The testing of I.C. Engines is the process of assessing the performance and operation of the engine in the efficient manner. There are various parameters which can be measured for testing of I.C. Engines. The testing of the engine is necessary for understanding the efficient operation of the engine and the engine components.
The operation of the new engine depends upon the surface finish, tolerance and lubrication & cooling system. The engine performance can be improved by adjustment of the various parameters and proper settings of all components of the engine.
The engine condition can deteriorates as the engine operates for length of the service and some components wear out. The condition of these components due to wear and tear can be assessed during testing and such components can be replaced by new components.
The testing of the engine provides improvement in the performance of the engine by increasing the efficiency and fuel economy of the engine. The continuous testing of engine can maintain fuel efficient operation of the engine and help in diagnosis of the failures of parts or components.
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During the testing of I.C. Engines, there are various instrument used for measurement of engine parameters. The testing results can be analyzed for knowing the performance of the engine.
The various parameters measured during the test are:
(i) Speed
(ii) Power
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(iii) Air consumption
(iv) Fuel consumption
(v) Efficiency of engine
(vi) Energy balance sheet
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(vii) Exhaust gas analysis
Testing of Constant Speed Internal Combustion Engines According to Indian Standard:
The Indian Standard code used is 1600-1960.
This code is used for testing of constant speed reciprocating internal combustion engines like:
(I) Compression ignition engines
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(II) Carburetor type engines or spark ignition engine
(III) Gas engines.
This code is not applicable to pressure-charged engines, engines for road or rail traction, engines for ship propulsion or for marine auxiliaries, and engines for aircraft propulsion or aircraft auxiliaries.
The engine shall be subjected to preliminary run of 49 hours at rated speed under operating temperatures us specified by the manufacturer in non-stop cycle of hours each, conforming to the following cycle, the period of each run being a minimum one cycle:
Performance Test According to Indian Standard:
After the preliminary run the performance test shall be conducted in accordance with the methods specified in IS 1601-1960:
The complete performance test for a variable speed engine will comprise of the following determinations at suitable speeds over the engine speed range of the engine specified by the manufacturer.
(1) At each suitable speed-
(i) Maximum brake power
(ii) Maximum torque
(iii) Frictional power.
(2) Specific fuel consumption at 100, 80, 60, 40, and 20 percent of the maximum load at the corresponding speed.
(3) Fuel consumption rate in grams per hour at recommended no load minimum speed.
In every test, a sufficient number of runs shall be made throughout the speed range. A run shall be made at the lowest steady speed at which the engine operates.
Performance data shall be obtained under stabilized operating conditions.
Durations of the experimental run depends upon two principles:
(a) No data shall be taken until load, speed and temperature have been stabilized.
(b) Recorded data shall be average sustained values maintained over a period of atleast one minute, with no significant change occur during that time.
Power Testing:
For all power tests with results to be plotted versus speed, a single series of stabilized runs at ascending speed is sufficient. This series of runs should progress continuously from the lowest to the maximum. If the engine requires to be idled between run to avoid excessively high temperature, sufficient time should be allowed for the engine again to reach to its stabilized condition before taking readings. The brake load recorded should be steady and constant throughout the run.
Speed Testing:
Engine speed should be held as constant as far as possible by means of applied dynamometer load at wide open throttle, or by throttle adjustment at part load.
Frictional Power:
The friction power shall follow immediately after the power test and if not possible then rest shall be conducted under conditions those for the power test.
Fuel Consumption:
Fuel consumption shall be measured simultaneously with brake power; The fuel consumption measurement shall not be started until the engine is stabilized. A measuring interval of not less than 60 seconds shall be used when measuring speed and fuel consumption. All specific fuel consumption figures shall be based upon observed brake power.
Besides the tests mentioned above various other general and special purpose tests are carried out on I.C. engines according to IS: 1602-1960, IS: 1603-1960, IS:10000-1980 and IS:7347-1974.
Measurement of Speed:
The speed of the engine can be measured at the crankshaft of the engine in revolution per minute (RPM).
The instruments used for measurement of speed are:
(1) Tachometer
(2) Stroboscope
(3) Sensor.
(1) Tachometer:
These are electro-mechanical devices in which output voltage is measured, which are proportional to the velocity of the shaft. It is a transducer which converts velocity of the shaft into an electronic signal.
The tachometers are of two types:
(i) Contact type
(ii) Non-contact type.
The contact type tachometers are used to measure the speed of shaft by making contact. The probe of tachometer is attached to the shaft and the speed is measured by making contact.
The non-contact type tachometers are used for measurement of speed. The speed of the rotating shaft is sensed by a sensor placed on the rotating shaft and the light emitted by tachometer.
(2) Stroboscope:
The device measures the speed by variable frequency flashing brilliant light. The variable frequency oscillator controls the frequency of flashing light. A spot is placed on the rotating object and light of variable frequency is adjusted on the moving object to show the spot to be stationary. The frequency of light which shows spot stationary of rotating object measures the speed of object.
(3) Sensor:
The sensors are used to measure the speed by digital measurement and non-contact type system. The sensors use electro-mechanical techniques and pickups to measure the speed.
These are two types of sensor:
(i) Photo-electric
(ii) Magnetic.
(i) Photo-Electric Pick-Up:
It consists of light sensor emitting rays on an opaque disc having holes arranged systematically along the periphery, which is mounted on the shaft. When the device rotates light rays passes through the holes of the disc and light sensor produces a pulse which is given to a digital counter. Fig. 14-1 shows the sketch of photo-electric pick-up.
(ii) Magnetic Pickup:
It consists of permanent magnet on which a coil is wound. A toothed metallic wheel is fitted to measure the speed of the device. The toothed wheel is made to pass the air gap of a permanent magnet. Every time when tooth passes the air gap, it changes reluctance and flux of the coil which creates EMF in the coil. Fig. 14-2 shows the magnetic pickups.
Measurement of Power:
The power is developed by the engine due to combustion of fuel and this power is transferred to the crank shaft. The power from the engine is transferred to the load connected to the engine.
The indicated power of the engine is the useful power produced by the engine after the combustion of the fuel in the engine cylinder. The heat energy produced by the combustion of the fuel is converted into useful work in the engine cylinder which is known as indicated power of the engine.
Brake power is mechanical power which is available at the engine crank-shaft. Some power will be lost in overcoming the internal friction of the engine which is known as frictional power. The brake power of the engine is produced at the shaft when the frictional power is extracted from the indicated power of the engine produced.
IP = BP + FP
Where,
IP = Indicated Power of the engine
BP = Brake Power of the engine
FP = Frictional power of the engine.
The indicated power of an engine is the power usually developed in its cylinders. The indicated power is always greater than the brake power of the engine, because there will always be some power losses between the cylinder and output shaft mainly due to friction between the moving parts of the engine and the pumping power needed to exhaust and re-charge the cylinder. The frictional losses also include the power needed to drive the essential engine auxiliaries.
The difference between the indicated power and the brake power is termed the friction power.
The indicated power can be measured by following methods:
(i) Engine indicator
(i) Morse test
(iii) Willian line method
(iv) Motoring test
(v) Cathode ray tube.
(i) Engine Indicator:
The indicated power of the internal combustion engine may be estimated when the following data are available:
(a) Area of the positive loop of the indicator diagram
(b) Area of the pumping loop or negative loop of the indicator diagram
(c) Spring scale for each indicator diagram
(d) Nature of the cycle (two stroke cycle or four stroke cycle)
(e) Dimensions of the engine
(f) Speed of the engine.
(ii) Measurement of Indicated Power by Morse Test:
One method by which a close estimate of the indicated power of a multi-cylinder internal combustion engine can be made is by means of the Morse test. In this method, the engine under test is coupled to a suitable dynamometers and the brake power is determined and let its value be B.
The first cylinder is now cut out either by shorting out the spark plug in case of petrol engines or by interrupting the fuel supply in case of oil engines. The load is adjusted to keep the speed constant. The brake power B1 under new condition is now determined from the new brake load.
The first cylinder operation is now re-introduced and the second cylinder is now cut-out. The engine speed is kept at its original value by adjusting the brake load, and the brake power B2 with the second cylinder cut-out is determined. This procedure is adopted for each cylinder in turn.
The fundamental assumption is that the friction and pumping power of the shorted cylinder remain the same after shorting as they were when the cylinder was fully operative. This assumption is valid as long as the speed remains constant as the friction power varies as some power of the speed. It needs and should only take a few seconds to cut out one cylinder and adjust the load to keep the speed constant.
(iii) Willian Line Method:
In this method the indicated power is measured using the indirect method of plotting the graph. This method is suitable for the diesel engine.
In this method, the fuel consumption is measured by running the engine at different loading condition. In case of diesel engines, the fuel consumption is directly proportional to the power produced by the engine. The graph can be plotted by measuring the power produced against the fuel consumed by the engine.
Fig. 14-6 shows the graph of power versus the fuel consumption. The fuel consumption is proportional to the power produced by the engine. The graph can extended in the x axis to meet x axis at point C. This is the frictional power of the engine as point C from origin. The indicated power can be known by adding frictional power to the brake power of the engine. The test should be conducted at constant speed.
(iv) Motoring Test:
In the motoring test, the engine is connected to the electric motor and the engine is run at constant speed using the motor. The power required to drive the engine is given by the power consumed by the motor. The power is consumed by the motor in overcoming the friction of the engine moving parts. This is frictional power of the motor. The indicated power can be calculated by adding frictional power to the brake power.
(v) CRT Method:
In this method, the pressure crank angle diagram is drawn on the cathode ray tube. The signal of the crank angle movement is given to X-axis of CRT by the pickups installed on the crank shaft. The variation of pressure in the engine cylinder is detected by the piezo electric pick up in the engine cylinder.
The signal is given to the CRT to plot the pressure variation on Y-axis. This is known as pressure crank angle diagram.
The Brake Power (B.P.) of an engine is the useful power available at the crankshaft of the engine. It is measured by running the engine against some form of absorption brake, which is known as Dynamometer. The brake power of the engine is measured at the steady state condition of the engine when the speed of engine is constant.
This is possible if the power produced by the engine is equal to the load applied on the engine. The load is applied on the engine by the instrument known as Dynamometer and engine fuel supply is adjusted to run the engine in the steady state conditions. Then the power is measured by the Dynamometer. Fig. 14-10 shows the brake power measurements.
Measurement of Air Consumption:
The air consumption of a liquid fuel engine may be measured for the following reasons:
(I) To determine the air-fuel ratio
(II) To obtain the total mass of exhaust gases
(III) To obtain the combustion chamber efficiency especially in the case of petrol engines running on a rich mixture
(IV) To determine the volumetric efficiency.
The air taken into the cylinder of an internal combustion engine can be estimated in various ways.
The following are a few methods which are commonly employed in the laboratory:
(a) Gasometer
(b) Viscous air flow meter
(c) An orifice meter and air box
(d) Exhaust gas analysis.
The measurement of air by gasometer is exact one but the method is cumbersome.
Exhaust gas analysis method assumes that all the carbon present in the fuel finally appears in the carbon dioxide or carbon monoxide and the flue gas analysis difference term is always nitrogen.
The orifice method of air measurement is not reliable on pulsating flows. With single cylinder engine the method is very unsatisfactory while fairly reliable results are obtained with multi-cylinder engines running at high speed.
Types of Meters:
The alcock viscous flow air meter is used for measurement of flow rates. The air flows through a form of honey comb so that the flow is viscous. The resistance of the element is directly proportional to the velocity of air. The pressure difference can be measured across the honey comb by use of inclined manometer.
The felt pads are fitted in the manometer connections to damp out the fluctuations. A damping vessel can be fitted between the meter and engine to reduce the effect of pulsations. Fig. 14-20 shows viscous air flow meter.
The air box meter uses a very large tank fitted with sharp edge orifice. The engine suction pipe is connected to the tank and the orifice is fitted on the opposite side of the tank. The size of box is 200 times the engine cylinder volume so that the pulsating flow required by the engine can be converted to steady flow by the use of air box.
The air is sucked continuously by the orifice and the air is supplied to the engine through the suction pipe intermitantly. Due to continuous flow of air across the orifice, there is pressure difference on the two sides of orifice. This pressure head can be measured by use of the manometer. The water manometer is normally used for measurement of pressure head across the orifice.
Let A = Area of the orifice, m2
d = diameter of orifice, m
Cd = Coefficient of discharge of orifice
hw = head of water measured across the orifice using manometer, m
pa = density of air
pw = density of water
head of air across the orifice = ha
ha = hw pw / pa
Mechanical and Volumetric Efficiency:
Mechanical Efficiency:
The mechanical efficiency is the ratio of the brake power to the indicated power of the engine at same speed expressed in terms of percentage.
ηm = Brake Power / Indicated Power x 100 = BP/IP x 100.
Volumetric Efficiency:
During the suction stroke of the cycle, a certain quantity of air or charge is taken into the engine cylinder. The actual volume of air drawn into the cylinder of the naturally aspirated engine is less than the volume swept by the piston.
The quantity of charge or air taken into the cylinder depends upon:
(1) The temperature of charge entering the cylinder.
(2) The back pressure of the gases in the engine cylinder, and
(3) The resistance to the flow of fresh charge into the cylinder through the inlet valves and ports.
The volumetric efficiency is the ratio of the mass of actual air sucked in the engine cylinder during suction process to the theoretical mass of air which can be admitted in the cylinder. The theoretical mass of air can be calculated at the engine conditions or ambient temperature or STP/NTP conditions.
If the volumetric efficiency is at the engine conditions, then it is the ratio of volume of charge per cycle drawn in during the suction stroke to the swept volume of piston is known as the volumetric efficiency of the engine. Both the volumes should be measured at the same temperature and pressure of the atmosphere surrounding the engine.
Fuel Consumption:
The fuel consumption of engines is usually measured and expressed in terms of fuel consumed by the engine in kg of fuel per hour. When the run of the engine is known and the fuel consumed by the engine is known then fuel consumption in kg per hour is calculated.
The fuel consumed by the engine is measured by using the burette or graduated tube supplying fuel to the engine cylinder. The time for given quantity of fuel consumed is measured using the stop watch. Fig. 14-23 shows the burette for measurement of fuel.
The volumetric efficiency can be measured on NTP and STP conditions. The theoretical mass of air can be calculated on NTP and STP condition by considering density of air of STP for NTP conditions.
The average value of this efficiency is from 70 to 80 percent for the naturally aspirated engines. In the case of supercharged engines the volumetric efficiency may be more than 100 percent when air at about atmospheric temperature is forced into the cylinder at a pressure greater than that of the air surrounding the engines.
Fuel Consumption:
The fuel consumption of engines is usually measured and expressed in terms of fuel consumed by the engine in kg of fuel per hour. When the run of the engine is known and the fuel consumed by the engine is known then the fuel consumption in kg per hour is calculated. The fuel consumed by the engine is measured by using the burette or graduated tube supplying fuel to the engine cylinder. The time for given quantity of fuel consumed is measured using the stop watch Fig 14.23 shows the burette for measurement of fuel.
Specific Fuel Consumption:
It is the fuel consumption of the engine per unit time for unit power produced by the engine. It is measured in kg per unit power in kW per hour. When the fuel consumption per hour is known and the power of the engine is known.
The fuel consumption is measured on the basis of brake power or indicated power.
The indicated specific fuel consumption is the fuel consumed by the engine per unit time per unit indicated power produced by the engine. It is measured as kg/kW-hr.
The brake specific fuel consumption is the fuel consumed by the engine per unit time per unit brake power produced by the engine. It is measured as kg/kW-hr. When a series of value of the second are plotted for the range of power output (no load to full load) for an engine, a curve will be obtained which will give the information at which brake power kW the engine should be run to give least consumption of fuel.
When the specific fuel consumption is least, the thermal efficiency will be the maximum. Heavy oil engine has a specific fuel consumption of 0.22 kg to 0.242 kg of fuel oil and the same value for high speed compression ignition engine is 0.212 kg per brake power in kW per hr.
Thermal Efficiency:
The thermal efficiency of the engine is an indicator of efficient operation of the engine. It indicates the amount of energy converted to actual work. The thermal efficiency can be calculated on the basis of indicated power or brake power.
i. Indicated Thermal Efficiency:
It is the ratio of the heat converted to actual work in the engine cylinder to the heat supplied due to combustion of fuel.
ii. Brake Thermal Efficiency:
It is the ratio of the heat converted to actual work at the crank shaft to the heat supplied due to combustion of fuel.
The brake thermal efficiency is always less than the indicated thermal efficiency and the relationship is given by –
ηthb = ηthi x ηm
Effect of Parameter on Efficiency:
The efficiency of I.C. engines derived by assuming working substance as air is known as the air standard efficiency. This is the ideal efficiency of the engine.
The actual thermal efficiency will be about 60% of the air standard efficiency for the following reasons:
(i) Properties of the actual working fluid reduce the pressures and temperatures round the cycle due to variation of specific heats, dissociation and changes in the number of moles present during combustion.
(ii) Compression and expansion are not adiabatic, heat being lost to the jacket water.
(iii) Combustion and heat rejection are not at constant volume since they require a definite time.
The actual work delivered by the engine will be less than the work developed during the power loop due to pump loop, friction and other mechanical losses. The brake thermal efficiency is, therefore, defined as the actual work delivered to the output shaft, so called brake work divided by the energy released during combustion of the fuel used. The brake thermal efficiency for automobile engines averages around 25% and has increased very little in the last sixty years.
In the engines working on Diesel cycle or dual cycle, only air is taken in during the suction stroke, therefore, the compression ratio is made higher than that in engine working on Otto cycle because there is no danger of pre-ignition and so by increasing the compression ratio say 11 to 22, a higher efficiency is obtainable in Diesel cycle than is possible with Otto cycle.
It should be noted that for the same compression ratio, the efficiency of the Otto cycle is greater than that of the Diesel engines.
Fig. 14-24 shows variation of power versus speed at full load. The power is maximum at particular speed means it gives highest efficiency fig. 14-25 shows variation of torque versus speed. The torque becomes maximum at particular speed. Fig. 14-26 shows the actual indicator diagram which shows power loop and pumping loop.
In the engines working on Diesel cycle or dual cycle, only air is taken in during the suction stroke, therefore, the compression ratio is made higher than that in engine Working on Otto cycle because there is no danger of pre-ignition and so by increasing the compression ratio say 11 to 22, a higher efficiency is obtainable in Diesel cycle than is possible with Otto cycle.
It should be noted that for the same compression ratio, the efficiency of the Otto cycle is greater than that of the Diesel engines.
Due to higher compression ratio, the temperature at the end of compression is sufficient to ignite the fuel oil which is injected into the cylinder at the end of compression stroke. Hence such engines are known as Compression ignition engine.
There are many other criteria besides thermal efficiency that are used to evaluate the performance and suitability of an engine. For instance a high specific power based on engine weight would be desirable in transport application, but this index of performance is really not too satisfactory, since it is a function of engine speed.
Increasing the engine speed to increase the power has the detrimental effect of increasing mechanical friction and inertia loads. A more meaningful index of performance which is frequently used to compare engine outputs is the mean effective pressure (m.e.p.) which is defined as the work performed during the cycle divided by the piston displacement volume.
The brake mean effective pressure of Diesel engines are currently running from 5 to 18.6 bar and have exceeded 27.6 bar experimentally. Diesel engines are used in a broad range of heavy duty applications because of its high efficiency and reliability.
These range from 30 MW versions for power generation and marine applications to 3 MW Diesel-Electric locomotives, which dramatically increased the thermal efficiency of locomotives from 10% to 35% when the conversion from steam was made.
Diesel engines are also used in a few low weight applications, including some automobiles. The smaller engines generally use engines employing Otto cycle because it results in a lighter and less expensive engine than the Diesel engines.
Effect of Parameters on Volumetric Efficiency:
In a modern un-supercharged engine the volumetric efficiency very rarely exceeds about 80%.
Its actual value, however, depends upon a variety of factors, among the more important of which are:
(1) Engine Speed:
The volumetric efficiency reduces with increase in the speed of the engine, because less time is available for filling of the cylinder at higher speeds. Fig. 14-35 shows the variation of speed versus volumetric efficiency.
(2) Compression Ratio:
The volumetric efficiency depends upon compression ratio because the clearance volume reduces with the compression ratio. The reduction in clearance volume effects the amount of exhaust gases in the clearance volume and this increase the volumetric efficiency of the engine. Fig. 14-36 shows variation CR with respect to volumetric efficiency.
(3) Inlet Charge Temperature:
The volumetric efficiency is effected by the inlet charge temperature. If the charge temperature is higher than the density of charge is less and mass of charge admitted in the cylinder is less. This reduces the volumetric efficiency of the engine. Fig. 14-37 shows the variation of inlet charge temperature versus volumetric efficiency.
(4) Mixture Strength:
The mixture strength slightly effects the volumetric efficiency of the engine. If the mixture is rich mixture then the large amount of exhaust gases are produce which reduces the volumetric efficiency.
If the mixture is lean then the less amount of exhaust gases are produced, these increase the volumetric efficiency. Fig. 14-38 shows the variation of volumetric efficiency versus mixture strength.
Other influencing factors are those of induction and exhaust system lay out, valve area, valve timing, throttle opening and cylinder temperature.