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One of the primary reasons why hydrocarbons produce energy is the production of water during the combustion process.

I've been trying to find out why diesel is considered to have a higher energy density compared to gasoline and I can think of a couple of reasons:

  • Higher molar mass (more carbon/hydrogen to burn)
  • Easier to burn diesel since it is heavier (weaker bonds)

I think I'm on the right path but I don't have a clear picture of the situation.

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    $\begingroup$ If your question were about Diesel and Otto engines, from the perspective of physics I would like to add to your picture drawn that the compression/decompression ratio in a Diesel engine is much higher than a Otto engine; as the mix auto-ignites, there are no spark plugs necessary (glowing plugs are a different story). In terms of the Carnot cycle, this is one contribution why a Diesel engine may be more efficient than an Otto engine in terms of converting chemical energy into mechanical energy. $\endgroup$ – Buttonwood May 14 '17 at 15:25
  • $\begingroup$ related chemistry.stackexchange.com/questions/69425/… $\endgroup$ – Mithoron May 14 '17 at 19:19
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Diesel and gasoline have roughly the same energy per unit mass (lower heating value), about $41~\pu{MJ/kg}$).

The density of diesel is about $833~\pu{kg/m^3}$ compared to $740~\pu{kg/m^3}$ for gasoline. This gives diesel about 13% higher energy density per volume.

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  • $\begingroup$ The actual structure of the hydrocarbons themselves don't contribute to the energy per unit mass, but their willingness to undergo combustion? $\endgroup$ – Billy Jones May 14 '17 at 16:58
  • $\begingroup$ I don't know what you mean by the term "willingness to undergo combustion". But the structure of the hydrocarbons has influence on their heating values. E.g. methane > ethane > propane > butane > pentane (en.wikipedia.org/wiki/…) $\endgroup$ – aventurin May 14 '17 at 17:03

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