100% efficiency, its cold reservoir would have to be at absolute zero (which is
theoretically impossible), or its hot reservoir would have to be infinitely hot. e
ce = efficiency of Carnot engine
Tc = temperature of cold reservoir (K)
Th = temperature of hot reservoir (K)
21.9 - Otto cycle: internal combustion engine
The internal combustion engine shown above is used
in automobiles and is probably familiar to you. In this
section, we describe the operation and engine cycle
of this engine, and present an equation for its
efficiency. The cycle is called the Otto cycle, after the
German inventor of the “four-stroke” internal
combustion engine, Nikolaus August Otto. A “stroke”
is a movement in or out of a piston. In the complete
Otto cycle, a piston moves in and out twice, making
four strokes.
In an internal combustion engine, there is no heat
reservoir in the usual sense. Instead of heat flowing
into the engine, the working gas itself is the source of
energy. It is a mixture of air and gasoline vapor that is ignited inside a cylinder (hence the term “internal combustion”), and expands to drive a
piston that does work. The thermal energy that is used to do work is created by burning the gasoline mixture. You see the cylinder and piston in
the illustrations below.
The four steps of the basic thermodynamic cycle together require two strokes of the piston. The steps occur rapidly. Two additional strokes
drive the combustion products out of the cylinder and replace the air and gasoline mixture for the next cycle. For simplicity, we combine these
two strokes into one “exhaust” step.
Here are the steps in the Otto cycle.
First: The piston compresses a mixture of air and gasoline vapor inside the cylinder adiabatically.
Second: A spark plug ignites the mixture. This is a constant-volume process: The temperature and pressure of the mixture both increase, but
the volume stays the same.
Internal combustion engine.Otto cycle (internal combustion)
·Air-fuel compressed adiabatically
Otto cycle (internal combustion)
·Ignition: temperature and pressure
increase