ADDIS ABABA UNIVERSITY INSTITUTE OF TECHNOLOGY

Transcription

1 1 INTERNAL COMBUSTION ENGINES ADDIS ABABA UNIVERSITY INSTITUTE OF TECHNOLOGY MECHANICAL ENGINEERING DEPARTMENT DIVISON OF THERMAL AND ENERGY CONVERSION

2 IC Engine Fundamentals 2 Engine Systems An engine must at least include the following systems to operate Air supply and Exhaust system Fuel supply system Combustion system Ignition system (SI) Cooling system Lubricating system Starting system Electrical power supply (battery or generator)

3 3 IC Engine Fundamentals

4 IC Engine Fundamentals 4 pressure = force area force = pressure x area

5 IC Engine Fundamentals 5 pressure = force area force = pressure x area

6 IC Engine Fundamentals 6 Ignition system

7 IC Engine Fundamentals 7 Ignition system Crank mechanism

8 8

9 Spark Ignnition System 9 Distributer contains: - A rotating cam driven by the engine drive - A set of breaker points, - A condensor(capacitor) - A roter and distributer cap o The ignition coil consit of two transformer windings sharing a common megnetic core- the primary (200 to 300 turns) and secondary windings (21,000 turns). o An alternating current in the PW induces magnetic field in the coil s core so that it serves as a step up transformer.

10 Spark Ignnition System 10 For an ignition coil, one end of windings of both the primary and secondary are connected together. This common point is connected to the battry ( ussually through the current limiting ballast rsistor) The other end of the primary is connected to the point in the distributer The other end of the SW is connected via the the distributer cap and rotor to the spark plugs

11 IC Engine Fundamentals 11 Intake system Ignition system Crank mechanism

12 IC Engine Fundamentals 12 Intake system Exhaust system Ignition system Crank mechanism

13 IC Engine Fundamentals 13 Intake system Cooling system Thermostat Exhaust system Ignition system Crank mechanism

14 14

15 IC Engine Fundamentals 15 Intake system Cooling system Thermostat Exhaust system Ignition system Lubrication system Crankcase vent

16 16

17 IC Engine Fundamentals 17 Otto cycles Intake Compression, Combustion Power Exhaust

18 IC Engine Fundamentals 18 fuel air air + fuel pressure 1. INTAKE STROKE TDC volume BDC

19 19 IC Engine Fundamentals Stoichiometric mixture pressure TDC volume BDC

20 20 IC Engine Fundamentals pressure 2. COMPRESSION STROKE TDC volume BDC

21 IC Engine Fundamentals 21 pressure TDC volume BDC

22 IC Engine Fundamentals Power stroke pressure TDC volume BDC

23 23 IC Engine Fundamentals pressure TDC volume BDC

24 IC Engine Fundamentals Exaust stroke pressure TDC volume BDC

25 IC Engine Fundamentals 25 pressure TDC volume BDC

26 IC Engine Fundamentals 26 Wor k = (pressure x volume) Exhaust gas residual pressure positive work TDC volume BDC Negative work

27 27 IC Engine Fundamentals

28 IC Engine Fundamentals-Some Problems 28 Intake tuning Exhaust tuning Emissions Mixture preparation 1. Performance 2. Fuel economy 3. Emission Combustion, auto ignition Wear C x H x CO NO x Inertia lubrication

29 IC Engine Fundamentals-Diesel Engine 29 The Diesel Engine pressure TDC volume BDC Negligible negative work

30 IC Engine Fundamentals-Diesel Engine psi

31 31

32 IC Engine fundamental-si Engine 32 Intake Stroke The piston travels from TDC to BDC with the intake valve open and exhaust valve closed. Piston draws air and fuel mixture into cylinder

33 33 IC Engine fundamental-si Engine Compression Stroke When the piston reaches BDC, the intake valve closes and the piston travels back to TDC with all valves closed. Rising piston compresses mixture raising both the pressure and temperature in the cylinder. Near the end of the compression stroke, the spark plug is fired and combustion is initiated. (constant-volume combustion).

34 34 IC Engine fundamental-si Engine Expansion Stroke With all valves closed, the high pressure created by the combustion process pushes the piston away from TDC. As the piston travels from TDC to BDC, cylinder volume is increased, causing pressure and temperature to drop.

35 IC Engine fundamental-si Engine Exhaust Stroke With the exhaust valve remaining open, the piston now travels from BDC to TDC in the exhaust stroke. This pushes most of the remaining exhaust gases out of the cylinder into the exhaust system 35

36 36 IC Engine fundamental-si Engine Intake Stroke Intake stroke in Diesel engine is the same as the intake stroke in an SI engine. The major difference being no fuel is added to the incoming air.

37 IC Engine fundamental-diesel Engine 37 Compression Stroke The same as in SI engine except that only air is compressed and compression is to higher pressures and temperature. Late in the compression stroke fuel is injected directly into the combustion chamber where it mixes with the very hot air. This causes the fuel to evaporate and self-ignite causing combustion to start. Combustion is fully developed by TDC and continues at about constant pressure until fuel injection is complete (i.e. after Cut - off) and the piston has started towards BDC.

38 IC Engine fundamental-diesel Engine 38 Power Stroke o The power stroke continues as combustion ends and the piston travels towards BDC. Exhaust Stroke o The same as SI engine

39 IC Engine Fundamentals- 2 Stroke Engine 39 The two stroke engine employs the crankcase as well as the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston. One power stroke in one revolution of the crankshaft The main difference between two stroke and four-stroke engines is in the method of filling the fresh charge and removing the bunt gases from the cylinder.

40 IC Engine Fundamentals- 2 Stroke Engine 40 In a two-stroke engine, the filling process is accomplished by the charge compressed in crankcase or by a blower. The induction of the compressed charge moves out the product of combustion through exhaust ports. Therefore, no piston strokes are required for these two operations. Two strokes are sufficient to complete the cycle, one for compressing the fresh charge and the other for expansion or power stroke.

41 41 IC Engine Fundamentals-2 Stroke Engine A compression stroke (intake + compression) starts by closing the intake and exhaust ports, and then compresses the cylinder contents and draws fresh charge into the crankcase. The fuel/air mixture is drawn into the crankcase by the vacuum that is created during the upward stroke of the piston. As the piston approaches TDC, combustion is initiated.

42 42 IC Engine Fundamentals- 2 Stroke Engine A power or expansion stroke (combustion + exhaust) At the end of compression stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. At the same time, another crankcase compression stroke is happening beneath the piston.

43 43 IC Engine Fundamentals-2 Stroke Engine During downward movement of the piston first the exhaust ports and then the intake ports are uncovered. When the inlet ports are uncovered, the fresh charge which has been compressed in the crankcase flows into the cylinder. (Scavenging) The piston and the ports are generally shaped to deflect the incoming charge from flowing directly into the exhaust ports and to achieve effective scavenging of the residual gases

44 IC Engine Fundamentals-2 Stroke Engine 44 Upward stroke of the piston During the downward stroke Intake Valve open Crank Case compression Transfer port Opening Scavenging

45 45 IC Engine Fundamentals -2 Stroke Engine Upward stroke of the piston Down ward stroke of the piston Cylinder compressionanother intake stroke is happening beneath the piston Power Stroke- Crankcase compression Exhaust Stroke- Scavenging

46 IC Engine Fundamentals 46 Comparison of SI & CI Engines Description SI Engine CI Engine Basic Cycle Otto Cycle Heat Constant volume Diesel cycle, Heat constant pressure Fuel Introduction of Fuel Load control Ignition Gasoline, Highly volatile fuel, self ignition temp is high Fuel-air mixture introduced during suction stroke (Carburetor is necessary) Throttle controls the quantity of mixture introduce Required an ignition system with spark plug Diesel oil, non volatile fuel, selfignition temp is comparatively low Fuel directly injected to the combustion chamber at high pressure (fuel pump and injector is necessary) The quantity of fuel is regulated in the pump. Air quantity is not control Ignition system & sparkplug are not necessary

47 IC Engine Fundamentals 47 Comparison of SI & CI Engines Description SI Engine CI Engine Compression ratio Speed Thermal Efficiency 6-10, upper limit is fixed by anti knock quantity of fuel They are high speed engines (light weight & homogeneous combustion) Lower thermal Efficiency (lower compression ratio) upper limit is limited by weight increase of the engine They are low speed engines ( heavy weight & heterogeneous combustion) Higher thermal efficiency (higher compression ratio) weight Lighter due to lower peak pressure Heavier due to higher peak pressure

48 IC Engine Fundamentals 48 Comparison of 2-Stroke & 4-Stroke Engines 4-Stroke Engines The cycle completed in 4 strokes of piston or 2-revolution of crankshaft Turning moment is not uniform & hence a heavier flywheel is needed The power produced for the same size engine is less (2 revo gives 1 power) Lesser cooling & lubrication requirement Lower rate of wear & tear 2-Stroke Engine The cycle is completed in twoa strokes of the piston or one power strokes obtained in every revo. of crankshaft Turning moment is more uniform & hence a lighter flywheel can be used Power produced for the same size of engine is more (Theoretically twice, actually 1.3 times) due to 1 Power stroke in 1 revo. Of Cra Sh) Greater cooling & lubrication requirements Higher rate of wear & tear

49 49 IC Engine Fundamentals 4-S Engines 2-S Engine Contains valve & Valve mechanisms to open & close valves No valves but ports (some 2-s engines are fitted with exhaust valves or reed valve) The initial cost is very high (Coz of heavy wt. & complicated valve mechanism) Volumetric Efficiency is more (Coz of more time for induction) Thermal Efficiency is higher, part load efficiency is better Used where Efficiency is important ( in Cars, Buses, aero planes, etc Initial cost of the engine is less (Coz od light wt. & simplicity) Volumetric Efficiency is low (Coz of lesser time for induction) Thermal Efficiency is less, part load efficiency is poor compared 4-s engine Used where low cost, compactness & Light Wt. are important (in Scooters, Motorcycles

50 IC Engine Fundamentals-Wankel Engine 50 Alternative to the reciprocating engine geometry The intake, compression, combustion and exhaust process happen in different cavities created between the rotor and the part of the housing. In Wankel engine there are two rotating parts: the triangular shaped rotor and the output shaft with its integral eccentric (lobe).

51 51 The rotor revolves directly on the eccentric The rotor has an integral timing gear which meshes with the fixed timing gear on one side of the housing to maintain the correct pulse relationship between the rotor and the eccentric shaft rotations. As the rotor makes one complete rotation, during which the eccentric shaft rotates through three revolutions, each chamber produces one power stroke.

52 IC Engine Fundamentals-Wankel Engine 52 A rotary engine has an ignition system and a fuel-delivery system that are similar to the ones on piston engines. The Rotor The rotor has three convex faces, each of which acts like a piston. Each face of the rotor has a pocket in it, which increases the displacement of the engine, allowing more space for air/fuel mixture. At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber

53 IC Engine Fundamentals-Wankel Engine 53 The rotor has a set of internal gear teeth cut into the center of one side. These teeth mate with a gear that is fixed to the housing. This gear mating determines the path and direction the rotor takes through the housing.

54 IC Engine Fundamentals-Wankel Engine 54 The housing is roughly oval in shape. its shape is designed so that the three tips of the rotor will always stay in contact with the wall of the chamber, forming three sealed volumes of gas. Each part of the housing is dedicated to one part of the combustion process.

55 55 IC Engine Fundamentals-Wankel Engine The four sections are: Intake Compression Combustion Exhaust The intake and exhaust ports are located in the housing. There are no valves in these ports. The exhaust port connects directly to the exhaust, and the intake port connects directly to the throttle.

56 IC Engine Fundamentals-Wankel Engine 56 Output Shaft The output shaft has round lobes mounted eccentrically, meaning that they are offset from the centerline of the shaft. Each rotor fits over one of these lobes. The lobe acts sort of like the crankshaft in a piston engine. As the rotor follows its path around the housing, it pushes on the lobes. Since the lobes are mounted eccentric to the output shaft, the force that the rotor applies to the lobes creates torque in the shaft, causing it to spin.

57 IC Engine Fundamentals-Wankel Engine 57 ADVANTAGES DISADVANTAGES higher output for similar displacement and physical size simple and contain far fewer moving parts the shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot spots from forming seals exposed to heating and cooling cycles in use, led to a very high incidence of loss of sealing incomplete combustion of the airfuel charge, with the remaining unburned hydrocarbons released into the exhaust. Is difficult to expand the engine to more than two rotors