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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 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 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 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 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 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 – 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,