Basic Terminology used for a Engine

Basic Terminology used for a Engine
BORE AND STROKE
The diameter of the cylinder is known as bore and the displacement of the piston, i.e. its travel from
top dead centre (TDC) to bottom dead centre (BDC) is called stroke (Fig. 1). In old engines, the stroke
was always greater than the bore but the recent trend is towards a shorter piston stroke. This is
because in the short piston stroke, the less of power due to friction is minimized. Also, the inertia and
centrifugal load on the bearings are reduced. In the square engine, which is the latest in technology,
the bore and strokes are equal.
PISTON DISPLACEMENT (SWEPT VOLUME)
This is the volume that the piston displaces during its movement from BDC to TDC (Fig. 2).
Suppose D is the bore dia. and L is the stroke length. The piston displacement is given as
(π/4) D2L.
Example: For a four-cylinder engine with a 10 cm bore dia. and 8 cm stroke length, the piston
displacement (v) will be = (π/4) x 102 x 8 x 4= 2512cm3
COMPRESSION RATIO
The compression ratio (CR) of an engine is a measure of how much the air/air-fuel mixture is
compressed in the cylinder. It is the volume of air when the piston is at BDC divided by its
volume of air when the piston is at TDC. The volume above the piston is called clearance
volume.
The CR of an engine is an important factor in its performance. By increasing only the
compression ratio, the engine power increases, the other factors remaining unchanged.
Fig. 1. Bore and stroke of a heat engine
Fig. 2 Piston displacement and clearance volume
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HORSE POWER
Power is the rate at which work is done. The rate at which the engine can do work is measured in
horse power (HP). One HP is equivalent to 4500 kg m per min. The various methods of defining
horsepower are described below.
Indicated Horse Power
The amount of power that can be measured on the flywheel is always less than the power
generated in the engine on account of expansion of the combusted fuel. The power that is actually
developed in the cylinder is called indicated horse power and is given by:
where
P - means effective pressure in kg/cm2
L - stroke length in m
A - area of cylinder in cm2
N - power stroke per min (for a four stroke engine N = rpm/2 and for a two-stroke engine N =
rpm)
Brake Horse Power
It is the horsepower available on the crankshaft and is measured by a suitable dynamometer.
SAE Horse Power (Taxable Horse Power)
The SAE (Society of Automotive Engineers) horse power rating is used to compare engines on a
uniform basis, usually for tax purposes. The formula is
2.5
2 D N
HP =
where
D - diameter of cylinder in inches
N- number of cylinders
Belt Horse Power
It is the power of the engine. measured at the end of a suitable belt, receiving drive from the PTO
shaft of the tractor.
Power Take Off Horse Power
It is the power delivered by a tractor through its PTO shaft. In general the belt and PTO horse power
of a tractor will approximately be the same and is measured by either a hydraulic or an electrical
dynamometer.
Drawbar Horse Power
It is the power of a tractor measured at the end of the drawbar. It is that power which is available to
pull loads.
4500
PL A N
IHP =
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Maximum Horse power and Net Horse power
The maximum HP is measured at the engine flywheel without any of the power consuming
accessories being attached. This is not a practical rating as it does not represent "usable" HP.
Net HP is measured at the engine flywheel in the same manner as the maximum HP. The difference
in the two is because the engine is equipped with accessories. Net HP is the basis for rating the HP
of industrial and farm tractors.
Effect of Environment on Horse power.
HP is affected by barometric pressure and atmospheric temperature. Therefore, whatever actual
horsepower is observed on the dynamometer is called the observed HP, whereas the corrected HP is
the observed HP corrected to standard atmospheric conditions.
The standard operating conditions are:
1. Mean barometric pressure of 736 mm of mercury corresponding to an altitude of 300 m above
mean sea level.
2. Water vapour pressure of 27.4 mm of mercury corresponding to a relative humidity of 65 per
cent at 35°C.
3. Intake air temperature of 35°C.
For decreases in the atmospheric pressure, a deduction from the rated output of the engine shall
be made at the rate of 1.4 per cent per 100 m of altitude above 300 m. This de-rating is valid up to
an altitude of 2500 m.
Also, for any increase of the intake air temperature above 35°C, a further deduction shall be
made at the rate of 0.25 per cent per 0C.
MEAN EFFECTIVE PRESSURE
The mean effective pressure (MEP) is the average pressure during the power stroke, minus the
average pressure during the other three strokes (Fig. 3). In fact, the MEP is the pressure that
actually forces the piston down during the power stroke.
A- starting of suction stroke; B -start of compression; C-start of injection; D-start of
combustion; E-peak firing pressure; F-start of exhaust; G-ignition delay
Fig. 3 Typical diesel engine pressure indicator diagram:
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VOLUMETRIC EFFICIENCY
The amount of air entering the cylinder due to the vacuum created by the downward motion of the
piston is always less than the actual displacement of the piston because of the constriction of the air
intake system. Therefore, the actual air taken into the cylinder divided by the swept volume is known
as volumetric efficiency.
TORQUE
Like power, torque is another important measure of engine performance. Any force applied on some
point to cause a turning effect is called torque (T), mathematically represented by force (F) multiplied
by the distance of force from the centre of shaft (r), i.e.
T = F x r
In an engine, the piston applies a torque to the crankshaft through the connecting rod and crank when
it is moving down on the power stroke. The amount of torque depends on the pressure exerted by the
piston and the length of the crank arm. The greater the push on the piston, the greater the torque.
Torque should not be confused with power. Torque is the twisting effort that the engine
applies through the crankshaft, whereas power is the rate at which the engine does work. The unit of
torque is kg m.
RELATION OF HORSE POWER, SPEED AND TORQUE FOR ENGINES
An engine is capable of running faster than the speed at which it reaches maximum HP and HP
decreases after reaching its maximum point. The reason for this is that an engine is designed to
operate most efficiently up to a certain speed, depending upon its design.
Fig. 4 Torque compared with BHP
The HP curve continues to rise as the engine speed increases until maximum power is attained.
This is also true with the torque curve, but the torque curve will reach its maximum point much
earlier. An engine develops more torque at intermediate speeds than at the maximum speed. This
is because the volumetric efficiency is higher at intermediate speeds. Thus, there is a greater
amount of air to burn during the power stroke. This results in higher combustion pressure and
therefore, a greater torque is applied to the crankshaft. At higher speeds the volumetric efficiency
and combustion pressure are both lower and thus, the torque is also lower.
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In farm tractor engines, the rated operating speed may be as much as 800-1000 rpm higher than
the peak torque rpm. As the rated operating speed is higher than the peak torque speed, it
becomes possible for the engine to take advantage of the "torque reserve" when the engine is
loaded with an unexpected increase in the torque due to overloads. The operating speed in such a
state has a tendency to drop immediately. As the speed decreases towards the peak torque, the
speed torque increases and enables the engine to overcome the overload, thus maintaining its
power. If the overload is so great as to reduce the engine speed too below the peak torque speed,
the engine will lose its capacity to carry the overload. The engine speed will in such a case
continue to drop unless the load is reduced.
MEASUREMENT OF HORSE POWER
Dynamometers are used to measure the horse power. Dynamometers are classified as brake,
drawbar or torsion according to the manner in which the work is applied. Also, they may be
classed as absorption or transmission, depending on the nature of energy.