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Thread: question about a crankshaft

  1. #1

    question about a crankshaft

    The crankshaft gathers the momentum power from 4 pistons.
    So that the momentum power increases as we go closer to the end of the crankshaft.

    1. Is it true?

    2. The diameter of the crankpin is large, and the big end of the con rod is large.

    Can the crankpins and big end diameter be gradual?

    For example, the first is 20 mm, the second 25 mm, the third 30 mm and the fourth 35 mm?

    Only from the mechanical aspect. not oil etc.
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  2. #2
    Principle Engineer
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    No and no.
    The torque input from cylinder #1 is the same as the torque from cylinder #4. These torque inputs are cyclic and not cumulative in the manner you suggest.

    There are four problems that you need to understand in designing a crankshaft:

    1) The bending stresses on the individual crankshaft components such as the pins, journals and crank webs. These are the result of the piston and rod assembly loads and the centrifugal force in the crank bits themselves. There are gas forces and inertia forces to consider.

    Peak stress in the filet area is a big consideration.

    2) After you determine the loads on one crank throw and size the bits accordingly, the next step is to select a firing order and calculate the necessary counterweights to counteract the primary imbalance and/or to reduce the bending moments in the main bearing areas.

    3) The cyclic nature of the firing pulses and the flexibility of the crankshaft and driveline can result in the accumulation of harmful levels of torsional stress when the engine is operated at a speed where there is a resonance. Engines of more than two cylinders should have an estimate made of the resonant points in the engine operating range. This is where vibration damper designss and flywheel modifications are made to move these resonant points or eliminate them.

    4) The lubrication of the crankshaft and associated bearings should be considered. Sufficient oil for hot idle will determine the pump size and a review of the way in which the load moves around the journals will show if there are any half speed vectors in dangerous locations.

    You will repeat these stages a few times as the design process is iterative.

    If, when you complete all of your calculations you still feel that progressive journal diameters are the hot setup, a savage beating by the manufacturing engineers will straighten you out. You see, they will not want to tool up four different sized connecting rods. Purchasing will not want to buy four different bearings and will stall your project until you are replaced.

  3. #3
    Hi Hudson

    The last sentence in my question was to ignore the oil or other factors.

    I ask about the mechanical average power.

    Lets say that the engine gives 40 horse powers.
    Each of the pistons give 10 HP.
    The power increases as the crankshaft forward.
    The crankpin needs to suffer the pressure from the piston that is 10 HP.
    The crankpin needs also to suffer the pressure from the previous pistons.
    The first crankpin needs to suffer only the pressure from the first piston, because near it there is no axial force. The axis is without rotational pressure.
    That is why the first crankpin can be weaker than the last crankpin.



    Quote Originally Posted by Hudson View Post
    No and no.
    The torque input from cylinder #1 is the same as the torque from cylinder #4. These torque inputs are cyclic and not cumulative in the manner you suggest.

    There are four problems that you need to understand in designing a crankshaft:

    1) The bending stresses on the individual crankshaft components such as the pins, journals and crank webs. These are the result of the piston and rod assembly loads and the centrifugal force in the crank bits themselves. There are gas forces and inertia forces to consider.

    Peak stress in the filet area is a big consideration.

    2) After you determine the loads on one crank throw and size the bits accordingly, the next step is to select a firing order and calculate the necessary counterweights to counteract the primary imbalance and/or to reduce the bending moments in the main bearing areas.

    3) The cyclic nature of the firing pulses and the flexibility of the crankshaft and driveline can result in the accumulation of harmful levels of torsional stress when the engine is operated at a speed where there is a resonance. Engines of more than two cylinders should have an estimate made of the resonant points in the engine operating range. This is where vibration damper designss and flywheel modifications are made to move these resonant points or eliminate them.

    4) The lubrication of the crankshaft and associated bearings should be considered. Sufficient oil for hot idle will determine the pump size and a review of the way in which the load moves around the journals will show if there are any half speed vectors in dangerous locations.

    You will repeat these stages a few times as the design process is iterative.

    If, when you complete all of your calculations you still feel that progressive journal diameters are the hot setup, a savage beating by the manufacturing engineers will straighten you out. You see, they will not want to tool up four different sized connecting rods. Purchasing will not want to buy four different bearings and will stall your project until you are replaced.
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  4. #4
    In other worlds:
    If I have an engine with only one piston, can the crank pins of it be thinner than the pins of 4 pistons crankshaft?

  5. #5
    Principle Engineer Cragyon's Avatar
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    Quote Originally Posted by gilteva View Post
    In other worlds:
    If I have an engine with only one piston, can the crank pins of it be thinner than the pins of 4 pistons crankshaft?
    Sure you can - just do the analysis and verify conformance.

  6. #6
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    The answer to your question #1 (Is it true?) is no. The pistons do not supply continuous power through out the cycle. You will rely on the flywheel and drivetrain inertia to carry the engine through from firing pulse to firing pulse. The firing pulses of a four cylinder, four stroke engine do not overlap.

    Your vision of the piston engine as some sort of equivalent to four electric motors hooked together end to end is not accurate.

    Since each piston delivers the same power the bearing loads on the crankpin are equal. So based on bearing requirements all crankpins are the same. The bending loads at the filet radii at the end of the crankpin are the same. If you "ignore other factors" the single cylinder and the four cylinder crankpin diameter can be the same.

    But wait! Things aren't the same for you because you show a three main bearing crank with a long, unsupported, section between one and two and three and four. This changes the bending moments and stress calculations compared to your single cylinder. Creating a five main bearing crank reduces the axial length available for the crankpin which reduces bearing area unless you increase the crankpin diameter. The crankpin is limited to about 60% of the bore diameter if you want to install the piston from the top of the block unless you angle the opening on the rod cap. And so, the compromises begin.

  7. #7
    Hudson,
    The big end of the con rod, is big because of the crankshaft, or because of the bearing oil?
    And, I want to send you a patent application of mine to your email, not in a forum. You agree?
    mine is the username and ----email removed----
    Last edited by Kelly_Bramble; 11-29-2016 at 01:04 PM.

  8. #8
    Principle Engineer
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    The crankpin is large to resist the bending stresses, to increase the torsional rigidity and to support the oil film.

    I do not wish to see your patent application.

  9. #9
    Lets say that I use a super strong material for the crankshaft. The crankshaft structure can be thin and lightweight. The crankpin, journals and bearings can be smaller. The connecting rods will have two small ends. There will be an advantage to this? Or I will need artificially to increase the diameter of the bearings in order to increases the oil resistance in the bearings?

  10. #10
    Principle Engineer Cragyon's Avatar
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    The best approach to understand your requirements is to analyze the stress using FEA.. Have you done an analysis?

    Quote Originally Posted by gilteva View Post
    Lets say that I use a super strong material for the crankshaft. The crankshaft structure can be thin and lightweight. The crankpin, journals and bearings can be smaller. The connecting rods will have two small ends. There will be an advantage to this? Or I will need artificially to increase the diameter of the bearings in order to increases the oil resistance in the bearings?

  11. #11
    I do not have mechanical engineering education. I am electronics engineer.
    I invented an engine with 3 con rods to every piston. Each connected to a crankshaft so that the con rod goes from the cylinder axis to outside and back. There are 3 con rods to each piston so the pressure to each is divided to 3. There is no crankshaft.

    It seems that the con rod can have two small ends.
    I now think that maybe this can be applied to an inline engine.
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  12. #12
    Kelly_Bramble's Avatar
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    More parts, more friction, more mechanical tolerance challenges, more cost, less efficient..

    You're just reciprocating pistons mechanically different..

  13. #13
    Gears transmission does not waste much energy. The crankshaft wastes much. The bearings in the crankshaft are large. The oil absorbs the heat, especially their heat increase from the pulse of pressure. small bearings means less heat.
    Laser can cut the gears at low cost. Cheaper than the expensive crankshaft.

  14. #14
    Kelly_Bramble's Avatar
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    Quote Originally Posted by gilteva View Post
    Gears transmission does not waste much energy.
    You still have bearings and those bearings are friction.. Fewer bearings = less friction. Also, gear meshing introduces friction. You need need to analyze the mechanical efficiency of your design and compare to a conventional hydrostatic bearing - crankshaft design.

    Fancy and more parts never means better or more efficient..

  15. #15
    Also the effective radius of the crankshaft is about half of its physical radius, because if the large bearings.

  16. #16
    Lead Engineer Cake of Doom's Avatar
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    Have you prototyped your 3 con rod piston?

  17. #17
    I am waiting for a government fund to build the prototype of the 3 con rod piston.
    The examiner was not very much impressed, saying that there are more unnecessary parts.
    That is why I opened the forum conversation, to get a second opinion.
    Patent US7434562 describes an engine with two opposed pistons that have three connecting rods. The connecting rods connect to two crankshafts that rotate around two axes.
    The piston is held by the 3 rods, but in order to balance the masses, the individual rods need to have the mass of the two other rods together. So it should be 2 times wider.

  18. #18
    Lead Engineer Cake of Doom's Avatar
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    Just wondered if you'd try any sort of working model because it looks unnecessarily complicated; too many moving parts to achieve the end goal = long term inefficiency and higher running costs due to maintenance etc.

  19. #19
    The advantages:
    1. There are no vibrations due to the symmetry. Meaning no noise even at 6000 RPM.
    2. There is no piston side thrust, like with one con rod. The rods move the piston up and down like an elevator.
    3. The piston can have spherical edges, so that mechanical mismatch will not destroy the sealing, even without sealing rings.
    4. There is no piston skirt, because the piston is kept horizontal by the rods, and it does not need the cylinder to be horizontal. Less weight for the piston. less friction. no oil maybe. Without skirt you can use iron with low thermal expansion, and not aluminum alloy with high thermal expansion.
    5. No piston skirt means shorter cylinder, and shorter engine.
    6. The patent enables to use sealing rings.
    7. The gears that hold the rods can have a diameter that is about the diameter of the piston movement. A crankshaft needs about double diameter because the crank pins are big.
    8. A crankshaft cost $700. 12 stainless steel gears cut by CO2 laser cost $200.
    9. The piston pressure falls on 3 rods, so each has 1/3 of the pressure.
    10. 6000 RPM without vibrations, and without problems from the friction, enables to replace a 0.5 liter engine with 1.5 liter engine.

  20. #20
    Technical Fellow jboggs's Avatar
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    You are not going to get the kind of detailed and thorough mechanical engineering analysis you need on a free online forum. You need to find a competent and experienced mechanical engineer to work through all the MANY details of your design. There are many man-hours of work to do here. Once you have completed your analysis and design, you can build a prototype. Then maybe you can demonstrate it for some private investors and you won't have to "wait for a government fund".

    Just my opinion.

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