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Proper RPM for cool down on air cooled engines?

Discussion in 'Lawn Mowing' started by sedge, Oct 17, 2007.

  1. sedge

    sedge LawnSite Fanatic
    Messages: 34,764

    This question came to my mind when reading and posting about air cooled vs liquid cooled motors.

    In your opinions, what is the correct rpm to run the motor during cool down for an air cooled motor?

    Length of time needed?

    I have the guys run them at half throttle or so from the back of the yard to the trailer, then they shut them off.

  2. Juan_Deere

    Juan_Deere LawnSite Senior Member
    Messages: 301

    I have never seen any special cool down requirements for air-cooled engines in the manuals of the mowers I have used.
  3. gmlcinc

    gmlcinc LawnSite Member
    Messages: 87

    I have a Toro z560 and the manual states set throttle to half and let idle for 60 seconds. What your doing is probably good as long as it's at least a minute. I always do that and then let it idle while I put up the gate and get the trimmer started.
  4. Lawdog302

    Lawdog302 LawnSite Member
    Messages: 197

    Full throttle no load is best for cool down. The slower the engine runs means less air flow over fcooling fins. Slowing engine down for cool down is a myth on air cooled, it builds heat.
  5. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    It may be a myth but it works for me. Every time I shut an engine down at full throttle bad things happen. Big back fires, smoke at re-start. Stuff like that. I let mine idle for a minute or so if I can.
  6. puppypaws

    puppypaws LawnSite Fanatic
    Messages: 9,148

    You are absolutely correct, the principle of more air movement means cooler a engine.

    The same principal applies to a liquid cooled engine as well, the more air movement through the radiator the cooler the engine runs. When you have enough mechanical air movement (by fan) over the radiator the lower the engine temperature. When you are sitting at a stop light if you do not have enough air circulating through the radiator you can watch the temperature gage rise, once you are moving at 60 mph again the engine temperature will drop.

    They build vehicles with just enough radiator surface to keep the engine at the designed operating temperature. When and if the radiator gets a certain amount of debris covering it and cuts the cooling surface by a very small percentage the engine will start gaining heat from low air flow.

    This scenario states why an air cooled engine cools best with the cooling fans at maximum rpm's, it is just a common sense thing.

    When you let a turbo charged diesel engine idle to cool down temperature, you are not just letting the engine cool but you are cooling the turbo charger also. The diesel applications use a larger radiator with more flues to give extra room for surface restriction because they are normally running in a dirtier environment.

    When the engine is taken from under blade rotation load it is cooling itself to the best of it's ability with the mower stationary at full throttle.
  7. Lawdog302

    Lawdog302 LawnSite Member
    Messages: 197

    That is why newer engines are designed with fuel shut off solenoids. Once you kill the engine at full throttle, it instantly stops fuel from entering the cylinders. The momentum of the engine coasting to a stop purges all extra fuel out of the system/muffler so there is nothing left to ignite(backfire).
  8. puppypaws

    puppypaws LawnSite Fanatic
    Messages: 9,148

    Did you and Bruce get together, he said he left you a message on your phone.
  9. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    I'm calling him tomorrow afternoon.
  10. Richard Martin

    Richard Martin LawnSite Fanatic
    Messages: 14,699

    That only works on engines that have those solenoids. Neither of my bigger Kaws (two 15s) that are only 2 years old have them. My EFI Kohler doesn't need one but it will smoke pretty badly on restart if I don't let it idle for a minute.

    Let me put it this way....

    Nothing bad has ever happened because I did let them idle.

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