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I. C. ENGINES By Deepa M S Varuna Tandon Dept. of Aeronautical Engineering MVJCE 81

I. C. ENGINE TESTING INTRODUCTION: The basic task in the design and development of engines is to reduce the cost and improve the efficiency and power output. In order to achieve the above task, the ‘development engineer’ has to compare the engine developed with other 80

I. C. ENGINE TESTING INTRODUCTION: -engines in terms of its output and efficiency. Towards this end he has to test the engine and make measurements of relevant parameters that reflect the performance of the engine. I. C. engine generally operates within a useful 79

I. C. ENGINE TESTING INTRODUCTION: -range of speed. Some engines are made to run at fixed speed by means of speed governor, which is its rated speed. The performance of the engine depends on the interrelationship between the power developed, speed and the specific fuel 78

I. C. ENGINE TESTING INTRODUCTION: -consumption at each operating condition within the useful range of speed and load. 77

I. C. ENGINE TESTING The following factors are to be considered in evaluating the performance of an engine: (i) Maximum power or torque available at each speed within the useful range of speed. (ii) The range of power output at constant speed for stable operation of the engine. The different speeds should be related at equal intervals within the useful speed range. 76

I. C. ENGINE TESTING (iii) Brake specific fuel consumption at each operating condition within the useful range of operation. (iv) Reliability and durability of the engine for the given range of operation. 75

I. C. ENGINE TESTING Purpose of Testing an I. C. Engine: In general the purpose or significance of testing an I. C. engine is to determine the following: (i) To determine rated power output with respect to the fuel consumption in Kg per Kw-hr of brake power output. 74

I. C. ENGINE TESTING Purpose of Testing an I. C. Engine: (ii) To determine the mechanical and thermal efficiencies of the engine. (iii) To see the performance of the engine when loaded at different loads. 73

I. C. ENGINE TESTING Purpose of Testing an I. C. Engine: (iv) To determine the quantity of lubricating oil required per bp Kw hr. (v) To determine the quantity of cooling water required per bp Kw hr. (vi) To determine the overload carrying capacity of the engine. (vii) To prepare the heat balance sheet of the engine. 72

I. C. ENGINE TESTING Some Important Terms as per ISI Standard: 1. Speed : The speed of an engine is the mean speed of its crank shaft in revolutions per minute (RPM), except in case of ‘free piston’ engines where the speed is the number of cycles per minute , of the reciprocating components. 2. Steady Load Speed Band: It is the maximum total variation in speed expressed as a %age of the mean speed, which may occur while there is no change in external load conditions. 71

I. C. ENGINE TESTING Some Important Terms as per ISI Standard: 3. Continuous Power: The power which the engine is capable of delivering continuously between the normal maintenance intervals stated by the manufacturer, at stated speed and under stated operating conditions. 4. Indicated Power: It is the total power developed in the working cylinder by the gases on the combustion side of the working pistons. 5. Friction Power: It is the power consumed in frictional resistance. 70

I. C. ENGINE TESTING Some Important Terms as per ISI Standard: 6. Brake Power: It is the total power measured at the driving shaft. 7. Fuel Consumption: The quality of fuel consumed by the engine per unit time of the stated power and under stated operating conditions. 8. Specific Fuel Consumption: It is the quantity of fuel consumed per unit of power per unit of time. It is generally expressed in gms of fuel consumed per k. W hr or B. H. P. /bp. 69

I. C. ENGINE TESTING Some Important Terms as per ISI Standard: 9. Standard Operating Conditions : The following are the standard operating conditions: (i) Mean Barometric Pressure: It is taken as 736 mm of mercury (Hg). (ii) Intake Air Temperature : It is taken as 3000 k or 270 C 68

I. C. ENGINE TESTING Engine Power : The energy flow through the engine is expressed in three distinct terms. They are ‘indicated power’, ip, ‘friction power’, fp and ‘ brake power’, bp. ‘Indicated power’ can be computed from the measurement of forces in the cylinder and ‘break power’ from the measurement of forces at the crank shaft of the engine. The ‘friction power’ can be estimated by motoring the engine or from the difference between ip and bp. i. e. fp=ip-bp 67

I. C. ENGINE TESTING Indicated Mean Effective Pressure (Pim): It may be defined as, the constant pressure acting over the full length of the stroke and capable of producing the same amount of work, as is actually produced during the complete cycle of the engine. It is generally denoted by ‘Pim’ or i. m. e. p. As, the pressure in the cylinder varies throughout the cycles and the variation can be expressed with respect to the volume or crank angle rotation to obtain p-V or p-θ diagrams, 66

I. C. ENGINE TESTING Indicated Mean Effective Pressure (Pim): respectively. However, such a continuous variation does not readily lend itself to simple mathematical analysis in the computation of ip. If an average pressure for one cycle can be used, then the computations become far less difficult. Refering figure, as the piston moves back and forth between TDC and BDC, the process lines on the p-V diagram indicated the successive states of the working fluid through the cycle. 65

I. C. ENGINE TESTING Indicated Mean Effective Pressure (Pim): The indicated network of the cycle is represented by the area 1 -2 -3 -4 enclosed by the process lines for that cycle. If the area of rectangle A-B-C-D equals, the area 1 -2 -3 -4, the vertical distance between the horizontal lines AB and CD respectively gives the ‘indicated mean effective pressure’, imep. It is a mean value expressed in N/m 2, which when multiplied by the displacement volume or swept volume, Vs gives the ‘same indicated net work’ as is actually 64

I. C. ENGINE TESTING Indicated Mean Effective Pressure (Pim): p-V diagram for an ideal four-stroke cycle engine

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I. C. ENGINE TESTING Brake Mean Effective Pressure (Pbm): It may be defined as the mean effective pressure acting on the face of piston, which would develop brake power equivalent to that during actual varying pressure condition. It is generally denoted by Pbm or b. m. e. p. 61

I. C. ENGINE TESTING Brake Mean Effective Pressure (Pbm): Friction mean effective pressure (Pfm) is that portion of mean effective pressure (Pim), which is required to overcome friction losses and brake mean effective pressure is the portion, which produces the useful power delivered by the engine. 60

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I. C. ENGINE TESTING Piston Speed : It is the average or mean distance travelled by the piston of the engine in one minute. i. e. , Piston speed = 2 L. N. m/min Where, L = Length of stroke (m) And N= Revolutions per minute of the crank shaft. 58

I. C. ENGINE TESTING Fuel-Air (F/A) or Air-Fuel (A/F) Ratio: The relative proportions of the fuel and air in the engine are very important from the stand point of combustion and efficiency of the engine. This is expressed either as a ratio of the mass of the fuel to that of the air or vice versa. In the SI engine the fuel-air ratio practically remains constant over a wide range of operation. In CI engines at a given speed the air flow does not vary with load, it is the fuel flow that varies directly with load. Therefore, the term fuel-air 57

I. C. ENGINE TESTING Fuel-Air (F/A) or Air-Fuel (A/F) Ratio: -ratio is generally used instead of air-fuel ratio. A mixture that contains just enough air for complete combustion of all the fuel in the mixture is called a ‘chemically correct’ or ‘stoichiometric fuel-air ratio’. A mixture having more fuel than that in a chemically correct mixture is termed as ‘rich mixture’ and a mixture that contains less fuel or excess air is called a ‘lean mixture’ or ‘weak mixture’. The ratio of actual fuel-air ratio to the chemically correct 56

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I. C. ENGINE TESTING Calorific Value (CV) : ‘Calorific value’ of a fuel is thermal energy released per unit quantity of the fuel when the fuel is burned completely and the products of combustion are cooled back to the initial temperature of the combustion mixture. Other terms used for the calorific value are ‘heating value’ and ‘heat combustion’. When the products of combustion are cooled to 250 C practically, all the water vapour resulting from the combustion process is 54

I. C. ENGINE TESTING Calorific Value (CV) : -condensed. The heating value so obtained is called the ‘higher calorific value’ or ‘gross calorific value’ of the fuel. The ‘lower or net calorific value’ is the heat released when vapour in the products of combustion is not condensed and remains in the vapour form. 53

I. C. ENGINE TESTING Measurement of Air Supply of an I. C. Engine: To measure air supply, the orifice method can be used if pressure pulsations could be damped out by some means. The usual method of damping out the pressure-pulsations is to fit an air box of suitable volume (500 to 600 times the swept volume in single cylinder engines and less in case of multi-cylinder engines) to the engine with an orifice placed in the side of the box, remote from the engine as shown in figure. 52

I. C. ENGINE TESTING Measurement of Air Supply of an I. C. Engine: Measurement of Air by Air Box Method. 51

I. C. ENGINE TESTING Measurement of Air Supply of an I. C. Engine: Let a= area of orifice in m 3. 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. 50

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I. C. ENGINE TESTING Determination of Indicated and Brake Power: In this method, to find out ip an indicator is used to find out the mean effective pressure. This method is used for slow speed engines. An indicator is an instrument which produces a graphic record of the pressure inside the engine cylinder for every position of the piston as it reciprocates. It consists of a small cylinder fitted with a piston, the under side of which is placed in communication with the cylinder. The upper side of the indicator piston is kept in 34

I. C. ENGINE TESTING Determination of Indicated and Brake Power: communication with the atmosphere. A helical spring on the top of the piston has one end attached to the piston and the other to the cover of the indicator cylinder through which passes the piston rod which carries a pencil at its upper end. This pencil traces out the indicator (p-V) diagram on the paper. In working with actual engines, it is often desirable to compute ip from a given Pim, i, . e. mean effective pressure and given engine 33

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I. C. ENGINE TESTING Brake Power (bp): Indicated power is based on indicated net work and is thus a measure of the force developed within the cylinder. More practical interest is the rotational force available at the delivery point, i. e. at the engine crank shaft also termed as drive-shaft and the power corresponding to it. This power is interchangeably referred to as ‘brake power’, ‘shaft power’ or delivered power’. In general, only the term brake power, bp, has been used here to indicate the power actually 29

I. C. ENGINE TESTING Brake Power (bp): -delivered by the engine. The power, bp, is usually measured by attaching a power absorption device to the drive-shaft of the engine. Such a device sets up measurable forces counteracting the forces delivered by the engine and the determined value of these measured forces is indicative of the forces being delivered. 28

I. C. ENGINE TESTING Measurement of bp, using Rope Brake Arrangement: The fig shows rope brake arrangement for the measurement of brake power, bp. Rope brake Dynamometer 27

I. C. ENGINE TESTING Measurement of bp, using Rope Brake Arrangement: 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 26

I. C. ENGINE TESTING Measurement of bp, using Rope Brake Arrangement: The other end carries the dead weights. The engine is thus run at a ‘constant speed’ which is measured with the help of a tachometer. Now, Let W= Force due to dead weight of the rope brake (N) S= Spring balance reading (N) D= Dia. Of the brake drum (m) d= Dia. of the rope (m) N= RPM of the crank shaft (given by tachometer) 25

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I. C. ENGINE TESTING MORSE TEST: It is the method of determining indicated power (ip) of each cylinder individually, of a multi cylinder IC engine, without the use of an indicator and thus computing the ‘total ip of the engine’ by summing up ip of all the cylinders. This method is adopted to calculate ip of high speed engines, i. e. where the indicator method is unsuitable. 22