Gas torque vs Diesel torque

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

  1. 7228sedan
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    7228sedan Senior Member

    Gonzo, as the gear in question is a V-Drive do you think that the weak link would be the V-drive portion or the clutches in the transmission itself?

    I know that Mercruiser used the 72 series velvet transmission with their TRS performance outdrives. Those drives were used on some pretty high HP performance boats back in the day. It is my assumption that the v-drive portion is the weaker link based on the transfer of the power through the gears.
     
  2. gonzo
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    gonzo Senior Member

    I think the V drive would get the same torque whether the engine is a diesel or gas. The reduction gear will match the torque and RPMs to the propeller so it is the same regardless of input.
     
  3. broke_not
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    broke_not Junior Member

    Sorry, but that's ridiculous. Vibration you can "see" is one thing....a modern diesel doesn't shake as much as earlier ones do if we're talking about what we can observe as far as the engine dancing about on its motor mounts goes, but there are still the characteristic diesel harmonics to contend with.

    Do you deal with diesels much on a day-to-day basis? I do, and if the same piece of machinery is available with gas or diesel power, the diesel versions always have more issues with hardware coming loose, brackets breaking, welds failing etc.

    The equipment I'm talking about comes in all shapes and sizes, but has one thing in common: It's all equipment that is designed and built by the manufacturer, and has had the power unit sort of added on *almost* as an afterthought. As a result, some of the same brackets/mounts/hardware are common among the gas and diesel versions. Those same components that will live happily for years without being re-torqued or replaced on a gasoline machine, fail/break/loosen on the diesel machines on a fairly regular basis. None of the components I'm referring to are withstanding the differences in torque or horsepower or the way it's ultimately delivered by the output shaft, I'm just talking about the harmonics/vibrations acting upon everything bolted to the engine itself....
     
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  4. discovery
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    discovery Junior Member

    If you look up the load ability of the bearings used in these transmissions, they are usually expressed x amount of torque per rpm. The reason this is so is each individual roller or ball in the bearings can spread the load over a larger area if they are spinning faster. If you think of a ball bearing, the individual balls have to spread whatever loads applied to them to an inner and an outer surface. stationery, the load is only on a small line (line contact), slow speed, that line is now moved a little and so its a small area contact, and as the speed rises, the area of contact grows. As the area of contact grows, so does the amount of torque able to be transmitted, right up until the mechanical strength of the bearing as a unit is compromised.

    In a marine gear, the torque is trying to separate the reduction gears (spur gears) as it turns the propshaft. The load on the bearings trying to keep those gears in correct position is the same whether it be petrol or diesel provided the torque level is the same. The difference comes when you realise the petrol is giving the torque at twice the revs as the diesel allowing the bearings to spred the load over twice the area as the diesel would because the diesel is only spinning half as fast.

    A bit long winded but I hope you get the jist.
     
  5. jonr
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    jonr Senior Member

    When you look at torque on an instantaneous basis, it's not the same, even at the same HP and RPM. It also varies with the number of cylinders. See http://en.wikipedia.org/wiki/Torsional_vibration and consider that when most people talk about torque, they actually mean "average torque" over one revolution or more.
     
    Last edited: Sep 15, 2013
  6. powerabout
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    powerabout Senior Member

    they then went to 71's on the 454, that was marginal...
    They also used 73's on early III drive packages
     
  7. powerabout
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    powerabout Senior Member

    I think he means torsional vibration
    A diesel can chew a drive train up that a petrol at twice the horsepower lasts forever
     
  8. El_Guero

    El_Guero Previous Member

    Not quite, in this example you are discussing 1800 rpm and 4900 rpm.

    With the same number of cylinders, you are distributing that torque impact much smoother at 4900 rpm than at 1800 rpm ....

    So, each impact of the diesel cylinders would have about 3 times the impact force of the gasser.

    Wayne
     
  9. sdowney717
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    sdowney717 Senior Member

    Consider a diesel and gas engine of same torque rating with diesel at 2000 rpm = gas at 4000 rpm.

    Diesel engine running slower but with same torque at the lesser 2000 rpm means each piston firing is hitting down with more force because engine is turning slower but it has the same torque as the gas engine at 4000 rpm. Which makes some sense if you think about it.

    Another way to visualize engine firing pulses is as an impact wrench which hits or whacks itself around and around.
    So each power stroke in a diesel will hit the gears harder than in a gas motor.
     
  10. fredrosse
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    fredrosse USACE Steam

    Diesel (CI) vs. Spark ignition (SI) engines, with the same number of cylinders, same output torque, same horsepower. Output torque is the aggregate of several processes:

    1. Intake stroke, not much happening with respect to torque requirements, consumes a little energy to bring in the charge of combustion air.

    2. Compression stroke, consumes some significant energy, and much more for the CI engine, which has a compression ratio far above the SI engine, typically 22:1 vs. 9:1, more or less. At this point the CI engine has a deficit in work energy, that has to be repaid during the overall cycle.

    3. Power Stroke, provides the only stroke that is not consuming energy, but the CI engine must provide more power on this stroke than the SI engine, to make up for the defict created during the compression stroke. The forces on the piston and rods, crank journals, etc are generally much higher for the Diesel.

    4. Exhaust Stroke, not much happening with respect to torque requirements, consumes a little energy to push the burnt gasses out of the engine.

    Looking at the overall cycle, the Diesel with its very high compression ratio has to do much more energy shuffling during the cycle process. That is why Diesels generally have to be built more heavy duty, why they generally have heavier flywheels to keep RPM variations during the cycle within check, and why they tend to shake stuff apart more than spark ignition engines.

    Remember that in the technology, the average torque output is the sum of the four strokes, and the Diesel will always have larger variations going around the cycle than a spark ignition engine will. Multiple cylinders ease the problem some, special balancers can ease the problem some, but the poor gear teeth feel the extra push on the power strokes of a Diesel, somewhat higher, so the same gearbox will have different "average output torque" ratings for Diesel vs. Spark Ignition.

    One way around this is to have a fluid coupling between the engine and the gearbox, (like an auto trans in an automobile) but these are so inefficient that they are not generally used in marine transmissions.
     
  11. powerabout
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    powerabout Senior Member

    Nice description
     
  12. jonr
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    jonr Senior Member

    Not only do diesels turn slower (more torque @ same HP) and have to make up for the compression stroke, they produce a higher/sharper peak pressure during the power stroke.
     
  13. FAST FRED
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    FAST FRED Senior Member

    , they produce a higher/sharper peak pressure during the power stroke.

    And that causes the transmission to need to be bigger heavier and more costly.
     
  14. powerabout
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    powerabout Senior Member

    On ignition and then a long high pressure blow down hence the torque they produce
     

  15. powerabout
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    powerabout Senior Member

    how about a CI engine with very low comp ratio and huge boost
    The intake stroke becomes a powerstroke and the pumping losses might go down on the compression stroke?
     
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