Q: Petrol or diesel for cars is purchased by volume rather than weight. The
weight of a litre of petrol on a hot day in summer must be less than on a
freezing cold day in winter. Does this mean that motoring in winter is more
economical? And is this why airline companies purchase their fuel by weight
rather than volume?
A: The density of any given fuel decreases with increase of temperature. So
the mass will generally be less for a given volume in summer. For an engine
operating at a fixed efficiency the energy produced is proportional to the mass
of fuel burned. So, all things being equal, winter motoring should be more
Because the fuel companies adjust the composition of their petrol or diesel
by season to ensure easy starting in winter and to avoid vaporisation in summer,
it is almost impossible to tell how the energy content of a volume of fuel will
vary over the year. The fuel consumption also depends on the efficiency with
which the carburettor or injection system copes with temperature changes and, in
winter, the car will take longer to warm up and the oil will be thicker, all of
which will reduce economy. So the actual seasonal difference will be negligible
in monetary terms.
Airlines buy their fuels by volume, the same as motorists. It does not seem
possible to get a meter which will give an accurate direct reading of the mass
of a liquid delivered. Aircraft fuel gauges read in kilograms and consumption is
calculated in kilograms per hour.
All flight planning is done using fuel in kilograms. It is also necessary to
know the aircraft weight, including the fuel, to calculate takeoff performance
The gauges are checked by converting the volume uplift into kilograms using
the density of fuel. The density of JET A1 fuel varies from 0.80 at around
freezing to about 0.775 at 35 °C, a difference of about 3 per cent. When it
is necessary to carry maximum fuel this can make quite a difference. On a Boeing
767, a full tank of fuel weighs about 80 500 at 30 °C and 83 000 kilograms
at 0 °C.
Captain Richard King
Castelletto Ticino, Italy
A: Yes, petrol is less dense in summer than in winter and so you get fewer
fuel molecules for your money—although the effect is less than you might
expect as fuel at petrol stations is stored in underground tanks and so the
temperature range is quite small. However, the aerodynamic drag on the car is
less in hot weather (the air is less dense) and the engine is more efficient
(particularly during the initial start from cold). My experience is that
motoring is normally slightly more economical in summer than in winter.
A: Yes, a litre of petrol contains less energy because it has less mass if
the temperature is high. The effect of this on the value of our purchase at the
pump has always been minimised by the storage of the fuel in underground tanks
which maintain the fuel at a reasonably constant temperature. An interesting
aside is that early record-breaking aviators used to cool their fuel with dry
ice or something similar in the tankers so that they could get the maximum mass
of fuel, and thus energy, into the fixed volume tank in their aircraft.
A: In this country, and throughout most of Europe, fuel is dispensed by
volume rather than mass, so it is correct that less fuel will be dispensed on a
hot summer’s day than in winter.
In practice however, the temperature of the fuel in the underground storage
tanks does not vary much. The problem is further complicated by differences in
where the suction lines from the tanks are situated. Suction lines near the
surface and in sunlight will contain hotter fuel than suction lines that are
buried deep, or are in the shade. The effect can be much more significant with
diesel tanks which are sometimes above ground and in sunlight, whereas petrol is
always stored underground.
However, the problem is greater for the petrol station than for the consumer.
Fuel delivered on a hot day from a road tanker in the Sun can shrink
considerably when it gets to the colder underground storage tanks.
A: In Canada all petrol and diesel pumps display a sign that the volume
dispensed is corrected to 15 °C. This gets around the problem that we have
in Canada of density changes caused by the extreme temperatures. It ranges here
in Ottawa from typically −28 °C to 38 °C.
It is interesting to observe that if you use natural gas as fuel for your car
it is sold by weight not volume. What this means is that when you fill your
natural gas cylinders to the maximum permitted pressure of 3000 pounds per
square inch in winter you will have a greater weight of natural gas than in
summer due to the gas law effect. For my car, this is typically 14 kilograms of
natural gas in winter and 12.5 kilograms in summer.
Q: How do crispy rice breakfast cereals make snap, crackle and pop sounds
when you pour milk on them?
A: While being made, the cereals are cooked into dry, crispy froths of
syrupy, starchy stuff. In the process, and on cooling, they harden, and stresses
induced during drying distort them. If you let them slowly absorb humidity form
the air, they soften and the stresses relax gently, evenly and quietly. The
cereals go leathery and sullen, and pass gently into milk and stomach.
However, if you wet the fresh, crisp cereals their stresses are relieved
suddenly and unevenly and thousands of minute shells suddenly find themselves
with one side hard and stressed and the other side soft. They are now
unsupported and break noisily as they rage against the coming of the bite.
Dennesig, South Africa
A: The processing of cereals for some breakfast foods creates bubbles in what
is a brittle but weak material which swells when it absorbs water. As liquid
touches one side of the bubble it swells and distorts so that the opposite side
splits. The sound of this splitting is amplified by the air cavity in the same
way as a stringed musical instrument vibration is amplified by its soundbox.
Coventry, West Midlands
This week’s questions
Sound barriers: The other day my wife and I were standing on our balcony on a
cool damp evening. We could hear noises from quite far away. My wife asked if
sound travels better in cool, damp air than in dry air. Does anyone know the
Summer spectrum: Why do the flowers that bloom in the spring such as
snowdrop, crocus, daffodil, forsythia, broom and hawthorn tend to white or
yellow, while the flowers of summer are all colours?
C. R. Jessel