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four stroke engine

  1. 1. Four Stroke Engine History of the FourStroke Automobile Engine 1
  2. 2. Four Stroke Engine History of the Four-Stroke Automobile Engine Perhaps the most well known engine type in the world, the automotive fourstroke engine has become the power plant of choice for today's consumers due to itsgreater efficiency and cost effectiveness over alternate reciprocating engines.The story of the internal combustion engine began in 1680 with a Dutch physi-cist, Christian Huygens, who conceptually designed an engine fueled by gun powder. However, the first internal combustion engine was actually built by a Sweetish inventor by the name of Francios Isaac de Rivaz in 1807. Through the combustionof a hydrogen and oxygen mixture, his engine, with some difficulty, powered acrudely constructed automobile. As the years went on, other inventors modified the design to be fueled by anything from gasoline to coal. The next greatest leap came in 1862 when a French engineer, Alphonse Beau de Rochas, designed and patented the first four-stroke engine. In 1864 an Austrian engineer, Siegfried Marcus, build the first gasoline powered vehicle, which was comprised of a cart and a one cylinder engine. But the biggest break through came in 1876 when Nikolaus August Otto invented the first successful four-stroke engine, aptly nick-namingthe four-stroke cycle the ”Otto Cycle.” [1] The next great milestone in the development of the four stroke engine was achieved by Gottlieb Daimler in 1885, who invented an engine with a vertical positioned cylinder, fueled by gasoline injected into a cylinder chamber through a carburetor. The innovations from these important inventors over the years culminated in Daimler's engine which is commonly referred to as the ”blue print” to the modern day internal combustion engine. [1] From the inception of the four-stroke internal combustion engine, many paths have been explored and followed to create the superior design, especially in the configuration of the cylinders. In general, there are seven types of reciprocating engine designs, an engine that employs one or more cylinders in which a piston(s) reciprocates back and forth. The first of these designs was the single cylinder engine. After its success, designers began to play with twin engines, or two cylinder engines which lead to the In-Line Engine, the V Engine, the Opposed Cylinder Engine, the W engine, the Opposed Piston Engine, and the Radial Engine. These different setups where further explored with engines such as the Vauxhall Wyvern and Velox engines to the Ford V-Four engine, utilizing an even greater numbers of cylinders than the original twin engines. 2
  3. 3. Four Stroke Engine Today's four-stroke engine manufacturer's mainly build In-Line or V configured engines. Perhaps the most widely recognized engine today is the Chevrolet Small Block V8 Engine. This engine was made popular through their depend ability and through hobbyists and the performance market because of the inter changeability of parts. After 35 years, General Motors discontinued their infamous engine, replacing it with the new Generation II engine in 1992. Although Chevroletseemingly has dominated today's after market industry, other manufacturers have successfully made engines for their vehicles from the Ford V-Eight, to the Cadillac North Star, to the Porsche In-Line Six as well as mainly others. 3
  4. 4. Four Stroke Engine Basic Components of Four-Stroke Engines 4
  5. 5. Four Stroke Engine Basic Components of Four-Stroke Engines A. Intake Valve- opens at the proper time to let in air and fuel. B. . Valve Cover- Protects the valves and the valve springs. Keeps dirt out and lubricating oil in. C. . Intake Port- the passageway in a cylinder head for the fuel and air to pass through. 5
  6. 6. Four Stroke Engine D. Head- a platform containing most of the parts of the combustion chamber. E. Coolant- circulating water and antifreeze to keep the temperature regulated. F. . Engine Block- cast in one piece. The basis for most of the parts of the engine. G. Oil Pan- where the oil is collected and recirculated. H. Oil Sump- the collected oil primarily for lubricating the crankshaft and rod bearing I. Camshaft- a round shaft with lobes, that rotates to open and close the fuel and exhaust valves. J. . Exhaust Valve- open at the proper time to release the exhaust K. Spark Plug- a device, inserted into the combustion chamber for firing an electrical spark to ignite air-fuel mixture L. Exhaust Port- the passageway in a cylinder head, for the exhaust to pass through M. . Piston- the part of the engine that moves up and down in the cylinder converting the gasoline into motion N. Connecting Rod- links the piston to the crankshaft. O. Rod Bearing- used to reduce friction to the rod and crankshaft P. Crankshaft- converts the up and down motion of the piston into a turning, or rotating motion 6
  7. 7. Four Stroke Engine Engine-Operating Cycle of Most Petrol and Diesel Engines 7
  8. 8. Four Stroke Engine Engine-Operating Cycle of Most Petrol and Diesel Engines Each movement of the cylinder up or down the cylinder is one stroke of the four stroke combustion cycle or Otto cycle. Most modern internal combustion engines use the four stroke cycle. The four stroke cycle consists of an induction stroke where air and fuel are taken into the cylinder as the piston moves downwards, a compression stroke where the air and fuel are compressed by the upstroke of the cylinder, the ignition or power stroke where the compressed mixture is ignited and the expansion forces the cylinder downwards, and an exhaust stroke where the waste gases are forced out of the cylinder. The intake and outlet ports open and close to allow air to be drawn into the cylinder and exhaust gases to be expelled. During the intake stroke the inlet valve opens at the top of the cylinder, as the piston moves down air and fuel are drawn into the cylinder. As the piston reaches its lowest position the inlet valve closes and the piston travels upwards compressing the air -fuel mixture. As the pi ston reaches its highest position at maximum compression a spark ignites the mixture causing a rapid expansion of gas raising the pressure in the cylinder and forcing the piston downwards. Once the cylinder has reached its lowest position the outlet port o pens and as the piston rises up the cylinder the exhaust gases are forced out. The valves which open and close the port are sprung to make them naturally close. The valves are opened by a system of rotating cams and pushrods driven by a camshaft which in t urn is timed and driven from the crankshaft. The valve timings vary between engines depending on the setup, generally there is some overlap to speed the flow of gases 8
  9. 9. Four Stroke Engine Intake 9
  10. 10. Four Stroke Engine Intake The intake stroke of the combustion cycle is when the piston travels down the cylinder with the intake port/ports open. A mixture of air and explosive fuel are drawn into the cylinder, the proportions of which are called the air -fuel ratio. Both the air-fuel ratio and the quality of the mixture (dispersion, droplet size etc.) is i mportant for an efficient combustion process. There are two methods of mixing air and fuel in a combustion engine, using a carburettor or fuel injection system. In a carburetted engine, during the intake stroke of the piston a vacuum is created in the inlet manifold. With a multi cylinder engine the vacuum is almost constant. The carburettor is located at the top of the manifold and air is drawn through it by the vacuum created in the manifold. The carburettor has a small fuel chamber supplied from the fuel tank by a pump, fuel passes through the carburettor to small fuel jets positioned in the air flow. The flow of air past the jets creates a pressure difference causing the fuel to be drawn out. The fuel vaporises in the air flow and passes through the manifold and into cylinders on their intake stroke. The diagram below shows the basic operation of a fixed jet carburettor. Electronic fuel injection systems spray fuel at high pressure either directly into the combustion chamber or into the intake port of the cylinder during the intake stroke. Using fuel injection enables improved control over the air -fuel mixture and reduces the power required to draw fuel from the jets. The diagram below shows a typical electronic fuel injection system. Diesel engines typically use direct injection which injects fuel directly into the combustion chamber during the compression stroke. The intake stroke on a diesel engine only draws air into the cylinder. 10
  11. 11. Four Stroke Engine 11
  12. 12. Four Stroke Engine Compression Compressi 12
  13. 13. Four Stroke Engine The compression stroke is the upwards movement of the piston in the cylinder with the valves closed following the intake stroke. This upwards motion compresses the fuel air mixture inside the combustion chamber raising the pressure. The difference between the initial volume of the cylinder and the final volume at the top of the compression stroke is known as the compression ratio. Typically this is approximately 9:1 in spark ignition engines and 15:1 for diesel engines. The compression ratio is particularly important in compression fired engines such as diesel engines. The fuel - air mix and compression ratio is critical to avoid pre -ignition which is the abnormal ignition of fuel in the combustion chamber before the combustion stroke. In diesel engines the fuel is injected under high pressure towards the top of the compression stroke. The distribution of fuel before combustion is also of interest because it affects the efficiency of combustion. 13
  14. 14. Four Stroke Engine Combustion 14
  15. 15. Four Stroke Engine Combustion Spark plugs are used to generate the spark which ignites the compressed fuel and air mixtu re in the spark ignition engine. To generate the spark a high voltage of around 20,000 Volts is applied. Low voltage current is fed through the primary winding of an inductor coil generating a magnetic field. The high voltage is generated when the low volt age supply is interrupted and the magnetic field breaks down generating a high voltage in the secondary winding which has a much larger number of coils. The low voltage supply to the coil is controlled by the distributor which also controls the spark plug that the high voltage surge is sent to. The distributor timing is critical and usually is timed mechanically from the engine. The diagram below shows the typical set - up of an ignition system for a spark ignition engine. Compression ignition engines such as the diesel engine do not use spark plugs to ignite the fuel - air mix. When the piston reaches the top of the compression stroke the temperature and pressure in the combustion chamber is sufficient to ignite the mixture. Controlled ignition in both spark ignition and diesel engines is essential for efficient combustion and avoid uncontrolled combustion effects such as pre- ignition, auto - ignition and engine knock. 15
  16. 16. Four Stroke Engine Exhaust 16
  17. 17. Four Stroke Engine Exhaust Exhaust gases are pushed out of the cylinder by the upwards motion of the piston following the ignition stroke. The exhaust gases are passed into the exhaust manifold and channelled into the exhaust pipe where they are released into the atmosphere. The exhaust system may contain a smoke box to trap the larger soot particles, it may also be fitted with a catalytic converter which removes some of the harmful components from the exhaust gases. On newer cars some of the exhaust gases are recycled back into the inlet system (typically at the manifold or air filter), this is known as exhaust gas re - circulation EGR. The efficiency of the combustion process and the design of the engine determine the exhaust constituents. Typically exhaust gases contain oxygen, nitrogen, water vapour, carbon dioxide, carbon monoxide, hydrogen, nitrous oxides, particulates and unburned hydrocarbons. 17
  18. 18. Four Stroke Engine Exhaust and Inlet Valve Overlap 18
  19. 19. Four Stroke Engine Exhaust and Inlet Valve Overlap Exhaust and inlet valve overlap is the transition between the exhaust and inlet strokes and is a practical necessity for the efficient running of any internal combustion engine. Given the constraints imposed by the operation of mechanical valves and the inertia of the air in the inlet manifold, it is necessary to begin opening the inlet valve before the piston reaches Top Dead Centre (TDC) on the exhaust stroke. Likewise, in order to effectively remove all of the combustion gases, the exhaust valve remains open until after TDC. Thus, there is a point in each full cycle when both exhaust and inlet valves are open. The number of degrees over which this occurs and the proportional split across TDC is very much dependent on the engine design and the speed at which it operates. 19
  20. 20. Four Stroke Engine Four Stroke Petrol Engine 20
  21. 21. Four Stroke Engine The cycle of operation in a four stroke petrol engine is completed in two revolutions of crank shaft or four strokes of piston. Stroke is defined as the distance traveled by the piston from one of the dead centers to the other dead centre. It is also equal to two times the crank radius. Hence in a four stroke engine work is obtained only during one stroke out of the four strokes of the piston required to complete one cycle. This engine works on Otto or constant volume cycle. 1. Suction stroke: To start with the piston is at or very near T.D.C. and the inlet valve is open and exhaust valve is closed. As the piston moves from T.D.C. to B.D.C. rarefaction is formed in the cylinder which causes the charge to rush in and fill the space vacated by the piston. The charge consists of a mixture of air and petrol prepared by the carburetor. The admission of charge inside the engine cylinder continues until the inlet valve closes at B.D.C. 2. Compression stroke: Both the valves are closed and the piston moves from B.D.C. to T.D.C. The charge is compressed up to a compression ratio of 5:1 to 9:1 and pressure and temperature at the end of compression are about 6 to 12 bar and 250º C to 300º C respectively. 3. Working, Power or Expansion stroke: When the piston reaches T.D.C. position, or just at the end of compression stroke, the charge is ignited by causing an electric spark between the electrodes of a spark plug, which is located some where in the walls of cylinder head. During combustion the chemical energy of fuel is released and there is rise in temperature and pressure of gases. The temperature of gases increases to about 1800º C to 2000º C and the pressure reaches 30 to 40 bar. Up till now the volume of gases formed however remains almost constant with both valves closed. Now the combustion products expand and push the piston down the cylinder. The reciprocating piston motion is converted into rotary motion of crankshaft by a connecting rod and crank. During expansion the pressure drops due to increase in the volume of gases and absorption of heat by cylinder walls. 4. Exhaust stroke: Theoretically exhaust valve opens at the end of working stroke when the piston is at B.D.C. position. But actually exhaust valve begins to open when about 85 % of the working stroke is completed. A pressure of 4 to 5 bar at this instant forces about 60 % of the burnt gases into the exhaust manifold at high 21
  22. 22. Four Stroke Engine speed. The remaining burnt gases are cleared off the swept volume when the piston moves from B.D.C. to T.D.C. During this stroke the pressure in side the cylinder is slightly above the atmospheric value. Some of the burnt gases are however left in the clearance space. The exhaust valve closes shortly after the piston reaches T.D.C. The inlet valve opens slightly before the end of exhaust stroke and the cycle repeats. Four Stroke Engine 22
  23. 23. Four Stroke Engine Working Of Four Stroke Engine 23
  24. 24. Four Stroke Engine Four Stroke Diesel Engine 24
  25. 25. Four Stroke Engine The cycle of operation in a four stroke diesel engine is completed in two revolutions of crankshaft or four strokes of piston using diesel oil as fuel. This engine works on diesel cycle. 1. Suction Stroke: Starting of engine is done by an electric motor or manually. In both cases the energy is supplied to the engine. In this stroke the inlet valve opens and the outlet valve remains closed. Piston moves from T.D.C. to B.D.C. and in this way a vacuum is created in the cylinder. This vacuum is filled by air alone and piston reaches to B.D.C. 2. Compression Stroke: Both valves are closed. This time piston moves from B.D.C. to T.D.C. Air is compressed in this stroke up to a compression ratio of 15:1 to 22:1 and a very high temperature is produced due to high pressure. The high temperature is the only cause of combustion of the fuel. The piston takes the power in this stroke from the flywheel. During this stroke the pressure and temperature attain a high value of 40 to 60 bar and 600º C to 700º C. 3. Working Stroke: At the end of compression stroke or when the piston reaches the T.D.C. position, a fine spray of diesel is injected in the cylinder through injector. The fuel burns by the heat of compressed air and due to its burning the power is produced. This power pushes the piston downward i.e. from T.D.C. to B.D.C. The excess energy of the piston is stored in the flywheel of the engine, which is further used for the remaining three strokes of the engine. The reciprocating motion of the piston is converted into the rotary motion of the crankshaft by connecting rod and crank. During expansion the pressure drops due to increase in volume of gases and absorption of heat by cylinder walls. 4. Exhaust Stroke: The exhaust valve begins to open when about 85% of the working stroke is completed. The force of piston coming from B.D.C. to T.D.C. forces the burnt gases into the exhaust manifold. Some of the gases are forced out due to higher pressure in the cylinder and the remaining gases are forced out by the piston. Some of the burnt gases are however left inside the clearance space. The exhaust valve closes shortly after T.D.C. The inlet valve opens slightly before the end of exhaust and in this way the cycle repeats. 25
  26. 26. Four Stroke Engine Four Stroke Disele Engine Working Of Four Stroke Disele Engine 26
  27. 27. Four Stroke Engine Comparison between petrol and diesel engine 27
  28. 28. Four Stroke Engine 28
  29. 29. Four Stroke Engine REFERENCES 29