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Works and how power is used And how to go about ut
Typology: Study notes
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Lecturer :
Dr. Kojo Atta Aikins
Course Objectives
To provide an understanding of:
✓ the sources and applications of different forms of power used in
agriculture.
✓ the basic principles of the types of power sources used in agriculture.
✓ the criteria for selecting appropriate machinery for different agricultural
operations.
✓ the operation of the various components of an agricultural tractor.
Course Outline
(manual, draught and mechanical).
Recommended Text Books
Egbo
and John Kilgour
th edition) by John B. Liljedahl, Paul K.
Turnquist, David W. Smith and Makoto Hoki
is known as the power. Therefore, power is the time rate at which work is done or
energy is transferred. That means more power does not necessarily imply more
work done, but the same work done within a shorter period.
Where P is power, W is work done, t is time taken to do the work, F is the force
applied and v is velocity. The SI unit of power is N m/s or Watt (W).
Motion may be defined as the speed or how fast something is moving. It is a
change in position of an object with respect to time and its reference point. The
SI unit of speed is metres per second (m/s). Speed is a scalar quantity, meaning it
does not include direction (only magnitude). Speed is normally thought of in two
ways: instantaneous speed and average speed. Instantaneous speed is the speed at
any given instant. It is the speed measured and displayed on the speedometer of a
vehicle. Average speed refers to the total distance covered divided by the total
time taken.
Velocity is a vector quantity (having magnitude and direction). Example: a car
travelling 60 km/h heading north. Moving in a straight line at the same speed is
called constant velocity. However, in real life, both speed and direction change
during motion. This change is velocity is called acceleration. Acceleration is the
rate at which velocity changes. Positive acceleration means increase in velocity
(speeding up), while negative acceleration means decrease in velocity (slowing
down) which is also called deceleration. The SI unit of acceleration is m/s
2 .
Timetaken
Distancetravelled Speed =
Timetaken
Finalvelocity-Initialvelocity
Timetaken
Changein velocity Acceleration
Heat, also called thermal energy , is defined as the form of energy that is
transferred between two objects by virtue of a temperature difference. That is, an
energy interaction is heat only if it takes place because of a temperature difference.
1.5.1 Effects of Heating
Different objects require different amount of heat to change their temperatures by
similar amounts. The knowledge about various thermal properties is therefore
important.
Heat Capacity (C) of an object is the heat required to raise its temperature by 1
Kelvin (K). SI unit for C is JK
Specific Heat Capacity (c) gives the heat capacity per unit mass of substance. SI
unit is J kg
When an amount of heat, Q , is transferred to an object to raise its temperature
from T 1 to T 2 , the quantity of the heat transferred can be calculated using:
2 1
2 1
Other thermal properties are thermal diffusivity, thermal conductivity etc.
Combustion can be defined as rapid oxidation generating heat, or both light and
heat. It occurs in the presence of air (oxygen), fuel and a source of heat.
Combustion has a wide variety of uses. The
most relevant use with respect to this course
is the combustion of fuel which is used for
energy production in power plants, gas
turbines and engines.
Assignment
Figure 1 - 2 : Components of
combustion
The higher rate of demand of power on the farm with respect to human output,
necessitates periods of rest to be taken by the farm worker. The required rest
period can be estimated using the expression:
Where Tr is the required rest period in min/h and P is the actual rate of energy
consumption in W.
Generally, man performs the following functions in mechanisation:
In performing the function as a source of power man acts as an energy converter
by consuming the chemical energy in foods and converting it into output
mechanical work.
Human as an Energy Converter
Figure 2 - 1 : Human as an energy converter
Advantages
sources (animal & mechanical).
Disadvantages
The table below shows some actual manual energy expenditure (consumption)
rates for certain field operations.
Table 2 - 1 : Actual manual energy expenditure (consumption) rates for certain field
operations in Africa
Activity Gross Power Consumed (Watts)
Grass cutting 300
Clearing bush and shrub 400 - 600
Hoeing 300 - 550
Planting 200 - 300
Ridging or deep digging 400 - 1000
Tree feeling 600
Head panning 200 - 400
Ploughing with single axle tractor in
irrigated field
Rotovating with single axle tractor 350 - 500
Ploughing with single axle tractor upland 300 - 650
Animals are also able to provide power for both tractive (mobile) and stationary
operations. Some of these operations include ploughing or seedbed preparation as
well as other farm activities like water pumping, threshing, harvesting and
transportation. It is estimated that nearly 80% of the total draft power used in
agriculture throughout the world is still provided by animals. In Ghana, bullock
and now donkey farming is practiced in the grassland areas of the Savannah,
Northern, North East, Upper East and Upper West Regions. The average force a
bullock can exert is nearly equal to one tenth of its body weight. Power developed
by an average pair of bullocks is about 750 W for usual farm work.
2.2.1 Types of Draught Animals
The main species of draught animals include the:
Bovine species - cattle (ox, bullock, cow, buffalo)
Equine species – horse
Asinine species – donkey, mule
Cameline species – camel, dromedary
Others include llamas, yaks, elephants, etc.
and donkeys are strong on their breasts but the neck becomes weak. They are
therefore harnessed with straps and collars across the breasts.
Figure 2 - 2 : Harnessed pair of horses
Figure 2 - 3 : A plough hitched to a donkey
2.2.4 Yokes
A yoke is a wooden crosspiece to which the animal applies a pushing force from
the forehead or neck. There are three types of yokes discussed here which include
head yoke, neck yoke and withers yoke. The design of yokes vary from region to
region across the world. For instance, the tradition of the English was to use neck
yoke. Head yokes could be found in Austria, Germany and Switzerland. The
withers yoke can also be found on numerous continents and could originate from
India.
The head yoke requires the animals to lower their heads and push forward, with
their head and horns bearing the entire load. It is used on non-humped cattle and
buffalo. This type is not commonly used in Ghana. If a head yoke is used, the
cattle must have:
Figure 2 - 4 : Head yoke
The neck yoke , worn on top of the middle of the neck at rest, with a set of bows
around the animals’ necks, requires oxen to push with their shoulders, neck, and
chest.
Figure 2 - 5 : Neck yoke
The withers yoke have wooden staves and rope or leather to hold it in place. It
rests against the hump, in front of the withers of the animal, and the staves do not
interfere with the shoulders, but instead actually turn forward away from the
shoulder. If a withers yoke is used, then the cattle must have:
malnutrition.
Mechanical power technology is the highest form of mechanisation used in
agriculture. Mechanical power is available through tractors, power tillers and oil
engines. The oil engine is a highly efficient device for converting fuel into useful
work. The efficiency of diesel engine varies between 32 and 38%, whereas that
of the carburettor engine (Petrol engine) is in the range of 25 and 32%. In recent
years, diesel engines, tractors and power tillers have gained considerable
popularity in agricultural operations. Normally, stationery diesel engines are used
as power source for water pumps, flour mills, chaff cutter, sugarcane crusher,
threshers, winnowers, etc.
Advantages
Disadvantages
Electricity has become a very important source of power on farms nowadays.
Electrical power is used mostly for running electrical motors for pumping water,
dairy industry, cold storage, farm product processing, and cattle feed grinding. It
is a clean source of power and runs smoothly. The operating cost remains almost
constant throughout its life. Its maintenance and operation need less attention and
care.
Advantages
Disadvantages
It is the energy mainly obtained from nature. Biomass, biogas (Figure 2 - 7 ),
sunlight (solar) and wind are mainly used in agriculture for power generation and
various agricultural processing operations. They can be used for lighting, power
generation, water heating, drying, greenhouse heating, water distillation,
refrigeration and diesel engine operation. This type of energy is inexhaustible in
nature.
Figure 2 - 7 : Production of biogas
Advantages
2.5.1 Solar Power
Using the sun to dry crops and grain is one of the oldest and most widely used
applications of solar energy. The simplest and least expensive technique is to
allow crops to dry naturally in the field, or to spread grain and fruit out in the sun
after harvesting. The disadvantage of these methods is that the crops and grain
are subject to damage by birds, rodents, wind, and rain, and contamination by
windblown dust and dirt. In recent times, solar energy has been applied in other
pastures, where electricity from power lines is unavailable. PV is often much less
expensive than the alternative of extending power lines into these remote areas.
Most tropical countries are blessed with a plentiful sunshine all year round. Ghana
receives good amount of solar energy (especially in the Northern part) with an
average annual solar radiation of 16-229MJ/m
2
. There is thus plenty of potential
for the development of solar energy. Conditions are therefore ideal throughout the
country for the exploitation of solar energy. The sun’s rays are received in a
collector and transmitted to a heat engine, which converts the solar energy into
mechanical power to run a water or irrigation pump. Solar can be used for
processing fruits and vegetable and for general drying of crops. Another
application is in solar – operated pumps (Figure 2 - 9 ). This is, however, not a
common source of farm power in most developing countries.
Figure 2 - 9 : Typical arrangement of the components of a solar-powered pumping system
2.5.2 Wind Power
Wind is created by the unequal heating of the Earth’s surface by the sun. Wind
turbines (Figure 2 - 10 ) convert the kinetic energy in wind into mechanical power
that runs a generator to produce clean electricity. The wind turns the blade of wind
turbines, which spin a shaft connected to a generator that makes electricity.
The energy of the wind, like that of flowing water, is more or less limited for farm
use, chiefly because it cannot be controlled and is seldom available when needed.
Consequently, the use of wind power on the farm is confined largely to water
pumping, because whenever the wind blows, even if but once or twice a week,
enough water can be pumped and conveniently stored to last several days or until
the wind blows again. The power of the wind is made available by means of the
common windmill (Figure 2 - 10 ). The power developed by this device depends
primarily upon the size of wheel and the wind velocity. However, a number of
other factors such as the type of wheel, design of wheel and mill and height of
tower affect the performance of a windmill.
Figure 2 - 10 : Wind Turbine (left) and wind mill (right)
Where the wind velocity is more than 32 km/h, wind mills can be used for lifting
water. A wind mill having 3.6 m diameter wheel mounted on 12.0 m tower is able
to produce from 0.08 to 0.66 kW with the wind velocity varying from 6.4 to 37
km/h. Thus the average capacity of a wind mill would be above 0.37kW.
2.5.3 Water Power
The power developed by flowing water depends upon two factors, namely, the
volume of water flowing per minute and the head or vertical distance the water
drops at the point where the power installation is located. The former can be
measured either by the float method or by a weir. The head is determined by
measuring the difference in surface level before the water falls and after. Devices
used for converting water power into useful form are generally classed as either
water wheels or turbines. The energy available from water falling from one level
to a lower level can be harnessed to run a few farm operations, such as a feed
grinding, or to operate a generating plant to provide electricity.
Heat engine is a machine for converting heat, developed by burning fuel into
useful work or it is an equipment which generates thermal energy and transforms
it into mechanical energy. Heat engine is of two types:
i. External combustion engine, and
ii. Internal combustion engine.
It is an engine designed to derive its power from the fuel, burnt outside the engine
cylinder. Steam is created when water is turned into a vapour as it is heated by
heat from a combustion chamber. The steam expands in volume to generate a high
pressure which works against a piston or turbine to generate power. This power is
used to drive a locomotive, ship, etc. and also generate electricity. Examples are
the steam engine and thermal power plants. Figure 3-1 is a schematic diagram of
the steam engine of a locomotive. Because the combustion is outside the engine
cylinder, they can use solid fuels like wood and coal.
Figure 3 - 1 : Schematic diagram of a locomotive head with steam engine
It is the engine designed to derive its power from the fuel, burnt within the engine
cylinder. Here combustion of fuel and generation of heat takes place within the
cylinder of the engine. Examples of IC engine include engines used in modern
automobiles, tractors, mowers, etc.
3.2.1 Principle of I.C. Engine
A mixture of fuel with correct amount of air is exploded in an engine cylinder
which is closed at one end. As a result of the explosion, heat is released, and this
causes the pressure of the burning gases to increase. This increase in pressure
forces a close-fitting piston to move down the cylinder. This movement of piston
is transmitted to a crankshaft by a connecting rod so that the crankshaft turns a
flywheel. To obtain continuous rotation of the crankshaft this explosion has to be
repeated. Before this, the burnt gases have to be expelled from the cylinder. At the
same time a fresh charge of fuel and air must be admitted, and the piston must
return to its starting position. This sequence of events is known as working cycle.
For more efficient combustion of the fuel, the fuel is vaporised to ensure that as
many fuel molecules as possible contact enough air for the combustion to be as
complete as possible. Another important thing is how fast the fuel burns to
produce that “explosive” force to get the full power from the engine. This can be
achieved by the vaporisation of the fuel and a preheating of the fuel. Therefore,
how far the air is compressed (heating up in diesel engine), amount of fuel used,
and the volatility of the fuel are very important.
3.2.2 Operation of I.C. Engine
I.C. engine converts the reciprocating motion (up-and-down or back-and-forth
motion) of the piston into rotary motion of the crankshaft by means of a
connecting rod. The piston which reciprocates in the cylinder is very close fit in
the cylinder. Rings are inserted in the circumferential grooves of the piston to
prevent leakage of gases from sides of the piston. Usually, a cylinder is bored in
a cylinder block. A gasket, made of copper sheet or asbestos is inserted between
the cylinder and the cylinder head. The combustion space is provided at the top
of the cylinder head where combustion takes place. There is a rod called
connecting rod for connecting the piston and the crankshaft. A pin called gudgeon
pin or wristpin is provided for connecting the piston and the connecting rod of the
engine. The end of the connecting rod which fits over the gudgeon pin is called
small end of the connecting rod. The other end which fits over the crank pin is
called big end of the connecting rod. The crankshaft rotates in main bearings
which are fitted in the crankcase. A flywheel is provided at one end of the
crankshaft for smoothening the uneven torque, produced by the engine. There is
an oil sump at the bottom of the engine which contains lubricating oil for
lubricating different parts of the engine (Figure 3 - 2 ).
