Purpose of High rpm Low Torque Engines

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

  1. Frosty

    Frosty Previous Member

    A longer stroke has a longer crank throw.

    At 90 degrees crank from TDC you have more leverage ,-thats why you have more torque. Nothing to do with changing bloody flywheels or air.
     
  2. Rik
    Joined: Dec 2007
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    Rik Senior Member

    Maybe the modern automotive engines are higher revving and derive their max hp at a relatively high rpm due to some of the (in addition to the ones mentioned) same theories as the aircraft industry.

    Lower torque equates to smaller drive line parts needed to harness this torque and smaller parts are in deed also lighter which enables the modern cars to be more fuel efficient, accelerate faster as they have less mass and makes the EPA's of the world happier.

    Not great for a boat motor but then again that's what diesel engines are for.
     
  3. Frosty

    Frosty Previous Member

    Rik I though you were going to say that Surface drives dont need torque, Ive found that with my set up.

    I asked Seafury what prop I needed and what engine to drive it with. Itook there advice and bought the high reving low torque yanmar 6lps', they were correct in doing so.

    Ive never seen the turbo boost meters anywhere near full boost, the engines seem ideally suited to the job.
     
  4. Joakim
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    Joakim Senior Member

    No, that is not the way it works. You get about the same torque from the same size of engine regardless of rpm (unless you have a charger). All modern engines have a torque about 100 Nm/liter (1.1 lbs/ft / CID) and the torque curve is rather flat, thus you get at least 80 Nm/liter for the whole usable rpm range. Which old low rpm engine has a higher torque? I think they have less torque due to poorer design.

    What is even the use of high torque in any other than planing boats? In a planing boat you really want to minimize weight, thus you don't want to have a low rpm engine which is big and heavy for its power. In all other types of boats you don't need the high torque at low rpm, since the torque to spin the propeller is very low at lower rpm.

    Many people have pointed out car industry and emission control. Those are important, but at the same time you have to remember that European and Japanese cars have used small volume high rpm (if max power at 5000-6000 rpm is high) engines for many decades.
     
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    That sounds reasonable, but it is not the whole truth. For the same displacement, a short stroke engine has larger piston diameter, so more surface and approx. the same torque.

    Two factors have led to design changes in favor of a short stroke: a large cylinder diameter accommodates larger valves, hence denser mixture and more efficiency. And the higher rpm allows the engine to be smaller, saving material, gross weight, flywheel mass etc.

    The longevity claim for dinosaur engines also isn't true. Modern engines have a better MTBF (Mean Time Between Failures) than the old fashioned ones.
     
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  6. Rik
    Joined: Dec 2007
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    Rik Senior Member

    My answer has nothing to do with surface drives, rather why automotive engines are what they are.. I see the new BMW, Ford, GM and other motors are spinning 7,500+ rpm and they do this so that they can produce a HP number (that will help sell the car) and they also use it to the drive trains advantage as I noted with a lower mass transmission, drive shaft and rear diff.

    Surface Drives and boats... Each is unique..
     
  7. baboonslayer
    Joined: May 2010
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    baboonslayer Junior Member

    rxcomposite, I'm not trying to start something, but do you think I am stupid or something? that's a major NO **** about the piston rings being in contact with the cylinder wall, and not the actual piston itself. I don't understand why you think I didn't know that when you consider the fact that I am a mechanic.

    Yes, those high revving motors are shorter and lighter, but, I mentioned above that we are talking about continuous duty engines where weight doesn't really matter, and I am sure that they have a better torque weight ratio than your short stroke. You can have all the horses you want, because maximum horsepower is produced at a higher RPM than maximum torque, and I prefer torque over horses.
     
  8. gonzo
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    gonzo Senior Member

    Torque and power are related. You can't have one without the other. Continuous duty engines are de-rated compared to intermitent use ones. Can you name an engine that is heavy and slow turning and has a better weight/torque ratio than, say, a Chevy8.1?
     
  9. baboonslayer
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    baboonslayer Junior Member

    The chevy 8.1 has 340 hp @ 4200 rpm and 450 lb ft of torque @ 3200 rpm. The dry weight is 730lbs. That gives it a weight to torque ratio of 7.3:4.5 which is pretty good, and so far I haven't found anything to beat those specs. (yet :D) but if there was a THEORETICAL engine that produced 12hp a@ 600 rpm then it would have 105.4 lb ft of torque, and lets leave the weight up to your imagination :) but I'm guessing it would be in the 700-800 lb range. What I'm saying is yes, you chevy 8.1 has a better weight torque ratio than a Listeroid diesel (the only slow speed engine still made even if it's not gas), but does it have a better torque hp ratio and a better rpm torque ratio?
     
  10. gonzo
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    gonzo Senior Member

    If anyone could make styrofoam blocks ;) then we would really have something.
     
  11. rxcomposite
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    rxcomposite Senior Member

    Why would you get pissed off if I add piston rings because I was talking about friction? How do I know you are a mechanic?

    I am posting for general information so do not get sensitive as i do not mean to make you look stupid!
     
  12. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The closest you can get is to buy styrofoam beads (filling for beanbag chairs) and mix that with acrylic resin like Jesmonite.

    That way you can make blocks or any other shape you want.
     
  13. baboonslayer
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    baboonslayer Junior Member

    Sorry rxcomposite, I thought you were trying to explain to me that the piston itself doesn't contact with the cylinder wall when you said, "It is not the piston itself that is in contact with the cylinder wall. it is the piston rings." so I was angry because I already knew this since I work as a mechanic to pay for college.

    very funny with the styrofoam engine blocks. I would rather use cast iron though.
     
  14. RonL
    Joined: Nov 2010
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    RonL Junior Member

    CDK came close to saying what my thoughts are, a large piston area applies more force for a shorter time because the volume pressure fades more rapidly. The smaller piston moves further down the bore before pressure fades, always the trade offs.
    Everything else is just dancing with details for reducing friction and increasing optimum air flow.

    High twisting power during high temperatures, will always require more mass for strength, in order to keep distortions to an acceptable tolerance.
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    The piston doesn't contact the cylinder wall only because there is an oil film between the wall and the pistion. That oil film does transmit substantial forces between the wall and pistion, and if the oil film breaks down piston scuffing and wear happens big time. The force created by the angle of the rod and the vertical makes big lateral forces on the piston and that is transmitted to the wall by the oil film.

    The rings pass oil in both directions and they ride on a very thin film of oil too. The fact that oil passes the rings in both directions is how dirt that gets past your air filter ends up in your crankcase.

    Longer stroke can mean higher rod angularity, and higher piston and wall forces. There is no free lunch, what you take away from one aspect you may have to give back on the other side.
     
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