Compression ratios : Need to learn more 1!

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  1. Compression ratios : Need to learn more 1! 
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    Hi guys.

    I'm trying to understand a lttle more about compression ratios , How it all works and effects it has on engines. I've read the sticky but want to get into a little more. A few sites i've looked at are very very technical and to be honest i'm strugglin' a little. Is there any idiot friendly sites..

    Thanks

    D.
     
     

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    In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

    Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.

    A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.

    Higher compression ratios will however make gasoline engines subject to engine knocking if lower octane rated fuel is used, also known as detonation. This can reduce efficiency or damage the engine if knock sensors are not present to retard the timing. However, knock sensors have been a requirement of the OBD-II specification used in 1996 Model Year Vehicles and newer.

    Diesel engines on the other hand operate on the principle of compression ignition, so that a fuel which resists autoignition will cause late ignition which will also lead to engine knock.


    shamelessley copied and pasted but thats the simplest explanantion i could find
     
     

  5. The Following 3 Users Say Thank You to Deano16v For This Useful Post:

    czkb (03-06-2014),mistreku (28-07-2010),MONSTER (15-06-2011)

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