As a matter of fact to power the aircraft and to provide required amount of thrust “Turbo” engines are used and there are basically “5 types” of turbo engines with varied applications. So let’s find out the differences.

What is a Turbo Engine?

An Internal Combustion engine equipped with turbo chargers is a turbo engine.

Turbos are composed of a shaft with a turbine wheel on one end and a compressor wheel on the other. These are covered by a snail-shaped housing featuring an inlet port, through which the wasted exhaust gases enter at a high pressure. As the air passes through the turbine, the turbine spins and the compressor turns with it, drawing in large amount of air which is compressed and passed out of the outlet port.

A turbocharger which can be fitted to an engine for better air intake

A pipe feeds this compressed air back into the engine cylinders via an intercooler, which cools the air before it reaches the cylinders because the air must be of the optimum temperature required for combustion. As turbos run at high speeds (up to 250,000 RPM), they typically have an oil cooling system to make sure they don’t run too hot.

The first turbocharger was produced in the late 19th century by German engineer, Gottlieb Daimler, but it did not come to prominence until after WWI, when aircraft manufacturers began adding them to aeroplanes to provide power to engines operating at higher altitudes, where the air is thinner.

Turbochargers weren’t added to car engines until 1961, when US manufacturer Oldsmobile, used a simple turbo to boost the power of a 3.5L V8 engine. In 1984, Saab developed a new, more efficient turbo system, and this design, with a few tweaks and modifications, remains the most popular turbocharger configuration today.

click the link below to get the idea..

The above system shown is generally used in vehicular engines. So, for an aircraft or heavier than air machine we have a different setup.

Parts of an Jet Engine

The above images shows the parts and the process of a jet engine, where….

Process 1 – Air Intake

Inlet at the front of a turbojet engine, while it may appear simple, it contributes a lot to the efficiency of an aircraft engine. Its role is to direct air into the blades of the compressor, and it can help overcome the lack of sufficient air into the engine when the aircraft moves at lower speeds. The air intake can help to slow down the flow of air when the aircraft is flying at high speed. No matter how fast a plane is moving, the air flowing into the engine should be subsonic.

Process 2 – Compression

The role of the turbine at the rear of the aircraft engine is to drive the compressor. It compresses the incoming air to increase the atmospheric pressure. The compressor comprises of a series of fans, with each containing small blades. The role of the compressor is to compress the air as it passes each stage of compression.

Process 3 – Combustion

The magic starts at the combustion chamber. The chamber combines the high pressure to ignite the mixture. Combustion continues as the mixture or fuel continues to flow through the engine to the compressor and turbine. Turbojet aircraft engines run lean (more air and less fuel) because the engine requires an extra flow of air to remain cool.

Process 4 – Exhaust

The air mixture and burned fuel shoots out of the engine through an exhaust nozzle. The engine produces thrust as the compressed air pulls out of the front side of the compressor, which then pushes the aircraft forward.

Process 5 – Turbine Rotation

These are a series of fans, which work the same as a windmill. Their role is to absorb energy as the high-speed air flows through the compressor. Turbines have blades that are attached to the shaft so that they can rotate it which eventually rotates the compressors and fan. Turbojet aircraft engines have an excellent design. The energy harnessed by the turbine is further utilised for other electrical uses inside the aircraft.

So in a nut shell the air is taken in from the inlet and then passes through the series of compressors after which the compressed air enters into the combustion chamber, where the mixing of fuel and burning takes place and at the last stage the thrust is produced by the help of the nozzel from where the gases moves out.

Now this basic concept is further developed to give 5 types of engines:

  1. Turbojet Engine
  2. Turbofan Engine
  3. Turboprop Engine
  4. Turboshaft Engine
  5. Ramjet Engine

1.Turbojet Engines

The concept of the turbojet aircraft engine is simple. It entails taking air in from the engine’s rear side and then compressing it in the compressor. But fuel has to be added to the combustion chamber and burned to raise the fluid mixture temperature to about 1000 degrees.

The hot air that is produced is then pushed through a turbine that rotates the compressor. The pressure at the discharge of the turbine should be twice the pressure in the atmosphere. However, that depends on the efficiency level of an aircraft engine. The excessive pressure then moves to the nozzle that then generates gas streams, which are responsible for creating a thrust. Now these engines are mainly found in fighter jets, all civil aviation aircrafts are shifting towards Turbofan

2. Turbofan Engines

Turbofan jet engines are equipped with a massive fan at the front for sucking in air. For turbofan jet engines, most of the air flow around the exterior of an aircraft engine to give the plane more thrust even at low speeds and make it quiet. It has two flows as shown, Primary Stream and Fan Stream. About 80% of the thrust is produced by the fan stream (by-pass stream) and the rest is generated by the primary stream hence the engine becomes more fuel efficient and produces higher thrust at low speeds. Rest all fuction are same as turbojet engines. Turbofan jet engines are powering most of todays airliners.

3. Turboprop Engines

In this engine the exhaust thrust is sacrificed in favour of the shaft power (propeller shaft). There are two shafts in this setup, one is attached to the propellar and other is the main shaft carrying the centrifugal compressors the main shaft of the engine (jet enhine) rotates at 20-25 thousand rpm, which is not favourable for the propeller as it will produce sonic shock waves, which are dangerous. So the setup is used with a Reduction Gearbox which controls the speed of the Propeller.

But when we have Turbofan and Turbojet engines then why do we have to go back to Turboprop engines. Because the turboprop engines are more fuel efficient at low speeds of about 500 knots. Now this becomes of immence importance to airlines when the route (leg) is short (UDAN routes). ATR 42-72 becomes much more efficient and a sensible purchase by an airline for this purpose instead of Airbus 320s or Boing 737s.

4. Turboshaft Engines

The turboshaft engine is a form of gas-powered turbine that operates the same as a turboprop engine. But unlike a turboprop engine, turboshaft engines don’t drive a propeller. Instead, it is used in helicopters to provide power to the rotor.

Turboshaft engines are designed in a way that makes the speed of a helicopter rotor to rotate independently of the gas generator’s speed. That allows the speed of a helicopter rotor to remain constant even when the gas generator’s speed declines. It also modulates the power that a helicopter produces.

5. Ramjet Engines

These are the lightest types of engines in aircraft and come with no moving components. The speed of an aircraft is responsible for forcing air into the engine. Ramjet operates the same as a turbojet, except that the rotating parts are not present.

Because ramjets cannot produce thrust at zero airspeed, they cannot move an aircraft from a standstill. A ramjet-powered vehicle, therefore, requires an assisted take-off like a rocket assist to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at supersonic speeds around Mach 3 (2,300 mph; 3,700 km/h). This type of engine can operate up to speeds of Mach 6 (4,600 mph; 7,400 km/h).Ramjets can be particularly useful in applications requiring a small and simple mechanism for high-speed use, such as missiles.

Innovation in Design

After many years of research there are developments taking place with regard to the development of engines that seek to not only increase efficiency but also reduce noise and carbon footprint.

Eg: PurePower PW1000G geared turbofan engine.

High gear - Aerospace America
Source: AIAA

The fan drive gear system (FDGS) is the fulcrum of the innovative design for the PW1000G. P&W the manufacturer behind this innovative engine took years and spent about a “billion dollars in research and development” to prefect this geared system. The robust “star gear system leverages the basic laws of physics to improve propulsive efficiency.” The reliable gearbox is, lightweight, about “20 inches in diameter and transmits about 30,000 horsepower.” The FDGS follows the fan shaft but separates the engine fan from the low pressure compressor and turbine. The “fan rotates at a slower speed and the low pressure compressor and turbine operates at a higher speed. This allows each engine module to operate for optimum efficiency.”

This engine has significant improvements in aerodynamics, application of lightweight materials and efficiency gains with the high-pressure spool, low-pressure turbine, the combustor, engine controls, and the engine health and maintenance monitoring systems. These improvements are made possible by the step change in the basic engine architecture of the fan drive reduction gear. The engine has a lightweight composite fan case and low-pressure fan that move more than 90 percent of inlet air around the core engine.

This design resulted in a “50 to 75 percent reduction in engine noise over current models, a remarkable 12:1 bypass ratio, and an expected 16 to 20 percent better fuel burn over today’s engines.” The compact, high-speed low-pressure system runs cooler and accomplishes more work with fewer stages, a design feature that helps reduce the number of airfoils and life-limited parts. “The high bypass ratio and the efficient combustor help reduce fuel burn and carbon and nitrogen oxide emissions.”

It is currently selected as the exclusive engine for the Airbus A220, Mitsubishi SpaceJet, and Embraer’s second generation E-Jets.

What the future of Engines look like?

A jet engine works by burning fuel to release hot exhaust gas. This gas is then forced through the blades of a turbine, making the turbine’s spinning wheels rotate and propelling the aircraft forward. Because conventional jet engines rely on fuel burn to operate, this technology is inextricably linked to CO2 emissions.

But what if little or no fuel was necessary to power engines?

This is the concept behind electric powered vehicles, here, the motor is composed of a rotor and a stator. With pulses of electricity from a power electronics device, the stator produces a magnetic field around the rotor which rotates and then turns a vehicle’s drive train, rotor shaft, etc. The energy is supplied by a hydrogen fuel cell or a battery pack, which is generally powered by lithium-ion cells. These are similar to the batteries in a laptop, but multiplied by several thousand.

But this comes with a challenge, the primary challenge of this electric energy is that it cannot be stored efficiently (from the perspective of both mass and volume), at least not with today’s technology. In the simplest terms, a large quantity of batteries is required to equal the performance of fuel. This means that the battery in an electric car (for example) can represent approximately a third of its empty weight.

Mass and volume storage also are problematic for hydrogen fuel cells. This is because the chemical hydrogen must either be stored at high pressure, as in a gas state, or as saturated liquid hydrogen – which needs to be kept at around -253°C and requires large and heavily insulated tanks.

So the solution is to combine the best of two worlds.

This is the idea behind hybrid-electric propulsion. It uses a combination of conventional internal combustion engine with an electric-propulsion system. Thus “Hybridization for optimization”, thereby reducing fuel burn.

Source: Airbus

The E-Fan X demonstrator, a model of which was on display at 2020 Singapore Air Show, is currently testing the potential of hybrid-electric technology to power a 100-passenger regional aircraft. Although the technology will not be mature until the 2030s timeframe (at the earliest), electric and hybrid-electric technology is expected to help the aviation industry to take a giant leap towards zero-emission flight over the long term.    

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