Name: R. G.
Date: Around 1999
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?
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
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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