Turbojet, Turbofan Engines

Early jet engines worked on the process where the majority of the air passing into the inlet of the engine was used in the combustion and passed through the core of the engine to the exit of the engine exhaust. 

On the figure above you can see turbojet engine with all air going through the turbine. 

On Figure below there are two types of turbojets: centrifugal and axial. Many early turbojets were centrifugal, such as early Rolls-Royce engines. The outlet of the compressor stage is perpendicular to the axis of the rotation of the impeller. The main disadvantage is their quite large size. Early jets were subsonic, so drag wasn't such a problem. However as aircraft got faster it was needed to develop engine with higher performance, which led to axial turbojets. Axial flow turbojets compress the air parallel to the axis of rotation of the shaft. While axial flow engines are rather longer than centrifugal flow engines, the cross-sectional areas are smaller, which leads to smaller drag. 

Two main types of turbojets: centrifugal (left) and axial (right).

This engines has zero by-pass which is going to be main part of this article. 

Turbofan engines are a development from the turbojet. They operate using the same principles and have the same sections: compression, combustion, and turbine. However, on top of it, there is large fan in front which is surrounded by a duct. 

On figure below, you can see two different schematic pictures of turbofan engines. The top figure shows high-bypass engine which has a large fan that routes much air around the turbine. The bottom figure shows the low-bypass engine  which has a smaller fan routing more air into the turbine.

Of course, early aircraft engines produced sufficient thrust, however they burned a lot of fuel producing excessive amount of emissions and engines were as well pretty noisy. And that led to improvement in turbine propulsion technology to produce cleaner, more efficient and quieter engines by development of by-pass technology. Generally, but not completely, the higher the by-pass ratio, the higher efficiency is achieved. 

In zero by-pass turbojet engine the high temperature and high pressure exhaust is accelerated by the expansion through the propelling nozzle and produces all the thrust. The compressor absorbs all the mechanical power produced by the turbine. 

The first turbofan engines still had basic structure, but very large fan was placed in front of the engine. A turbofan engine is so sometimes called as fan-jet engine or by-pass engine, as it is jet engine variant which produces thrust using a combination of jet core outflow and by-pass air which has been accelerated by ducted fan that is driven by the jet core. 

In all jet engines, a high velocity exhaust is produced. Simplified thrust is derived as mass flow rate times velocity difference of velocity of the gas at the nozzle minus the intake air velocity (velocity of the aircraft), or simply the increase of the velocity of the air: Thrust = M . (v_e - v_i)

In case of rocket engine the gases which leave the engine are the products of the combustion

of the rocket propellants carried onboard. And so there is no intake velocity term v_i required. The thrust is then only: Thrust = M . v_e

In case of turbofan the situation needs to take by-pass air flow into account: Thrust = M_fan . (v_tb - v_i) + M_core . (v_e - v_i), where M_fan is mass flow rate of air through the fan duct, and v_tb is the velocity of air flow through fan duct. 

The ratio M_fan/M_core is called the by-pass ratio and is cited for both mixing and non-mixing turbofans. Engines with by-pass ratios of less than 2 are termed low by-pass ratio engines, while those with ratios above 2 are considered high by-pass. In a high by-pass design, the ducted fan and nozzle produce most of the thrust.

The by-pass ratio of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. For example, 10:1 bypass ratio means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core.

Turbofans represent an intermediate stage between turbojets, which derive all their thrust from exhaust gases, and turbo-prop engines which derive minimal thrust from exhaust gases. Turboprop engines are generally another variant built on the turbojet engine, and use the turbine to produce shaft work to drive a propeller. See the following picture. 

Turbofan engines are usually described in terms of by-pass ratio, which together with engine pressure ratio, turbine inlet temperature and fan pressure ratio are important design parameters. By-pass design provides a lower fuel consumption for the same thrust, measured as thrust specific fuel consumption. Thrust specific fuel consumption is the fuel efficiency of an engine design with respect to thrust output. 

High by-pass designs are the dominant for commercial aircraft. Business jets use medium by-pass ratio engines. Combat aircraft use engines with low by-pass ratios to compromise between fuel consumptions and the requirements of combat: high power-to-weight ratios, supersonic performance, and the ability to use afterburners.

The airbreathing engine burns fuel to produce useful gas kinetic energy. Some of this energy is lost in

the form of heat in the jet outflow, by kinetic heating, conduction to engine components, and friction. Thermal efficiency (ηth ) of the engine, is given as ratio of the gas energy produced in the engine to the heat energy released by the fuel in unit time.

Propulsive efficiency comparison for various gas turbine engine configurations

The generation of thrust as M . (V_e – V_i), is always accompanied by the rejection

of power in the form of wasted kinetic energy (Ek = 1⁄2 . M . (V_e – V_i)^2) with a consequent effect on the propulsive efficiency (ηp). For a given thrust, this wasted kinetic energy can be reduced by choosing a high value of air mass flow rate (M) and a low value of (V_e – V_i), since kinetic energy is proportional to the square of the velocity. 

While the turbofan is just an extension of the turbojet, there are different reasons to use the different by-pass designs. For example, having a large by-pass ratio is preferable from a noise and efficiency viewpoint. A large fan, however, increases the amount of drag and decreases the maximum operating speed of the engine, limiting the exhaust velocity. Using low by-pass design minimizes these limitations, leading to increases in engine speeds and exhaust velocity, while retaining the extra efficiency of a turbofan. 

For example, among the fastest planes, such as the SR-71 Blackbird, MiG-25, and the Concorde, all used turbojets. The lack of drag from using a fan, and resultant lower cross-sectional area, led to that it was much easier to optimize the aerodynamics for supersonic applications.

SR-71 Blackbird

The engine pressure ratio (EPR) is  defined to be the total pressure ratio across the engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor. 

credit: NASA

The engine pressure ratio is simply the product of the pressure ratio across all of the engine components, so 

EPR = pt8 / pt2 = (pt3 / pt2) * (pt4 / pt3) * (pt5 / pt4) * (pt8 / pt5). For the numbers, see the Figure above. 

The engine temperature ratio (ETR) is defined similar to the engine pressure ratio and it is the total temperature ratio across the engine. ETR is the ratio of nozzle total temperature Tt8 to compressor face total temperature Tt2. The ETR is simply the product of the temperature ratio across all of the engine components: 

ETR = Tt8 / Tt2 = (Tt3 / Tt2) * (Tt4 / Tt3) * (Tt5 / Tt4) * (Tt8 / Tt5). Numbers are in the same order as the previous. 

If we know engine temperature ration, and the corresponding engine pressure ratio, we can determine the thrust of an engine using the nozzle performance information and the thrust equation.  The general thrust equation is given as: 

F = M_e . V_e - M_i . V_i + (p_e - p_i) * Ae,

where thrust F is equal to the exit mass flow rate M_e times the exit velocity V_e minus the free stream mass flow rate M_i times the free stream velocity V_i plus the pressure difference across the engine (p_e - p_i) times the engine area Ae. For gas turbine engines, the nozzle is usually designed to make the exit pressure equal to free stream, meaning that the p_e = p_i. In that case the thrust equation simplifies to: 

F = M_e . V_e - M_i . V_i

Specific thrust is another useful parameter. It is the thrust per unit air mass flowrate of a jet engine (e.g. turbojet, turbofan, etc.) and it can be calculated by the ratio of net thrust to total intake airflow: 

Fs = F / M_i

Low specific thrust engines tend to be more efficient of propellant  at subsonic speeds, but also have a lower effective exhaust velocity and lower maximum airspeed. High specific thrust engines are mostly used for supersonic speeds.

Turbofan engines come in a variety of engine configurations. The basic element of a turbofan is a spool, a single combination of fan/compressor, turbine and shaft rotating at a single speed. 

The single-shaft turbofan is probably the simplest configuration, comprising a fan and high-pressure compressor driven by a single turbine unit, all on the same spool. Mirage 2000 is an example of aircraft using single engine single shaft turbofan. 

Aft-fan turbofan was one of the earliest turbofans, which featured and integrated aft fan with low pressure turbine unit located in the turbojet exhaust jetpipe. 

Many turbofans have at least basic two-spool configuration, where the fan is on a separate low pressure  spool, running concentrically with the compressor or high pressure spool. The low pressure spool runs at a lower angular velocity, while the high pressure spool runs faster and the compressor compresses part of the air for combustion. Rolls-Royce BR-700 is an example of this configuration.

Boosted two-spool is used in order to achieve higher overall pressure ratio. It can be achieved by either raising the high pressure compressor pressure ratio or adding an intermediate pressure compressor between the fan and the fan and high pressure compressor to boost the latter unit to raise the overall pressure ratio of the engine cycle. 

Three-spool configuration was used by Rolls-Royce for their large civil turbofans, where the T-stages of the boosted two-spool configuration were separated into a separate intermediate pressure spool, driven by its own turbine.

The geared turbofan configuration is a type of turbofan, that has a gearbox between the fan and the low pressure shaft to spin each at optimum angular velocities.

1. Fan 2. Gearbox

In a geared turbofan, a planetary reduction gearbox is placed between the fan and the low pressure shaft allowing the latter to run at a higher rotational speed thus enabling fewer stages to be used in both the low pressure turbine and the low pressure compressor, increasing efficiency and reducing the weight. However, some energy will be lost as heat in the gear mechanism.  Also the weight which saved on turbine and compressor stages is partly offset by that of the gearbox.

One more type on the end. It belongs to the geared turboprop, however the flow is different. It is turboprop engine, PT6 engine. It consists of two basic sections: a gas generator with accessory gearbox and a free power turbine with reduction gearbox.  It often seems to be mounted backwards in an aircraft in so far as the intake is at the rear and the exhaust at the front. Many variants of the PT6 have been produced also for helicopters, land vehicles, hovercraft, boats, etc. 

PT6: see inlet and exhaust position

Comparison to common types: inflow left, exhaust right

So, how to conclude this article? What is the main difference between turbofan engine and turboprop engine? I may not have talked about it clearly in the article, but I'd like to make it clear:

1. In turbofan engines, a gas turbine engine is used to drive a fan to generate the thrust while, in turboprop engines, it is used to drive a propeller.

2. In turbofan engine, thrust generated is a combination of by-pass flow and gas turbine exhaust, while turboprop engines generate thrust almost completely by the propellers.

3. Turbofan engines perform with good efficiency at both supersonic and transonic flight, but a turboprop engine can be only used in subsonic flight.