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Branched Alkanes

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Regular, Premium Diesel
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Regular, Premium Diesel

                         

Credit: Rene Schwietzke
Source: http://www.flickr.com/photos/rene-germany/2455154999/
License: CC BY-NC 3.0

When you pull up to a gas station, you have choices to make. Do you go with regular or should you pay more for a higher grade? What is the difference in gas composition and value? Diesel is not an issue (unless your vehicle uses it) because the diesel pump nozzle is too big to fit in a regular gasoline-burning car. So we’re back to our question – what’s the difference?

News You Can Use

  • Gasoline is a complex mixture of hydrocarbons. Major fractions are alkanes and branched alkanes, along with aromatics. These fractions are obtained from crude oil through a process known as distillation. The different components are separated by heating the material to boiling. Lighter molecular weight materials boil at lower temperatures than heavier ones and can easily be separated.
  • Gasoline ignition is powered by spark plugs. When the vaporized gas enters the cylinder, a spark is fired from the plug to ignite the gas and drive the piston. Diesel engines do not have spark plugs. They use a glow plug for initial ignition, but the ignition in the cylinder occurs as a result of the heat already generated.
  • Credit: Andrew Napier
    Source: http://www.flickr.com/photos/andrewnapier/4354462705/
    License: CC BY-NC 3.0

    This is the step of the combustion cycle where a compressed mixture of air and fuel (red dots) is ignited to produce the force necessary to move the piston down [Figure2]

     

  • We are seeing an increasing use of “gasohol”, a mixture of ethanol and gasoline (usually about 10% ethanol and 90% gasoline). This approach decreases the overall consumption of gasoline. In the winter, the blended mixture seems to provide less air pollution since it burns more completely.
  • Watch a video on petroleum refining at the link below: https://www.youtube.com/watch?v=vscX_zawdQw

Explore More

Use the links below to learn more about the composition of gasoline. Then answer the following questions.

  1. How many hydrocarbons might be found in gasoline?
  2. What is the “octane number”?
  3. Is a premium grade of gasoline needed occasionally to clean out the engine?
  4. How are the viscosities of diesel and regular gasoline affected by lower temperatures?
  5. Which type of fuel produces more CO2 emissions?

Image Attributions

  1. [1]^ Credit: Rene Schwietzke; Source: http://www.flickr.com/photos/rene-germany/2455154999/; License: CC BY-NC 3.0
  2. [2]^ Credit: Andrew Napier; Source: http://www.flickr.com/photos/andrewnapier/4354462705/; License: CC BY-NC 3.0

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