Lecture Notes Engineering Sciences DEC1013

TOPIC 10HEAT AND COMBUSTION PROCESS

Learning Outcomes

At the end of this chapter, student should be able to :1. Define the first law of thermodynamics2. describe the isothermal, adiabatic and combustion processes3. 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.

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10.2 Adiabatic Process

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.

For this type of change T= constant and therefore PV = constant; the gas obeys Boyle’s law (Figure 1).

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1.2

0.6

0.8

1.0

Pressure1.2

0.6

0.8

1.0

Pressure

Lecture Notes Engineering Sciences DEC1013

(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 adiabatic curves for PV will be steeper than those for an isothermal change (Figure 2). True adiabatic changes are difficult to produce in reality, but the expansion of air from a burst tyre or balloon and the expansion and compression of air through which a sound wave is passing are very close to adiabatic changes.

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.

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Figure 3

Volume

1.40

1.20

1.00

0.80

0 2 4 6 8 10

T1

Pressure

Volume

1.40

1.20

1.00

0.80

0 2 4 6 8 10

To

Pressure

Lecture Notes Engineering Sciences DEC1013

10.2.3 Temperature variation in a reversible adiabatic change

When a gas in an insulated container is compressed or expanded it suffers a change in temperature, the molecules of gas gaining energy from, or losing energy to, the moving

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)).

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

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(b)

(c)(a)

Figure 4

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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.

Internal Combustion EnginesThe Internal combustion is a process in which the combustion of a fuel occurs within the system. Internal combustion engines are type of heat engine in which the fuel is burnt inside the engine or where fuel combustion takes place inside the engine.

WorkingMostly, pistons are used inside the internal combustion type heat engines. Pistons move up and then down inside the cylinders present in the heat engines. A single motion of a piston either in upward direction or in downward direction inside the cylinder is called a Stroke.

Examples of Internal Combustion Heat Engines:1. Gasoline engines are more efficient than steam engines. Around 33.33% of the

energy in a gasoline internal combustion engine gets converted to work.2. Cars mostly have four-stroke internal combustion heat engines consisting of the

following parts as shown in figure.1. Intake Stroke2. Compression Stroke3. Power Stroke4. Exhaust Stroke

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Lecture Notes Engineering Sciences DEC1013

A: Intake stroke:At the start of the cycle the piston will be at the TDC (Top dead center), as the piston moves towards BDC (Bottom dead center) the inlet valve open which allows the air-fuel mixture to gush inside the chamber (fig A).

B: Compression stroke:The piston moves towards TDC from BDC. While moving upwards the piston compresses the air-fuel mixture. As the air-fuel mixture is compressed it attains high pressure and temperature (fig B).

C: Power stroke:A spark is produced at the TDC by spark plug which ignites the compressed air-fuel mixture. This creates an explosion in the combustion chamber. This force pushes the piston towards BDC (fig C). This is also known as combustion stroke.

D: Exhaust stroke:Once the piston hits the BDC, due to inertia of the force produced in previous cycle it moves upwards. The exhaust valve opens and the waste gas is pushed outside the combustion chamber (fig D).

Thus to complete one cycle from intake stroke to exhaust stroke the fuel has to go through four stages. Hence it is called a four stroke engine. During this cycle the crankshaft makes two full rotations, half rotation in each cycle.

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:

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Lecture Notes Engineering Sciences DEC1013

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.

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)

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.

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Lecture Notes Engineering Sciences DEC1013

THANK YOU

Work Done by a Gas

When a gas expands it does work on its surroundings. That work is equal to the area under the curve on a P-V diagram which describes that expansion.

Isobaric process:

Process at constant pressure:

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Lecture Notes Engineering Sciences DEC1013

W = F D

W = ( F / A ) ( A D )

W = P ( AD )

W = P ( V)

Work = area under the curve on a P-V diagram.

Work = area under the curve on a P-V diagram -- even if the process is not isobaric.

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Lecture Notes Engineering Sciences DEC1013

Isometric processes:

Isochoric:

Constant volume:

No work is done in an isometric process.

Isothermal process:

Process at constant temperature:

We can keep the temperature constant by having the system in contact with a heat reservoir.

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Lecture Notes Engineering Sciences DEC1013

Adiabatic process:

Process with zero heat flow:

Insulated:

The amount of work done will be less than for an isothermal process between the same two volumes. Notice that an adiabat is steeper than an isotherm.

Example:

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Lecture Notes Engineering Sciences DEC1013

W = 2.5 atm-l

Now, what is an "atmosphere-liter"? It is a pressure (atm) multiplied by a volume (liter) so it must be some sort of work or energy. Now, this is just a units conversion question,

W = 2.5 atm-l [ 0.001 m3 / 1 l ] [ 1.013 x 105 Pa / 1 atm ]

W = 253 Pa-m3 [ (N/m2) / Pa ]

W = 253 N-m

W = 253 J

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