18-10-2016, 10:03 AM
1459526951-ICEnginetesting.pdf (Size: 1.57 MB / Downloads: 5)
INTRODUCTION
At a design and development stage an engineer would design an engine with certain aims
in his mind. The aims may include the variables like indicated power, brake power,
brake specific fuel consumption, exhaust emissions, cooling of engine, maintenance free
operation etc. The other task of the development engineer is to reduce the cost and
improve power output and reliability of an engine. In trying to achieve these goals he has
to try various design concepts. After the design the parts of the engine are manufactured
for the dimensions and surface finish and may be with certain tolerances. In order verify
the designed and developed engine one has to go for testing and performance evaluation
of the engines.
Thus, in general, a development engineer will have to conduct a wide variety of engine
tests starting from simple fuel and air-flow measurements to taking of complicated
injector needle lift diagrams, swirl patterns and photographs of the burning process in
the combustion chamber. The nature and the type of the tests to be conducted depend
upon various factors, some of which are: the degree of development of the particular
design, the accuracy required, the funds available, the nature of the manufacturing
company, and its design strategy. In this chapter, only certain basic tests and
measurements will be considered.
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.,
• evaluate the exhaust smoke and exhaust emission, and
• differentiate between the performance of SI engine and CI engines.
PERFORMANCE PARAMETERS
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.
© Specific Power Output.
(d) Specific Weight.
(e) Exhaust Smoke and Other Emissions.
The particular application of the engine decides the relative importance of these
performance parameters.
For Example : For an aircraft engine specific weight is more important whereas for an
industrial engine specific fuel consumption is more important.
For the evaluation of an engine performance few more parameters are chosen and the
effect of various operating conditions, design concepts and modifications on these
parameters are studied. The basic performance parameters are the following :
(a) Power and Mechanical Efficiency.
(b) Mean Effective Pressure and Torque.
© Specific Output.
(d) Volumetric Efficiency.
(e) Fuel-air Ratio.
(f) Specific Fuel Consumption.
(g) Thermal Efficiency and Heat Balance.
(h) Exhaust Smoke and Other Emissions.
(i) Specific Weight.
Power and Mechanical Efficiency
The main purpose of running an engine is to obtain mechanical power.
• Power is defined as the rate of doing work and is equal to the product
of force and linear velocity or the product of torque and angular
velocity.
• Thus, the measurement of power involves the measurement of force
(or torque) as well as speed. The force or torque is measured with the
help of a dynamometer and the speed by a tachometer.
The power developed by an engine and measured at the output shaft is called the
brake power (bp) and is given by,
2
60
π
=
NT bp . . . (7.1)
where, T is torque in N-m and N is the rotational speed in revolutions per minute.
The total power developed by combustion of fuel in the combustion chamber is,
however, more than the bp and is called indicated power (ip). Of the power
developed by the engine, i.e. ip, 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.
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
© Air consumption
(d) Smoke density
(e) Brake horse-power
(f) Indicated horse power and friction horse power
(g) Heat going to cooling water
(h) Heat going to exhaust
(i) Exhaust gas analysis.
In addition to above a large number of other measurements may be necessary depending
upon the aim of the test.
7.3.1 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.
7.3.2 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.
Accurate measurement of fuel consumption is very important in engine testing work. IC Engine Testing
As already mentioned two basic types of fuel measurement methods are :
• Volumetric type
• Gravimetric type.
Volumetric type flowmeter includes Burette method, Automatic Burrette flowmeter and
Turbine flowmeter.
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 three types of gravimetric type systems which are commercially
available include Actual weighing of fuel consumed, Four Orifice Flowmeter, etc.
7.3.3 Measurement of Air Consumption
One can say the mixture of air and fuel is the food for an engine. For finding out the
performance of the engine accurate measurement of both is essential.
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.
All kinetic flow-inferring systems such as nozzles, orifices and venturies have a square
law relationship between flow rate and differential pressure which gives rise to severe
errors on unsteady flow. Pulsation produced errors are roughly inversely proportional to
the pressure across the orifice for a given set of flow conditions. The various methods
and meters used for air flow measurement include
(a) Air box method, and
(b) Viscous-flow air meter.
7.3.4 Measurement of Exhaust Smoke
All the three widely used smokemeters, namely, Bosch, Hartridge, and PHS are basically
soot density (g/m3
) measuring devices, that is, the meter readings are a function of the
mass of carbon in a given volume of exhaust gas.
Hartridge smokemeter works on the light extinction principle.
The basic principles of the Bosch smokemeter is one in which a fixed quantity of
exhaust gas is passed through a fixed filter paper and the density of the smoke stains on
the paper are evaluated optically. In a recent modification of this type of smokemeter
units are used for the measurement of the intensity of smoke stain on filter paper.
In Von Brand smokemeter which can give a continuous reading a filter tape is
continuously moved at a uniform rate to which the exhaust from the engine is fed. The
smoke stains developed on the filter paper are sensed by a recording head. The single
obtained from the recording head is calibrated to give smoke density.
7.4 MEASUREMENT OF EXHAUST EMISSION
Substances which are emitted to the atmosphere from any opening of the exhaust port of
the engine are termed as exhaust emissions. If combustion is complete and the mixture is stoichiometric the products of combustion would consist of carbon dioxide (CO2) and
water vapour only.
However, there is no complete combustion of fuel and hence the exhaust gas consists of
variety of components, the most important of them are carbon monoxide (CO), unburned
hydrocarbons (UBHC) and oxides of nitrogen (NOx). Some oxygen and other inert gases
would also be present in the exhaust gas.
Over the decade numerous devices have been developed for measuring these various
exhaust components. A brief discussion of some of the more commonly used instruments
is given below.
7.4.1 Flame Ionization Detector (FID)
The schematic diagram of a flame ionization detector burner is shown in Figures 7.1(a)
and (b) shows burner