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VCR engines - the next frontier

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  • #46
    The chart posted by ikdor shows that heat from friction is only 8.4% of the total power consumption of an engine with about 18% total efficiency from fuel to the wheels. Most (64%) of the losses are heat through the walls of the combustion chamber and out of the exhaust. If you have an engine with only the pistons and crankshaft in the block and operate it by hand, it takes little effort to keep it moving by exerting pressure on the pistons, and the faster you turn it, to some degree, the easier it is, due to rotational inertia. A fully assembled engine (with plugs removed) takes more effort because of the valve springs and camshaft operation, but much of the energy absorbed in the springs is returned when they decompress.

    It may be difficult to envision, but the linear acceleration and force of the piston is converted to rotational inertia in the crank, and in a well designed engine the mass is balanced so that the conversion is very efficient. The entire mechanical process of using the pressure of the burned fuel-air mixture into rotational energy at the crankshaft is just conversion from potential to kinetic and back again, with the only mechanical losses being due to friction and stresses on the components (consider the heat generated by bending metal).

    I think the actual inefficiency is based on the way the fuel-air mixture burns in the confined space of the combustion chamber. At TDC, the pressure increases rapidly and exerts a powerful force on the cylinder head and the piston, which cannot move. But since the crankshaft is moving, the volume of the chamber increases and the pressure decreases. The exact (thermo)dynamics may be complicated, because the combustion does not take place evenly and the volume is changing in one direction (two directions in an opposed piston design), and the efficiency is a trade-off based on burning rate and compression ratio and speed.

    The idea I proposed would allow an immediate expansion of the combustion chamber to several times the volume at TDC, storing the energy in a spring rather than extremely high pressure and temperature as it is in a conventional engine. Then the spring would return this energy to the piston as it moved, so that the combustion chamber volume would remain relatively constant for a significant portion of the power cycle, and perhaps deliver the most power at the 90 degree crankshaft angle where there is maximum mechanical advantage.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #47
      Just because it moves back in the opposite direction with the same speed, does NOT mean it has the same velocity. In fact, it has exactly the opposite velocity. if it was originally 10m/s north, it will now be 10m/s South, or in other words, it goes from +10m/s to -10m/s velocity.
      Ummm, you have me thinking about that. I thought the piston went from 0m/s to +nm/s then back to 0m/s on each stroke. I dont see any carry over of piston inertia from one stroke to the next.

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      • #48
        Originally posted by PStechPaul View Post
        The idea I proposed would allow an immediate expansion of the combustion chamber to several times the volume at TDC, storing the energy in a spring rather than extremely high pressure and temperature as it is in a conventional engine.
        The camless Koenigsegg engine aims to be able to store braking energy as compressed gas , as the engine works equally aswell as a compressor.....so possible could use your idea aswell.

        F1

        The inclusion of the Motor Generator- Heat is a bit of a red herring for heat energy recovery.

        The size of the FIA mandated turbo is massive (as shown in the Renault engine PDF below)...and would have led to a turbo lag of 2-3 seconds..The MGH is being used to spool up the tubine to reduce the lag to acceptable limits.
        http://www.renaultsport.com/IMG/pdf/...-en_final2.pdf

        Rob

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        • #49
          Originally posted by The Artful Bodger View Post
          Ummm, you have me thinking about that. I thought the piston went from 0m/s to +nm/s then back to 0m/s on each stroke. I dont see any carry over of piston inertia from one stroke to the next.
          Sure, I was trying to simplify things. It does not go from full speed in one direction to full speed in the other instantaneously.

          Let's start with the piston at the bottom of it's stroke, and not moving. The work to accelerate the piston on the next upstroke needs to come from the crank. At some point in the stroke the piston reaches maximum speed. It has some kinetic energy. Now, we need to slow the piston down, and eventually stop it, before reversing direction. The crank is still trying to rotate in the same direction, but now it acts to slow the piston down---hmmm, so the piston is still trying to move at the same speed, but the crank is slowing that reciprocating motion down, meaning that the reciprocating motion is trying to speed up the crank rotary motion.......well, I think I have convinced myself that I was wrong, at least some of the energy due to piston motion will be returned to the crank....

          The only thing that can slow it down is the crank(besides friction), and if you think about it, the crank does this fairly gradually because of teh mechanis of teh conversion of rotary to reciprocating motion.

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          • #50
            Originally posted by andywander View Post
            Sure, I was trying to simplify things. It does not go from full speed in one direction to full speed in the other instantaneously.

            Let's start with the piston at the bottom of it's stroke, and not moving. The work to accelerate the piston on the next upstroke needs to come from the crank. At some point in the stroke the piston reaches maximum speed. It has some kinetic energy. Now, we need to slow the piston down, and eventually stop it, before reversing direction. The crank is still trying to rotate in the same direction, but now it acts to slow the piston down---hmmm, so the piston is still trying to move at the same speed, but the crank is slowing that reciprocating motion down, meaning that the reciprocating motion is trying to speed up the crank rotary motion.......well, I think I have convinced myself that I was wrong, at least some of the energy due to piston motion will be returned to the crank....

            The only thing that can slow it down is the crank(besides friction), and if you think about it, the crank does this fairly gradually because of teh mechanis of teh conversion of rotary to reciprocating motion.

            Good duty,,, plain and simple - you "get it"...

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