Gas torque vs Diesel torque

Discussion in 'Inboards' started by 7228sedan, Feb 19, 2013.

  1. 7228sedan
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    7228sedan Senior Member

    Can anyone explain to me why a marine gear manufacturer would have 2 different ratings (gas & diesel) based on torque? I could understand HP as a 300 HP gas engine makes roughly half of the torque of a 300 HP diesel engine. If the gear has a capacity of 495 lb-ft when coupled to a gas engine, why would the same gear only have a maximum capacity of 375 lb-ft when coupled to a diesel? The only reason I could think of is that the diesel torque occurs at such a lower RPM... I am confused.:confused:
     
  2. gonzo
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    gonzo Senior Member

    There are two basic ratings parameters for gears: torque and RPM. A gas engine will typically run at a higher RPM. If a gear has a rating of 495lb/ft at a lower RPM running diesel, it should be no difference. However, the 300HP gas engine will produce a much lower maximum torque than the lower turning 300HP diesel, so your torque specifications can't be right.
     
  3. jonr
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    jonr Senior Member

    Torque is torque no matter where it comes from. But vibration is another issue and I suspect that is the difference.
     
  4. DCockey
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    DCockey Senior Member

    I've heard of different torque ratings for automotive transmission when used with a Diesel engine vs a gasoline engine. The reason given was differences in the torque pulses as the cylinders fired.
     
  5. alan white
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    alan white Senior Member

    A gear is capable of a certain torque limit and a diesel will apply more torque at a lower RPM compared with a gas engine. Since the diesel will usually supply the power at a lower rpm range, the ratio of the gears should be different.
    The idea is to turn the prop at the same speed regardless of the engine type. The gas engine's speed has to be slowed down more to do this. If you had access to the driven gear and turned the prop by hand by using your finger to turn the gear, a gear with half the diameter would be harder to rotate than a bigger gear due to it having less leverage.
    Output gears on diesels would, all things being equal, be smaller in this way. The teeth, like your finger, would be harder to push around due to the smaller diameter. The gears would have more wear on the diesel because of this smaller diameter output gear diameter, just like your finger would be indented more trying to spin that gear.
    The diesel gearset would therefore have to be wider or larger in diameter or both. They could also be better, more durable gears. In any case, the same gears would not last as long on the diesel if they had been spec'd for the gas engine.
     
  6. 7228sedan
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    7228sedan Senior Member

    The gear in question is a Velvet Drive 72C V-Drive. Per the mfg spec, the torque rating for gas is 495 lb-ft, while in diesel trim it is 375 lb-ft. This is for the 1.51 & 1.21 ratio units. There is no mention of RPM, just lb-ft... I was under the impression that a lb-ft was a lb-ft whether it was generated by gas, diesel, or a gorilla with a big breaker bar. If the gears are able to withstand 495 lb ft say at 4200 RPM on a big block gas motor. What could be the reason why those same gears wouldn't be able to withstand the same 495 lb ft at 1800 RPM?
     
  7. 7228sedan
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    7228sedan Senior Member

    This is more or less what I was thinking is the problem...
     
  8. alan white
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    alan white Senior Member

    It doesn't make sense. A ft lb is a ft lb. At the same horsepower, a gas engine would have half the torque at twice the RPM.
     
  9. Number4

    Number4 Previous Member

  10. 7228sedan
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    7228sedan Senior Member

    Hence my confusion...

    Number4, yes that was the source of my data. There is an additional PDF out there with the specifics regarding the V-drive which is a bit lower in capacity.

    This being the case, has anyone heard of a way to increase the capacity of a velvet drive for heavier service?
    I guess it makes it a little difficult as I have no idea what the "weak link(s)" could be i.e. clutches, gears both...
     
  11. DCockey
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    DCockey Senior Member

    In automotive gearboxes the ultimate limiting factor is generally the loads on gear teeth with the distance between shafts being a major factor in determining those loads.
     
  12. jonr
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    jonr Senior Member

  13. 7228sedan
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    7228sedan Senior Member

    Jonr, do you mean if the diesel was balanced to a greater degree, the transmission would be able to handle the increased torque?
     
  14. gonzo
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    gonzo Senior Member

    The reduction gear on a Velvet Drive is added to the tail. It doesn't affect the torque or RPMs on the clutches or the input shaft. Modern diesels don't vibrate any more than gas engines.
     

  15. latestarter
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    latestarter Senior Member

    RPM is an average figure as is torque. It varies with the position on the 2 or 4 stroke cycle.

    On the power stroke it is accelerated and the torque increases enormously, on exhaust, induction and especially compression it is slowing down and the torque is negative. But for the stored energy in the heavy flywheel the engine would stop, this is less of a problem on a multi-cylinder engine were the next cylinder in the firing order takes over.

    Because a diesel engine's compression ratio is so much higher than a petrol/gas engine the higher negative torque to compress the air/fuel mixture means the peak torque on the power stroke has to be greater to give the same average torque.

    For example in the case of a racing motorbike, even at the back wheel the pulses affect the adhesion of the tyre. The V twins had different characteristics to the straight 4s even though their nominal BHP and torque were similar. A high speed video of some motor bikes shows the chain going slack at times during the cycle.
     
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