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Piston size and proportionate forces

  1. Oct 28, 2009 #1
    Given the same radial force and combustion pressure, would it be correct to say that the larger diameter piston would have a greater propensity to overcome the radial friciton of the piston rings? That is would a 50cm piston diameter overcome the friction than say a 25cm diameter piston easier (with the same combustion pressure).
  2. jcsd
  3. Oct 28, 2009 #2
    Yes. That is one reason why 4-piston engines are generally more efficient than 8-piston engines. Another reason is that the volume to surface ratio of the combustion chamber is higher, which relates to convection heat loss after ignition, while the burned gas is pushing the piston down. This might be one reason Cadillac dropped its V-16 engine, and the Lincoln Zephyr its V-12.
    Bob S
  4. Oct 28, 2009 #3

    Ranger Mike

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    the bigger piston is going to have more drag than the smaller one. regarding cc volume and pressure..if you have two IC with same comp ratio this means same force..right?
    so F x D /T = Power..would not the 4 cyl make less power simply by the number of cylinders?
  5. Oct 28, 2009 #4
    With the same CR you'd have the same pressure in the cylinder. You'd have more force with a larger piston.

    F= P*A

    If you had a 4 cylinder and 8 cylinder engine, both 2 litres.

    The 4 cylinder would make more torque, but big hefty pistons give huge stresses ad it wouldnt rev as high. The V8 could rev higher and make more top end power, but less overall torque.

    Back in the 90's F1 teams had a free reign over the number of cylinders they could use, but capped at 3.5L.
    Ferrari always made the most top end power from their V12, as it was also the highest revving engine on the grid at about 15000-17000 rpm.
    The Ford V8 was down on power to the ferrari as it couldnt rev nearly as high, but it was the most drivable engine on the grid due to a better torque curve. Revs ususally topped at 13000-14000rpm.
  6. Oct 28, 2009 #5

    Ranger Mike

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    exactly...Chris .. good one!!

    this is why i love this forum...the less cylinders will make more bottom end torque but rattle your teeth when revving.. firing a piston every 90 degrees the smoother V8 firing every 45 degrees will make more high end hp way smoother .. but will be less on torque...
  7. Oct 28, 2009 #6
    I have never understood for the uneven firing order of V8's. It is not left right left right etc. The exhaust manifold breathing sounds like xxxx. My old '41 Buick with a straight eight sounded much better. See table for V8's in
    Bob S
  8. Oct 28, 2009 #7
    I know you can slightly alter power output characteristics with odd firing patters (not entirely sure how though). Also odd firing patterns only really occur in crossplane V8's. With flatplane cranks you can have an even firing order, with 1 bank firing like a straight 4 and the other bank firing like the mirror image of that straight 4.

    Beyond that i'm with you wondering why they have an odd firing pattern.
  9. Oct 29, 2009 #8

    Ranger Mike

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    good info..my two cents is that in the old days..( here we go again) volume automotive manufacturers had to compromise regarding carbureation, smooth operation, power , longevity and production cost. the intake manifold layout has to consider single intake runner or siamese port design. The same goes for the exhaust ports. The central located carb means there will be a lean couple of runners and a rich couple of runners and half that are " optimum". Thing two was to develop a firing order with minimum stress induced on the crank and engine case (block). Note the weakest and highest stressed points on the crank are at either end. The middle throws are nested in the center of the case and have pretty good lateral support. These are the cylinders that light off ( cyl. 4) followed by the opposite bank piston (cyl3.) that is connected to that same crank pin. the end slugs like No. 1 fire followed by the opposite end No.8 slug. Manufacturing costs of single runner intake and exhaust ( like the Hemi) get real expensive compared to economy of scale mass production cost of siamese port design. Also crank balance shaft cost, assembly time, maintenance ..etc...figure in too.

    Opps..i just looked at the firing order on a small block Ford and this theory just got shot down..
    what do you think Chris?
  10. Oct 29, 2009 #9
    I really have no idea what im talking about regarding this it's all just an educated guess, but each of the points you made makes sense. As to how relevent the engine manufacturers think they are in deciding a firing order I don't know.

    I'd guess the uneven charge distribution in a plenum/single carb based intake would prbably be the most like course for them having odd firing orders. With reliabity of the crank coming in a close second.

    You never know though, all this talk of power, stress and engineering reasons maybe tosh. It may be the sound the designer finds sexy.

    The thundering rumble of a crossplane V8 vs the howl of a flat crank V8... it's a toss up as to which a prefer.
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