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Lesson 7

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  1. Lesson 7 • Text The Gas Exchange process(换气过程) Jimei university

  2. A basic part of the cycle of an internalcombustionengine is the supply of fresh air and removalof exhaust gases. This is the gasexchange. Jimei university

  3. Scavenging is the removal of exhaust gases by blowing in fresh air. Jimei university

  4. Charging is the fillingof the engine cylinder with a supply or charge of fresh air ready for compression. Jimei university

  5. Withsupercharging(增压) a larger mass of air is supplied to the cylinder by blowing it in under pressure. Jimei university

  6. Older engines were ‘naturally aspirated(自然吸气)’—taking fresh air only at atmospheric pressure. Jimei university

  7. Increasing the density of the charge air by the use of a suitable compressorbetween the air intake and the cylinder, the weight ofairinducedper working strokeis increased. Jimei university

  8. Thereby a greater weight of fuel can be burnt with a consequent increase in the cylinder power output. Jimei university

  9. This increasein charge air densityis accomplishedon most modern diesel engine typesby use of exhaust gas turbocharging, Jimei university

  10. in which a turbine wheel driven by exhaust from the engine is rigidlycoupledto a centrifugal type air compressor. Jimei university

  11. Both four-stroke and two-stroke cycle engines may be pressure charged. Jimei university

  12. It should be noted that supercharging is not simply a matter of adding equipment to non-supercharged engines. Jimei university

  13. The supercharged engine must be designed to withstand the increased pressures and thermal loads which results. Jimei university

  14. It is essentialto the scavenging process that the airentering the cylinderis at a higher pressure than the gasin the exhaust manifold. Jimei university

  15. On two-stroke diesels an electrically driven auxiliary blower is usually provided because the exhaust gas driventurbo-blower cannot provide enough air at low engine speeds. Jimei university

  16. High pressure supercharged engines use turbochargers inseries(串联的). The pressurizedair is usually cooled to increase the charge air density. Jimei university

  17. The turbo-blower or turbo-charger has on opposite ends of a single shaft an exhaust gas driven turbine and an air compressor. The compressor and the turbine are sealed from each other. Jimei university

  18. Exhaust gas Turbine Air Seal Shaft Exhaust gas Compressor Pressurize air Fresh air Fresh air Jimei university

  19. It is essential that each cylinder should be adequately scavenged of gases before a fresh charge of air is compressed, Jimei university

  20. otherwise this fresh air charge is contaminated by residual exhaust gasesfrom the previous cycle. Jimei university

  21. Further, the cycle temperature will be unnecessarily high if the air charge is heated by mixing with residual gases and by contact with hot cylinders and pistons. Jimei university

  22. In the exhaust turbocharged engines the necessary scavenging is achieved by providing satisfactory pressure difference between the air manifold and the exhaust manifold. Jimei university

  23. The air flow through the cylinder during the exhaust/scavenge overlap period has a valuable cooling effect; Jimei university

  24. it helps to increase the volumetric efficiency(容积效率)andto ensure a low cycle temperature and also to obtain a relatively cooler exhaust imposedon the turbine blades(叶片). Jimei university

  25. In the four-stroke cycle engine there is an adequate overlap between the air inlet valve opening and the exhaust valve closing. Jimei university

  26. With two-stroke cycle engines this overlap is limited by the engine design characteristics, and some slight mixing of exhaust gases and incoming air does occur. Jimei university

  27. A number of different scavenging methods are in use in slow speed two stroke engines. Jimei university

  28. In each the fresh air enters as the inlet port is opened by the downward movement of the piston andcontinues until the port is closed by the upward moving piston. Jimei university

  29. The flow path of the scavenge air is decided by the engine port shape and design and the exhaust arrangements. Jimei university

  30. Three basic systems are in common use: the cross flow(横流扫气), the loop(回流扫气) and the uniflow(直流扫气). Jimei university

  31. In cross (or transverse) scavenging the incoming air is directed upwards, pushing the exhaust gases before it. The exhaust gases then travel down and out of the exhaust ports. Jimei university

  32. Exhaust outlet (a) Cross scavenging Jimei university

  33. In loop scavenging the incoming air passes over the piston crown then towards the cylinder head. Jimei university

  34. The exhaust gases are forced before the air passing down and out of exhaust ports located just above the inlet ports. Jimei university

  35. (b) Loop scavenging Jimei university

  36. With uniflow scavenging the incoming air enters at the lower end of the cylinder and leaves at the top. Jimei university

  37. The outlet at the top of the cylinder may be ports (in the opposed piston engine对置活塞式发动机) or a large valve (in the poppet valve engine). Jimei university

  38. (c) Uniflow scavenging Jimei university

  39. Each of the systems has various advantages and disadvantages. Jimei university

  40. Cross scavenging requires the fittingof a piston skirt to prevent air or exhaust gas escape when the piston is at the top of the stroke. Jimei university

  41. Loop scavenge arrangements have low temperature air and high temperature exhaust gases passing through adjacent ports, causing temperature differential problems for the liner material. Jimei university

  42. Uniflow is the most efficient scavenging system but requires either an opposed piston arrangementor an exhaust valve in the cylinder head. Jimei university

  43. All three systems have the ports angled to swirl(使…成漩涡运动) the incoming air and direct it in the appropriate path. Jimei university

  44. It should be emphasized that opposed piston slow speed engines are no longer in production and poppet valves are used in the majority of new designs. Jimei university

  45. There are two distinctturbocharging methods in practical use, respectively termed the ‘constant pressure’ and ‘pulse’ systems. Jimei university

  46. It is the constant pressure system that is now used by all slow-speed two-stroke engines. Jimei university

  47. At one time almost all four-stroke engines operated on the pulse system, Jimei university

  48. though constant pressure turbocharging is now becoming more common as it provides greater fuel economy while considerably simplifying the arrangement of exhaust piping. Jimei university

  49. READING MATERIAL • A. CHARGE AIR COOLER Jimei university

  50. The engine is equipped with a charge air cooler. The insert type charge air cooler is mounted(安装) in a welded casing (air trunk). The trunk(扫气箱) is bolted to the engine block. Jimei university