Jet engines have been the successful drivers of aircraft for nearly a century now. In this article, we will explain the technology behind the jet engine in a logical, step by step manner. We will also understand what is a Turbofan engine and why it is preferred over a turbojet.
A jet engine keeps an aircraft moving forward using a very simple principle, the same that makes an air filled balloon move. Yes, Newton’s third law of motion. Just like the reaction force produced by the air moves the balloon, the reaction force produced by the high speed jet at the tail of the jet engine makes it move forward.
So the working of jet engine is all about producing a high speed jet at the exit. The higher the speed of the jet, the greater the thrust force. The thrust force makes an aircraft move forward.
In this section we will introduce the basic design and the basic components of a jet engine. Historically such basic designs have been the first successful design to be used practically. After introducing the concept of the basic design, we will move to modern and more commercial designs of the jet engines.
Such high speed exhaust is achieved by a combination of techniques. If you can heat the incoming air to a high temperature, it will expand tremendously and will create the high velocity jet. For this purpose a combustion chamber is used. An atomized form of the fuel is burnt in the combustion chamber. The combustion chamber also makes sure that air and fuel will get mixed in an efficient manner. In Fig.2 you can note the holes provided on the combustion chamber for this purpose.
Effective combustion require, the air which enters the combustion chamber to be at moderately high temperature and pressure. To bring the air to this condition, a set of compressor stages are used. The compressor consists of rotor and stator parts. The rotating blades of the compressor add energy to the fluid and its temperature and pressure rise to a level, suitable to sustain combustion. The stator blades (they are stationary)sit between rotor blades. Function of the stator blades is to guide the airflow at a proper angle to the next rotor stage. Stator blades also help to decrease the airflow velocity by its diverging design. This will result in pressure rise.
The compressor receives the energy for the rotation from a turbine, which is placed right after the combustion chamber. The compressor and turbine are attached to the same shaft. The high energy fluid that leaves the chamber makes the turbine blades turn. In short the only function of the turbine in a jet engine is to drive the compressor. The rotational mechanical power produced by the turbine is completely consumed by the compressor.
In Fig.4 you can see that, the turbine blades have a special airfoil shape. When the hot gas flows over it, such airfoil shape will create a lift force and make them turn.
As the turbine absorbs energy from the fluid, its pressure drops.
Through these steps, we have achieved our objective; a really hot and high speed air emitted from the exit of the engine. The engine case becomes narrower towards the outlet, which results in even greater jet velocity.
In short, the synchronized operation of the compressor, combustion chamber and turbine makes the aircraft move forward.
Modern aircraft use a slightly improved compressor-turbine arrangement called a two spool. Here 2 independent turbine-compressor stages are used. The shaft of the outer compressor-turbine passes concentrically through the inner one. The outer turbine is subjected to a low energy fluid and will run at a lower speed than the inner turbine.
With decrease in pressure the fluid expands. This means towards the outlet of the turbine volumetric flow rate of the exhaust is also high. This means at this region annulus area of the flow should also be higher. This is the reason why the low pressure blades of the turbine are longer compare to the high pressure blades. Since in a 2-spool engine, the low pressure blades have a low speed, it helps to reduce centrifugal stress induced at the root, thus improving the blade’s life. Some modern aircraft even use a 3 spool engine.
The engine we have discussed so far is more specifically called as Turbojet engine. Turbojet engines tend to produce high levels of noise. Historically tubojet engines are the first types of engines successfully developed. They are primarily used only in military aircraft nowadays. A revolutionary improvement was made to this engine by fitting a large fan with the low pressure spool. Such engines are called Turbofan engines, and almost every commercial aircraft run on them.
A turbofan engine bypasses a huge amount of air around its core engine area as shown in Fig.7 . This means that a good amount of mass flows over the engine without undergoing the combustion process.The ratio of bypassed air to the core mass flow air is called as Bypass Ratio(BPR). The ever narrowing bypass duct provides a good jet velocity to the bypassed air. In a turbofan engine the majority of the thrust force comes from the fan’s reaction force. Further, the fan greatly improves air flow in the system by sucking in more air. Thus it helps to improve the thrust. This means, high thrust creation with an expense of slightly more fuel. This is the reason why Turbofan engines are highly fuel economical. Today’s most civil jetliner’s fall under high bypass ratio category. To be considered as high bypass ratio category, the bypass ratio should be at least 5:1. There exists jet engines with BPR as high as 9:1.
The noise produced by a jet engine is highly dependent on the exit jet velocity. Since in a turbofan, the bypassed cold air gets mixed with the hot air, it is possible to keep the outlet temperature within a limit. This will lower the outlet jet velocity. Thus it overcomes the noise problem. The advantages of the turbofan engines are summarized below.
The advantages of a turbofan engine are listed below.
With quieter exhaust and better fuel economy, the turbofan engines continue to dominate aircraft propulsion systems. We hope, this provided a nice introduction to the working of jet engines.
This article is written by Sabin Mathew, an IIT Delhi postgraduate in mechanical engineering. Sabin is passionate about understanding the physics behind complex technologies and explaining them in simple words. He is the founder of Learn Engineering educational platform. To know more about the author check this link