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Name: R. G.
Status: other
Age: N/A
Location: N/A
Country: N/A
Date: Around 1999 


Question:
What is the typical volumetric ratio of gasoline to combustion products? i.e. how much "gas" do you get for a unit volume of liquid gasoline by burning it?


Replies:
Gasoline is a mixture of many hydrocarbons; for the purposes of this answer I will estimate that it consists of isooctane, which has the composition C8H18. Each molecule of isooctane that burns completely produces 8 molecules of CO2 and 9 molecules of H2O.

C8H18 + 12.5 O2 --> 8 CO2 + 9 H2O

All that we need to know to answer your question is the volume of one mole (a standard number of molecules) of liquid isooctane, and the volume of the combustion products resulting from the isooctane. One mole of isooctant has a volume of 165 mL, or 0.165 liters. As a pretty good approximation, a mole of any gas at room temperature and atmospheric pressure occupies 24.4 liters.

So, if you assume that all of the combustion products will be in vapor form, this amounts to 17 moles of gas, or 381 liters. If you assume that the CO2 is in vapor form and the water is liquid, that's 8 moles of gas at 180 liters, plus 9 moles of liquid water at 18 mL/mole, or 0.16 liters.

These numbers are a little deceptive for two reasons. The first is that 12.5 moles of oxygen (280 liters) are being consumed in the combustion. The second is that the combustion products are released at higher than room temperature, because heat is produced in the combustion. This boosts the amount of push that powers the engine. If heat weren't produced, the combustion would overall not result in any volume increase at all - in fact, gas volume would decrease. You would be converting 280 liters oxygen + .16 liters isooctane to 180 liters carbon dioxide + 0.16 liters water, for an overall volume LOSS of 100 liters per mole. In other words, burning 1 unit volume of isooctane will cause a net reduction in gas volume of 1125 unit volumes. So, the power for an internal combustion comes not from producing more molecules of gas, but from the heat of combustion.

Richard E. Barrans Jr., Ph.D.
Assistant Director, PG Research Foundation
Darien, IL USA



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