Auto$mart
Learn the facts: Fuel consumption and CO
2
What is the issue?
For an internal combustion engine to move a vehicle down the
road, it must convert the energy stored in the fuel into
mechanical energy to drive the wheels. This process produces
carbon dioxide (CO
2
).
What do I need to know?
Burning 1 L of gasoline produces approximately 2.3 kg of CO
2
.
This means that the average Canadian vehicle, which burns
2000 L of gasoline every year, releases about 4 600 kg of CO
2
into the atmosphere.
But how can 1 L of gasoline, which weighs only 0.75 kg,
produce 2.3 kg of CO
2
? The answer lies in the chemistry!
Î The short answer:
Gasoline contains carbon and hydrogen atoms. During
combustion, the carbon (C) from the fuel combines with
oxygen (O
2
) from the air to produce carbon dioxide (CO
2
).
The additional weight comes from the oxygen.
Î The longer answer:
Gasoline is composed of hydrocarbons, which are hydrogen
(H) and carbon (C) atoms that are bonded to form
hydrocarbon molecules (C
X
H
Y
). Air is primarily composed of
nitrogen (N) and oxygen (O
2
).
A simplified equation for the combustion of a hydrocarbon
fuel may be expressed as follows:
Fuel (C
X
H
Y
) + oxygen (O
2
) + spark g water (H
2
O) +
carbon dioxide (CO
2
) + heat
In this combustion reaction, we see that the hydrogen from
the gasoline combines with oxygen from the air to produce
water (H
2
O). Similarly, the carbon from the fuel combines
with the oxygen from the air to produce carbon dioxide
(CO
2
). The combustion process also produces heat that is
converted into the mechanical energy that propels the
vehicle.
So it’s the oxygen from the air that makes the exhaust
products heavier.
Now let’s look specifically at the CO
2
reaction. This reaction
may be expressed as follows:
C + O
2
g CO
2
Carbon has an atomic weight of 12, oxygen has an atomic
weight of 16 and CO
2
has a molecular weight of 44
(1carbon atom [12] + 2 oxygen atoms [2 x 16 = 32]).
Therefore CO
2
is 3.67 times heavier than carbon
(44÷12= 3.67).
This is how 1 L of gasoline, which contains about 0.63 kg of
carbon, can produce about 2.3 kg of CO
2
(3.67 × 0.63 kg = 2.3 kg).
The fine print
Î Gasoline is a complex mixture of several types of
hydrocarbon molecules. The composition varies according to
the crude source, refinery, time of year, age of the product
and storage conditions. Accordingly, gasoline’s density and
exact carbon content will vary. The figures used here are
averages.
Î In typical combustion processes, some of the hydrocarbons
escape the cylinder unburned, yielding volatile organic
compounds. Also, some of the carbon does not fully oxidize,
which produces carbon monoxide (CO).
CO
2
produced by other fuel types
Environment Canada publishes factors to estimate CO
2
and other
emissions from fuel combustion. Tailpipe CO
2
emissions vary by
fuel type because of their different densities. Denser hydrocarbon
fuels, such as diesel, contain more carbon and will therefore
produce more CO
2
for a given volume of fuel. The tailpipe CO
2
emissions for various transportation fuels are listed in the
following table.
Fuel type CO
2
tailpipe emissions
(kg/L)
Gasoline 2.29
E10
(10% ethanol + 90% gasoline)
2.21
E85
(85% ethanol + 15% gasoline)
1.61
Diesel 2.66
B5
(5% biodiesel + 95% diesel)
2.65
B20
(20% biodiesel + 80% diesel)
2.62
How can I help?
You can reduce CO
2
emissions by:
• purchasing the most fuel-efficient vehicle that meets your
needs
• using fuel-efficient driving techniques
• driving less often
Visit vehicles.nrcan.gc.ca to find out more about buying and
driving your vehicle to save fuel, save money and reduce your
impact on the environment.
References
Pollution Probe 2009, Primer on Automobile Fuel Efficiency and
Emissions, 2009. www.pollutionprobe.org
Canada’s Action on Climate Change, www.climatechange.gc.ca
Environment Canada, www.ec.gc.ca
Natural Resources Canada, www.nrcan.gc.ca
U.S. Environmental Protection Agency, www.epa.gov and
www.fueleconomy.gov
U.S. Department of Transportation, www.italladdsup.gov
National Association of Clean Air Agencies, www.4cleanair.org
The International Council on Clean Transportation,
www.theicct.org
For information regarding reproduction rights, please contact
Natural Resources Canada at: [email protected].
Aussi disponible en français sous le titre : Les faits : Consommation de carburant et CO
2
© Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources Canada, 2014
