Internal combustion engine (ja304) chapter 3


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Internal combustion engine (ja304) chapter 3

  2. 2. INTERNAL COMBUSTION ENGINE (JA304) INTRODUCTION : This topic covers the understanding of combustion process in spark ignition as well as compression ignition engine. It also includes knocking phenomenon and fuel characteristics.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  3. 3. INTERNAL COMBUSTION ENGINE (JA304) COMBUSTION PROCESS TERMS Normal Combustion A combustion process which is initiated solely by a timed spark and in which the flame front moves completely across the combustion chamber in a uniform manner at a normal velocity. Abnormal Combustion A combustion process in which a flame front may be started by hot combustion-chamber surfaces either prior to or after spark ignition, or a process in which some part or all of the charge may be consumed at extremely high rates. Surface Ignition Spark knock Hot spots-combustion- chamber depositsCHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  4. 4. INTERNAL COMBUSTION ENGINE (JA304) Spark Knock A knock which is recurrent and repeatable in terms of audibility. It is controllable by the spark advance; advancing the spark increases the knock intensity and retarding the spark reduces the intensity. Knock is the name given to the noise which is transmitted through the engine structure when essentially spontaneous ignition of a portion of the end gas—the fuel, air, residual gas, mixture ahead of the propagating flame—occurs. There is an extremely rapid release of most of the chemical energy in the end-gas, causing very high local pressures and the propagation of pressure waves of substantial amplitude across the combustion chamber.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  5. 5. INTERNAL COMBUSTION ENGINE (JA304) Surface Ignition Surface ignition is ignition of the fuel-air charge by any hot surface other than the spark discharge prior to the arrival of the normal flame front. It may occur before the spark ignites the charge (pre- ignition) or after normal ignition (post-ignition). Surface Ignition is ignition of the fuel-air mixture by a hot spot on the combustion chamber walls such as an overheated valve or spark plug, or glowing combustion-chamber deposit: i.e., by any means other than the normal spark discharge. Following surface ignition, a flame develops at each surface- ignition location and starts to propagate across the chamber in an analogous manner to what occurs with normal spark-ignition.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  6. 6. INTERNAL COMBUSTION ENGINE (JA304) Ignition The ignition system is designed to ignite the air and fuel that have been mixed in the fuel system. It is important to improve this system. Each year, ignition is becoming more and more computerized. Today’s ignition systems are almost totally computer controlled for improved combustion. Pre- Ignition This Is one process where the spark is heated up before the ignition begins. It causes rough running and in extreme cases, can do damage to the engine. Causes of pre-ignition include the following: 1. Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition. 2. Glowing carbon deposits on a hot exhaust 3. A sharp edge in the combustion chamber or on top of a piston 4. Sharp edges on valves that were reground improperly 5. An engine that is running hotter than normal due to a cooling system problem 6. Auto-ignition of engine oil droplets.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  7. 7. INTERNAL COMBUSTION ENGINE (JA304) Ignition Delay Ignition delay is defined as the time (or crank angle interval) from when the fuel injection starts to the onset of combustion.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  8. 8. INTERNAL COMBUSTION ENGINE (JA304) Delay period is the commencement of injection and it is indicated by the dot on the compression line 15° before dead centre. The period 1 is the delay period during which ignition is being initiated, but without any measurable departure of the pressure from the air compression curve which is continued as a broken line in the diagram as it would be recorded if there were no injection and combustion. 6000 1 5000 4000 kN/m2 3000 2000 1000 75° 50° 25° 0° 25° 50° 75° Degrees of crank angle Figure of Delay periodCHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  9. 9. INTERNAL COMBUSTION ENGINE (JA304) Combustion Combustion is one process in which air and fuel are burned after being mixed at a correct ratio of 14.7 parts of air to 1 part of fuel. The specific requirements for combustion including such as air, fuel and ignition requirements, timing, air-fuel ratios, compression ratio and engine efficiency. The internal combustion engine has certain requirements for efficient operation. There must be sufficient air for combustion, correct amounts of fuel mixed with the air, and ignition to start combustion. AIR FUEL COMBUSTIONCHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  10. 10. INTERNAL COMBUSTION ENGINE (JA304) Timing *One process of identifying when air, fuel and ignition occur in relation to the crankshaft rotation. This is a relationship between the position of the piston and crankshaft. For the engine to operate efficiently, the air and fuel mixture must enter the cylinder at the correct time. This mean that the intake valve must be opened and closed at the correct time. The exhaust valve must be opened and closed at the correct time too. The ignition must also be timed. The timing of the ignition can change with speed and load; if the process is correctly timed, maximum power will be obtained in converting chemical energy into mechanical energy.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  11. 11. INTERNAL COMBUSTION ENGINE (JA304) Air-Fuel ratio Air –fuel ratio is defined as the ratio of air to fuel mixed by the carburetor or fuel injectors. The term air-fuel ratio is often called the stoichiometric ratio. The air and fuel must be thoroughly mixed. Each molecule of fuel must have enough air surrounding it to be completely burned. If the two are not mixed in the correct ratio, engine efficiency will drop, and exhaust emission level will increase.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  12. 12. INTERNAL COMBUSTION ENGINE (JA304) Advance Timing Timing advance is required because it takes time to burn the air-fuel mixture. Igniting the mixture before the piston reaches TDC will allow the mixture to fully burn soon after the piston reaches TDC. If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder with the greatest force. Ideally, the time at which the mixture should be fully burnt is about 20 degrees ATDC. This will utilize the engines power producing potential. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston still moving up, causing knocking (pinging) and possible engine damage.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  13. 13. INTERNAL COMBUSTION ENGINE (JA304) If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston is already traveling too far down the cylinder. This results in lost power, high emissions, and unburned fuel.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  14. 14. INTERNAL COMBUSTION ENGINE (JA304) Retarded Timing Retarded timing can be defined as; changing the timing so that fuel ignition happens later than the manufacturers specified time. For example, if the timing specified by the manufacturer was set at 12 degrees BTDC initially and adjusted to 11 degrees BTDC, it would be referred to as retarded. In a classic ignition system with breaker points, the basic timing can be set statically using a test light or dynamically using the timing marks and a timing light.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  15. 15. INTERNAL COMBUSTION ENGINE (JA304) Graph engine pressure Vs crank angle Refer to the graph of engine pressure Vs crank angle, this graph shows how the pressure in the combustion chamber, between the compression rings, and in the crankcase varies with crank angle in an engine cycle. Pressure ( P ) P1= cylinder pressure P1 P2= pressure between Compression rings. P3 = crankcase pressure. Reverse blow by P2 Occur here P3 BDC TDC BDC Crank angle (θ) Figure 4.2 : engine pressures Vs crank angleCHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  16. 16. INTERNAL COMBUSTION ENGINE (JA304) There is a time delay in the pressure change from one chamber to the next due to the restricted flow passage created by the compression rings. Later in the power stroke, when the exhaust valve opens, pressure between the compression rings will be greater than in the combustion chamber, and some gases will be forced back into the chamber. This is called reverse blow by. Point of engine pressure Vs crank angle diagram. The process of relationship between engine pressures as a function of crank angle, is shown in Figure 4.2 the cylinder pressure ( P1), pressure between piston compression rings ( P2 ), and pressure in the crankcase ( P3 ). There is a time delay for pressure change from one chamber to the next due to the restricted flow passage past the pistons. When the exhaust valve opens and blow down occurs, pressure in the combustion chamber decreases quickly and opens P2 > P1. At this point, reverse blow by occurs. The need for crankcase ventilation can be seen by the pressure buildup in the crankcase.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  17. 17. INTERNAL COMBUSTION ENGINE (JA304) Knocking This is one process that happens within the combustion chamber. It sounds like a small ticking or rattling noise within the engine. In long term, the piston and ring can be damaged as well as the spark plug and valve. Another name for knocking is detonation. Valves Spark plug Second flame front from Knocking area ignition of low-octane Flame front gasoline Figure 4.4.1: knocking results when two flame fronts hit each other. One is from the spark plug combustion, and one is from early fuel ignition caused by poor-quality fuel.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  18. 18. INTERNAL COMBUSTION ENGINE (JA304) Figure 4.4.1 shows what happens inside a combustion chamber when knocking occurs. When the spark fires to ignite the air-fuel mixture, it produces a flame front. Shortly after ignition, a second explosion (flame front) and ignition of fuel takes place on the other side of the combustion chamber. This flame front is produced from low-octane fuel combusting, or burning, too early. When these two energy fronts hit each other, they cause a pinging or knocking within the engine. Knocking is usually caused by poor-quality fuel. The temperatures within the combustion chamber are high enough to cause the air mixture to ignite without the spark plug. Fuel is made to have anti-knocking characteristics. Actually, anti-knocking fuel needs higher temperature to start burning.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  19. 19. INTERNAL COMBUSTION ENGINE (JA304) In detail, the knocking for the cylinder pressure as a function of time in typical SI engine combustion chamber is shown in the following graphs ( Figure 4.4.5, 4.4.6, 4.4.7.) Cylinder Cylinder Pressure Pressure Time Time Figure 4.4.5 : Normal Combustion Figure 4.4.6 : Combustion with with no Knocking light Knocking Cylinder Pressure Time Figure 4.4.7: Combustion with Heavy KnockingCHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  20. 20. INTERNAL COMBUSTION ENGINE (JA304) Effects of knocking during engine process The effects of knocking during engine process are ;- 1. a drop in engine performance. 2. pollution of gases from the combustion is incomplete. 3. high consumption of fuel. Reduce knocking problem during engine process In this case we have three options to reduce knocking during engine process :- 1. Increase the ignition combustion engine. 2. Reduce the heat the final combustion. 3. Use high quality fuel.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  21. 21. INTERNAL COMBUSTION ENGINE (JA304) Compression Ignition Combustion The combustion process in a compression ignition engine starts when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression. Ricardo Diagram Ricardo diagram is another important term in combustion. This diagram is important because it is used to study about the time ignition of one combustion. If the fuel in the chamber burned very fast, slow, or late; we can see this problem in the Ricardo diagram (Figure 4.8.)CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  22. 22. INTERNAL COMBUSTION ENGINE (JA304) Pressure ( P ) Line 2 Line 1 Line 3 Crank angle ( TCD ) Figure 4.8 : Ricardo Diagram Line 1 explains the good condition of combustion. Line 2 explains the overhead and the curve in Line 3 explains late ignition.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  23. 23. INTERNAL COMBUSTION ENGINE (JA304) Catane Number There is a delay between the time that fuel is injected into the cylinder and the time that the hot gases ignite. This time period or delay is expressed as a catane number. Catane number ranges from 30 to 60 on diesel fuel. Catane number is an indication of the ignition quality of the diesel fuel. The higher the catane number, the better the ignition quality of the fuel. High catane numbers should be used to start an engine in cold weather. A catane number of 85 to 96 is often used for starting diesel engines in cold weather. If a low catane number is used in diesel engine, some of the fuel may not ignite. The fuel will then accumulate within the cylinder. When combustion finally does occur, this excess fuel will explode suddenly. This may result in a knocking sound as in gasoline.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)
  24. 24. INTERNAL COMBUSTION ENGINE (JA304) Exercise Question 1 Explain the term ―Knocking‖. Question 2 List the effects of knocking during engine process Question 3 List 3 options on how to reduce knocking problems during engine process. Question 4 What is combustion? Give three factors required to make combustion? Question 5 Draw and explain the Ricardo Diagram.CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)