# Gasoline has a very high molar heat capacity. Is this a good thing for its use in engines?

Wikipedia states that the molar heat capacity of gasoline is $$228 \, J/mol\cdot K$$ (https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities). This is more than twice that of liquid water ($$75.327 J/mol\cdot K$$) which is regarded as having a very high molar heat capacity. This got me thinking whether the high heat capacity of gasoline has any positive or negative affects on its use in engines or in other industries?

The fact that gasoline has a high molar heat capacity would mean that it takes a lot of heat energy to warm up gasoline to a given temperature relative to other working fluids. So surely this would make gasoline an inefficient fuel in a heat engine relative to something with a lower molar heat capacity like water or even a simple monatomic gas like helium? If this is the case then why is gasoline used extensively in heat engines? Does the large amount of energy stored in its chemical bonds act as a positive that simply outweighs the negative of its high molar heat capacity or is the high molar heat capacity of gasoline even a negative to begin with?

Any help on this issue would be most appreciated!

• The relevant quantity for engines is the specific heat per kilogram not per mole, and the specific heat per kg for gasoline is about half that of water. Feb 16 at 8:51
• @JohnRennie Thanks for response! Okay I was thinking that would be the case. However does that mean that the molar heat capacity of gasoline is completely inconsequential insofar as heat engines are concerned or does is play at least a small role? Feb 16 at 16:09

## 1 Answer

The heat capacities of liquid fuels for engine use per kilogram have only a tiny influence on engine operation; here is why.

On a per-kilogram basis, the specific heat of the fuel is a minor fraction of the energy content of the fuel itself.

Note also that internal combustion engines throw off about 60% of the heating value of the fuel as waste heat. Among other things, this heats up the intake pipes that conduct the fuel/air mixture into the valve assembly for each cylinder, which tends naturally to vaporize the fuel before it gets into the combustion chamber "for free".

In the case of port-injected engines, the fuel spray from the injector is typically aimed at the underside of the (hot) intake valve to guarantee complete vaporization even if the rest of the air induction plumbing upstream of it is not yet up to full temperature. A useful side effect of this is to cool the valve head via the heat of vaporization of the gasoline when the engine is running at full load and generating lots of heat.

• Thanks for the response! Okay this clears up pretty much everything for me! Feb 17 at 5:21
• very good then! -NN Feb 17 at 6:36