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Compression Ratios. What do they mean?

Discussion in 'Technical and Troubleshooting Torque' started by N2O, Sep 5, 2006.

  1. This is my example. A compression ratio of 10:1.

    Does this mean that when the piston is at top dead center, the pressure in the gap between the top of the piston and the top of the head is 10 times the atmospheric pressure?

    Or does it mean that the pressure is 10 times greater than Fuel/Air mix when the piston is at bottom dead center just after inlet, and before compression?

    This has been bugging me.

    I imagine that the higher the compression ratio, the greater the power you get out of your motor, until you compress it so much that it won't explode at all.


  2. This is more an educated guess, but I think it's relative to outside air pressure.
  3. From Wikipedia:

    The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine. It is a ratio between the volume of a combustion chamber and cylinder, when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.

    The ratio is calculated by the following formula:

    CR = ((pi* b^2 * s) / 4 + Vc) / Vc

    b = cylinder bore (diameter)
    s = piston stroke length
    Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.

    * Due to pinging (detonation), the CR in a gasoline/petrol powered engine will usually not be much higher than 10:1.
    * In engines running exclusively on LPG or CNG, the CR may be higher, due to the higher octane rating of these fuels.
    * In engines with a 'ping' or 'knock' sensor and an electronic control unit, the CR can be as high as 13:1 (2005 BMW K1200S)
    * In a turbo charged or super charged engine, the CR will be around 8.5:1
    * In a diesel engine, the CR will be 14:1 and higher.
  4. It doesn't need to be relative to outside air pressure - it is simply the ratio of the maximum volume of the cylinder (Bottom Dead Centre) to the minimum volume (Top Dead Centre). If your 500cc single has a volume of 50cc at TDC, you have compression ratio of 10:1

    As a predictor of performance, it strikes me as a very rough measure indeed.
  5. You are 100 % correct.
  6. Thanks wanderer for the maths, but i already know it :D

    Yeah I was just curious about what it's relative to.

    I'm still not sure if i'm 100% conviced that its just the volume ratio... :D:D:D
  7. Be convinced....
  8. Ok.
    I'm happy with it now. Done some more reading.

    Also, curiously, higher octane fuel, generally means it has a higher activation energy (to reduce the chances of detonation of the FA mix), meaning that it would be harder to run your car/bike on it, but apparently they use a different mixture to give a more powerful 'bang'
  9. the higher the compression ratio, the higher the octane rateing must be in fuel to stop knocking/detonation. Or lower the red line.

    Bikes generally being very much like a high perfomance race engine really hate poor low quality, low octane fuel.

    The higher the compression, the more efficient the engine burns the fuel resulting in improved economy/performance. A big reason for this is heat, the closer the air/fuel mix is to combustion temperature the better it burns.

    Hence diesel engines run compression up around 20:1, this takes the heat from a large area and compress's it into a small area creating very high temperatures, ready ro explode violently as soon as fuel is added.

    So running to much compression for your fuel, the compression alone creates temperatures high enough to ignite the fuel way before the spark plugs go zap. ( so thats what it sounds like when I burn a hole through my piston)

    The next step would be to look at "flame fronts" and "volumetric efficency" to further understand where the horsepower is made. Has any one made a true "hemi" yet in bikes?

    edit~top fuelers (nitro burners/funny cars) pump massive amounts of raw fuel, a richer mix does not detonate as easily as a good or lean mix so nitro is flowing out the zoomies (headers).

    With only 2 gears (forward), shortly after hitting top gear they hit the "lean burn", as the name suggests it leans out the fuel/air mix so it's a lot closer to where it should be. The extra kick is like thier 3rd gear and hurts engines.
  10. The 4.2L turbo in the Nissan Patrol has a CR of 22.7:1.. :shock:
  11. Yes, higher octane fuel requires more energy to ignite however increasing the compression ratio increases the pressure and temperature of the fuel/air mix - reducing the amount of energy needed from the sparkplug for ignition. Increase compression too far and a sparkplug isn't needed at all (as in a diesel). The extra power from increased compression comes from the fact that the more you compress a fuel/air mix (ie the closer you bring it to it's autoignition point) the more rapidly it will combust - therefore the force of the explosion will be greater and the efficiency of the engine will increase.
  12. It's also my understanding that the smaller the bore, the higher the compression ratio may be before pre-detonation occurs. This is another reason why bikes often get away with higher compression ratios than cars. It's because the cylinder bores are smaller and so the ratio of the cylinder metal to the volume being compressed means that the mixture gets cooled a little better during the compression cycle, and so is less likely to pre-detonate.

    This is why you'll see small 4-cyl 250-600cc bikes with ratios of up to 13:1 or ever higher, while a big V-twin or single won't get away with anything much more than the standard 10:1 unless it's using a high-octane fuel.
  13. Yep, that and the fact that compressing a larger volume by say 10:1 produces more pressure than compressing a smaller volume by the same ratio (which reduces the temperature needed for pre-ignition).
  14. No they dont use a different mixture to give it more bang. High octane fuels have a higher activation energy so they dont detonate prematurely.
    High octane fuels can therefore be used with higher compression ratios without detonating early or 'knocking" (either combusting before the mixture is ignited by spark or mixture ignites from points other than that from the spark). It is in fact the high compression ratio that gives a more powerful bang.

    Power is increased in an engine by increasing the Brake Mean Effective Pressure (BMEP). One of the factors that influences BMEP is compression reatio, hence why compression ratio is an indicator of (potential) engine performance but is subject pumping (induction/exhaust systems) and mechanical losses, combustion chamber shape, installation engineering (heating/ cooling/vibration) etc to realise.

    Check out http://www.epi-eng.com/ET-BMEP.htm
  15. Yes higher octane fuels do allow a higher compression ratio, but they also contain more chemical energy.

    This is because they use not only different chemicals in the blend, but longer chain carbohydrates that have been cracked to achieve the chain length witnesed in petrol.
  16. Not necessarily - there's fuel on the market now using ethanol (which has less energy than petrol) as an octane booster . So the octane rating is higher but the amount of energy is less. This means an engine can still produce more power, but fuel consumption will increase. Drag cars are a good example - they produce more power than could be achieved with petrol despite using a fuel with half the energy. But they use a lot of it.
  17. True. Ethanol also runs cooler, which allows higher compression ratios.

    I personally suspect that the 95 petrol is reached by chracking and from there up by adding chemicals. But that's just my guess based on the way my bikes have run.
  18. High octane fuels are made from larger aromatic hydrocarbon mixtures.
    This means they are harder to begin igntion (higher activation energy) or they have a large octane rating, but they also give a higher yield of energy per mol of fuel burnt.

    The enthalpy is for determining the energy yield per mol (23,000,000 molecules)

    Example. Toulene has a knock rating of 112, meaning it has a high activation energy, but it also has an energy yield of 116 kJ/mol

    n-pentane has a knock rating of 63 (very easy to ignite) but an energy yield of 48576kj/ kg. so divide this by the molar mass of pentane (72g/mol) and some other fancy stuff and you get a heat of combustion of 7.2kj/mol

    Optimax contains ethanol as a volumetric booster, but it also contains large hydrocarbons that affect both the ping and the total energy yield.
    Remember that "petrol" is a mixture of often hundreds of different chemicals. you can add shit to it to VERY GREATLY change both the octane rating AND the energy per ml.

  19. I didn't know that, I'd have thought there wasn't enough time to allow for any real cooling benefit from the increased ratio to get that big an effect. Smaller is also stronger/higher rpm so small pot screamers sound fun.

    I know you have trouble going to big and haveing trouble with hot spots in the middle of the pistons, but thats my lesson for the day.