Unit 5. Pressure-charGfiYg
PRE-READING TASKS
1. Pay attention to the correct pronunciation of the following words:
utilisation [ju:tilai'zeifen]
augmentation [agmen'teifen] residual [n'zidjual]
appreciable [э'ргкЬЫ] interference [jnts'fisrsns]
2. Try to understand the meaning of the following words and word combinations used in the text Pressure-charging from the definitions given below, induced (air) p.p. introduced, delivered (air)
ambient (air) a. surrounding atmospheric (air)
augmentation n. increase, raising, rise
a governor n. a regulating or controlling device, a controller
obtainable adj. which can be obtained/received
substantial adj. considerable, great enough
appreciable adj. that can be seen
less-exacting conditions less stressful or less severe conditions
sustain v. suffer (from), to be subjected to
resolution n. decision, solving and, as a result, removing or eleminating
3. Translate the words and word combinations using the definitions given above.
Induced air charge; ambient air density; augmentation of power; an external governor; an increase in power is obtainable; a substantial increase in power output; an appreciable reduction; less-exacting conditions at cylinders; the components which sustain the increased loads; resolution by a careful analysis.
READING
PRESSURE-CHARGING
In an engine which draws its combustion air direct from the atmosphere the density of the induced air charge is approximately the same as the ambient air density. As this air density determines the maximum weight of fuel that can effectively be burned per working stroke in the cylinder, it also determines the maximum power that can be developed by the engine. If, therefore, the charge air density is increased by the interposition of a suitable compressor between the ambient air and the cylinder, it follows that the weight of air per working stroke is increased and thereby a greater weight of fuel can be burned in the same cylinder, with proportionate augmentation of power. This is a self-acting process, which doesn't require an external governor.
The power for driving the compressor has an important influence on the operating efficiency of the engine. For example, it is relatively uneconomical to drive the compressor direct from the engine by chain or other mechanism because some of the additional power is absorbed thereby and there is thus an increase in specific fuel consumption for the extra power obtained. But if the compressor is driven by the heat energy in the engine exhaust gases - about 35 % of the total heat energy in the fuel is discharged to the exhaust gases - then an increase in power is obtainable which is almost proportional to that of the charge-air density. This is the essential principle of exhaust turbo-charging.
The turboblower comprises a gas turbine, driven by the engine exhaust gases, direct-coupled to a compressor, i.e. a blower, which draws air from the ambient atmosphere and discharges to the air inlet manifold on the engine. The power generated in the turbine must equal that required by the compressor. The advantages of pressure-charging by means of an exhaust turboblower system are:
1. a substantial increase in power output for any stated engine size and piston speed, alternatively, a substantial reduction in engine dimensions and weight for any stated horse-power;
2. an appreciable reduction in the specific fuel consumption rate at all engine loads;
3. a reduction in initial cost;
4. increased reliability and reduced maintenance costs, resulting from less-exacting conditions at cylinders.
In pressure-induction engines the components which sustain the increased piston loads and reactions may or may not be heavier than those in non-P.I. engines. This is a problem for resolution by a careful analysis of the working stresses in the respective components.