Chemistry AS DF DF3 DF4

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  • Created by: A.B.
  • Created on: 01-12-13 12:13
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  • DF Auto-Ignition and Octane Numbers SL DF3  and DF4
    • Definition
      • Auto-ignition is the explosion of a fuel without a spark.
      • The octane number is a measure of the tendency of petrol to auto-ignite.
        • High number, low tendency
      • Auto-ignition in a petrol engine causes:
        • A knocking sound
        • Reduced engine performance
        • Engine damage
    • Octane Numbers
      • 2,2,4-trimethylpentane = octane number 100
        • 80% 2,2,4-trimethylpentane with 20% heptane = octane number 80
      • Heptane octane number 0
        • 80% 2,2,4-trimethylpentane with 20% heptane = octane number 80
      • Comparing octane numbers
        • Short chain compounds have a higher octane number than long-chain compounds
        • Branched-chain compounds have a higher octane number than the corresponding unbranched-chain compounds
        • Cycloalkanes have higher octane numbers than the corresponding straight-chain alkanes
        • Arenes have higher octane numbers than cycloalkanes
        • Oxygenates have higher octane numbers than the corresponding alkanes and are added to petrol to increase the octane number.
      • The photochemical industry uses a number of different chemical reactions to increase the octane numbers of the components in petrol.
        • Cracking
        • Reforming
        • Isomerisation

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