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Effective Compression Ratio Definition?

  1. May 17, 2016 #1
    hello everyone,
    i would like please to know what is Effective Compression Ratio for an RC 2stroke Engine that i have ? the value in the info is 9.7, so i need to know the meaning and what values are the best ? what is the range ?
  2. jcsd
  3. May 17, 2016 #2
    The effective compression ratio is the actual (rather than geometric) compression ratio, taking into account intake efficiency, scavenging and boosting events (where boost is used). Geometric or theoretical compression ratio is the volume of the space in the cylinder above the piston at the point where the cylinder is closed vs. the volume present when the piston is at top dead center.

    What that 9.7 means is that the actual engine in operation compresses the intake charge volume at a ratio of 9.7:1.

    I don't fool around with RC engines, so I can't say if 9.7:1 is "good" or "bad".
  4. May 17, 2016 #3
    thank you OldYat47 for your reply, so as i understood you mean it's like the total volume of the cylinder which can contain fluid, is 9.7 times the volume when the piston is at the top ?

    ok but do you think for an engine 9.7 is something good ? or first can it be a factor that says if the engine is good, average or bad ?
  5. May 17, 2016 #4
    I must be clear in that it's not the geometric volume of the cylinder. It is the volume of the cylinder charge, taking into account that volume will be less than the geometric volume. You can think of it as the quantity (let's say mass) of fuel/air mixture at something less than atmospheric pressure drawn into the cylinder at intake. An effective compression ratio of 9.7 to 1 in a car engine would be admirable. As I said, I don't work with RC engines so I can't make a judgement on that.
  6. May 18, 2016 #5
    yes exactly what i understood the volume of quantity as you said, but is the ratio the way i said it before ?
  7. May 19, 2016 #6
    No. What you are referring to is the geometric compression ratio. That assumes that the entire volume will fill with mixture at existing atmospheric pressure. In reality that doesn't happen because of intake inefficiencies, turbulence, etc.
  8. May 19, 2016 #7
    ah ok i think i got the idea, so what i said is let's say the perfect volume that must be filled, what it refers to exactly is the real volume in real life
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