So far we have explained how the pressure volume and temperature of a gas are
related. Temperature and pressure changes can result in a volumetric change and
this can be used to do work.
Before we further investigate the detail of useful work,
It is fairly obvious that for any engine to operate it requires a source of energy,
but now we have discovered that its efficiency is dependent not just on how hot
the energy source is but on how hot it is relative to the normal environment of
Thermodynamics applies to all systems but there is a tendency to focus its use with respect to gases.
You have probably already seen and or used equations relating volume pressure and temperature.
It would make sense to do some revision on this
The last idealised cycle that we need to look at is when all the heat exchanges
are carried out under constant pressure. This is what typically happens in a gas
Fresh air fed in to the engine is compressed adiabatically; fuel is then b
In many situations we find that a system goes through a series of thermal
processes which ultimately takes the system back to its original starting point.
This is called a cyclical process and it is commonly found in engines of various
The engines that we have looked at so far are all ideal and do not suffer from
friction, heat loss and vibration that absorb the energy produced by a physical
When analysing the practical performance of a real engine rather than creatin
By studying closed systems we have developed an equation for work and found
relationships between some of the key parameters of gases.
Closed systems do exist in practical situations but they do not exist alone. Many
engineering situations are
To start the process of analysing the transfer of energy it makes sense to follow
the tradition of simplifying the process wherever possible. This can be achieved
by reducing the number of variables. By creating situations that keep the
Having looked at many aspects of thermodynamics and investigating how they
impact on the ideal Carnot cycle we can now explore a cycle which can be put
to more practicable purposes. The Otto cycle forms the basis of spark ignition
In the previous lesson we have discovered that
P1 V 1 P 2 V 2
So we can say that
This equation is known as the general gas equation, and the value of the
constant is dependent on the mass (m) and nature of the gas involve