Purpose of High rpm Low Torque Engines

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

  1. RonL
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    RonL Junior Member

    http://www.boatdesign.net/forums/attachment.php?attachmentid=51595&stc=1&d=1293223160


    This removes any side thrust on the piston and can offer lots of mechanical options.
     

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  2. Yellowjacket
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    Yellowjacket Senior Member

    That's a heavy and expensive way to address a problem that doesn't exist. While it does eliminate side thrust on the piston, that thrust occurrs when the piston is moving at high speed and the lube film between the piston and wall can handle the piston thrust. No practical advantage to that type of mechanism and that's why it isn't used on high speed engines. If you have a very low piston speed it could be an advantage, but in the real world it doesn't have any real use.
     
  3. mark775

    mark775 Guest

    What part of a piston would touch the wall were it not for a thin film of oil? And on skirtless pistons?
     
  4. baboonslayer
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    baboonslayer Junior Member

    merry Christmas... or whatever you celebrate...
     
  5. Yellowjacket
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    Yellowjacket Senior Member

    The connecting rod is what is called a "two force member" that means, since the rod is on bearings, that the only forces in the rod act along the length of the rod. That means the only force in the rod is in compression or tension along the length of the rod. Draw a line down the center of the rod and that is the line of action of the forces in the rod. As the rod angles over, there is a force on the piston, from the rod, that changes in angle, at top dead center the force is pulling directly down on the rod and at bottom dead center it is pushing upward on the rod. In between those points the rod force sweeps, based on the rod angle, and the resultant force is reacted by the piston. This side force has to be reacted by the piston. This force is in the plane of the rod and pushes the pistion toward the wall as the rod sweeps in angle. The maximum piston side force occurs when the crank throw is half way between top dead center and bottom dead center and the rod is at the highest angle from the vertical.

    Note that even with a "skirtles" piston, there is some force trying to rock the pistion in the bore. So long as the angle of the force sweeps along a section of the piston that is in contact with the wall, the piston won't rock enough to have a problem. Even F1 pistons have some skirt in the plane of the crank, it is just cutaway on the sides where it doesn't do anything. Think of a line along the center line of the rod that extends to the wall across from the pistion. As long as there is piston material there in contact with the wall, the piston won't rock. You need to make sure that you have enough area to support the oil film too, so if you look at the piston, there is still a section of material opposite of the pin that is there to prevent rocking of the piston in the bore. Cutting out the sides of the skirts removes weight (reduces stress and allows higher rpm). The designer needs to make sure he has sufficient area to prevent the piston from overpowering the oil film and the faster he runs the engine the less skirt material he needs.
     
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  6. mark775

    mark775 Guest

    I am not schooled in this but it is counter-intuative: "The maximum piston side force occurs when the crank throw is half way between top dead center and bottom dead center and the rod is at the highest angle from the vertical." Seems to me the maximum side force would be just after BDC. Oh, oh, oh... TDC and BDC are referenced to crank throw. Yes, that all makes sense. I just assumed that the rings did all the touching. What an ugly thought, that piston touching the cylinder bore on a cold, unoiled engine.
     
  7. FAST FRED
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    FAST FRED Senior Member

    """"The driving force behind smaller more fuel efficient engines is the CAFE standard. That pushes auto makers towards smaller, lighter, and unfortunately lower powered cars. In order to keep performance up, these smaller engines spin at higher RPM's to get the necessary power.""""

    However the air polution NAZIS have forced engines into a generally inefficient setup , by requiring "clean" at start up first 15 seconds etc.

    This is LOVED by the various rulers that charge tax per gallon.

    AS it takes a 15% overall hit to fuel economy for all this last .0001% clean , the higher revenues are delightful to our masters.

    Long stroke slower speed seems to be making a comeback on big ship motors , now longer stroke and 88 RPM,for efficiency.


    FF
     
  8. Joakim
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    Joakim Senior Member

    What is your "continuous duty" application? Why would torque be the most important value for it? If you want to optimize torque weight ratio, use a transmission with high gear ratio.
     
  9. RonL
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    RonL Junior Member

    A problem that doesn't exist:?: I have seen too many scarred pistons and cylinders to agree with that thought. I'll admit it is a heavy and expensive solution, but it offers much more than a solution to side thrust on a piston.

    As for speed, I love the 1,000 to 3,000 rpm range (I think that is what you refer too) but by splitting the mass of the rotating parts and ELIMINATING side thrust on the piston, safely producing more power at higher speeds just seems logical to me.

    As for the oil, I agree with you and think, starvation of supply, or heat is the killer.

    I wonder how many people will look at that picture and only see one machine ? or just one possible power function ?

    On a side note, I hope everyone is having the merry holiday they prefer.
    Best wishes to all.
    Ron
     
  10. rxcomposite
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    rxcomposite Senior Member

    Apology accepted, no problem. Bear in mind that in this forum, there are members who are just starting to know the bow of the boat from the stern but there are engineers, mathematicians, doctorates, lawyers, ect., who are reading this post.

    Back to topic. “I honestly don't see the point in modern 4000+ rpm gas engines that are complicated and have a lot of electronics. The high rpm wears down the cylinder sleeve way quicker than any low rpm high torque heavy flywheel engine does. They are too complicated for the average person to work on. Also, they suck down fuel like a jumbo jet. You could compare their torque output to that of an electric toothbrush motor. So, I ask again, why are we using these motors? Theoretically speaking, they are manufactured to break, right?”


    As I posted in *10 , high rpm engines has specific use and are limited to the proportion of the time it is tasked to full power.

    Marine engines are generally classed into three categories:

    Light duty rating
    Medium duty rating
    Continuous duty rating

    Continuous duty ratings are engines that are tasked 70 to 100% of their power the moment they pull off the harbor. These are trawls, ferries, freighters, carriers..

    The medium duty ones use maximum power intermittently 40 to 60% of the time. Max power is called for about half of the time such as going out of the harbor empty but fully laden when returning such as a fishing/lobster boat.

    Light duties are boats/ships that are used occasionally 20 to 30% of the time like leisure boats sitting in the docks most of the time. Patrol boats are also light duty as it spends most of its time patrolling at low speed (80% of its lifetime) and goes up to pursuit speed only when engaged.

    These rating are tied to Time Between Overhaul (TBO). So a continuous duty engine with a TBO of five years, a light duty engine that sees only 20% usage per year will add up to 100% in five years. It is not exactly a throw away as it depends on the usage.

    Other engine manufactures have up to five different ratings because they sell engines that are of the same block, but detuned, say 100 Hp to 80 Hp, so that it will have a higher rating.

    Light duty engines are therefore analogous to a car use. You don’t use a 300 hp. car to commute daily to work. You use it occasionally to accelerate, to show off, or for the pure pleasure of using that much Hp. Most of the time it is sitting in your garage. Besides, it takes only 25 horses or so to cruise at 55 m.p.h.
     
  11. rxcomposite
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    rxcomposite Senior Member

    May I anwer it?

    As a general design rule, maximum efficiency is achieved by swinging the largest prop at the lowest rpm and engine torque it can handle. Prop diameter is limited by the depth of the harbor it will be servicing.

    On the other hand, high speed boat design requires a low displacement to length ratio. Low displacement translates to a shallow draught hence the requirement for a smaller diameter propeller.

    Assume a large propeller requires x number of turns to go a certain distance (without slip for illustration) but a smaller propeller will need more number of turns to reach the same distance.

    Let us borrow from mechanical engineering for a moment. A large radius will have a greater moment to turn but a smaller radius has a smaller moment so less torque. Thus it follows, low rpm, high torque; high rpm low torque. The same power will be dissipated from the formula P=work/time or P=force x distance/time
     
  12. baboonslayer
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    baboonslayer Junior Member

    Thanks rxcomposite. So, the conclusion is that if you are pushing around a heavy full displacement boat then you want low RPM and high torque coupled with a big prop so you can go farther on less fuel.
    Like my new avatar and signature? (I'm thinking of changing my name also in case it offends any animal rights people :D)
     
  13. rxcomposite
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    rxcomposite Senior Member

    That's correct. The biggest prop that you can swing as dictated by the hull clearances. This even true with shallow draft vessel. Swing the biggest prop without protruding from the baseline.

    Like your avatar but it does not represent you. You said you are a mechanic? right? But it does not matter. So far the forum had never had an issue with animals but politics.;):p and religion.
     
  14. Joakim
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    Joakim Senior Member

    But why the engine needs to be low rpm? All the propeller sees is the propeller shaft rpm and torque. Transmissions with different gear ratios are available and you are going to use some transmission anyway. Or are you using a CPP directly connected to the crankshaft?

    Yes big and slow propellers are more efficient at displacement speeds, but at the same time propeller diameter is quite restricted by other factors. If the propeller is not that big, you will not have better efficiency at lower rpm.

    The key to low fuel consumption is low boat speed and a diesel, which is much more fuel efficient than gasoline. So why do you want a gasoline engine, if you are interested in low fuel consumption?
     

  15. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Large props on modern heavy ferries are powered by fast spinning turbines.
    That is what gearboxes are for. Engine rpm never dictates prop speed.
     
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