Lecture Notes
Engineering Sciences
BTE1013
TOPIC 10
HEAT AND COMBUSTION PROCESS
Learning Outcomes
At the end of this chapter, student should be able to :
1. Define the first law of thermodynamics
2. describe the isothermal, adiabatic and combustion procesess
3. explain volumetric efficiency of an engine combustion
10.1 HEAT AND COMBUSTION PROCESS
 First law of thermodynamics
 Isothermal process, adiabatic process, combustion process
 Volumetric efficiency
The first law of thermodynamics is an expression of the principle of conservation of
energy.
The law states that energy can be transformed, i.e. changed from one form to another,
but cannot be created or destroyed. It is usually formulated by stating that the change in
the internal energy of a system is equal to the amount of heat supplied to the system,
minus the amount of work performed by the system on its surroundings.
10.2 Adiabatic Process
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An adiabatic process is one in which no heat is gained or lost by the system. The first law
of thermodynamics with Q=0 shows that all the change in internal energy is in the form
of work done. This puts a constraint on the heat engine process leading to the adiabatic
condition shown below. This condition can be used to derive the expression for the work
done during an adiabatic process.
10.2.1 Adiabatic and isothermal changes
When a gas is compressed or expanded there are many possible connections between
the changes of pressure, volume and temperature. We will restrict ourselves to looking
at just two basic variations.
(i) Isothermal expansion or compression
In an isothermal change the temperature of the gas is kept constant during the change in
pressure and volume by adding or removing heat energy from the system. For this
reason isothermal changes should take place in thin-walled, conducting containers.
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Lecture Notes
Engineering Sciences
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For this type of change T= constant and therefore PV = constant; the gas obeys Boyle’s
1.2
1.2
Pressure
Pressure
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
0
5
Volume
10
0
5
Volume
10
Isothermal
Figure 1
Figure 2
Adiabatic
law (Figure 1).
(ii)Adiabatic expansion or compression
In an adiabatic change the total heat content of the system is kept constant and
therefore the temperature of the gas will alter; no heat must enter or leave the system.
This type of change should occur in an insulated container.
Since the temperature of the gas changes the adiacurves for PV will be steeper than those for an
isothermal change (Figure 2). True adiabatic changes
difficult to produce in reality, but the expansion of air
burst tyre or balloon and the expansion and compression of air through which
wave is passing are very close to adiabatic changes.
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batic
are
from a
a sound
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Lecture Notes
Engineering Sciences
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10.2.2 Isothermal graphs at different temperatures
The following graphs show the PV curves for isothermal changes for a given mass at two
different temperatures To and T1 where T1>To.
1.40
1.40
Figure 3
Pressure
Pressure
1.20
1.20
To
1.00
T1
1.00
0.80
0.80
0
2
4
6
8
10
Volume
0.60
0
2
4
6
8
10
Volume
0.60
10.2.3 Temperature variation in a reversible adiabatic change
When a gas in an insulated container is0.40compressed or expanded it suffers a change in
temperature, the molecules of gas gaining energy from, or losing energy to, the moving
0.40
0.20
0.20
Figure 4
(b)
0.00
0.00
(a)
(c)
walls of the container.
Consider a volume of gas enclosed in an insulating container by a frictionless piston. Let
the initial velocity of a molecule moving in the x-direction be u.
(a) If it collides with a stationary wall of the container its velocity after collision will be -u
(see Figure 4 (a)).
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Now consider the case when the wall is moving at velocity v, first an expansion and then
a compression (Figures 4(b) and (c)).
(b) The velocity of the molecule relative to the wall = u - v.
After collision:
velocity relative to the wall = - (u - v)
velocity relative to the Earth = - (u - 2v)
The final velocity is less than the initial velocity by 2v, and so the gas has cooled. Notice
that this cooling only takes place while the wall of the container is moving.
(c) Similarly for the compression we can say that, after collision, the velocity relative to
the Earth = - (u + 2v)
This shows an increase in velocity, and therefore the temperature of the gas will be
raised
10.3 THE COMBUSTION PROCESS
Combustion is what we do to power our vehicles. If combustion is perfect and complete,
we get the highest possible power output, the best fuel economy and the least amount
of pollutants.
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The power of a gasoline engine is derived from controlled combustion. As with any
burning process three things are required: fuel, oxygen and heat. In an internal
combustion engine, the same three things that are required are:
 Fuel (gasoline)
 Oxygen (from the air)
 Heat (from the spark plug)
When combustion is perfect, exactly three things are produced:
 Heat
 Carbon dioxide
 Water vapor
Carbon atoms from the hydrocarbon fuel (HC) combine with oxygen from the
atmosphere to form water. Hydrogen atoms from the HC’s combine with oxygen to form
water. There is exactly the right amount of fuel to consume the existing oxygen.
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Unfortunately, perfect combustion RARELY takes place. The ratio of air to fuel, although
controlled more precisely than ever before is not always perfect, the fuel we use has
numerous impurities, and the air that is drawn into the engine is 78% nitrogen, which can
cause NOx (nitrous oxide) emission under certain conditions.
Even if the air/fuel ratio is perfect, all the gas may not be burned: the heat of combustion
may not be sufficient, the spark timing could be off, or the spark could be less than
perfect. These conditions lead to incomplete combustion and the production of
vehicular emissions.
There are five exhaust gases observed in today’s vehicle. Three of them are considered
polluting gases:
 Carbon monoxide (CO)
 Hydrocarbons (HC)
 Oxides of Nitrogen (NOx)
The other two gases are considered informational gases and are used as diagnostic aides
they are:
 Carbon dioxide (CO2)
 Oxygen (O2)
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Lecture Notes
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The automobile is said to be responsible for approximately 50% of the hydrocarbons,
over 75% of the carbon monoxide and nearly 50% of the oxides of nitrogen that pollutes
our atmosphere.
Vehicle manufacturers are being forced to lower the emissions of their products every
year, and legislation such as the clean air act employs programs like Drive Clean to
maintain a vehicle’s pollution control system’s integrity well into its usable life.
THANK YOU
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