Purpose of High rpm Low Torque Engines

Discussion in 'Gas Engines' started by baboonslayer, Dec 21, 2010.

  1. rxcomposite
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    rxcomposite Senior Member

    The tip of the prop is fastest as it is away from the center of the shaft. If it is too fast tip cavitation develops and lessen the effectiveness of the prop. There are reversible engines that run at 800 to 1000 rpm and can be direct coupled eliminating the 3% loss on gearboxes.

    If the correct engine match cannot be found a marine gear is used. Modern gearboxes now claims losses down to 1.5% so engine with gearbox is common.

    As CDK has mentioned, turbines are used with a gearbox. Turbines are more efficient than diesel but develops power at high rpm. It also has less parts to wear so the benefits offsets the losses. A ferry is a workhorse. It goes up to full speed and maintains it as soon as it clears the harbor.
     
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  2. Steve W
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    Steve W Senior Member

    This is a great thread, (unlike many where the knowitalls revert to name calling to try to enhance their stature) Im not a gearhead and there are a lot of things that confuse me, like the relationship of hp and torque. I have a car,a VW TDI (sorry,i know this thread is about gas engines) that produces a whopping 90hp at 4000rpm and 149ft/lbs at 1900rpm,this has been the most satisfying car i have ever driven,why? one word,torque. If i mention 90hp to gearheads they would laugh as do i,i mean i dont care what hp it puts out at 4000 rpm, i dont think ive ever run it to 4000, maybe 3500 winding it through the gears but usually more like 3000 so why do i even care what the hp is? Where i spend most of my time is between 1600 and 2200 and in this range it is powerful and supremely economical. It seems to me that hp is all about marketing as in the saying "you buy horsepower but you drive torque". It would be more meaninful to me if the manufacturers would give me the HP and Torque both at the rpm that i will drive at.I recently smashed the car and it was totaled so i ,of course bought it back from the insurance company with the intent of putting the engine in something, another car or perhaps a boat (where else could i buy a known diesel engine with all the electronics for $300? even though it has a quarter million miles on it)
    So,why dont they give me the hp at, in this case 2000rpm and perhaps someone can give me the math for me to compute it myself.
    Thanks,Steve.
     
  3. RonL
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    RonL Junior Member

    Can you clear that last part up ? I think I know what you meant, but cannot think of any way that engine RPM can be nonrelated to prop speed.:confused:

    Ron
     
  4. gonzo
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    gonzo Senior Member

    They are called gears. You can get any prop RPM regardless of engine RPM
     
  5. CDK
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    CDK retired engineer

    Sorry, I sometimes take large steps to keep a message short.
    What I intended to convey is that a slow spinning prop isn't necessarily powered by a slow revving engine. But of course, once they are mated with a gearbox between them, engine rpm determines prop rpm.

    There is an old, smaller ferry here that is used occasionally in wintertime when there is little traffic. It has a medieval large engine, directly coupled to the prop shaft.
    The helmsman has to stop the engine while approaching the harbor, then at the proper moment start it again in opposite direction to slacken the vessel's speed and kill it when the ramp touches the shore.
    A tricky business judging from the way the bow (or stern) is dented.
     
  6. CDK
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    CDK retired engineer

    (Torque x Engine speed) / 5,252 = Horsepower
    (torque in ft/lbs, hence the conversion factor)

    What you so particularly liked in the TDI is the almost flat torque curve between 1600 and 2200 rpm. Only diesels behave like that.
     
  7. Steve W
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    Steve W Senior Member

    Thanks CDK, so at peak torque at 1900rpm i would be using about 54 hp, doing 58mph and getting about 58mpg (or about 1gph) these are real fuel consumption figures based on 50000 miles of use. I would love to use this engine in a boat, just dont know what yet.
    Steve.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Yes, they are probably dinosaurs and I thought extinct but about 4 years ago, I surveyed a ship with a direct coupled engine that was huge and heavy. The ownwer was probably just trying to squeeze the most life out of it.

    My first job was an apprentice marine engineer in a ship with a direct coupled triple expansion steam engine. It was short lived as it was delivered to the scrapyard that year.

    The reason I mentioned direct coupled engine is that it still exist in Principles of Naval Architecture and thus is still part of the design spiral.
     
  9. Joakim
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    Joakim Senior Member

    No, the key is the charger, not diesel. I have a Peugeot gasoline turbocharged engine, which produces flat torque from 2000 to 4500 rpm and then flat power to about 6000 rpm. It's all about the charger and settings of the charging pressure. Almost all current car diesels are turbocharged and not for maximum power, but for much power (and torque) at lower rpm.
     
  10. Submarine Tom

    Submarine Tom Previous Member

    Our passenger Ferry here a (~175 passengers, no vehicles) 35 knot catamaran, recently changes out their two turbines for diesels. It's their flag ship (out of three) and makes the 2 1/2 hour run, two - four times a day depending on the season. This would seem to be in direct constrast to post #46. I am curious to understand why they would go to the expense and inconvenience if they were not to gain from the change...

    -Tom
     
  11. rxcomposite
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    rxcomposite Senior Member

    Here is something I prepared for the other thread. I think it is dead so I will post it here.

    Volumetric Efficiency

    Theoretically, the power of the engine is developed by the burning of the liquid fuel. Fuel is rated by its BTU content. In IC engine, the liquid fuel is vaporized and mixed with air. To burn properly, a correct proportion of fuel and air must be maintained. This is called fuel to air mixture. Too much air and the mixture is “lean”, too little air and the mixture is “rich”.

    Air (and fuel) is drawn in by the piston during the intake stroke. Volumetric efficiency is a ratio (or percentage) of what quantity of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Efficiency is about 85 to 90% for IC engines. It also means the volume of fuel/air coming in and the volume of burnt fuel coming out.

    To increase power, more fuel should enter the chamber. To maintain proper fuel/air mixture, more air needs to be drawn. The volumetric efficiency needs to be improved.

    There are several ways to increase the VE;

    1. Increase valve size or use multiple valves with twin cam
    2. Reduce air resistance by streamlining the air flow (porting intake and exhaust port)
    3. Use high lift cams so the valves stay open longer
    4. Use variable valve timings, valve opens more at high rpm

    Ambient air pressure is one atmosphere (14.7 lb.). To increase the pressure inside the cylinder at more than 100% of the atmosphere, forced induction method is needed by;

    1. Injecting pressurized air- limited by compressed air supply.
    2. Supercharging- linear pressure from 1 rpm up but bulky, great for quarter mile run
    3. Turbo charging- have turbo lag but efficient at high rpm
    4. Injecting nitrous oxide- limited by supply of nitrous, great for acceleration.

    Or use special fuel blends with high octane or high oxygen content.

    My apology Baboonslayer, I know you are well versed in this.
     
  12. rxcomposite
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    rxcomposite Senior Member

    I can only guess that it is not efficient for the short run. At 2 1/2 hours run plus 15 minutes slowing down and 15 minnutes going out, thats about 20% of the time it is not at at full speed. Turbine is not so fuel efficient at low speed.

    Another reason I can think of is that turbine is not in the mainstream anymore and parts are expensive.

    So it is either the operator has found out he is losing money on fuel or parts.
     
  13. Joakim
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    Joakim Senior Member

    Are turbines really more fuel efficient than diesels? There are diesels with clearly better than 50% efficiency. I don't know marine turbines well, but I thought that turbines are only around 40% without a recovery boiler + steam turbine (I guess marine turbines do not have them).

    Here are some efficiencies: http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption
     
  14. RonL
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    RonL Junior Member

    The lower heating value LHV is used for internal combustion engines efficiency calculations because the heat at temperatures below 150 °C (300 °F) cannot be put to use.

    Can someone put that statement in layman terms, it just makes no sense to me, especially, because water boils at 212 F.:confused:
    Also BTU values for the different fuels are higher than most of my books state.
    I know I'm getting old, metric is tough for me, lots of things are new and improved, it seems that new words and acronyms are making it much harder to learn because older terms and words are no longer being used in some context.

    Ron
     

  15. RonL
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    RonL Junior Member

    That is just wrong, gears are a direct lock between increasing or reducing speed between two objects. The ratio will determine an exact speed of one based on speed of the other.

    Fluid, magnetic, electrical, and conical friction, can give variable speeds between engine and final component.

    Sorry gonzo, my left side brain didn't talk to my right side brain, before making my post. The same reason I can't comprehend the term "waste heat".
     
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