MECHANICAL ENGINEERING EXPERIMENTATION AND LABORATORY II EXPERIMENT ENGINE PERFORMANCE TEST

Transcription

1 MECHANICAL ENGINEERING EXPERIMENTATION AND LABORATORY II EXPERIMENT ENGINE PERFORMANCE TEST 1. Objectives To determine the variation of the brake torque, brake mean effective pressure, brake power, and brake specific fuel consumption of an automotive gasoline engine at constant throttle valve opening and variable engine speed. 2. Theoretical background 2.1 Types of test The test on combustion engines can be divided into two types: (1) Variable speed test Such as for automotive and marine engines. (2) Constant speed test Such as for generator and pump drive. Variable speed tests can be divided into full load test which is usually referred to as wide open throttle (WOT) test, where maximum power and minimum specific fuel consumption at each different speed are the objectives and part load tests for determining variations in the specific fuel consumption. The constant speed test is run mainly for determining the specific fuel consumption. For an automotive gasoline engine, the test is done by adjusting the brake or external load so that the engine is at the lowest desired speed while opening the throttle valve until it is wide (or fully) open and then leave it there. The ignition timing is adjusted (if manual) to give maximum torque at this speed. The engine is run for a period of time until approximate temperature equilibrium is reached. The fuel consumption is then measured. During this interval of time, the average speed, brake load, etc. are recorded. Record items include all data necessary to calculate the required results as well as all data necessary to reproduce the test. After the completion of this run, the brake or load is adjusted until the speed has changed by the desired amount while the spark is adjusted for maximum torque (unless automatic control of the spark is specified). Equilibrium conditions of temperature are again obtained and the procedure is repeated. 2.2 Performance characteristics (a) Power and mechanical efficiency The power from an engine is called brake power because the early type of loading device is called a prony brake. It is in fact the power measured directly at the output shaft and is sometimes called shaft power. The net power is measured with all engine components; the gross power allows removal of fan, exhaust pipe, alternator, etc. The power actually developed on the piston heads in the engine is called indicated power, since it is calculated from a p-v (pressure-volume) diagram obtained from a device called a spring indicator. A part of this indicated power is used for overcoming the friction of the moving parts of the engine and also in the induction of the fuel-air charge and the delivery of the exhaust gases. Therefore:

2 IP = BP + FP (2-1) The ratio of the power delivered by the engine to the total power developed within the engine is known as the mechanical efficiency, ηm; η = BP FP m 1 IP = IP (2-2) (b) Mean Effective Pressure (mep) The performance characteristic is defined as the theoretical constant (or mean) pressure which can be imagined exerted during each power stroke of the engine to produce power equal to the brake or indicated power. Therefore BP = Bmep x A x L x n x N P (2-3) where BP = brake power (kw) Bmep = brake mean effective pressure (kpa) A = piston head area (m 2 ) L = stroke (m) N = number of cylinder = number of power stroke per second per cylinder (1/sec) N P Similary IP = Imep x A x L x n x N P (2-4) where IP = indicated power Imep = indicated mean effective pressure Note that: np where N n R N = n R = engine speed (rev./sec) = number of engine rev. per (power stroke per cylinder) (rev.) = 2 for 4-stroke-engine (c) Torque and mep A general relation between torque and mep is P = 2πNT = mep x A x L x n x N P (2-5) where P T = power (kw) = torque (kn-m)

3 The equation is valid for either brake or indicated mean effective pressure depending on whether values of brake or indicated torque and power are substituted. Torque and power are not the good comparative performance indices because they depend on the size of the engine. Mean effective pressure may in fact be thought of as specific torque. Thus, one objective in designing good engine is to build engine with high mean effective pressure. (d) Specific Fuel Consumption and Thermal Efficiency If the test results show a consumption of m (gm) of fuel in t (minutes) at a power of P (kw), Then Fuel mass flow per unit power-hr = 60m Pt (gm/(kw-hr)) (2-6) This equation defines the specific fuel consumption and this may be either the brake or indicated specific fuel consumption depend on which power is used in the formula above. This specific fuel consumption is a comparative parameter that shows how efficiently an engine is converting fuel into work. This performance characteristic is more desirable than thermal efficiency, because all quantities are measured in standard and accepted physical units. While Power output Thermal efficiency, η H = (2-7) Rateof Heat or Energy Input In using this parameter, it becomes necessary to evaluate the heat of combustion (the heating value) of the fuel which is open to question. (e) Correction Factor The power output of the engine is directly related to atmospheric conditions; for example, low barometric pressure, high temperature and high humidity will reduce the engine output. Therefore, it is desirable to correct the engine performance to some standard environment. 3. Apparatus Fig. 1 Schematic Diagram of the Engine Test

4 1) The test set consists of 4 stroke, 4 cylinder automotive gasoline engine coupled directly to Heenan - Froude DPX-2 hydraulic dynamometer via cardan shaft. Fig. 2 Heenan Froude Hydraulic Dynamometer Fig. 3 Cross-section through casting of Heenan Froude Hydraulic Dynamometer

5 2) Engine control is at the left of the test bed. The throttle control can be locked in any of ten marked positions with the last position marking the fully open throttle, making the maximum air flow available to aspirate the maximum amount of fuel. WARNING!! If you open the throttle too wide without enough load on the output shaft, the engine speed will increase beyond the maximum allowable speed This can cause serious damage and injury. Removing heavy load off the engine output shaft without closing down the throttle will also give the same result. 3) Dynamometer operation: The dynamometer is used to put a load on the engine and absorb the power by churning up and heating the cooling water which flows between sets of moving and stationary vanes. The water supply should be fully opened before the engine is started. The load is increased by turning the hand wheel clockwise while watching the torque meter and tachometer. 4) Torque measurement: The dial must be set before the engine is started. In order to set it, you have to release the clamp at the back of the dial and adjust the handwheel at the top to bring the two pointers (between the spring and the heavy weight) together.then setting the hand on the dial to ZERO position by turning the adjusting screw at the top of the dial. The clamp is used to prevent creeping of the handwheel. When the engine starts there will be a reading under no load corresponding to friction and windage in the system. To take a reading under dynamic conditions, we firstly have to turn the handwheel above the dial to bring the two pointers in line. Then read the dial. 5) Speed measurement: Plug the digital revolution counter into the wall outlet. Close the switch to turn the unit on. Read RPM directly. 6) Fuel Supply: In normal operation, flow from the overhead fuel tank bypasses the measuring burette (see Fig. 4) When a measurement is to be made, firstly fill the burette by opening its vent valve to let the air out and the gasoline in. Then close the fuel line valve ahead of the tank so that the engine will only consume the fuel from burette. At the end of measurement open the fuel line valve, and close the vent valve to present overflow of fuel. Fig. 4 Fuel Measuring Burette

6 The burette has 3 divisions 50.0 cc: 50.0 cc: cc so that you have a choice of 0.050, and litre depending on what is necessary to give a measurement of time not less than 50 seconds. Given that the specific gravity of the fuel is References 1) OBERT, E.F.; Internal Combustion Engines & Air Pollution, Intext Educational Publishers, 1973, Chapter 2. 2) TAYLOR, C.F.; The Internal Combustion Engine in Theory & Practice, Vol.1, 2nd edition, MIT press, 1966, Chapter 12.

7 Supplement for Engine Performance

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31