Basics of Internal Combustion Engines by Indranil Mandal
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The document discusses internal combustion engines. It begins by defining heat engines and classifying them as either external or internal combustion engines. The key difference is whether combustion occurs outside or inside the engine cylinder. It then focuses on internal combustion engines, describing their basic design and classifying them in various ways, such as by fuel type, number of strokes, ignition method, and application. The main components of internal combustion engines like the cylinder, piston, crankshaft, valves, and flywheel are also explained. Finally, the four-stroke operating cycle of a typical spark-ignition internal combustion engine is summarized in three steps: intake, compression, and power strokes followed by the exhaust stroke.
Basics of Internal Combustion Engines by Indranil Mandal
- 1. INTERNAL COMBUSTION ENGINE Module – I INTRODUCTION Heat engine: A heat engine is a device which transforms the chemical energy of a fuel into thermal energy and uses this energy to produce mechanical work. It is classified into two types- (a) External combustion engine (b) Internal combustion engine Classification of Heat Engines: L: Large over 10000 KW M: Medium between 1000- 10000 KW S: Small under 1000 KW
- 2. External combustionengine: In this engine, the products of combustion of air and fuel transfer heat to a second fluid which is the working fluid of the cycle. Examples: In the steam engine or a steam turbine plant, the heat of combustion is employed to generate steam which is used in a piston engine (reciprocating type engine) or a turbine (rotary type engine) for useful work. Figure 1: External Combustion Engine In steam engine or steam turbine the heat generated due to combustion of fuel and it is employed to generate high pressure steam, which is used as working fluid in a reciprocating engine or turbine (From Figure 1). In a closed cycle gas turbine, the heat of combustion in an external furnace is transferred to gas, usually air which the working fluid of the cycle. Internal combustion engine: In this engine, the combustion of air and fuels take place inside the cylinder and are used as the direct motive force. It can be classified into the following types: 1. According to the basic engine design- (a) Reciprocating engine (Use of cylinder piston arrangement), (b) Rotary engine (Use of turbine) 2. According to the type of fuel used- (a) Petrol engine, (b) diesel engine, (c) gas engine (CNG, LPG), (d) Alcohol engine (ethanol, methanol etc) 3. According to the number of strokes per cycle- (a) Four stroke and (b) Two stroke engine 4. According to the method of igniting the fuel- (a) Spark ignition engine, (b) compression ignition engine and (c) hot spot ignition engine 5. According to the working cycle- (a) Otto cycle (constant volume cycle) engine, (b) diesel cycle (constant pressure cycle) engine, (c) dual combustion cycle (semi diesel cycle) engine. 6. According to the fuel supply and mixture preparation- (a) Carburetted type (fuel supplied through the carburettor), (b) Injection type (fuel injected into inlet ports or inlet manifold, fuel injected into the cylinder just before ignition). 7. According to the number of cylinder- (a) Single cylinder and (b) multi-cylinder engine 8. Method of cooling- water cooled or air cooled 9. Speed of the engine- Slow speed, medium speed and high speed engine 10. Cylinder arrangement-Vertical, horizontal, inline, V-type, radial, opposed cylinder or piston engines. 11. Valve or port design and location- Overhead (I head), side valve (L head); in two stroke engines: cross scavenging, loop scavenging, uniflow scavenging.
- 3. 12. Method governing- Hit and miss governed engines, quantitatively governed engines and qualitatively governed engine 13. Application- Automotive engines for land transport, marine engines for propulsion of ships, aircraft engines for aircraft propulsion, industrial engines, prime movers for electrical generators. Comparison between external combustion engine and internal combustion engine: ADVANTAGES OF INTERNAL COMBUSTION ENGINES: 1. Greater mechanical simplicity. 2. Higher power output per unit weight because of absence of auxiliary units like boiler, condenser and feed pump 3. Low initial cost 4. Higher brake thermal efficiency as only a small fraction of heat energy of the fuel is dissipated to cooling system 5. These units are compact and requires less space 6. Easy starting from cold conditions DISADVANTAGES OF INTERNAL COMBUSTION ENGINES: 1. I C engines cannot use solid fuels which are cheaper. Only liquid or gaseous fuel of given specification can be efficiently used. These fuels are relatively more expensive. 2. I C engines have reciprocating parts and hence balancing of them is problem and they are also susceptible to mechanical vibrations.
- 4. INTERNAL COMBUSTION ENGINE PARTS AND THEIR FUNCTION Cylinder: It is the main part of the engine inside which piston reciprocates to and fro. It should have high strength to withstand high pressure above 50 bar and temperature above 2000°C. The ordinary engine is made of cast iron and heavy duty engines are made of steel alloys or aluminum alloys. In the multi-cylinder engine, the cylinders are cast in one block known as cylinder block. Cylinder head: The top end of the cylinder is covered by cylinder head over which inlet and exhaust valve, spark plug or injectors are mounted. A copper or asbestos gasket is provided between the engine cylinder and cylinder head to make an air tight joint. Piston: Transmit the force exerted by the burning of charge to the connecting rod. Usually made of aluminium alloy which has good heat conducting property and greater strength at higher temperature. Figure 2 and 3 shows the different components of IC engine: Figure.2: Different parts of IC engine
- 5. Figure.3: Different parts of IC engine Piston rings: These are housed in the circumferential grooves provided on the outer surface of the piston and made of steel alloys which retain elastic properties even at high temperature. 2 types of rings- compression and oil rings. Compression ring is upper ring of the piston which provides air tight seal to prevent leakage of the burnt gases into the lower portion. Oil ring is lower ring which provides effective seal to prevent leakage of the oil into the engine cylinder. Connecting rod: It converts reciprocating motion of the piston into circular motion of the crank shaft, in the working stroke. The smaller end of the connecting rod is connected with the piston by gudgeon pin and bigger end of the connecting rod is connected with the crank with crank pin. The special steel alloys or aluminium alloys are used for the manufacture of connecting rod. Crank: - It is a lever between connecting rod and crank shaft. Crankshaft: It converts the reciprocating motion of the piston into the rotary motion with the help of connecting rod. The special steel alloys are used for the manufacturing of the crankshaft. It consists of eccentric portion called crank. Crank case: It houses cylinder and crankshaft of the IC engine and also serves as sump for the lubricating oil. Flywheel: It is big wheel mounted on the crankshaft, whose function is to maintain its speed constant. It is done by storing excess energy during the power stroke, which is returned during other stroke. IC ENGINE – TERMINOLOGY 1. Cylinder bore (D): The nominal inner diameter of the working cylinder. 2. Piston area (A): The area of circle of diameter equal to the cylinder bore. 3. Stroke (L): The nominal distance through which a working piston moves between two successive reversals of its direction of motion. 4. Dead centre: The position of the working piston and the moving parts which are mechanically connected to it at the moment when the direction of the piston motion is reversed (at either end point of the stroke).
- 6. (a) Bottom dead centre (BDC): Dead centre when the piston is nearest to the crankshaft. (b) Top dead centre (TDC): Dead centre when the position is farthest from the crankshaft. 5. Displacement volume or swept volume (Vs): The nominal volume generated by the working piston when travelling from the one dead centre to next one and given as, Vs=A × L 6. Clearance volume (Vc): the nominal volume of the space on the combustion side of the piston at the top dead centre. 7. Cylinder volume (V): Total volume of the cylinder. V= Vs + Vc 8. Compression ratio (r): r= Vs/ Vc FOUR-STROKE CYCLE S-I ENGINE - PRINCIPLE OF OPERATION: Figure.3: cross section of a SI Engine In Four-stroke cycle engine, the cycle of operation is completed in four-strokes of the piston or two revolutions of the crankshaft. Each stroke consists of 180°, of crankshaft rotation and hence a cycle consists of 720°of crankshaft rotation. The series of operations of an ideal four-stroke SI engine are as follows (see Figure.4 & Figure.5) 1. Suction stroke Suction stroke 0-1 starts when the piston is at top dead centre and about to move downwards. The inlet valve is open at this time and the exhaust valve is closed. Due to the suction created by the motion of the piston towards bottom dead centre, the charge consisting of fresh air mixed with the fuel is drawn into the cylinder. At the end of the suction stroke the inlet valve closes.
- 7. 2. Compression stroke The fresh charge taken into the cylinder during suction stroke is compressed by the return stroke of the piston 1-2. During this stroke both inlet and exhaust valves remain closed. The air which occupied the whole cylinder volume is now compressed into clearance volume. Just before the end of the compression stroke the mixture is ignited with the help of an electric spark between the electrodes of the spark plug located in combustion chamber wall. Burning takes place when the piston is almost at top dead centre. During the burning process the chemical energy of the fuel is converted into sensible energy, producing a temperature rise of about 2000°C, and the pressure is also considerably increased. 3. Expansion or power stroke Due to high pressure the burnt gases force the piston towards bottom dead centre, stroke 3-4, and both the inlet and exhaust valves remaining closed. Thus power is obtained during this stroke. Both pressure and temperature decrease during expansion. 4. Exhaust stroke. At the end of the expansion stroke the exhaust valve opens, the inlet valve remaining closed, and the piston is moving from bottom dead centre to top dead centre sweeps out the burnt gases from the cylinder, stroke 4- 0. The exhaust valve closes at the end of the exhaust stroke and some 'residual' gases remain in the cylinder. Each cylinder of a four-stroke engine completes the above four operations in two engine revolutions. One revolution of the crankshaft occurs during the suction and compression strokes, and second revolution during the power and exhaust strokes. Thus for one complete cycle, there is only one power stroke while the crankshaft turns by two revolutions. Most of the spark-ignition internal combustion engines are of the four- stroke type. They are most popular for passenger cars and small aircraft applications. The construction and working of a four-stroke petrol engine is shown below: Figure.4:The four-stroke spark-ignition (SI) engine cycle (Otto cycle or constant volume cycle)
- 8. Figure.5: Ideal and actual indicator diagrams for four-stroke SI engine Figure.6: Four-stroke petrol engine valve timing diagram in relation to the pressure volume diagram
- 9. Valve Timing Diagram: Valve timing is the regulation of the points in the cycle at which the valves are set to open & close. In the ideal cycle inlet & exhaust valve open & close at dead centers. Figure.7: Theoretical valve timing diagram But in actual cycles, they open or close before or after dead centers as explained below: - There are two factors, one mechanical & other dynamic, for the actual valve timings tot be different from the theoretical valve timing. a) Mechanical factor: - The puppet valves of the reciprocating engines are opened & closed by cam mechanisms. The clearance between cam, tappet and valve must be slowly taken up & valve slowly filled, at first, if noise and wear is to be avoided, for the same reasons the valve cannot be closed abruptly, else it mill bounce on its seat. (Also the cam counters should be so designed as to produce gradual and smooth changes in directional acceleration.) Thus, the valve opening & closing periods are spread over a considerable number of crankshaft degrees. As a result, the opening of valve must commence ahead of time at which it is fully opened ie before dead centers.) The same reasoning close after the dead centers. b) Dynamic factor: - Besides mechanical factors of opening & closing of valves, the actual valve timing is set taking into considerations, the dynamic effects of gas flow. Valve timing Diagram for 4 stroke S.I. Engine:- 1) Intake valve timing: - Intake valve timing has a bearing on the actual quantity of air sucked during the suction stoke, i.e. if affects the volumetric efficiency. From the actual valve timing diagram for both low and high speed S.I. engine it is seen that the intake valve opens 10° before the arrival of the piston at TDC on the exhaust stock. This is to ensure that the valve will be fully open the fresh charge starting to flow into the cylinder as soon as possible after TDC. As the piston moves out in the suction stroke, the fresh charge is drawn in through the intake valve. When the piston searches BDC and starts to move in the compression stroke, the inertia of the entering fresh charge tends to cause it to continue to move into cylinder. To take advantage of this, the intake valve is closed after TDC so that maximum air is taken in. This is called ram effect. The time the intake valve should remain open after TDC is decided by the speed. The charge speed is low and so the air inertia is low and hence the intake valve should close relatively early, after BDC for a slow speed engine as compared to a high speed engine.
- 10. Figure.8: Valve timing for low and high speed four-stroke SI engine 2) Exhaust valve timing: - The exhaust valve is set to open before BDC (say about 25° before BDC in low speed engines & 55° before BDC is high speed engines). If the exhaust valve did not start to open until BDC, the pressures in the cylinder would be considerably above the atmospheric pressure during the first portion of the exhaust stroke, increasing the work required to except the exhaust gases. But opening the exhaust valve earlier reduces the pressure near the end of the power stroke & thus, causes some loss of useful work on this stoke. However, the overall effect of opening the valve prior to the timing the piston reaches BDC results in overall gains in output. The closing timing of exhaust valve effects the volumetric efficiency. By closing the exhaust valve a few degrees after TDC (after 15° for low speed and 20° for high speed), the inertia of exhaust gases tends to scavenge the cylinder by conveying out a greater mass of the gas left in the clearance volume. This results in increased volumetric efficiency. The period when both the intake & exhaust valves are open at the same time is called as valves over lap.
- 11. Table 1–Typical valve timings for four-stroke SI engines Valve timing is different for different makes of engines. b-before, a-after TDC-Top dead centre BDC-Bottom dead centre.