Turbojet and Turbofan Jet Engine Overview

Part of a series of Jet Engine Overviews

Turbojet/Turbofan engines are the most common type of jet engines in use today. From commercial airliners to jet fighters, the vast majority of production jet aircraft utilize at least one of these types of engines. These engines are relatively simple air breathing designs which follow the basic layout of a jet engine.

Turbojet Engines

The turbojet engine is considered to be the most basic jet engine design. It consists of an intake, a compression chamber, a combustion chamber, a turbine and an exhaust nozzle.

Air enters the intake and immediately passes into the compression chamber. In the compression chamber air is compressed by spinning fans as it passes through a narrowing duct into the combustion chamber. Just before the air reaches the combustion chamber, it is mixed with fuel. Inside the combustion chamber, the fuel/air mixture is ignited. The hot, high-pressure air expands out of the combustion chamber where it passes through a set of turbines. The passing air spins these turbines which, in turn, spin the compressive fans at the fore of the engine, sustaining the flow of air. After passing the turbines, the air exits the back of the engine through the exhaust nozzle. In engines with an afterburner, air is reheated between the turbine and the exhaust nozzle. Afterburners are typically about 4 times less efficient than the primary combustion chamber due to the decreased air pressure aft of the turbine.

The following graphs allow an easier visualization of the air temperature, pressure and velocity during the operation of a turbojet engine.

Turbofan Engines
The turbofan engine is an augmentation of the turbojet engine. As the name would imply, the turbo fan adds a fan which functions similarly to a prop on a propeller driven plane. The turbofan engine contains a standard turbojet engine at its core but adds a duct around the outside of this engine for some air to pass through.

The flow that passes through the integral turbojet is called core airflow while the air that moves through the outer duct is called bypass flow.
In General, there are 2 different types of turbofan engines, they are…

Low Bypass Turbofan Engines
In low bypass engines, a relatively large portion of the air that passes through the engine is core airflow. To put it another way, there is a lower amount of bypass flow. The ratio of bypass airflow to core airflow is usually around 2:1 or less, with low bypass engines being optimized for efficient flight at higher Mach numbers. Most modern fighter jets utilize low bypass engines. Fighters like the F-16 and F/A-18 have bypass ratios of around .35:1.
High Bypass Turbofan Engines
Conversely, high bypass engines allow lots of air to bypass the integral turbojet. High bypass engines usually have ratios between 5:1 and 10:1. These engines are optimized for efficient subsonic flight, usually around Mach .8. Most modern commercial airliners and some military transports use this type of engine.

Turbojet/Turbofan History

In 1939, the He 178 was the first turbojet aircraft to fly.
Shortly thereafter, in 1944, the Me 262 became the first operational jet fighter, followed closely by the Gloster Meteor in the same year.
By the 1950s, jet engines were nearly ubiquitous on military aircraft, and some had even been approved for civilian use, such as those mounted on the de Havilland Comet.
By the 1970s, jet engines had become all but ubiquitous in the commercial air industry due to the introduction of the high bypass turbofan.
In 2003, the Concord makes its last flight, marking the end of the turbojet engine in the commercial airline industry.
As of writing, all current production fighter planes use some form of turbofan engine, along with most bombers and most commercial airliners.

Turbojet/Turbofan pros and cons

Turbojet Pros