Potter & Somerton - Chapter 8: Gas Power Cycles

Section 8.2 - Gas Compressors. This section gives an interesting account and description of gas compressors, however we will not consider interstage cooling in this course. Note that both the centrifugal and multiple axial flow compressor stages are used in the T700 Gas Turbine engine (Chapter 6) that Dr Tom Scott presented to us in the ME 321 course.

Section 8.3 - The Air-Standard Cycle. This section introduces our approach to analyzing the two types of internal combustion engine that we consider - the spark ignition Otto cycle engine and the compression ignition Diesel cycle engine. We will treat the entire cycle as equivalent to a closed cycle using air, the combustion being treated as external heating of the air, and the exhaust and air intake strokes being treated as constant volume cooling of the air. This allows us to obtain a closed form approximation of the cycle performance without being concerned with the complexities associated with the combustion process. In this section Potter introduces the concepts of compression ratio (r) and mean effective pressure (MEP). We will discuss Example 8.3 in class.

Sections 8.5 (The Otto Cycle) and 8.6 (The Diesel Cycle). These two sections form the major areas of concentration for this chapter, and we will cover the equation development and examples in detail. Note that we previously did an initial discussion of the Diesel cycle engine in ME321 (refer back to Chapter 3 and Chapter 4). We will review this as well as Examples 8.4 and 8.5 in class. We recommend that you attempt as many as possible of the following Supplementary Problems for these sections:

• Otto Cycle: 8.36, 8.37, 8.38, 8.40b. (For Problem 8.40b
• Diesel Cycle: 8.41, 8.42, 8.43, 8.45, 8.46a.
  Additional Exercise 8.1 An ideal air-standard Diesel cycle engine has a compression ratio of 18 and a cutoff ratio of 2. At the beginning of the compression process, the working fluid is at 100 kPa, 27°C (300 K). Determine the temperature and pressure of the air at the end of each process, the net work output per cycle [kJ/kg], and the thermal efficiency.
  (Answers: T₂ = 889 K, P₂ = 5336 kPa, T₃ = 1778 K, T₄ = 884 K, P₄ = 295 kPa, w_net = 612 kJ/kg, Thermal efficiency h_th = 58%)

Note that the nominal values used for air at 300K are C_P = 1.00 kJ/kg.K, C_v = 0.717 kJ/kg.K,, and k = 1.4. However they are all functions of temperature, and with the extremely high temperature range experienced in Diesel engines one can obtain significant errors. For this exercise (as well as 8.40b and 8.46a above) you should use the temperature dependence table of Specific Heat Capacities of Air.

Section 8.8 (The Stirling and Ericson Cycles). We covered the Stirling cycle in ME321 in Chapter 3 and Chapter 4. We will review the Ericson cycle if time permits.

• Supplementary Problems 8.49, 8.51

Sections 8.9 (The Brayton Cycle) and 8.10 (The Regenerative Gas Turbine Cycle). Again, only if time permits. Recall the T700 Gas Turbine engine which we covered in Chapter 6.

• Supplementary Problems 8.52, 8.53, 8.55, 8.56, 8.57, 8.62, 8.64

Sections 8.12 (The Turbojet Engine). Review this section including Example 8.13

All answers to the Supplementary Problems are given at the end of the chapter.

Back to the Thermodynamics Home Page