Tiêu đề Unit5 - Heat engine-notes1-1.pdf ✅

Steam Engine 1. INTRODUCTION The reciprocating steam engine is one of the oldest types of heat engines which converts heat energy into mechanical energy Like the reciprocating I.C. engine, it is essentially a piston and cylinder arrangement, but whilst in the former, it is the hot gaseous products of combustion which expand in the cylinder doing work on the piston, in the latter, the working fluid is steam. Although the importance and usefulness of the steam engine has been greatly encroached upon, in the recent years, by the development of steam turbines and I.C. engines, it has still many specialised applications. It is widely used in marine work because of its ability to use any fuel, low cost of machinery and easy repair. In general sense, the piston engine is superior to the turbine for variable speeds and heavy starting torque, while turbine has superseded the engine for large central power station units and for auxiliaries, requiring high rotative speeds. 2. CLASSIFICATION OF STEAM ENGINES Steam engines are classified according to their various constructions and operating features. The various classifications are as follows: 1. According to the 'position of cylinders: (a) Vertical engine (b) Horizontal engine (c) Inclined engine. 2. An important classification of steam engine is based upon the No. of working strokes/revolution: (a) Single acting engine (b) Double acting engine In single acting engine, steam acts on one side of the piston or there is one working stroke/revolution whereas in case of double acting steam engine, the steam acts on the piston during every stroke or there are two working strokes/revolution of the crank shaft. WORKING OF A SIMPLE STEAM ENGINE Fig. 1 shows the section through the cylinder of an engine in which steam can be admitted alternately to one side of the piston and then the other, thus making the engine double acting. The piston is made a loose fit in the cylinder and the rings shown serve to prevent the leakage of steam from one side of the piston to the other side. The piston rod is usually fastened into the piston head by a taper ended rod nut and is then earned through the cylinder head, the gland and packing serving to make a steam tight joint. The other end of the piston rod is connected to the cross head which allows it a straight line motion. The connecting rod and crank convert reciprocating motion of the piston into rotary motion of the flywheel thus transferring power developed in the engine cylinder to the main shaft The main shaft also carries an eccentric which converts the rotary motion of the main shaft into reciprocating motion of the slide valve. In other words, eccentric drives the valve through the agency of the eccentric rod and valve stem. Like piston rod the valve rod passes through the cover of the steam chest. Fig.1 Section of a steam engine
In Fig 1, the slide valve is shown in a position, admitting steam on the crank end side of the cylinder. On the cover end, the cylinder is open to exhaust which may take place in a condenser or in atmosphere depending upon whether the engine is condensing or non-condensing. As the steam enters one side of the piston, the steam in space on the other side is forced out through the space under the valve and out of the stroke, the valve will have been moved to a similar position at the other end. Steam will then he admitted at that end and the end previously receiving steam will be open to exhaust. RANKINE CYCLE APPLIED TO STEAM ENGINE PLANT In actual practice, all the operations of the Rankine cycle take place in the steam engine plant as a whole and not in the cylinder alone. A schematic diagram of a steam engine plant is shown in Fig. below. The various operations of Rankine cycle in this case are shown on p-v and T-s in the below figure. Operation de. The feed water from the condenser, at a pressure p2 and temperature T2 is raised to the boiler pressure p1 by adiabatic compression in the feed pump. The increase in temperature consequent upon this compression may be of the order of a few degrees centigrade and is represented by de on the T-s diagram (b), the equivalent work being represented by area dhge on the p-v diagram. Operation ea. During this process, liquid receives, sensible heal at constant pressure p1 to raise its temperature to the saturation value corresponding to the boiler pressure p1. This operation may take place in an economizer or may be partially carried out in special feed heaters between the feed pump and the boiler or more commonly between the hot well and the feed pump the necessary heat being frequently supplied by bleeding live steam from the boiler or turbine. Operation ab. The evaporation of water takes place in the boiler at pressure p1. The evaporation may be partial at b, or complete at b1 or frequently a superheat is imparted to raise the temperature at constant pressure to ‘Ts’ (given by point b2). From b, b1 or b2 the vapour expands. Operation bc. The steam having its condition given by b, b1 or b2 passes from the boiler to the engine cylinder where it expands adiabatically to c, c1 or c2. Operation cd. The steam is exhausted to the condenser where its condensation takes place at constant temperature and pressure till it is returned to the original stale at d. The cycle is thus completed. The hatched areas on both diagrams represent the work done in this closed cycle. The process dea has been greatly exaggerated on the T-s diagram to illustrate it. If plotted actually, it is very difficult to differentiate between dea and line da. Hence the triangular stip dea in the liquid region is too small to merit consideration so it may be neglected and in that case the expression for work done and the Rankine efficiency shall be the same as compared to theoretical values.
INTRODUCTION TO INTERNAL COMBUSTION ENGINES HEAT ENGINES Any type of engine or machine which derives heat energy from the combustion of fuel or source and converts this energy into mechanical work is termed as a heat engine Heat engines may be classified into two main classes as follows : 1. External Combustion Engines. 2. Internal Combustion Engines. External combustion engines (E In this case, combustion of fuel takes place outside the cylinder as in case of steam engines the heat of combustion is employed to generate steam which is Other examples of external combustion engines are hot air engines, steam turbine and gas turbine. These engines are generally used for dri power etc. Internal combustion engines (I.C. engines) In this case, combustion of the fuel with oxygen of the air occurs within the cylinder of the The internal combustion engines group includes engines and air, known as gas engines, those using lighter liquid fuel or spirit known as petrol those using heavier liquid fuels, known as oil compression ignition or diesel engines. The detailed classification of heat engines is given below: INTRODUCTION TO INTERNAL COMBUSTION ENGINES Any type of engine or machine which derives heat energy from the combustion of fuel or source and converts this energy into mechanical work is termed as a heat engine Heat engines may be classified into two main classes as follows : ombustion Engines. External combustion engines (E.C. engines) In this case, combustion of fuel takes place outside the cylinder as in case of steam engines the heat of combustion is employed to generate steam which is used to move a piston in a Other examples of external combustion engines are hot air engines, steam turbine and gas turbine. These engines are generally used for driving locomotives, ships, genera combustion engines (I.C. engines) In this case, combustion of the fuel with oxygen of the air occurs within the cylinder of the The internal combustion engines group includes engines employing mixtures of combusti ines, those using lighter liquid fuel or spirit known as petrol those using heavier liquid fuels, known as oil compression ignition or diesel engines. The detailed classification of heat engines is given below: Fig,1. Classification of heat engines. INTRODUCTION TO INTERNAL COMBUSTION ENGINES Any type of engine or machine which derives heat energy from the combustion of fuel or any other In this case, combustion of fuel takes place outside the cylinder as in case of steam engines where used to move a piston in a cylinder. Other examples of external combustion engines are hot air engines, steam turbine and closed cycle ving locomotives, ships, generation of electric In this case, combustion of the fuel with oxygen of the air occurs within the cylinder of the engine. employing mixtures of combustible gases ines, those using lighter liquid fuel or spirit known as petrol engines and those using heavier liquid fuels, known as oil compression ignition or diesel engines.
Advantages of reciprocating internal combustion engines over external combustion engines : Reciprocating internal combustion engines offer the following advantages over external combustion engines : 1. Overall efficiency is high. 2. Greater mechanical simplicity. 3. Weight to power ratio is generally low. 4. Generally lower initial cost. 5. Easy starting from cold conditions. 6. These units are compact and thus require less space. Advantages of the external combustion engines over internal combustion engines: The external combustion engines claim the following advantages over internal combustion engines : 1. Starting torque is generally high. 2. Because of external combustion of fuel, cheaper fuels can be used. Even solid fuels can be used advantageously. 3. Due to external combustion of fuel it is possible to have flexibility in arrangement. 4. These units are self-starting with the working fluid whereas in case of internal combustion engines, some additional equipment or device is used for starting the engines. DEVELOPMENT OF I.C. ENGINES Brief early history of development of I.C. engines is as follows : • Many different styles of internal combustion engines were built and tested during the seond half of the 19th century. • The first fairly practical engine was invented by J.J.E. Lenoir which appeared on the scene about 1860. During the next decade, several hundred of these engines were built with power upto about 4.5 kW and mechanical efficiency upto 6%. • The Otto-Langen engine with efficiency improved to about 11% was first introduced in 1867 and several thousands of these were produced during the next decade. This was a type of atmospheric engine with the power stroke propelled by atmospheric pressure acting against a vacuum. • Although many people were working on four-stroke cycle design. Otto was given credit when his prototype engine was built in 1876. • In the 1880s, the internal combustion engines first appeared in automobiles. Also in this decade the two-stroke cycle engine became practical and was manufactured in large number. • Rudolf Diesel, by 1892, had perfected his compression ignition engine into basically the same diesel engine known today. This was after years of development work which included the use of solid fuel in his early experimental engines. • Early compression engines were noisy, large, slow, single cylinder engines. They were however, generally more efficient than spark ignition engines. • It wasn't until the 1920s that multicylinder compression ignition engines were made small enough to be used with automobile and trucks. • Wakdle's first rotary engine was tested at NSV, Germany in 1957. • The practical Stirling engines in small number are being produced since 1965. — These engines require costly material and advanced technology for manufacture. — Thermal efficiencies higher than 30% have been obtained. — The advantages of Stirling engine are low exhaust emission and multi-fuel capability.