ZIMSEC O Level Combined Science Notes: Fuel efficiency

  • Fuel efficiency measures the amount of energy that can be obtained from a given quantity of fuel
  • It is often useful to know which fuel gives the most energy(heat) per given unit
  • In a laboratory fuel efficiency can be determined by measuring the temperature rise of water when it is heated using a known mass of fuel
  • Usually when a fuel burns in a plentiful oxygen supply
  • It produces carbon dioxide and water
  • This is known as complete combustion
  • When a fuel is burnt in an insufficient oxygen supply environment
  • The word equation for complete combustion is:
  • \text{octane (fuel)+oxygen} \rightarrow \text{carbon dioxide + water}
  • There is incomplete combustion
  • Carbon monoxide and soot(which is unburned carbon) are produced
  • The word equation for incomplete combustion is:
  • \text{octane (fuel)+oxygen} \rightarrow \text{carbon monoxide + carbon+water}
  • Carbon monoxide is a poisonous gas
  • It is thus not recommended to make a fire in a room that is not well ventilated
  • Soot is often observed in the exhaust pipes of motor vehicles
  • It is also observed as the dark colored part of smoke that comes out of motor vehicle exhausts ans well as factory chimneys
  • When the carbon in fuels in completely oxidized to carbon dioxide
  • That is when there is complete combustion
  • More energy is released than when only
  • Carbon dioxide is partly oxidized to carbon monoxide
  • That is when combustion is incomplete
  • Complete combustion results in more fuel efficiency than incomplete combustion

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