The generic definition of a heat engine is any device which takes heat
for its input and turns part of it into work. That part of the heat energy
which is not turned into work is returned to the environment as waste heat.
Practical devices include some obvious ones, as well as some not so obvious
Power station, in which a fuel is used to heat water, which in turn drives
a turbine to generate electricity
A hurricane, whiich uses the thermal energy of warm tropical waters to
develop from a small disorganized low pressure system into a raging destructive
The gasoline engine as a heat engine
The gasoline engine is a four stroke engine which burns a mixture of gasoline
and air to produce very hot gas in any one of a set cylinders (usually
either 4, 6, or 8) and uses the expansion of the very hot gas to produce
useful work. At the end of the process the hot gas is exhausted to the
atmosphere. The term 'four stroke' refers to the four different parts which
make up the one cycle of the engine
After the completion of the full cycle the engine returns to original state,
ready to repeat the cycle again. The temperature has not changed, and so
there is no change in the internal energy of the engine. There has been
heat put into the engine (Q1) and some heat taken out again
(Q2). We can then write from the First Law of Thermodynamics
The piston is drawn down, drawing in a fuel/air mixture through an entry
The entry valve closes, and the piston is drawn back up, compressing the
A spark ignites the fuel/air mixture. The production of heat caused by
the burning of the fuel is the heat input to the engine. As a result of
the explosive burning of the gas the piston is forced down, creating the
work that the engine produces.
Finally the exit valve opens, and the piston rises again, forcing the spent
fuel out of the cylinder. The gas is still quite hot, and so carries the
waste heat away from the engine.
Efficiency of a heat engine
Efficiency is defined as the ratio of the work that is extracted from the
engine to the heat energy that has to be put in
Using the equation above from the First Law of Thermodynamics we can write
||Q1 - Q2
|| = 1 -
Suppose that an engine is built which for each four stroke cycle 25
MJ of heat (Q1) produces 8 MJ of work.
The amount for heat exhausted to the atmosphere would be Q2
= Q1 - W = 25 MJ - 8 MJ = 17 MJ
The efficiency would then be W/Q1 = 8/25 = 0.32 (32 %)
The Carnot Engine
The Carnot Engine is not a real engine. It is a mathematical model, the
limit representing the best possible efficiency that a heat engine could
possibly have. Although it can never be built it is useful in giving us
an upper limit to the best engine that we can hope to build.
Its efficiency is determined by the Second
Law of Thermodynamics, and depends on only two temperatures, the temperature
of the hot gas (TH) and the temperature of the atmosphere where
the exhaust gases go (TC). The efficiency is equal to
Suppose that our Carnot engines operates between the two temperatures
= 900 oC = 1173 K and 50 oC = 323 K. The efficiency
of the Carnot engine would be
|Efficiencyc = 1 - 323/1173 = 1 - 0.275 = 0.725 (72.5 %).
A real engine using these two temperatures must have an efficiency less
that this (often much less).