Unit-3.1 Testing and Performance.pptx

Testing and Performance
Unit 3
Internal Combustion Engine
B. Tech. VII Semester, 2020-21
Department of Mechanical Engineering
Swami Keshvanand Institute of Technology, M & G, Jaipur
Dr. Ashish Nayyar
Mr. Ankit Agarwal
Mr. Chandan Kumar

Objectives
After studying this unit, you should be able to
 understand the performance parameters in
evaluation of IC engine performance,
 calculate the speed of IC engine, fuel
consumption, air consumption, etc.,
 differentiate between the performance of SI
engine and CI engines
SKIT_DME_ICE_AN/AA/CK

Introduction
 Engine performance is an indication of the
degree of success of the engine performs its
assigned task.
 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.

The basic performance parameters are
the following :
 Power and Mechanical Efficiency.
 Mean Effective Pressure and Torque.
 Volumetric Efficiency.
 Fuel-air Ratio.
 Specific Fuel Consumption.
 Thermal Efficiency and Heat Balance.
 Relative efficiency
 Exhaust Smoke and Other Emissions.
 Specific Weight.

Power and Mechanical Efficiency
Brake Power
 The power developed by an engine and
measured at the output shaft is called the
brake power (BP) and is given by,
𝐵𝑃 =
2𝜋𝑁𝑇
60
Where, T is torque in N-m and N is the
rotational speed in revolutions per minute.

Indicated Power(IP)
The total power developed by combustion of fuel in the
combustion chamber is called indicated power (IP), however,
it is more than the BP.
It is due to
● some power is consumed in overcoming the friction
between moving parts,
● some in the process of inducting the air and removing the
products of combustion from the engine combustion
chamber.

 It is the power developed in the cylinder at the
top of the piston
𝐼𝑃 =
𝑃𝑖𝑚𝑒𝑝 × 𝐿 × 𝐴 × 𝑁 × 𝑘
60
where,
Pimep = Mean effective pressure, N/m2,
L = Length of the stroke, m,
A = Area of the piston, m2,
N = Rotational speed of the engine, rpm (It is
N/2 for four stroke engine), and
k = Number of cylinders.
Cont...

Mechanical Efficiency
 Mechanical Efficiency =
𝐵𝑃
𝐼𝑃
 The difference between IP and BP is
called friction power (FP).
FP = IP − BP
 Mechanical Efficiency=
𝐵𝑃
(𝐵𝑃+𝐹𝑃)
Cont...

Mean Effective Pressure and Torque
 Mean effective pressure is defined as a
hypothetical/average pressure which is assumed to
be acting on the piston through out the power
stroke.
𝑃𝑖𝑚𝑒𝑝 =
𝐼𝑃 × 60
𝐿𝐴𝑁𝑘
, 𝑃𝑏𝑚𝑒𝑝 =
𝐵𝑃 × 60
𝐿𝐴𝑁𝑘
 If the mean effective pressure is based on BP it is
called the brake mean effective pressure.
 If the mean effective pressure is based on IP it is
called the indicated mean effective pressure.

 The torque is related to mean effective pressure by
the relation
𝐵𝑃 =
2𝜋𝑁𝑇
60
𝐵𝑃 =
𝑃𝑏𝑚𝑒𝑝 × 𝐿𝐴𝑁𝑘
60
By equating these relations:
2𝜋𝑁𝑇
60
=
60
𝑇 =
𝑃𝑏𝑚𝑒𝑝 × 𝐴𝐿𝑘
2𝜋
Cont...

 Thus the torque and mean effective pressure are
related by the engine size.
 A larger engine produces more torque for the same
mean effective pressure.
 Higher the m.e.p., higher the power will be
developed by the engine for given displacement.
◦ Power of engine is dependent on size and speed, hence it
is not possible to compare engines on the basis of power
developed.
◦ Mean effective pressure is the true indication of the
relative performance of different engines.
Cont...

Specific Output
 Specific output of an engine is defined as the brake power (output)
per unit piston displacement and is given by,
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑂𝑢𝑡𝑝𝑢𝑡 =
𝐵𝑃
𝐴 × 𝐿
=
60 × 𝐴𝐿
=
𝑘
60
× 𝑃𝑏𝑚𝑒𝑝 × 𝑁 = 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × 𝑃𝑏𝑚𝑒𝑝 × 𝑁
 The specific output consists of two elements – the bmep (force)
available to work and the speed with which it is working.
 That means, for the same piston displacement and bmep an engine
running at higher speed will give more output.

Volumetric Efficiency
 Volumetric efficiency of an engine is an indication
of the measure of the degree to which the engine
fills its swept volume
Vol. 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑉𝑜𝑙. 𝑜𝑓 𝑎𝑖𝑟 𝑖𝑛𝑑𝑢𝑐𝑡𝑒𝑑 𝑎𝑡 𝑖𝑛𝑡𝑎𝑘𝑒
𝑆𝑤𝑒𝑝𝑡 𝑉𝑜𝑙𝑢𝑚𝑒
 The amount of air taken inside the cylinder is
dependent on the volumetric efficiency of an
engine
𝜂𝑣 =
𝐴𝑖𝑟 𝑡𝑎𝑘𝑒𝑛 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑
𝑆𝑤𝑒𝑝𝑡 𝑣𝑜𝑙𝑢𝑚𝑒 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑
=
𝐶𝑑𝐴0 2Δ𝑃/𝜌𝑎𝑖𝑟
𝐴𝑐 𝐿𝑁𝑘

Fuel-air Ratio
 Fuel-air ratio (F/A) is the ratio of the mass of
fuel to the mass of air in the fuel-air mixture.
 Relative fuel-air ratio is defined as the ratio of
the actual fuel-air ratio to that of the
stoichiometric fuel-air ratio required to burn the
fuel supplied
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑓𝑢𝑒𝑙 𝑎𝑖𝑟 𝑟𝑎𝑡𝑖𝑜, 𝐹𝑅 =
𝐴𝑐𝑡𝑢𝑎𝑙 𝑓𝑢𝑒𝑙 𝑎𝑖𝑟 𝑟𝑎𝑡𝑖𝑜
𝑠𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑢𝑒𝑙 𝑎𝑖𝑟 𝑟𝑎𝑡𝑖𝑜

Specific Fuel Consumption
 Specific fuel consumption is defined as the
amount of fuel consumed for each unit of
brake power developed per hour.
𝑆𝐹𝐶 =
𝑚𝑓
𝑃𝑜𝑤𝑒𝑟
𝐵𝑆𝐹𝐶 =
𝑚𝑓
𝐵𝑟𝑎𝑘𝑒 𝑃𝑜𝑤𝑒𝑟
𝐼𝑆𝐹𝐶 =
𝑚𝑓
𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑃𝑜𝑤𝑒𝑟

Thermal Efficiency
 Thermal efficiency of an engine is defined
as the ratio of the output to that of the
chemical energy input in the form of fuel
supply.
𝐵𝑟𝑎𝑘𝑒 𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝐵𝑃
𝑚𝑓 × 𝐶𝑉
where, CV = Calorific value of fuel, kJ/kg, and
mf = Mass of fuel supplied, kg/sec.

Relative efficiency or Efficiency ratio
 Relative efficiency or efficiency ratio is the ratio
of thermal efficiency of an actual cycle to ideal
cycle.
𝜂𝑟𝑒𝑙 =
𝐴𝑐𝑡𝑢𝑎𝑙 𝑡ℎ𝑒𝑟𝑚𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
𝐴𝑖𝑟 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
 It is the very useful criterion which indicates the
degree of development of the engine.

Specific weight
 Specific weight is defined as the weight of the
engine in kilogram for each brake power
developed and is an indication of the engine bulk.
 Specific weight plays an important role in
applications such as power plants for aircrafts.

Numerical

Numerical Example 3
 A six-cylinder, gasoline engine operates on the four-
stroke cycle. The bore of each cylinder is 80 mm and the
stroke is 100 mm. The clearance volume per cylinder is
70 cc. At the speed of 4100 rpm, the fuel consumption is
5.5 gm/sec. [or 19.8 kg/hr.) and the torque developed is
160 Nm.
 Calculate : (i) Brake power, (ii) The brake mean
effective pressure, (iii) Brake thermal efficiency if the
calorific value of the fuel is 44000 kJ/kg and (iv) The
relative efficiency on a brake power basis assuming the
engine works on the constant volume cycle r = 1.4 for
air.

Solution:

Basic Measurements
Measurement of Speed
 Measuring speed is to count the number of revolutions
in a given time.
Revolution counters are:
 Mechanical tachometer,
 Digital tachometer,
For accurate and continuous measurement of speed,
◦ a magnetic pick-up placed near a toothed wheel coupled to the
engine shaft can be used.
◦ It will produce a pulse for every revolution and a pulse counter
will accurately measure the speed.

Fuel Consumption Measurement
Fuel consumption is measured in two ways :
 Volumetric type: 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.
 Gravimetric type: to measure the time required for
consumption of a given mass of fuel.

Volumetric type Fuel Flow Measurement:
Volumetric type flow meter includes Burette method,
Automatic Burrette flow meter and Turbine flow meter.
Burette method:
 A glass burrette having bulbs of known volume and
having a mark on each side of the bulb.
 Time taken by the engine to consume this volume is
measured by stop watch.
 Volume divided by time will give the volumetric flow
rate.
Cont...

Gravimetric Fuel Flow Measurement:
 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 two types of gravimetric type systems as:
 Actual weighing of fuel consumed,
 Four Orifice Flowmeter
Cont...

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 box method
 For damping out pulsation of air is to fit an air box of
suitable volume (500 to 600 times the swept volume in
single cylinder engine & less in the case of multi cylinder
engine) to the engine with an orifice.
 Now initial velocity of air is zero. The velocity across orifice
is given by
𝐶 = (2𝑔ℎ𝑎𝑖𝑟)
Where, hair = differential head of air causing the flow. The
differential head is measured by water column and hence must
be converted into equivalent air column.
𝜌𝑎𝑖𝑟𝑔ℎ𝑎𝑖𝑟 = 𝜌𝑤𝑔ℎ𝑤 ⇒ 𝜌𝑎𝑖𝑟ℎ𝑎𝑖𝑟 = 𝜌𝑤ℎ𝑤 ⇒ ℎ𝑎𝑖𝑟
=
𝜌𝑤
𝜌𝑎𝑖𝑟
ℎ𝑤
Cont...

Now,
𝐶 = 2𝑔 ×
𝜌𝑤
𝜌𝑎𝑖𝑟
ℎ𝑤
Volume flow of air 𝑄 = 𝐴 2𝑔 ×
𝜌𝑤
𝜌𝑎𝑖𝑟
ℎ𝑤
Theoretical mass flow = 𝑄 × 𝜌𝑎 ⇒ 𝐴 2𝑔 ×
𝜌𝑤
𝜌𝑎𝑖𝑟
ℎ𝑤 × 𝜌𝑎
⇒ 𝐴 2𝑔 × 𝜌𝑤𝜌𝑎ℎ𝑤
Actual mass flow = 𝐶𝑑 × 𝐴 2𝑔 × 𝜌𝑤𝜌𝑎ℎ𝑤
Also we can express it in terms of pressure at orifice inlet i.e.
𝑝 = 𝜌𝑎𝑅𝑇 ⇒ 𝜌𝑎 =
𝑝
𝑅𝑇
Mass flow ⇒ 𝐶𝑑 × 𝐴 2𝑔 × 𝜌𝑤ℎ𝑤
𝑝
𝑅𝑇
Cont...

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,
(a) Power absorption dynamometers and
(b) Transmission dynamometer.
A rotor driven by the engine under test is electrically,
hydraulically or magnetically coupled to a stator.

 Figure shows the basic principle of a dynamometer.
 For every revolution of the shaft, the rotor periphery
moves through a distance 2𝜋R against the coupling force
F.
 Hence, the work done per revolution is W = 2𝜋RF
 The external moment or torque is equal to P × L where, P
is the scale reading and L is the arm.
 This moment balances the turning moment R × F, i.e.
P × L = R × F
Work done/revolution
= 2 𝜋 𝑃L
Work done/minute
= 2𝜋PLN
where, N is rpm.
Hence, power is given by
Brake power ⇒
BP = 2𝜋NT
P
Cont...

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.
Cont...

Measurement of Friction Power
 The brake power measurement involves the determination
the difference between indicated power and the brake
power output of an engine is the friction power.
 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.
 The bsfc rises with an increase in speed and at some speed
it renders the use of engine prohibitive.
 Thus, the level of friction decides the maximum output of
the engine which can be obtained economically.

The various methods to measurement of Friction Power are
as:
1. Willan's Line Method
2. Morse Test
3. Motoring Test
4. Difference between I.P & B.P
5. Retardation Test
Cont...

Willian's Line Method
• Graph between fuel consumption (gm/sec) v/s b.p at a
constant speed & variable load (5% to 40%) is plotted.
• Then the graph is extra plotted back to zero fuel
consumption.
• When this graph cuts
the b.p axis is an
indication of friction
power of engine at
that speed.
• This negative work
represents the
combined loss due to
mechanical friction,
pumping & blowby.
Cont...

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.
 Under this condition all other cylinders ‘motor’ this cut-out
cylinder.
 The output is measured by keeping the speed constant at its
original value.
 The difference in the outputs is a measure of the indicated
horse power of the cut-out cylinder.
 Thus, for each cylinder the ip is obtained and is added
together to find the total ip of the engine.
(IP)nth = (BP)nth + FP
Cont...

Motoring Test:- The engine is first run-up to the desired speed
by its own power and allowed to remain under the given
speed and load conditions for some time.
 The power of engine is absorbed by swinging field type
electric dynamometer.
 The fuel supply is then cut-off and by suitable electric-
switching devices the dynamometer is then converted to
run as a motor to drive for 'Motor' the engine at the same
speed at which it was previously running.
 The power supply to the motor is measured which is a
measure of the friction power of the engine. During the
motoring test the water supply is also cut-off so that the
actual operating temperatures are maintained.
Cont...

Measurement of Indicated Power
 There are two methods of finding the IP of an engine:
(a) By taking a indicator diagram with the help of a
suitable engine indicator by
 Pressure volume (p-v) plot
 Pressure crank angle (p-𝜃) plot
(b) By measuring brake power and friction power
separately and adding the two

Performance of SI engine
The performance of an engine is usually studied by 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.

Figure shows the
heat balance for a
petrol engine run
at full throttle over
its speed range.
Table gives the approximate percentage values of various losses in SI and CI engines.
Cont...

Figure shows the brake thermal efficiency, indicated thermal efficiency,
Mechanical efficiency and specific fuel consumption for the above SI
engine
Cont...

Performance of CI engine
 The bmep, bp and torque directly increase with load, as
shown in Figure (on next slide).
 Unlike the SI engine bhp and bmep are continuously
raising curves and are limited only by the load.
 The lowest brake specific fuel consumption and hence
the maximum efficiency occurs at about 80 percent of
the full load.

Performance Curves of a Six Cylinder Four-stroke Cycle Automotive Type
CI Engine at Constant Speed
Cont...

Testing of Engine
 Full throttle & variable performance
by Throttle Valve by Dynamometer
Curve between heat balance & rpm
Curve between 𝜂𝑚, 𝜂𝑖, 𝜂𝑏 & 𝑟𝑝𝑚
Curve between bsfc, ifsc & rpm
 Constant Speed & Variable load
by throttle valve by dynamometer
Curve between heat balance & BP
Curve between BP & 𝜂𝑚𝑡ℎ, 𝜂𝑖𝑡ℎ, 𝜂𝑏𝑡ℎ
Curve between BP & BSFC

Thank You…
I.C. Engine Laboratory
Department of Mechanical Engineering,
SKIT, Jaipur.

Unit-3.1 Testing and Performance.pptx

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