspark Member

    Messages:
    27,029
    Location:
    yarm
    you could try the 8hp it might be ok or as said gear it down a touch . it will give it more torque but not extra hp :)
     
  16. 8ob

    8ob Forum Supporter

    Messages:
    4,422
    Location:
    moscow on thames
    I would run with what you have and use two unloaders set at different release pressures and differentials, the larger of the two pumps could unload @90psi and the smaller one at @120. That would give the petrol motor a fair chance of coping with demand rather than both pumps banging in/out at the same time.

    Bob
     
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  17. armalites Member

    Messages:
    4,123
    Herefordshire
    I fitted a clutch pulley to my Honda powered unit, works really well, when the pressure is up it the revs drop and the clutch releases so the engine can sit ticking over with no load. When the pressure drops the revs rise and the clutch engages to drive the pump.

    It's the same sort of thing used on Honda powered karts but it has a pulley instead of a sprocket.
     
  18. MCKDAVID Member

    Yep, the compressor head on mine is the equivalent of a 3hp electric in terms of air delivery but uses the 5.5hp petrol honda...

    For mechanical pressure control, as above you'll need some form of unloader, but all that does is dump the air pressure at the set point, the engine will continue to run at whatever speed it's set to. To reduce (control) the engine speed you need throttle control as well. My petrol engined compressor was a lash up of bits to kind of make it work when I got it. To make it usable, I stripped and cleaned the unloader valve which was blocked totally and not dumping at all. That done, it worked, but the engine was still running at higher revs all the time, so I bought a pneumatic throttle control valve off ebay. Think it was around thirty quid including delivery from across the pond. Piped it up to suit, now the pressure ramps up to 8 bar, and the unloader dumps the surplus air, and the pneumatic actuor pushes the throttle arm down to low idle speed, and conversly ramps up on demand when the air pressure drops. Works fine.

    Only problem I had was initially I connected the throttle actuator to the unloader port directly, but when it started dumping the volume of air the pressure at the point of connection dropped slightly. Dropped enough to "fool" the actuator into thinking it was calling for more air, so it ramped the engine up to full speed again, basically defeating the purpose of the actuator, obviously that was no good.... I plugged the unloader port off and put a tee under the PSV at the other end of the tank. There the pressure was stable whether or not the unloader dumped so the engine reduced speed accordingly.
    Agreed, connecting under the PSV isn't the pefect solution, but everything connected to it is rated over the PSV rating so if I ever get to the point where the PSV lifts, all the connected bits are still over rated to be on the safe side...



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  19. MCKDAVID Member

    might work, but will be a lot of wasted air, depending on the capacity of the compressors, it might not reach the second set point? :dontknow:and effectively be running full time?
     
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