spark ignition engines comb s5 ind .ppt
- 1. Combustion in SI Engines Dr BAIJU B
- 2. Combustion may be considered as a relatively rapid chemical combination of the hydrogen and carbon in the fuel with the oxygen in the air, resulting in liberation of energy in the form of heat. The energy released is utilized to drive the IC engine.
- 3. The conditions necessary for combustion are the presence of a combustible mixture and some means of initiating the combustion process. In a SI engine, the combustible mixture is generally supplied by the carburator, and a spark plug is utilised to initiate the combustion.
- 4. There are two general phases in the combustion process, the preparation phase, and the actual burning phase.
- 5. Normal combustion and flame front propagation Combustion in the SI engine roughly divided into two general types, normal and abnormal. Normal combustion will be discussed first.
- 6. In the ideal and air cycles, heat is assumed to be added instantaneously. In an actual SI cycle, combustion occurs over a finite period of time. It normally begins at the spark plug and progresses through the combustible mixture with a rather definite flame front separating the unburned charge from the products of combustion.
- 8. The rate of motion of the flame front across the combustion chamber is determined by the reaction rate and the transposition rate. The reaction rate is the result of a purely chemical combination process in which the flame eats its way into the unburned charge, it is comparable , to a forest fire progressing through the combustible dry leaves under no wind condition.
- 9. The transposition rate is the result of a physical movement of the flame front relative to the cylinder walls , and is due to the pressure differentials set up between the burning gases and the other gases in the combustion chamber.
- 10. Fig 8-1 (a) is assumed to be a combustion chamber hypothetically divided into three equal volumes and masses of combustible mixture. Assume that the flame proceeds across the chamber from left to right. If the mass of mixture in section (A) is completely burned, it will expand and compress sections (B) and ( C ) into smaller volumes of increased density.
- 11. The flame front has progressed across section A due to the reaction rate but has been moved to the right further due to the transposition rate. Now suppose the flame front progresses through section B. This section will expand and compress section C into a still smaller volume, and will also compress section A to a lesser extent.
- 12. The flame front will again be transposed to the right. If the number of sections is assumed to be infinite, the net result is a rather smoothly progressing flame front moved forward not only by the reaction effect, but also by the transposition effects of the expanding burning gases.
- 13. The flame velocity across the chamber follows a pattern similar to that in fig 8-2. The flame travel appears to pass through three distinct stages.
- 15. Initially (area I), the flame front progresses relatively slowly due primarily to a low transpositon rate and low turbulence. Since there is a relatively small mass of charge burned at the start, there is very little transposition of the flame front. It is thus propagated almost entirely by the reaction rate, resulting in a slower advance.
- 16. As the flame front proceeds into more turbulent areas and commences to consume a greater mass of mixture, it progresses more rapidly and at a rather constant speed (area II). The average velocity of the flame during this period of travel is referred to as the flame velocity or flame speed
- 17. Towards the end of flame travel, the volume of unburned charge is reduced appreciably, and the transposition rate again becomes negligible, thereby reducing flame speed. Also the reaction rate is again reduced since the flame is entering a zone of relatively low turbulence.
- 18. Variables affecting flame speed 1. Turbulence: The most important single factor affecting flame speed is turbulence. The flame speed increases with increasing turbulence. This is due to the additional physical intermingling of the burning and unburned particles at the flame front, which expedites reaction by increasing the rate of contact. Increasing engine speed generally increases turbulence and therefore , exerts considerable effect on flame speed.
- 19. 2. Air-fuel ratio : Another variable which affects the flame speed is A/F ratio. The highest flame speeds will be obtained with an A/F ratio somewhat richer than chemically correct. If the A/F ratio is increased or decreased from this value, the flame speed is reduced.
- 20. 3. Other factors: The flame speed is also affected by variables as the pressure and temperature of the entering charge, humidity, amount of residual gas, spark timing, and compression ratio. The effect of these variables is not generally large.
- 21. Rate of pressure rise: The rate of pressure rise is dependent upon the mass rate of combustion of the mixture in the cylinder.
- 22. Illustration of various combustion rates
- 23. Figure shows the relationship between pressure and crank angle for three different rates of combustion , namely, a high, a normal, and a low rate. With lower rates of combustion, it becomes necessary to initiate burning at an earlier point on the compression stroke because of the longer time necessary to complete combustion.
- 24. Also, note that higher rates of pressure rise, as a result of the higher rates of combustion, generally produce higher peak pressures at a point closer to TDC, which is a generally desirable feature. Higher peak pressures closer to TDC produce a greater force acting through a larger portion of the power stroke, and hence, increase the power output.
- 25. If the rate is too high, the forces exerted on the piston tend to cause rough operation. Designing and operating the engine in such a manner that approximately one half of the pressure rise has taken place as the piston reaches TDC.
- 26. Abnormal combustion Important types of abnormal combustion are preignition and detonation
- 27. Preignition: If some portion of the boundary of the combustion chamber, such as a spark plug, exhaust valve or carbon particle, becomes overheated under certain operating conditions, it is possible for this part to act in the same manner as the regular spark, and ignite the adjacent fresh combustible charge. An entirely distinct flame front is thus produced, and the process is termed preignition.
- 28. Such a condition is undesirable since combustion becomes both erratic and uncontrollable. Preignition tends to raise the temperature and pressure in the chamber which cause the temperature of the hotspot to rise further , and encourage still earlier preignition on succeeding cycles.
- 29. Preignition may advance peak pressures to such a point that they occur before the piston reaches TDC on the compression stroke. In such a case , the peak pressure in those cylinders which are preigniting will oppose piston movement during the last part of the compression stroke, thus decreasing total output as well as causing rough engine operation.
- 30. Detonation: A combustible mixture of fuel and air, under certain conditions of temperature, pressure, and density, has the chance of igniting without the assistance of an initiating flame or spark. Such an event is known as auto-ignition.
- 31. In SI engines, the main factor in the auto-ignition phenomenon is the last portion of the unburned charge in the combustion chamber. As the normal flame front proceeds across the chamber, it raises the pressure and temperature of the remaining portion of the unburned charge.
- 32. Under certain conditions of pressure, temperature, and density of the unburned charge, this charge may auto-ignite and burn almost instantaneously, thus releasing energy at a much greater rate than during the normal combustion process.
- 33. The extremely rapid release of energy causes pressure differentials of considerable magnitude in the combustion chamber which give rise to radical vibrations of the gaseous products, producing an audible knock. This condition is known as detonation.
- 34. Detonation is a most important aspect in the operation of SI engine, since it is the major factor limiting the compression ratio of an engine.
- 35. In order to auto-ignite, the last unburned portion of the charge must reach and remain for a definite amount of time above a certain critical temperature which is dependent upon conditions of pressure and density of the unburned charge.
- 36. Once these conditions are reached, a preparation phase commences, followed by the actual burning phase. The preparation phase is known as the ignition delay.
- 38. Fig 8-4 represents a normal flame front travelling across a combustion chamber from A towards D, and increasing the pressure, temperature, and density of the unburned charge (area BB’D). If this unburned charge does not reach its critical temperature for auto ignition, it will not auto-ignite, and the flame front BB’ will proceed on through the unburned charge to point D in an orderly manner.
- 39. If the unburned charge (area BB’D) reaches and remains above its critical conditions for auto-ignition, there is a possibility of detonation. In essence, a race develops between the flame front and the ignition delay. If the flame front can proceed from BB’ to D and consume the unburned charge in a normal manner, prior to completion of the ignition delay period, there will be no detonation.
- 40. If the flame front is able to proceed only as far as, say CC’, during the ignition delay period, then the remaining portion of the unburned charge (area CC’D) will detonate. Detonation occurs, in a SI engine, near the end of combustion.
- 41. In summary, if the unburned charge does not reach its critical temperature, there can be no detonation. If the ignition delay period is longer than the time required for the flame front to burn through the unburned charge, there can be no detonation. But if the critical temperature is reached and maintained, and the ignition delay is shorter than the time it takes for the flame front to burn through the unburned charge, then the charge will detonate.
- 42. In order to inhibit detonation, a high critical temperature for auto- ignition and a long ignition delay, are desirable qualities in SI engine fuels.
- 43. Variables affecting detonation 1. Temperature pressure and density factors: Any action which tends to reduce the temperature of the unburned charge will tend to prevent detonation by reducing the possibility for the charge to reach its critical temperature for auto ignition.
- 44. The following actions will reduce the possibility of detonation (a) Lowering the compression ratio lowers the pressure and temperature (b) Lowering the inlet temperature of the mixture (c) Retarding spark timing (d) Use excessively rich or lean mixtures to reduce flame temperature.
- 45. 2. Time factors: any action which tends to increase the normal flame speed, or length of the ignition delay period, will tend to reduce detonation. The following actions tend to reduce the tendency to detonate: (a) increasing turbulence and thus increasing flame speed (b) increasing engine speed (c) using a rich A/F ratio to obtain maximum flame speed