FARM POWER should be made between increasing the rotor area and increasing the
tower height to maximize the power output of the windmill. It is
Scientific definition for power is the rate at which work is done.
however important to note that certain topographical features
Work is expressed in Joule (J) and time is expressed in Seconds(s).
augment wind speed. Wind speed increases if wind is funneled into a
Power therefore is expressed in Joules per Second (J/s). A power of
valley and will also be slightly higher near the ground at the top of a
one Joule per second is equivalent to one Watt (W), the SI unit for
smooth hill. Wind power find large applications in pumping water in
power. Power may also be expressed in kilowatts (kW) or megawatts
ranches, irrigation systems and in homes. Windmill generators (aero
(MW). The common unit for expressing engine power is the
generators) are now widely used to convert wind energy in to
Horsepower (Hp). One horsepower is approximately equal to 746
electrical energy in rural areas. Wind power is
Watts. It is however important to note that farm power in this context
inexhaustible/renewable.
is taken to mean energy. Energy is the ability to do work. Energy is
expressed in Joules. Energy is required in the farm to do both tractive 3. Solar:
work (i.e. involving pulling or drawing) such as cultivation,
Solar energy from the sun comes in two forms; heat and light. These
harvesting, transportation, etc. and stationary work that involves
energies may be used in processing of crops (drying) and lighting up
powering from the PTO shaft or belt drive such as silage making,
farm structures. Light energy from the sun may be allowed into
feed grinding, threshing, winnowing, pumping water, etc.
structures such as a stores or machinery shed by installing
Sources of farm power transparent sheets during roofing. The light energy from the sun may
also be converted into electrical energy using photovoltaic cells. The
1. Animal:
electrical energy is then stored in accumulators as chemical energy.
Man is the most important source of power at the farm. Man can be The unique advantages of the photovoltaic cells have prompted great
the source of power, supervise work or control draft animals or interest in them, not only in industrialized nations but also in the
machinery. The following animals have been used by man to do developing nations. Photovoltaic cells have no waste products,
work; oxen, donkeys, horses, mules, camels, water buffalos and require little maintenance and operate almost indefinitely without
elephants. Animals may be used to carry luggage on their backs, degradation. The chemical energy stored in the accumulators is
draw a cart or pull plough. As long as the animals are kept in good converted into electrical energy to light the farm or power
health, this source of power is inexhaustible/renewable. The use of telecommunication, refrigeration and audio-visual equipment. It is
animals as a source of power is common in the developing world also used to power other small electrical appliances.
(i.e. Latin America, Asia and African countries). Animal power has
The heat energy from the sun may be used to dry farm produce by
gained popularity in these regions due to:
spreading the produce out in the sun or using various solar dryer
The farm sizes. systems. It can be used to heat water both for domestic and dairy
Land topography. work, water distillation and cooking using the various solar cookers.
Types of crops grown. Heat energy from the sun may also be used to produce steam by
Lack of suitable fuel and machines at reasonable costs. using large concave reflectors to concentrate heat onto a boiler. The
Initial cost of mechanical equipment is prohibitive. steam produced from the boiler is used to drive a turbine connected
Plentiful supply of low cost labour. to a dynamo to produce electricity. Solar energy is inexhaustible/
Advantages of animal power renewable.
(i) Easily available. 4. Water:
(ii) Capable of working on steep slopes and irregular terrains Water flowing from a higher to a lower level can be used to turn a
(iii) Used for all types of work. generator. This action is called hydroelectric generation. Large
(iv) Low initial investment cost. streams or rivers are dammed to create reservoirs to provide a
(v) Supplies manure to the field and fuels to farmer. controlled water flow. The hydroelectric generation system can also
(vi) Live on the farm. be installed at a water fall so that as the water falls from high to low
Disadvantages of animal power level, the water turns a turbine connected to a dynamo to produce
electricity. The amount of power derived from water depends on the
(i) Seasons and weather affect their efficiency. volume of water per second and the head (the vertical distance the
(ii) Their work output is low. water drops at the point where the power installation is located).
(iii) Require full maintenance when there is no work. Water has also been used for transport as well as driving mills.
(iv) Creates unhealthy atmosphere near the residence. Energy from flowing water is inexhaustible/renewable as long as the
(v) Slow in doing work. water catchment area for the rivers are conserved, otherwise is
2. Wind: exhaustible/non-renewable.
Wind power has not been extensively used because it is not easy to 5. Biomass:
control and is seldom available when most needed. Wind power is
harnessed using a windmill. The major factors responsible for the Biomass is plant materials and animal waste used as fuel. Biomass as
power output from a particular windmill are; the area of windmill a source of energy can be both exhaustible/non-renewable and
rotor, windmill rotor design, height of the windmill tower and wind inexhaustible/renewable depending on how it is managed. There are
speed. Doubling the area will double the power output while two major ways to obtain biomass for use as an energy source. This
doubling the wind speed increases the power output put eight times. can be either by collecting by-products or grow a crop for that
It is important that the windmill be positioned so as to take best specific purpose.
advantage of the prevailing wind. The wind speed is affected by Wood is probably the first fuel used by man. It is either used
topography (hills and valleys) and by surface roughness (buildings directly as fuel after natural drying or is transformed into
and tress). Surface roughness tends to reduce wind speed near the charcoal by burning the wood in limited air. Wood by-products
ground. Wind speed increases with elevation. A cost comparison such as barks and branches are obtained during wood
Eng. Wanzetse Moses (BSc. AGEN)
0725945231
, processing. Wood processing also produces large quantities of operational cost remains almost constant throughout its life and the
saw dust which can also be used as fuel. maintenance/operation needs less attention and care.
By-products from food processing industries such as bagasse
Advantages of electrical power
(the dry dusty pulp that remains after juice is extracted from
(i) Highly efficient.
sugar cane or similar plants) may be used as fuel.
(ii) Can work for long hours.
Crop residues such as straw from rice, wheat, barley, sorghum
(iii) Maintenance and operational costs are constant.
and maize provide good possibilities for energy production.
(iv) Not affected by weather.
Their conversion to energy is currently limited to alternative
uses such as for manure and animal feeds. Disadvantages of electrical power
Animal excreta are often mixed with agricultural by-products to (i) Initial capital investment is high.
form compost which is returned into the soil to improve on its (ii) Requires good amount of technical knowledge.
physical and chemical structure. However, in many developing (iii) Dangerous if not handled with care.
countries with serious wood shortages, cow dung and other non- 8. Heat engines (Mechanical Power):
dried excreta are burned as fuel.
A heat engine is a device that converts heat into mechanical energy
Unfortunately, the burning of biomass is the cause of a great deal of that can be used to do work. Many kinds of heat engines have been
pollution and has contributed to the so-called "greenhouse effect" invented like the external combustion engines that once drove
and as a consequence global warming. steamships and the current internal combustion engine that drives
Through fermentation, organic residues can be converted into liquid cars. Heat engines provide mechanical power in tractors, power
or gaseous fuels. The two most common means are the anaerobic tillers and several different kinds of stationary engines used for
fermentation and alcoholic fermentation to produce methane gas (a pumping water, driving flour mills, chaff cutters, threshers,
major component of natural gas) and ethyl alcohol (ethanol) winnowers, etc.
respectively. Advantages of heat engines
6. Fossil fuels: (i) They have high efficiency.
(ii) They are not affected by weather.
Fossil fuels are fuels formed by natural processes such as anaerobic
(iii) They require less space.
decomposition of buried dead organisms. Fossil fuels contain high
percentages of carbon and include coal, petroleum, and natural gas. Disadvantages of heat engines
Fossil fuels are exhaustible/no-renewable sources of power. (i) Initial capital investment is high.
(ii) High maintenance and running costs.
Coal is a combustible black or brownish-black sedimentary rock
(iii) Requires technical knowledge.
composed primarily of carbon along with variable quantities of
other elements such as hydrogen, sulphur, oxygen, and nitrogen. Factors considered when choosing a farm power
Coal may be used to provide heat for cooking or generation of (i) Scale of operation: The amount of work will determine the
steam to drive an engine or for electricity generation. choice of power source.
Petroleum products are useful materials derived from crude oil (ii) Timeliness of the operation: Some operations require great
(petroleum) as it is processed in oil refineries. Petroleum is timing and must be finished within a specified time limit.
converted to petroleum products through a refining process to (iii) Financial requirement: Power sources require financial
produce several classes of fuels. These include: investment; some are fairly cheap while others require heavy
o liquefied petroleum gas (LPG) financial investment in purchasing and maintaining.
o aviation gasoline (iv) Technical knowhow: Power sources require skills to use and
o motor gasoline maintain for maximum efficiency. Consider the skills an
o jet fuels operator requires to use and maintain a tractor and the skills that
o kerosene would be required to use draft animals.
o diesel other oil products (v) Nature of land: The land topography (slope, unevenness, etc.)
or the presence of obstacles (Tree stumps or stones) will
Natural gas is associated with liquid petroleum; it either occurs
determine the choice of power source. Animal power would be
above the liquid petroleum in the earth or dissolved in it. Natural
ideal for very sloppy lands as a tractor is likely to lose stability.
gas is a major source of electricity generation. It also capable of
(vi) Availability: The presence or absence of a source of power is
generating powerful heat for domestic cooking and heating fuel. In
the first step to choosing a source of power.
much of the developed world it is supplied to homes via pipes
(vii) The nature of the soil. The nature of the soil may have also a
where it is used for ovens, natural gas-heated clothe dryers, and
direct influence on the power source for a farm. Heavy soils
heating or cooling. Compressed natural gas (CNG) is used in rural
(e.g. clays) are very hard in dry weather and this is when land
homes without pipe connections.
preparation is recommended. This demands that if a tractor is to
7. Mains electricity: be bought, then it has to be of a high horsepower to be able to
pull the plough.
Electricity is a type of energy associated with electron flow.
Electricity generation is the process of generating electrical power (viii) The nature of the crop: The crop under cultivation
from other sources of primary energy. Electrical power is used influences the choice of power. If the tractor will be required to
mostly for running electrical motors for pumping water, dairy weed, then a row-crop tractor is recommended.
industry, cold storage, farm product processing and cattle feed
Ergonomic considerations in farm power and machinery design
grinding. It is also used by electric lamps for producing light, electric
and selection
heaters or cookers to produce heat and to operate electrical
equipment. It is a clean source of power that runs smoothly. Its Ergonomics is the study of designing equipment and devices that fit
the human body and its cognitive abilities. The two terms "human
Eng. Wanzetse Moses (BSc. AGEN)
0725945231
,factors" and "ergonomics" are essentially synonymous. The πd
2
International Ergonomics Association defines ergonomics or human ¿
xl
factors as; the scientific discipline concerned with the understanding 4
of interactions among humans and other elements of a system, and Where d =diameter of bore , l=Stroke length
the profession that applies theory, principles, data and methods to
2
design in order to optimize human well-being and overall system 22 x 100
performance. Hence , Piston displacement ( PD)= x 124
7x4
When using animal power, the harnessing yokes should also have 3
cushions at all points of contact with the animal body. ¿ 974 285.71 mm
Design proper work attire and gear.
Designing tools that promote good working posture. 974 285.71 3
¿ cm
Machines should have smooth and rounded edges. 1000
Increasing machine automation.
Vehicles (Tractors) should have enclosed cabin with air ¿ 974.3 cm3
conditioning system.
Vehicles (Tractors) should have seats with cushions and 1
(ii) Clearance volume ( CV ) = xPiston displacem ent
suspended on springs or hydraulics reduce vibrations felt by the 6
operator.
1
HEAT ENGINES ¿ x 974.3 cm3
6
Engine terminologies 3
¿ 162.4 cm
1. Bore: This is the diameter of the cylinder. (iii)
2. Stroke: Thisis the maximum length of travel of the piston from
one extreme position to the otherin one direction. Total cylinder volume=Pistondisplacement +Clerance v
3
3. Top Dead Centre (TDC): This is the position of the piston at ¿( 947.3+162.4)cm
the end of its travel when moving towards the cylinder head. 3
4. Bottom Dead Centre (BDC): This is the position of the piston ¿ 1 109.7 cm
at the end of its travel when moving towards the crank case.
5. Piston displacement (PD): This is the volume displaced by the (iv)
piston when it moves from TDC to BDC. It is also known as the Total cylinder volume
swept volume. Compressionratio=
6. Clearance volume (CV): Thisis the space or volume between Clearance volume
the top of the piston and the engine cylinder head when the 1109.7
piston is at TDC. It is also called combustion chamber. ¿
162.4
By default, clearance volume is a sixth of piston displacement:
1 ¿ 6.833
Cv= xPD How engine power is developed
6
7. Total Cylinder volume (TCV):This is the sum of the piston Although engine systems vary with different fuels, all develop power
displacement and the clearance volume. in the same manner. For an engine to operate, a definite series of
TVC =PD +CV events must take place. These are;
Fill the combustion cylinder with combustible mixture (SI
8. Compression ratio(CR):The ratio of total cylinder volume to engines) or only air (CI engines).
clearance volume Compress the fuel-air mixture or air only in the combustion
TCV cylinder into a smaller space.
CR= Ignite the mixture by introducing a spark (SI engines) or
CV spraying fuel (CI engines) causing expansion in the combustion
Alte rnatively , CR=TCV :CV cylinder which pushes the piston hence producing power.
9. Engine size:This is given by the bore and stroke of the engine e.g. Drain the combustion cylinder off the burned gas in readiness to
a 100mm x 124mm engine has a bore of 100mm and a 124mm refilling it with a fresh charge.
stroke.
Aspects affecting an engine’s power output
Example
1. Displacement: Other factors constant, engine output increases
Given a one cylinder engine of size 100mm x 124mm, determine: with engine displacement. A given displacement can be obtained
(i) The piston displacement with fewer cylinders of greater individual displacements or several
(ii) Clearance volume smaller cylinders of smaller individual displacements.
(iii) The total cylinder volume and 2. Compression ratio:Other factors held constant, a higher
(iv) The compression ratio compression ratio results in a higher engine output. A high
compression ratio is desirable because it allows an engine to
Solution extract more mechanical energy from a given mass of air-fuel
mixture due to its higher thermal efficiency.
(i)
Piston displacement=Volume swept by the piston∈one stroke
3. Gas flow: Since the cylinder must be filled, it is important that the
fuel-air mixture (SI engine) or air (CI engine) is able to flow in
Eng. Wanzetse Moses (BSc. AGEN)
0725945231
, freely through the intake system into the combustion cylinder. In 3. Compression ratio: Diesel engines have compression ratios of
naturally aspirated engines, the only moving force is the partial between 15:1 and 20:1, while petrol engines have compression
vacuum created by the intake stroke of the piston. It is therefore ratios of between 5.5:1 and 7:1. The higher compression ratio in
important that the intake system be shaped and arranged so that it diesel engines raises the temperature of the air to about 600 oC
does not retard the flow. The intake valve must be large and open which is well above the flash point of the atomized diesel. This
wide enough to permit free flow. Equally important, free flow gives diesel more efficiency as the higher compression ratio
through the exhaust system is desirable if the burnt gases are to be results in greater expansion of gases in the cylinder following
removed efficiently. combustion. The result is a more powerful stroke.
4. Engine speed: In a 4-stroke engine, there is only one power stroke 4. Design of engine parts: Engine parts for petrol engine are
in the 4-stroke cycle while in the 2-stroke engine; there is one generally lighter in weight compared to those for diesel engine.
power stroke in the 2-stroke cycle. The engine power output Diesel engines have beefed up parts such as the piston pins, piston
therefore tends to increase with engine speed. rods, crank shafts and bearing to support the crank shaft.
5. Grade and type of fuel: Petrol engine only use petrol, while
Expresing engine power
diesel engines also uses diesel. It is not possible to interchange the
The horsepower is the unit of measure for rating engine capacities. fuels.
Categories of power are necessary for conveying information about
Comparison of 4-stroke and 2-stroke engines.
an engine’s performance. The most common ones include:
A 4-stroke engine is one where one power cycle of the engine is
Indicated horsepower: The total power developed by all
completed in two revolutions of the crankshaft or four strokes of the
cylinders and received by pistons without friction and losses in the
piston.
engine. It is the maximum theoretical power output of the engine
developed based on gas pressure occurring in the cylinder after Intake stroke: This occurs when the piston is moving from the
combustion. It is a value of primary interest to engine designers. TDC to BDC. The intake valve is open while the exhaust valve is
Friction horsepower: This is the power required to run the engine closed. In SI engines, fuel air mixture enters the cylinder while in
at a given speed without producing any useful work. It represents CI engines, air only enters.
the friction and pumping losses of the engine. This friction
represents power loss and heat production.
Flywheel/Brake horsepower: This is the power output at the
engine flywheel and is the maximum power obtained from an
engine. It is the power delivered by an engine and is available at
the end of the crank shaft at the engine flywheel. It is measured
using a suitable dynamometer.
Draw bar horsepower: The power developed at the hitch or draw
bar available for pulling, dragging or similar tractive effort.
PTO horsepower: This is power measures at the PTO shaft while
the engine is operating under a load. It is a function of torque and
speed measured in revolutions per second (rps) or revolutions per
minute (rpm). There is usually some gear reduction between Compression stroke: Occurs immediately after the intake stroke
engine speed and PTO shaft. The reduction increases torque value when the piston moves from the BDC to TDC. At the start of the
but reduces speed. compression stroke, the intake valve closes and the exhaust valve
Rated horsepower: This is used by engine manufacturers to remains closed. The piston moves to TDC compressing the air fuel
indicate the power an engine should be expected to produce under mixture (in SI engine) or only air (in CI engine).
normal operating conditions. Rated power is not necessarily the
operation point for best fuel consumption. It is expressed in term
of recommended power and speed.
Classification of internal combustion engines
Internal combustion engines may be classified based on the fuels
used as diesel also known as compression ignition (CI) engine or
petrol also known as spark ignition (SI) engine. They may also be
classified based on the number of strokes made by the piston in one
power cycle as 4-stroke or 2-stroke engine.
Comparison between diesel and petrol engines.
1. Fuel supply: In petrol engines, fuel and air are mixed outside the Power/Expansion stroke: This occurs immediately after the
engine (in the carburettor) before moving into the combustion compression stroke. Power is developed from the fuel-air mixture.
chamber. In Diesel engines there is no pre-mixing of fuel and air Fuel is sprayed into the cylinder (in CI engine) or spark is
outside the combustion cylinder. Only air is taken into the produced by a spark plug (in SI engine) at the end of the
combustion cylinder, compressed and then sprayed with diesel. compression stroke. Both the intake and exhaust valves remain
2. Ignition: Petrol engine depend on a spark produced by a spark closed. The fuel-air mixture burns, expands forcing the piston to
plug to ignite the fuel-air mixture after compression. The diesel move from the TDC to BDC.
engines use the heat derived from compressed air in the cylinder to
cause ignition when a diesel spray is introduced from the injector
nozzle.
Eng. Wanzetse Moses (BSc. AGEN)
0725945231
tower height to maximize the power output of the windmill. It is
Scientific definition for power is the rate at which work is done.
however important to note that certain topographical features
Work is expressed in Joule (J) and time is expressed in Seconds(s).
augment wind speed. Wind speed increases if wind is funneled into a
Power therefore is expressed in Joules per Second (J/s). A power of
valley and will also be slightly higher near the ground at the top of a
one Joule per second is equivalent to one Watt (W), the SI unit for
smooth hill. Wind power find large applications in pumping water in
power. Power may also be expressed in kilowatts (kW) or megawatts
ranches, irrigation systems and in homes. Windmill generators (aero
(MW). The common unit for expressing engine power is the
generators) are now widely used to convert wind energy in to
Horsepower (Hp). One horsepower is approximately equal to 746
electrical energy in rural areas. Wind power is
Watts. It is however important to note that farm power in this context
inexhaustible/renewable.
is taken to mean energy. Energy is the ability to do work. Energy is
expressed in Joules. Energy is required in the farm to do both tractive 3. Solar:
work (i.e. involving pulling or drawing) such as cultivation,
Solar energy from the sun comes in two forms; heat and light. These
harvesting, transportation, etc. and stationary work that involves
energies may be used in processing of crops (drying) and lighting up
powering from the PTO shaft or belt drive such as silage making,
farm structures. Light energy from the sun may be allowed into
feed grinding, threshing, winnowing, pumping water, etc.
structures such as a stores or machinery shed by installing
Sources of farm power transparent sheets during roofing. The light energy from the sun may
also be converted into electrical energy using photovoltaic cells. The
1. Animal:
electrical energy is then stored in accumulators as chemical energy.
Man is the most important source of power at the farm. Man can be The unique advantages of the photovoltaic cells have prompted great
the source of power, supervise work or control draft animals or interest in them, not only in industrialized nations but also in the
machinery. The following animals have been used by man to do developing nations. Photovoltaic cells have no waste products,
work; oxen, donkeys, horses, mules, camels, water buffalos and require little maintenance and operate almost indefinitely without
elephants. Animals may be used to carry luggage on their backs, degradation. The chemical energy stored in the accumulators is
draw a cart or pull plough. As long as the animals are kept in good converted into electrical energy to light the farm or power
health, this source of power is inexhaustible/renewable. The use of telecommunication, refrigeration and audio-visual equipment. It is
animals as a source of power is common in the developing world also used to power other small electrical appliances.
(i.e. Latin America, Asia and African countries). Animal power has
The heat energy from the sun may be used to dry farm produce by
gained popularity in these regions due to:
spreading the produce out in the sun or using various solar dryer
The farm sizes. systems. It can be used to heat water both for domestic and dairy
Land topography. work, water distillation and cooking using the various solar cookers.
Types of crops grown. Heat energy from the sun may also be used to produce steam by
Lack of suitable fuel and machines at reasonable costs. using large concave reflectors to concentrate heat onto a boiler. The
Initial cost of mechanical equipment is prohibitive. steam produced from the boiler is used to drive a turbine connected
Plentiful supply of low cost labour. to a dynamo to produce electricity. Solar energy is inexhaustible/
Advantages of animal power renewable.
(i) Easily available. 4. Water:
(ii) Capable of working on steep slopes and irregular terrains Water flowing from a higher to a lower level can be used to turn a
(iii) Used for all types of work. generator. This action is called hydroelectric generation. Large
(iv) Low initial investment cost. streams or rivers are dammed to create reservoirs to provide a
(v) Supplies manure to the field and fuels to farmer. controlled water flow. The hydroelectric generation system can also
(vi) Live on the farm. be installed at a water fall so that as the water falls from high to low
Disadvantages of animal power level, the water turns a turbine connected to a dynamo to produce
electricity. The amount of power derived from water depends on the
(i) Seasons and weather affect their efficiency. volume of water per second and the head (the vertical distance the
(ii) Their work output is low. water drops at the point where the power installation is located).
(iii) Require full maintenance when there is no work. Water has also been used for transport as well as driving mills.
(iv) Creates unhealthy atmosphere near the residence. Energy from flowing water is inexhaustible/renewable as long as the
(v) Slow in doing work. water catchment area for the rivers are conserved, otherwise is
2. Wind: exhaustible/non-renewable.
Wind power has not been extensively used because it is not easy to 5. Biomass:
control and is seldom available when most needed. Wind power is
harnessed using a windmill. The major factors responsible for the Biomass is plant materials and animal waste used as fuel. Biomass as
power output from a particular windmill are; the area of windmill a source of energy can be both exhaustible/non-renewable and
rotor, windmill rotor design, height of the windmill tower and wind inexhaustible/renewable depending on how it is managed. There are
speed. Doubling the area will double the power output while two major ways to obtain biomass for use as an energy source. This
doubling the wind speed increases the power output put eight times. can be either by collecting by-products or grow a crop for that
It is important that the windmill be positioned so as to take best specific purpose.
advantage of the prevailing wind. The wind speed is affected by Wood is probably the first fuel used by man. It is either used
topography (hills and valleys) and by surface roughness (buildings directly as fuel after natural drying or is transformed into
and tress). Surface roughness tends to reduce wind speed near the charcoal by burning the wood in limited air. Wood by-products
ground. Wind speed increases with elevation. A cost comparison such as barks and branches are obtained during wood
Eng. Wanzetse Moses (BSc. AGEN)
0725945231
, processing. Wood processing also produces large quantities of operational cost remains almost constant throughout its life and the
saw dust which can also be used as fuel. maintenance/operation needs less attention and care.
By-products from food processing industries such as bagasse
Advantages of electrical power
(the dry dusty pulp that remains after juice is extracted from
(i) Highly efficient.
sugar cane or similar plants) may be used as fuel.
(ii) Can work for long hours.
Crop residues such as straw from rice, wheat, barley, sorghum
(iii) Maintenance and operational costs are constant.
and maize provide good possibilities for energy production.
(iv) Not affected by weather.
Their conversion to energy is currently limited to alternative
uses such as for manure and animal feeds. Disadvantages of electrical power
Animal excreta are often mixed with agricultural by-products to (i) Initial capital investment is high.
form compost which is returned into the soil to improve on its (ii) Requires good amount of technical knowledge.
physical and chemical structure. However, in many developing (iii) Dangerous if not handled with care.
countries with serious wood shortages, cow dung and other non- 8. Heat engines (Mechanical Power):
dried excreta are burned as fuel.
A heat engine is a device that converts heat into mechanical energy
Unfortunately, the burning of biomass is the cause of a great deal of that can be used to do work. Many kinds of heat engines have been
pollution and has contributed to the so-called "greenhouse effect" invented like the external combustion engines that once drove
and as a consequence global warming. steamships and the current internal combustion engine that drives
Through fermentation, organic residues can be converted into liquid cars. Heat engines provide mechanical power in tractors, power
or gaseous fuels. The two most common means are the anaerobic tillers and several different kinds of stationary engines used for
fermentation and alcoholic fermentation to produce methane gas (a pumping water, driving flour mills, chaff cutters, threshers,
major component of natural gas) and ethyl alcohol (ethanol) winnowers, etc.
respectively. Advantages of heat engines
6. Fossil fuels: (i) They have high efficiency.
(ii) They are not affected by weather.
Fossil fuels are fuels formed by natural processes such as anaerobic
(iii) They require less space.
decomposition of buried dead organisms. Fossil fuels contain high
percentages of carbon and include coal, petroleum, and natural gas. Disadvantages of heat engines
Fossil fuels are exhaustible/no-renewable sources of power. (i) Initial capital investment is high.
(ii) High maintenance and running costs.
Coal is a combustible black or brownish-black sedimentary rock
(iii) Requires technical knowledge.
composed primarily of carbon along with variable quantities of
other elements such as hydrogen, sulphur, oxygen, and nitrogen. Factors considered when choosing a farm power
Coal may be used to provide heat for cooking or generation of (i) Scale of operation: The amount of work will determine the
steam to drive an engine or for electricity generation. choice of power source.
Petroleum products are useful materials derived from crude oil (ii) Timeliness of the operation: Some operations require great
(petroleum) as it is processed in oil refineries. Petroleum is timing and must be finished within a specified time limit.
converted to petroleum products through a refining process to (iii) Financial requirement: Power sources require financial
produce several classes of fuels. These include: investment; some are fairly cheap while others require heavy
o liquefied petroleum gas (LPG) financial investment in purchasing and maintaining.
o aviation gasoline (iv) Technical knowhow: Power sources require skills to use and
o motor gasoline maintain for maximum efficiency. Consider the skills an
o jet fuels operator requires to use and maintain a tractor and the skills that
o kerosene would be required to use draft animals.
o diesel other oil products (v) Nature of land: The land topography (slope, unevenness, etc.)
or the presence of obstacles (Tree stumps or stones) will
Natural gas is associated with liquid petroleum; it either occurs
determine the choice of power source. Animal power would be
above the liquid petroleum in the earth or dissolved in it. Natural
ideal for very sloppy lands as a tractor is likely to lose stability.
gas is a major source of electricity generation. It also capable of
(vi) Availability: The presence or absence of a source of power is
generating powerful heat for domestic cooking and heating fuel. In
the first step to choosing a source of power.
much of the developed world it is supplied to homes via pipes
(vii) The nature of the soil. The nature of the soil may have also a
where it is used for ovens, natural gas-heated clothe dryers, and
direct influence on the power source for a farm. Heavy soils
heating or cooling. Compressed natural gas (CNG) is used in rural
(e.g. clays) are very hard in dry weather and this is when land
homes without pipe connections.
preparation is recommended. This demands that if a tractor is to
7. Mains electricity: be bought, then it has to be of a high horsepower to be able to
pull the plough.
Electricity is a type of energy associated with electron flow.
Electricity generation is the process of generating electrical power (viii) The nature of the crop: The crop under cultivation
from other sources of primary energy. Electrical power is used influences the choice of power. If the tractor will be required to
mostly for running electrical motors for pumping water, dairy weed, then a row-crop tractor is recommended.
industry, cold storage, farm product processing and cattle feed
Ergonomic considerations in farm power and machinery design
grinding. It is also used by electric lamps for producing light, electric
and selection
heaters or cookers to produce heat and to operate electrical
equipment. It is a clean source of power that runs smoothly. Its Ergonomics is the study of designing equipment and devices that fit
the human body and its cognitive abilities. The two terms "human
Eng. Wanzetse Moses (BSc. AGEN)
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,factors" and "ergonomics" are essentially synonymous. The πd
2
International Ergonomics Association defines ergonomics or human ¿
xl
factors as; the scientific discipline concerned with the understanding 4
of interactions among humans and other elements of a system, and Where d =diameter of bore , l=Stroke length
the profession that applies theory, principles, data and methods to
2
design in order to optimize human well-being and overall system 22 x 100
performance. Hence , Piston displacement ( PD)= x 124
7x4
When using animal power, the harnessing yokes should also have 3
cushions at all points of contact with the animal body. ¿ 974 285.71 mm
Design proper work attire and gear.
Designing tools that promote good working posture. 974 285.71 3
¿ cm
Machines should have smooth and rounded edges. 1000
Increasing machine automation.
Vehicles (Tractors) should have enclosed cabin with air ¿ 974.3 cm3
conditioning system.
Vehicles (Tractors) should have seats with cushions and 1
(ii) Clearance volume ( CV ) = xPiston displacem ent
suspended on springs or hydraulics reduce vibrations felt by the 6
operator.
1
HEAT ENGINES ¿ x 974.3 cm3
6
Engine terminologies 3
¿ 162.4 cm
1. Bore: This is the diameter of the cylinder. (iii)
2. Stroke: Thisis the maximum length of travel of the piston from
one extreme position to the otherin one direction. Total cylinder volume=Pistondisplacement +Clerance v
3
3. Top Dead Centre (TDC): This is the position of the piston at ¿( 947.3+162.4)cm
the end of its travel when moving towards the cylinder head. 3
4. Bottom Dead Centre (BDC): This is the position of the piston ¿ 1 109.7 cm
at the end of its travel when moving towards the crank case.
5. Piston displacement (PD): This is the volume displaced by the (iv)
piston when it moves from TDC to BDC. It is also known as the Total cylinder volume
swept volume. Compressionratio=
6. Clearance volume (CV): Thisis the space or volume between Clearance volume
the top of the piston and the engine cylinder head when the 1109.7
piston is at TDC. It is also called combustion chamber. ¿
162.4
By default, clearance volume is a sixth of piston displacement:
1 ¿ 6.833
Cv= xPD How engine power is developed
6
7. Total Cylinder volume (TCV):This is the sum of the piston Although engine systems vary with different fuels, all develop power
displacement and the clearance volume. in the same manner. For an engine to operate, a definite series of
TVC =PD +CV events must take place. These are;
Fill the combustion cylinder with combustible mixture (SI
8. Compression ratio(CR):The ratio of total cylinder volume to engines) or only air (CI engines).
clearance volume Compress the fuel-air mixture or air only in the combustion
TCV cylinder into a smaller space.
CR= Ignite the mixture by introducing a spark (SI engines) or
CV spraying fuel (CI engines) causing expansion in the combustion
Alte rnatively , CR=TCV :CV cylinder which pushes the piston hence producing power.
9. Engine size:This is given by the bore and stroke of the engine e.g. Drain the combustion cylinder off the burned gas in readiness to
a 100mm x 124mm engine has a bore of 100mm and a 124mm refilling it with a fresh charge.
stroke.
Aspects affecting an engine’s power output
Example
1. Displacement: Other factors constant, engine output increases
Given a one cylinder engine of size 100mm x 124mm, determine: with engine displacement. A given displacement can be obtained
(i) The piston displacement with fewer cylinders of greater individual displacements or several
(ii) Clearance volume smaller cylinders of smaller individual displacements.
(iii) The total cylinder volume and 2. Compression ratio:Other factors held constant, a higher
(iv) The compression ratio compression ratio results in a higher engine output. A high
compression ratio is desirable because it allows an engine to
Solution extract more mechanical energy from a given mass of air-fuel
mixture due to its higher thermal efficiency.
(i)
Piston displacement=Volume swept by the piston∈one stroke
3. Gas flow: Since the cylinder must be filled, it is important that the
fuel-air mixture (SI engine) or air (CI engine) is able to flow in
Eng. Wanzetse Moses (BSc. AGEN)
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, freely through the intake system into the combustion cylinder. In 3. Compression ratio: Diesel engines have compression ratios of
naturally aspirated engines, the only moving force is the partial between 15:1 and 20:1, while petrol engines have compression
vacuum created by the intake stroke of the piston. It is therefore ratios of between 5.5:1 and 7:1. The higher compression ratio in
important that the intake system be shaped and arranged so that it diesel engines raises the temperature of the air to about 600 oC
does not retard the flow. The intake valve must be large and open which is well above the flash point of the atomized diesel. This
wide enough to permit free flow. Equally important, free flow gives diesel more efficiency as the higher compression ratio
through the exhaust system is desirable if the burnt gases are to be results in greater expansion of gases in the cylinder following
removed efficiently. combustion. The result is a more powerful stroke.
4. Engine speed: In a 4-stroke engine, there is only one power stroke 4. Design of engine parts: Engine parts for petrol engine are
in the 4-stroke cycle while in the 2-stroke engine; there is one generally lighter in weight compared to those for diesel engine.
power stroke in the 2-stroke cycle. The engine power output Diesel engines have beefed up parts such as the piston pins, piston
therefore tends to increase with engine speed. rods, crank shafts and bearing to support the crank shaft.
5. Grade and type of fuel: Petrol engine only use petrol, while
Expresing engine power
diesel engines also uses diesel. It is not possible to interchange the
The horsepower is the unit of measure for rating engine capacities. fuels.
Categories of power are necessary for conveying information about
Comparison of 4-stroke and 2-stroke engines.
an engine’s performance. The most common ones include:
A 4-stroke engine is one where one power cycle of the engine is
Indicated horsepower: The total power developed by all
completed in two revolutions of the crankshaft or four strokes of the
cylinders and received by pistons without friction and losses in the
piston.
engine. It is the maximum theoretical power output of the engine
developed based on gas pressure occurring in the cylinder after Intake stroke: This occurs when the piston is moving from the
combustion. It is a value of primary interest to engine designers. TDC to BDC. The intake valve is open while the exhaust valve is
Friction horsepower: This is the power required to run the engine closed. In SI engines, fuel air mixture enters the cylinder while in
at a given speed without producing any useful work. It represents CI engines, air only enters.
the friction and pumping losses of the engine. This friction
represents power loss and heat production.
Flywheel/Brake horsepower: This is the power output at the
engine flywheel and is the maximum power obtained from an
engine. It is the power delivered by an engine and is available at
the end of the crank shaft at the engine flywheel. It is measured
using a suitable dynamometer.
Draw bar horsepower: The power developed at the hitch or draw
bar available for pulling, dragging or similar tractive effort.
PTO horsepower: This is power measures at the PTO shaft while
the engine is operating under a load. It is a function of torque and
speed measured in revolutions per second (rps) or revolutions per
minute (rpm). There is usually some gear reduction between Compression stroke: Occurs immediately after the intake stroke
engine speed and PTO shaft. The reduction increases torque value when the piston moves from the BDC to TDC. At the start of the
but reduces speed. compression stroke, the intake valve closes and the exhaust valve
Rated horsepower: This is used by engine manufacturers to remains closed. The piston moves to TDC compressing the air fuel
indicate the power an engine should be expected to produce under mixture (in SI engine) or only air (in CI engine).
normal operating conditions. Rated power is not necessarily the
operation point for best fuel consumption. It is expressed in term
of recommended power and speed.
Classification of internal combustion engines
Internal combustion engines may be classified based on the fuels
used as diesel also known as compression ignition (CI) engine or
petrol also known as spark ignition (SI) engine. They may also be
classified based on the number of strokes made by the piston in one
power cycle as 4-stroke or 2-stroke engine.
Comparison between diesel and petrol engines.
1. Fuel supply: In petrol engines, fuel and air are mixed outside the Power/Expansion stroke: This occurs immediately after the
engine (in the carburettor) before moving into the combustion compression stroke. Power is developed from the fuel-air mixture.
chamber. In Diesel engines there is no pre-mixing of fuel and air Fuel is sprayed into the cylinder (in CI engine) or spark is
outside the combustion cylinder. Only air is taken into the produced by a spark plug (in SI engine) at the end of the
combustion cylinder, compressed and then sprayed with diesel. compression stroke. Both the intake and exhaust valves remain
2. Ignition: Petrol engine depend on a spark produced by a spark closed. The fuel-air mixture burns, expands forcing the piston to
plug to ignite the fuel-air mixture after compression. The diesel move from the TDC to BDC.
engines use the heat derived from compressed air in the cylinder to
cause ignition when a diesel spray is introduced from the injector
nozzle.
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