文档介绍：PART 2
POWER AND PROPULSION CYCLES
PART 2 – POWER AND PROPULSION CYCLES
2A – Gas Power and Propulsion Cycles
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In this section we analyze several gas cycles used in practical applications for propulsion
and power generation, using the air standard cycle. The air standard cycle is an approximation to
the actual cycle behavior, and the term specifically refers to analysis using the following
assumptions:
• Air is the working fluid (the presence bustion products is neglected)
• Combustion is represented by heat transfer from an external heat source
• The cycle is ‘completed’ by heat transfer to the surroundings
• All processes are internally reversible
• Air is a perfect gas with constant specific heats
The bustion engine (Otto Cycle)
The different processes of an idealized Otto cycle (bustion engine) are shown
in Figure 2A-1:
3
P
Adiabatic reversible
QH
2
4
QL
5
P0 1
V = V
2 3 V1 = V4 V
Figure 2A-1: Ideal Otto cycle
i. Intake stroke, gasoline vapor and air drawn into engine (5 -> 1)
ii. Compression stroke, P, T increase (1->2)
iii. Combustion (spark), short time, essentially constant volume (2->3)
Model: heat absorbed from a series of reservoir at temperatures T2 to T3
iv. Power stroke: expansion (3 ->4)
v. Valve exhaust: valve opens, gas escapes
vi. (4->1) Model: rejection of heat to series of reservoirs at temperatures T4 to T1
vii. Exhaust stroke, piston pushes bustion products out of chamber 1->5
2A-1
The actual cycle does not have these sharp transitions between the different processes and might be
as sketched in Figure 2A-2
Not
P isentropic
Spark Exhaust
valve
opens
P0
Exhaust valve
closes
V
Figure 2A-2: Sketch of actual Otto cycle
Efficiency of an ideal Otto cycle
The starting point is the general expression for the thermal efficiency of a cycle:
work QQ+ Q
η==HL=+
heat input QH QH
The convention, as previous