Approximation of Jet Engine Thermodynamic Cycle – Joule (Brayton) Cycle

15 question-and-answer flashcards that review definitions, processes, formulas, engine types and simulation results from the provided lecture notes on the Joule (Brayton) cycle and its application to jet engines.

What thermodynamic cycle describes the working principle of gas turbines, turbojets and ramjets?

The Joule (Brayton) cycle.

Which two kinds of thermodynamic processes form the ideal Joule cycle?

Two isobaric processes (heat addition and rejection) and two adiabatic processes (compression and expansion).

During which stage of the Joule cycle does adiabatic compression occur and what happens to pressure and temperature?

Stage 1→2; pressure and temperature both rise while no heat is exchanged with the surroundings.

What type of process takes place in the combustion chamber during stage 2→3 of the Joule cycle?

Isobaric heat addition at nearly constant pressure.

In an open Brayton cycle, what leaves the engine during stage 4→1 instead of returning to the compressor?

The exhaust gases (working fluid) are expelled into the atmosphere.

Write the ideal thermal efficiency equation (η) of the Joule cycle in terms of compression ratio ε and specific-heat ratio k.

η = 1 − 1 / ε^(k−1).

Why must engineers choose an optimal compression ratio for a jet engine?

Because excessively high ratios can cause mechanical damage while too low ratios give poor thrust; the optimal value maximizes efficiency with minimal losses.

Which Newtonian principle explains how a jet engine produces thrust?

By accelerating mass rearward, Newton’s second and third laws create an equal and opposite forward thrust on the engine/aircraft.

What distinguishes a two-stream turbofan (fanjet) from a single-stream turbojet in terms of airflow?

A turbofan has a large fan that directs part of the inlet air around the core (bypass flow), improving overall efficiency and lowering fuel consumption.

Name two fighter aircraft that employ two-stream turbofan engines mentioned in the notes.

The F-22 Raptor and the Sukhoi Su-57.

How does a propfan (propfan/prop-fan jet) differ from a turboprop in generating thrust?

A propfan produces thrust from both its propeller (prop-fan) and the exhaust jet, whereas a turboprop gains almost all thrust from the propeller alone.

Why are rocket engines able to operate in outer space while air-breathing engines cannot?

Rocket engines carry both fuel and oxidizer, so they do not depend on atmospheric oxygen and can work in a vacuum.

What auxiliary devices are commonly used to start a jet engine before it becomes self-sustaining?

Electric or pneumatic starters, or an Auxiliary Power Unit (APU).

In the Octave simulation of the Joule cycle, which four main thermodynamic variables are calculated at each state point?

Temperature, pressure, specific volume, and the work/heat associated with the compressor and turbine.

According to the sample Octave output (compression ratio = 30, T3 = 1700 K), what was the calculated thermal efficiency of the cycle?

About 62 % (specifically 62.16 %).