Working Principles of Jet Engines: Turbojet and Turbofan

This article explains the working principles of turbojet and turbofan engines, which are the primary types of jet engines. It discusses how these engines generate thrust based on Newton's 3rd law of motion and the combustion of air and fuel. The article also explores the components of turbojet and turbofan engines, such as compressors, combustion chambers, turbines, and nozzles. Additionally, it explains the differences between net thrust and gross thrust.

Question 10
a) The very first jet engine was the turbojet. Turbojet is primary turbine engine that
generates all of its propulsion from the exhaust from the turbo section. Jet engines are
reaction engines that provides propulsion or thrust by expelling a reaction mass. Its
working principles are based on Newton’s 3rd law of motion (Cumpsty, 2003). This law
states that every action has an opposite and equal reaction. It extracts energy from a flow
of combustion gases. The ambient air is taken at the engine intake made of centrifugal
and or axial compressor which increases both the air temperature and pressure before it is
fed into the combustion chamber. After ignition, the entire process is self-sustaining
because of the constant flow of air and fuel provides for continuous combustion.
A high reaction mass (with high energy) is thus generated by burning the air/fuel mixture
and leaves the combustion chamber via one or more turbine with drives the compressor
(s). What is left of the exhaust gas is ejected through the nozzle thus providing the thrust
to propel the aircraft forward (Dickson & Chadwick, 2014). From the thrust equation of a
turbojet, three things occur.
The engine is flown through via the air intake (change in momentum)
Fuel’s momentum
Pressure difference is capable of generating a force experienced in the nozzle’s exhaust to
multiply by nozzle’s area.
The thrust is thus defined in two ways
Net thrust that is experienced by the jet in flight. Both the air momentum at the entrance
and exit to the engine are considered.
Gross thrust which is generated by the turbojet engine while still on the ground. The
momentum at the inlet is zero.
b. The thrust produced by a Turbofan combines jet-core efflux. Duct fan accelerate
bypassed air. Duct fan is driven by the jet core. Propulsion of Turbofan is obtained by
driving the turbine fan at the engine in the engine and its front in order to drive the
turbine. There two or more separate shafts in order to allow the fan rotate slower
compared to the inner engine core (Roux, 2007). The fan has a case that directs air flow
through the fan. A proportion of the air finds its entrance to the turbine engine while the
other proportion bypasses around the engine. The fan performs two roles. The proportion
of the air stream from the fan goes through the core thus allowing oxygen to burn fuel in
order to produce a thrust. The remaining air stream bypasses engine core in order to mix
the fast flowing stream form engine’s rear core. The efficient propulsion depends on the
relative airspeed ratio of exhaust and the surrounding air.
Thrust is mainly provided by a large proportion of air that only flow via the fans and
bypasses the rest of the engine in high-bypass engines.
Functions of Turbofan
Fans
The fans increases air velocity as air enters the engine while the remaining air is bypassed
around the engine.

Compressors. The remaining air is compacted via amalgamation of various fans plus a
converging duct. This process increases the core air pressure by more than 10000%
making the core air to have a lot of energy (Morris, Geiselhart, Coen, & Center, 2009).
Combustion chamber. The air that flows through the small combustion chamber mixes
with the jet fuel and then subjected to ignition. This is a significant part of the jet engine.
Turbine. The hot air that has high energy flows into the turbine section made of diverging
duct and more fans. The turbine pushes out the energy form the air making it to blow out
the turbine fans spin them with massive power. This is measured via Back Work Ratio.
Nozzle
The air at the core has high energy. The nozzle concentrates this air and then blows it out
of the rare engine. The thrust associated with the bypass air thrust propels the air forward.
Fthrust=mbypass × ∆ vbypass + mcore × ∆ vcore
mbypass refers to the mass flow-rate for the air that is bypassed
mcore the mass flow rate for the combusted air
∆ vcore is the velocity variation due to the compressor, fan, combustion chamber and
turbine.
∆ vbypass the air velocity changes as a result of the fan.
c. The pistons are functioned either one or more cylinders. Each of this made of fitting
piston that can move independently forming a repeated cycle within the cylinder. The
piston connects to a crankshaft through a connected rod. The hot gasses generated expand
because of the powerful force derived from by igniting the fuel inside the cylinder. This
drives the corresponding piston forward thus moving the connecting rod forward causing
crankshaft to turn. In a complete cycle, the piston is pushed back into the cylinder
forming a continuous and repeated cycle (Morris, Geiselhart, Coen, & Center, 2009).
The reciprocating action and linear motion is converted to rotary motion by the
crankshaft thus driving the propellers.
Engine power output. Power is a product of force and time. Torque on the other hand is
the force and engine speed. It is measured in crankshaft revolutions per minute (rpm).
Torque is rotational force that acts on the engine crankshaft.

References
Cumpsty, N. A. (2003). Jet Propulsion: A Simple Guide to the Aerodynamic and
Thermodynamic Design and Performance of Jet Engines. Cambridge, England:
Cambridge University Press.
Dickson, E. D., & Chadwick, D. L. (2014). Observations on Disturbances of Equilibrium and
Other Symptoms Induced by Jet-Engine Noise. The Journal of Laryngology & Otology,
65(03), 154-165.
Morris, S. J., Geiselhart, K. A., Coen, P. G., & Center, L. R. (2009). Performance potential of an
advanced technology Mach 3 turbojet engine installed on a conceptual high-speed civil
transport.
Roux, E. (2007). Turbofan and Turbojet Engines: Database Handbook. Elodie Roux.

Appendices
Jet engine thrust (turbojet) and equation
Turbofan thrust and equation
Front view of an aircraft on a runaway