Journal of Physics Special Topics
P4 4 Could McDonalds fries power a steam engine?
K. E. Wright, R. R. Norisam, J. S. Baker, T. W. Buggey
Department of Physics and Astronomy, University of Leicester. Leicester, LE1 7RH.
December 4th, 2015
Abstract
We determine whether a steam engine could be powered by the energy used per day to heat up 4 million
kilograms of McDonalds French fries [1]. We calculate the energy radiated by one fry as it cools down to
room temperature after it has been cooked. In doing so, we found that the energy radiated from all of
the fries produced by McDonalds in one day is 4.4 × 1014 J. We found that this energy could be instead
used to power a 1800HP internal combustion engine for 3018 days, using the potato starch of the fries as
a fuel.
Given that McDonalds cook 4 million kg of fries per
day, the energy radiated by McDonalds’ fries per day is
4.4 × 1014 J. We find that one chip will radiate 634.97J
of energy during this cooling process.
Introduction
McDonalds currently cook 4 million kilograms of
French fries per day [1], using an excessive amount of
energy to do so. We investigate how much energy this
process uses per day to heat up the fries and determine
whether that energy could be instead used to power a
standard 1800HP steam train engine. In doing so, we
aim to highlight the astonishing amount of energy used
to cook fast food by companies such as McDonalds.
Steam trains use internal combustion engines, obtaining heat from a heat reservoir, doing work and releasing
heat to a low temperature reservoir [5]. Heat reservoirs
require a fuel that has a high specific heat capacity, ie
coal. Potato starch has a high specific heat capacity
[7] and so it can be investigated whether potato starch
could be used as a fuel within the heat reservoir of an
internal combustion engine. It is investigated whether
it can power a steam engine for a sustained period of
time.
Investigating the energy required to power a
steam engine
Firstly we calculate the energy provided to a cold reservoir of an engine that uses potato starch as a fuel. In
order to do so, the efficiency of the engine must be
known. The 2nd law of thermodynamics states that
it is impossible to have a 100 percent efficient engine,
thus the maximum efficiency, the carnot efficiency, of
an internal combustion engine is calculated.
=1−
Tc
273
= 20.4%
=1−
Th
343
(2)
Where Tc and Th are the temperatures of the cold and
hot reservoir respectively. We can now calculate the
amount of energy that reaches the cold reservoir in
Finding the energy radiated away by a chip dur- this system:
ing its cooling process
Qh − Qc
First we calculate the energy radiated away by one
=
(3)
Qh
McDonalds fry as it cools from cooking temperature to
room temperature.We assume a cuboidal chip of length Where Qh and Qc are the energies of the hot and cold
58.9mm [2] and approximate a 10mm x 10mm frontal reservoirs respectively. Rearranging this equation for
surface area. It is also assumed that the McDonalds Qc :
chips are cooked to a temperature of 93 degrees [3] and
then left to cool to room temperature. Lastly it is asQc = Qh − Qh
(4)
sumed the fries are made from only potato starch [4].
14
where Qh = 4.4 × 10 J (Total energy radiated from
Applying the heat transfer equation [5]:
McDonalds’ fries per day) and = 0.204. We obtain a
value for Qc as 3.5 × 1014 J.
Q = mc∆T
(1)
Powering the steam engine
where m = mass of one chip, calculated using m = A 1800HP engine needs 1342800 J per second to funcρV and ρ is the bulk density of fried strips of potato, tion (using the conversion that 1HP is equivalent to 746
449kgm−3 [6] and volume, V, is calculated from the Joules per second [9]). Comparing this figure to the
dimensions quoted above. c is the specific heat capac- value calculated above, it is estimated that a potato
ity of potato, 3.43 KJ/kg [7]. Change in temperature, starch fuelled internal combustion engine could power
∆T = 93 − 23 = 70K.
a steam engine for 3018 days.
1
Could McDonalds fries power a steam engine?, December 4th, 2015
Conclusion
We have found that the energy used in heating up McDonalds fries(made from potato starch) could be instead used to power an 1800HP internal combustion
engine for 3018 days. This assumes that potato starch
is cooked to approximately 70 degrees and takes into
account of no heat losses in the furnace. We also assumed a maximum efficiency of the engine in order to
yield the maximum amount of energy possible. In this
sense the potato starch would most likely power the
train for less than the quoted 3018 days, thus it is recommended that future research strives to amend these
assumptions. Nevertheless it is an astonishing resulthighlighting the excessive energy wasted on a daily basis by fast food companies such as McDonalds.
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