Upskill / Space & drones / Engine thermodynamics: the Brayton cycle, efficiencies and bypass ratio
Engine thermodynamics: the Brayton cycle, efficiencies and bypass ratio
The numbers behind a jet engine: thrust from the momentum equation, the ideal and real Brayton cycle, propulsive and overall efficiency, specific fuel consumption, ratings and flat rating, worked through one illustrative turbofan design study. Part 1 of the Propulsion engineering series.
By Aerospace Garage Desk · In practice · 45 minutes · 24 narrated pages
What you will be able to do
✓ Calculate gross thrust, ram drag, net thrust and specific thrust for a two-stream turbofan from the momentum equation.
✓ Calculate ideal and real Brayton cycle efficiency and specific work, and explain how pressure ratio, turbine entry temperature, component efficiencies and cooling air interact.
✓ Calculate propulsive efficiency and explain why lower fan pressure ratio drives bypass ratio upwards until fan size, weight and drag stop paying back.
✓ Combine thermal and propulsive efficiency into overall efficiency, convert thrust specific fuel consumption between units and relate it to flight speed.
✓ Explain thrust lapse with altitude and temperature, certified take-off and maximum continuous ratings, flat rating and the corner point, and estimate hot-and-high thrust.
✓ Draw the thermodynamic lessons of an official investigation report on high-altitude flame-out and core lock.
Course content
Session 1 · Thrust and the momentum equation
1.1
Thrust is momentum: the equation behind every number · free preview
Gross thrust, ram drag and specific thrust
Two streams, one engine: fan and core thrust
Worked example: Vltava Propulsion finds the cruise thrust
Session 2 · The Brayton cycle with real numbers
1.2
The ideal Brayton cycle: pressure ratio sets the efficiency
The real cycle: losses, work ratio and the best pressure ratio
Turbine entry temperature and the price of cooling air
Worked example: Vltava checks a cycle against its ideal
Session 3 · Propulsive efficiency and the bypass ratio
1.3
Propulsive efficiency: the energy left behind in the jet
Why bypass ratio keeps rising
The price of a big fan: weight, drag and the optimum
Worked example: bypass ratio five or ten?
Session 4 · Overall efficiency and specific fuel consumption
1.4
Overall efficiency: thermal times propulsive
Thrust specific fuel consumption and what it really measures
What moves specific fuel consumption
Worked example: from the cycle to fuel per hour
Session 5 · Off-design: altitude, temperature and ratings
1.5
Altitude and speed: why thrust lapses
Ratings: take-off, maximum continuous and the minimum engine
Flat rating, the corner point and the hot day
Worked example: a hot afternoon at a high airport
Session 6 · At the edge of the envelope
1.6
Near the ceiling: when thrust available meets drag
Flame-out, windmilling and rotor lock
Where the Propulsion engineering series goes next
From the investigation file: Pinnacle Airlines Flight 3701
Who this course is for
Propulsion, performance and design engineers at engine makers, airframers and design organisations who need to work cycle numbers, not just describe them
Powerplant and performance engineers in airlines, MROs and lessors who compare engines, judge fuel-burn claims and explain ratings and flat rating to colleagues
Engineering graduates and licensed engineers moving into engine design, certification or fleet technical roles who want the thermodynamics behind the data sheet
You need: Comfort with basic algebra and a calculator; How gas-turbine engines work is useful background.
Upskill courses are for professional development and are not approved by any aviation authority. Learner Terms