Turbofan - Introduction

Introduction

A single-spool (or single-shaft) turbojet (which was the most basic form and the earliest type of jet engine to be developed) has 4 main stages, popularly known as Suck-Squeeze-Bang-Blow :

• 1 (suck) air enters through an intake,
• 2 (squeeze) and is compressed by an axial compressor to a greatly increased pressure and temperature.
• 3 (bang) The hot compressed air exits the compressor into a combustion chamber, where it is mixed with a fuel (e.g. kerosene), and combusted, greatly increasing the temperature of the compressed gases (at a constant pressure).
• 4 (blow) The very hot, highly compressed, combustion gases then enters a turbine stage, where thermal energy is converted to mechanical work from the pressure and temperature difference between the turbine inlet and outlet.

The mechanical power from the turbine in turn is used to drive the compressor, completing the engine cycle. A higher combustion temperature leads to a higher efficiency according to the Brayton cycle.

It is important to note that in a turbojet the compressor and turbine taken together form a net-zero mechanical energy system, i.e. all the mechanical shaft power produced by the turbine is consumed by the compressor. The net output of a turbojet is not shaft power, instead it is the kinetic energy of the jet exhaust itself. Although the expansion process in the turbine reduces the gas pressure (and temperature), there remains considerable thermal energy and pressure in the gases leaving the turbine. These energy forms are partly converted into kinetic energy by expansion to ambient pressure through a propelling nozzle, forming a high-velocity flow which provides reactive propulsion.

After World War II, two-spool (or two-shaft) turbojets were developed to make it easier to throttle back compression systems with a high design overall pressure ratio (i.e., combustor inlet pressure/intake delivery pressure). Adopting the two-spool arrangement enables the compression system to be split in two, with a low pressure (LP) compressor supercharging a high pressure (HP) compressor. Each compressor is mounted on a separate (co-axial) shaft, driven by its own turbine (i.e., the HP turbine and LP turbine). Otherwise, a two-spool turbojet is much like a single-spool engine.

Modern turbofans evolved from the two-spool axial-flow turbojet engine, essentially by increasing the relative size of the low pressure (LP) compressor to the point where some (if not most) of the air exiting the unit actually bypasses the core (or gas-generator) stream passing through the main combustor. Civil-aviation high-bypass turbofans usually have a single large fan disk, whereas most military-aviation low-bypass turbofans (e.g. combat and trainer aircraft applications) have multi-disk compressors as a compromise between greater power-to-weight ratios, supersonic performance, and the capability of using afterburners, versus the higher fuel economy of a high-bypass design. Modern military transport turbofan engines are virtually identical to their civilian counterparts.

Turboprop engines are gas-turbine engines that deliver almost all of their power to a shaft to drive a propeller. Turboprops remain popular on very small or slow aircraft, such as small commuter airliners, for their fuel efficiency at lower speeds, as well as on medium military transports and patrol planes, such as the C-130 Hercules and P-3 Orion, for their high take-off performance and mission endurance benefits. Like reciprocating propeller engines, turboprops can be used with controllable pitch propellers which allow thrust to be adjusted independently of the engine rotation speed.

If the turboprop is better at moderate flight speeds and the turbojet is better at very high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls-Royce). Another name sometimes used is ducted fan, though that term is also used for propellers and fans used in vertical-flight applications.

The difference between a turbofan and a propeller, besides direct thrust, is that the intake duct of the former slows the air before it arrives at the fan face. As both propeller and fan blades must operate at subsonic inlet velocities to be efficient, ducted fans allow efficient operation at higher vehicle speeds. Some large modern turbofans, like the Trent, have blade tip speeds at 1730 km/h.

Depending on specific thrust (i.e., net thrust to intake airflow), ducted fans operate best from about 400 to 2,000 kilometres per hour (250 to 1,200 mph), which is why turbofans are the most common type of engine for aviation use today—in airliners as well as in subsonic and supersonic military fighter and trainer aircraft. It should be noted, however, that turbofans use extensive ducting to force incoming air to subsonic velocities (thus reducing shock waves throughout the engine).

Bypass ratio (bypassed airflow to combustor airflow) is a parameter often used for classifying turbofans; when the low-bypass Conway engine entered service in 1960, no one even called it a turbofan, that term first being applied to Pratt and Whitney's JT3D with its 1-to-1 bypass.

The noise of any type of jet engine is strongly related to the velocity of the exhaust gases, typically being proportional to the eighth power of the jet velocity. High-bypass-ratio (i.e., low-specific-thrust) turbofans are relatively quiet compared to turbojets and low-bypass-ratio (i.e., high-specific-thrust) turbofans. A low-specific-thrust engine has a low jet velocity by definition, as the following approximate equation for net thrust implies:

where:

intake mass flow

fully expanded jet velocity (in the exhaust plume)

aircraft flight velocity

Rearranging the above equation, specific thrust is given by:

So for zero flight velocity, specific thrust is directly proportional to jet velocity. Relatively speaking, low-specific-thrust engines are large in diameter to accommodate the high airflow required for a given thrust.

Although jet aircraft are loud, a conventional piston engine or a turboprop engine delivering the same thrust would be much louder.