Download Measurement of brake power

Introduction
 Engine performance is an indication of the degree
of success of the engine performs its assigned
task, i.e. the conversion of the chemical energy
contained in the fuel into the useful mechanical
work.
 The performance of an engine is evaluated on the
basis of the following;
(a) Specific Fuel Consumption.
(b) Brake Mean Effective Pressure.
(c) Specific Power Output.
(d) Specific Weight.
(e) Exhaust Smoke and Other Emissions.
Basic measurements
 The basic measurements to be undertaken to evaluate
the performance of an engine on almost all tests are the
following:
(a) Speed
(b) Fuel consumption
(c) Air consumption
(d) Smoke density
(e) Brake horse-power
(f) Indicated horse power and friction horse power
(g) Heat balance sheet or performance of SI and CI engine
(h) Exhaust gas analysis
Measurement of speed
 One of the basic measurements is that of speed. A wide
variety of speed measuring devices are available in the
market. They range from a mechanical tachometer to
digital and triggered electrical tachometers.
 The best method of measuring speed is to count the
number of revolutions in a given time. This gives an
accurate measurement of speed. Many engines are fitted
with such revolution counters.
 A mechanical tachometer or an electrical tachometer can
also be used for measuring the speed.
 The electrical tachometer has a three-phase permanent-
magnet alternator to which a voltmeter is attached. The
output of the alternator is a linear function of the speed and is
directly indicated on the voltmeter dial.
 Both electrical and mechanical types of tachometers are
affected by the temperature variations and are not very
accurate. For accurate and continuous measurement of speed
a magnetic pick-up placed near a toothed wheel coupled to
the engine shaft can be used. The magnetic pick-up will
produce a pulse for every revolution and a pulse counter will
accurately measure the speed.
Engine speed
Mechanical Tachometer
Electrical Tachometer
Engine speed
 Rope brake dynamometer also use for measuring engine
speed.
o A rope is wound around the circumference of the brake drum. One end of the rope
is attached with balance as shown in fig.
Rope brake
Dynamometer
Fuel consumption measurement
Fuel consumption is measured in two ways:
(a) The fuel consumption of an engine is measured by
determining the volume flow in a given time interval
and multiplying it by the specific gravity of the fuel
which should be measured occasionally to get an accurate
value.
(b) Another method is to measure the time required for
consumption of a given mass of fuel
Fuel Measurement
 As already mentioned two basic types of fuel measurement
methods are :
(1)Volumetric type:
Volumetric type flowmeter includes Burette
method, Automatic Burette flowmeter and Turbine flowmeter.
Turbine
Flowmeter
(2)Gravimetric type:
The efficiency of an engine is related to the kilograms of
fuel which are consumed and not the number of litres.
The method of measuring volume flow and then correcting
it for specific gravity variations is quite inconvenient and
inherently limited in accuracy.
Instead if the weight of the fuel consumed is directly
measured a great improvement in accuracy and cost can be
obtained.
There are three types of gravimetric type systems which are
commercially available include Actual weighing of fuel
consumed, Four Orifice Flowmeter, etc.
Measurement of air consumption
 In IC engines, the satisfactory measurement of air
consumption is quite difficult because the flow is
pulsating, due to the cyclic nature of the engine and
because the air a compressible fluid.
 Therefore, the simple method of using an orifice in the
induction pipe is not satisfactory since the reading will be
pulsating and unreliable.
 The various methods and meters used for air flow
measurement include,
(a) Air box method, and
(b) Viscous-flow air meter
Air supply to engine(carburetor)
 Generally u tube manometer is use to check inlet air
pressure and by equation air consumption also find out.
Measurement
of Air
by Air Box
Method.
Air Measurement:
 Measurement of Air Supply of an I.C. Engine:
Let a = area of orifice in m3.
Cd = Coefficient of discharge of the orifice.
ΔH= Difference of pressure as measured in cm.
of water.
Ma= Mass of one cubic metre of air, in kg.
Mw=Mass of one cubic metre of water, in kg.
H= Head causing flow through the orifice in m.
of air.
Air Measurement:
V= Velocity of air flowing through the orifice in metre
per sec.
Q= Discharge of air flowing through the orifice ,
in m3 per sec.
Now, consider one m3 of air at a pressure of ‘p’
N/m2 and absolute temperature ‘T’, Kelvin.
Then, applying gas=n
pv=Ma.RT
But v=1m3
∴
𝑝
Ma=
𝑅𝑇
Air Measurement:
𝑝
Ma=
287.𝑇
Now, H =
( ∵ R=287 J/kg-K) ----1
△𝐻 𝑀𝑤
X
100 𝑀𝑎
m of air
Also, V = Cd. 2𝑔𝐻
And
∴
Q= a.V
Q=a.Cd. 2𝑔𝐻
-----2
Now, using the equation 1 and 2 the mass of the air
supplied can be calculated as follows.
Mass of air supplied = Q.Ma
 The brake power measurement involves the
determination of the torque and the angular speed of the
engine output shaft.
 The torque measuring device is called a dynamometer.
 Dynamometers can be broadly classified into two main
types, power absorption
 dynamometers and transmission dynamometer.
 Figure 7.3 shows the basic principle of a dynamometer. A
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rotor driven by the engine
under test is electrically, hydraulically or magnetically
coupled to a stator. For every
revolution of the shaft, the rotor periphery moves through a
distance 2pr against the
coupling force F. Hence, the work done per revolution is .
W = 2 pRF
The external moment or torque is equal to S × L where, S is
the scale reading and L is the
arm. This moment balances the turning moment R × F, i.e.
S×L=R×F
\ Work done/revolution = 2p SL
Work done/minute = 2p SLN
where, N is rpm. Hence, power is given by
Brake power P = 2p NT
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Scale
S
Coupling
force
Stator
Rotor
Counter
balance
weight
L
R
Figure7.3 : Principle of a Dynamometer
Absorption Dynamometers
These dynamometers measure and absorb the power output of the engine to which
they are coupled. The power absorbed is usually dissipated as heat by some
means. Example of such dynamometers is prony brake, rope brake, hydraulic
dynamometer, etc.
Transmission Dynamometers
In transmission dynamometers, the power is transmitted to the load coupled to the
engine after it is indicated on some type of scale. These are also called
torque-meters.
Measurement of brake power
 The brake power measurement involves the determination
of the torque and the angular speed of the engine output
shaft. The torque measuring device is called a
dynamometer.
 Dynamometers can be broadly classified into two main
types,
1. Power absorption dynamometers and
2. Transmission dynamometer.
1. Absorption Dynamometers
 These dynamometers measure and absorb the power
output of the engine to which they are coupled. The
power absorbed is usually dissipated as heat by some
means.
 Example of such dynamometers is prony brake, rope
brake,
hydraulic
dynamometer,
eddy
current
dynamometer, etc.
2. Transmission Dynamometers
 In transmission dynamometers, the power is transmitted
to the load coupled to the engine after it is indicated on
some type of scale. These are also called torque-meters.
Prony brake dynamometer
 One of the simplest methods of measuring brake
power (output) is to attempt to stop the engine by
means of a brake on the flywheel and measure the
weight which an arm attached to the brake will
support, as it tries to rotate with the flywheel.
 It consists of wooden block mounted on a flexible rope
or band the wooden block when pressed into contact
with the rotating drum takes the engine torque and
the power is dissipated in frictional resistance. Springloaded bolts are provided to tighten the wooden block
and hence increase the friction.
 The whole of the power absorbed is converted into heat
and hence this type of dynamometer must the cooled.
The brake horsepower is given by
 BP = 2p NT
 where, T = W × l
 W being the weight applied at a radius l.
INDICATED POWER
 The power developed in the cylinder is known as
Indicated Horse Power and is designated as IP.
 The IP of an engine at a particular running condition is
obtained from the indicator diagram.
 The indicator diagram is the p-v diagram for one cycle at
that load drawn with the help of indicator fitted on the
engine. The construction and use of mechanical indicator
for obtaining p-v diagram is already explained.
 The areas, the positive loop
and negative loop, are
measured with the help of a
plani meter and let these be
Ap and An cm2 respectively,
the net positive area is (Ap –
An).
 h=(Ap-An)/L in centimetre.
 The height multiplied by
spring-strength (or spring
number) gives the indicated
mean effective pressure of
the cycle.
 Imep=(Ap-An)*S/L
 Pm=Ap*Sp/L-An*Sn/L
 Sp = Spring strength used for taking p-v diagram of
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positive loop, (N/m2 per cm)
Sn = Spring strength used for taking p-v diagram of
negative loop, (N/m2 per cm)
Ap = Area in Cm2 of positive loop taken with spring of
strength Sp
An = Area in Cm2 of positive loop taken with spring of
strength Sn
IP developed by the engine is given by
IP=PmLAn/L
FUEL CONSUMPTION
 Two glass vessels of 100cc and 200cc capacity are
connected in between the engine and main fuel tank
through two, three-way cocks. When one is supplying the
fuel to the engine, the other is being filled. The time for
the consumption of 100 or 200cc fuel is measured with
the help of stop watch.
 A small glass tube is attached to the main fuel tank as
shown in figure. When fuel rate is to be measured, the
valve is closed so that fuel is consumed from the burette.
 The time for a known value of fuel consumption can be
measured and fuel consumption rate can be calculated.
 Fuel consumption kg/hr = Xcc X Sp. gravity of fuel
1000 x t
Measurement of friction power
 The difference between indicated power and the brake
power output of an engine is the friction power.
 Almost invariably, the difference between a good engine
and a bad engine is due to difference between their
frictional losses.
 The frictional losses are ultimately dissipated to the
cooling system (and exhaust) as they appear in the form
of frictional heat and this influences the cooling capacity
required. Moreover, lower friction means availability of
more brake power; hence brake specific fuel
consumption is lower.
Willan’s Line Method
 This method is also known as fuel rate extrapolation
method.
 In this method a graph of fuel consumption (vertical axis)
versus brake power (horizontal axis) is drawn and it is
extrapolated on the negative axis of brake power.
 The intercept of the negative axis is taken as the friction power
of the engine at that speed.
 As shown in the figure, in most of the power range the relation
between the fuel consumption and brake power is linear when
speed of the engine is held constant and this permits
extrapolation. Further when the engine does not develop power,
i.e. brake power = 0,
 The main draw back
of this method is the
long distance to be
extrapolated from
data between 5 and
40 % load towards
the zero line of the
fuel input.
 The directional
margin of error is
rather wide because
the graph is not
exactly linear.
Morse Test
 The Morse test is applicable only to multi cylinder
engines.
 In this test, the engine is first run at the required speed
and the output is measured.
 Then, one cylinder is cut out by short circuiting the spark
plug or by disconnecting the injector as the case may be.
Heat balance sheet
 The performance of an engine is usually studied by
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heat balance-sheet. The main components of the
heat balance are:
Heat equivalent to the effective (brake) work of the
engine,
Heat rejected to the cooling medium,
Heat carried away from the engine with the exhaust gases,
and
Unaccounted losses.
 The heat supplied to the engine is only in the form of
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fuel-heat and that is given by
Qs = mf X CV
value of the fuel.
The various ways in which heat is used up in the system is
given by
(a) Heat equivalent of BP = kW = kJ/sec. = 0 kJ/min.
(b) Heat carried away by cooling water = Cpw X mw (Two
– Twi) kJ/min.
Where mw is the mass of cooling water in kg/min or kg/sec
circulated through the cooling.
 (c) Heat carried away by exhaust gases = mg Cpg (Tge –
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Ta) (kJ/min.) or (kJ/sec).
Where mg is the mass of exhaust gases in kg/min.
Tg = Temperature of burnt gases coming out of the
engine.
Ta = Ambient Temperature.
Cpg = Sp. Heat of exhaust gases in (kJ/kg-K)
 A part of heat is lost by convection and radiation as well as
due to the leakage of gases. Part of the power developed
inside the engine is also used to run the accessories as
lubricating pump, cam shaft and water circulating pump.
 These cannot be measured precisely and so this is known
as unaccounted ‘losses’. This unaccounted heat energy is
calculated by the different between heat supplied Qs and
the sum of (a) + (b) (c).
 The results of the above calculations are tabulated in a
table and this table is known as “Heat Balance Sheet”.
This method is also known as fuel rate extrapolation
method.
In this method a graph of fuel consumption (vertical
axis) versus brake power(horizontal axis) is drawn and it
is extrapolated on the negative axis of brake power (see
Fig. below).
The intercept of the negative axis is taken as the friction
power of the engine at that speed.
 As shown in the figure, in most of the power range the
relation between the fuel consumption and brake power is
linear when speed of the engine is held constant and this
permits extrapolation.
 Further when the engine does not develop power, i.e.
brake power = 0, it consumes a certain amount of fuel.
 This energy in the fuel would have been spent in
overcoming the friction.
 Hence the extrapolated negative intercept of
the
horizontal axis will be the work representing the
combined losses due to friction, pumping and as a
whole is termed as the frictional loss of the engine.
 This method of measuring friction power will hold good
only for a particular speed and is applicable mainly for
compression ignition engines.
 The main draw back of this method is the long distance
to be extrapolated from data between 5 and 40 % load
towards the zero line of the fuel input.
 The directional margin of error is rather wide because the
graph is not exactly linear.
Morse Test
 This method can be used only for multi – cylinder IC
engines.
 The Morse test consists of obtaining indicated power of
the engine without any elaborate equipment.
 The test consists of making, in turn, each cylinder of the
engine inoperative and noting the reduction in brake
power developed.
 In a petrol engine (gasoline engine),each cylinder is
rendered inoperative by “shorting” the spark plug of the
cylinder to be made inoperative.
 In a Diesel engine, a particular cylinder is made
inoperative by cutting off the supply of fuel.
 It is assumed that pumping and friction are the same
when the cylinder is inoperative as well as during firing.
 In this test, the engine is first run at the required speed
and the brake power is measured.
 Next, one cylinder is cut off by short circuiting the spark
plug if it is a petrol engine or by cutting of the fuel supply
if it is a diesel engine.
 Since one of the cylinders is cut of from producing power,
the speed of the engine will change.
 The engine speed is brought to its original value by
reducing the load on the engine.
 This will ensure that the frictional power is the same.
 If there are k cylinders, then
Total indicated power k when all the cylinders are working
= ip1 + ip2 + ip3 + …………...+ ipk =Σ ipj
We can write Σipj = Bt + Ft ………………………………..(1)
where ipj is the indicated power produced by j th cylinder, k
is the number of cylinders,