1. BASIC MEASUREMENTS

2. Measurement of Speed 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. 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.

3. 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. Fuel consumption kg/hr = X cc X Sp. gravity of fuel / 1000 x t (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. Two basic types of fuel measurement methods are : Volumetric type : Burette method, Automatic Burrette flow meter and Turbine flow meter. Gravimetric type : The efficiency of an engine is related to the kilograms of fuel which are consumed and not the number of litres.

4. Fuel Consumption Measurement Gravimetric Fuel Flow Measurement 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. The commercially available gravimetric fuel type systems are Actual weighing of fuel consumed and Four Orifice Flow meter etc.

5. Measurement of Air Consumption The various methods and meters used for air flow measurement include (a) Air box method, and (b) Viscous-flow air meter. (b) Viscous-flow air meter: An air flow meter is a device that measures air flow, i.e. how much air is flowing through a tube. It does not measure the volume of the air passing through the tube, it measures the mass of air flowing through the device per unit time. Alcock viscous-flow air meter is another design of air meter. It is not subjected to the errors of the simple types of flow meters. With the air-box the flow is proportional to the square root of the pressure difference across the orifice.

6. Measurement of Air Consumption Viscous-flow air meter: With the Alcock meter the air flows through a form of honeycomb so that flow is viscous. The resistance of the element is directly proportional to the air velocity and is measured by means of an inclined manometer. Felt pads are fitted in the manometer connections to damp out fluctuations. The meter is shown in fig.

7. Measurement of Air Consumption Viscous-flow air meter: The accuracy is improved by fitting a damping vessel between the meter and the engine to reduce the effect of pulsations.

8. MEASUREMENT OF HEAT CARRIED AWAY BY COOLING WATER The heat carried away by cooling water is generally measured by measuring the water flow rate through the cooling jacket and the rise in temperatures of the water during the flow through the engine. The heat carried away by cooling water is given by Q w = m w C p (T wo – T wi ) kJ/min. where m w = mass of water/min. T wi = Inlet temperature of water, o C T wo = Out let temperature of water, o C C p = Specific heat of water. The inlet and out let temperatures of the water are measured by the thermometers inserting in the pockets provided at inlet to and outlet from the engine. The quantity of water flowing is measured by collecting the water in a bucket for a specified period or directly with the help of flow meter in case of large engine.

9. 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) Absorption dynamometers and (b) Transmission dynamometer. below figure shows the basic principle of a dynamometer. A rotor driven by the engine under test is electrically, hydraulically or magnetically coupled to a stator.

10. MEASUREMENT OF BRAKE POWER 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 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 = 2πSL Work done/minute = 2πSLN where, Nis rpm. Hence, power is given by Brake power P = 2πNT

11. MEASUREMENT OF BRAKE POWER Absorption Dynamometers These dynamometers measure and absorb the power out put 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.

12. MEASUREMENT OF BRAKE POWER Absorption Dynamometers 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. This system is known as the prony brake and forms its use; the expression brake power has come. The Prony brake shown in below figure works on the principle of converting power into heat by dry friction.

13. MEASUREMENT OF BRAKE POWER