The Function of the Cone at the Exhaust of Jet Engines

If there’s one feature on commercial planes that has always intrigued me, it’s the cone-shaped structure on the exhaust pipes of some jet engines, like those on the 747-200 and even on today’s planes. I’ve always assumed that exhaust pipes should just be open, similar to what I’ve seen on fighter jets. But why do the majority of airliners have this cone-shaped structure? It wasn’t clear to me until recently.

I’m sure some of you have also wondered why the back of some jet engines has that cone protruding from the exhaust. Well, those cones are called “exit cones,” and they serve a specific purpose in jet engines. But to truly appreciate the function of the exit cone, let’s first delve into how air flows through a jet engine.

Atmospheric air enters the engine at the front. This air is progressively compressed by a series of rotating compressor blades interleaved with stationary stators. As the air gets compressed, its temperature also increases.

However, not all the compressed air is used for combustion. A portion of it is bled off for various purposes within the aircraft, such as:

• Cooling the combustors where fuel is mixed with the compressed air and ignited.
• Providing thrust by being directed into the jet pipe as bypass air.

The remaining compressed air enters the combustors, where it’s mixed with fuel and ignited. This combustion process generates a hot, high-pressure gas stream. This hot gas then passes through a series of turbine disks. Turbines operate on the same principle as compressors but in reverse. They convert the thermal and pressure energy of the exhaust gas into rotational energy to drive the turbine itself and, in many engines, an external load like a fan in a turbofan engine, a propeller in a turboprop engine, or even generators or pumps in industrial applications.

The Role of the Jet Pipe and Exit Cone

Once the hot exhaust has passed through the turbine, it has served its purpose in terms of powering the engine’s internal components. However, in an aircraft engine, there’s still some valuable energy that can be extracted from this exhaust stream. This is where the jet pipe or nozzle comes in. The jet pipe is a specially shaped duct positioned behind the engine core. Its design aims to smoothly accelerate the exhaust gas out of the back of the engine, thereby generating thrust.

While the jet pipe plays a significant role in thrust generation, in most modern engines, the majority of the thrust comes from the bypass air mentioned earlier. The jet pipe typically contributes a smaller portion. Nevertheless, every bit of efficiency counts in engine design, hence the importance of optimizing the flow of exhaust gases.

Guiding the Flow

The exit cone forms a component of the jet pipe’s geometry. Here’s where the design gets particularly interesting. It’s important to understand that the exhaust can only pass through the space around the periphery of the turbine. Each successive turbine stage extracts energy from cooler, lower-pressure exhaust. Consequently, these later stages have larger turbine blades but a proportionally smaller rotor. However, this reduction in size has its limits.

Every gas turbine engine ultimately terminates in a large, flat rotor with one or more central shafts and bearings. If this flat rotor were the direct end of the engine, the exhaust would whirl into the void behind it. This would disrupt the smooth flow of exhaust down the jet pipe and potentially overheat the turbine rotor and bearings due to the turbulent gas.

The exit cone addresses this precise issue. It acts as a smooth transition from the circular path of the exhaust around the turbine to the desired exit at the rear of the engine. The cone essentially guides the exhaust gas out of the engine, preventing it from swirling back into the void behind the turbine rotor and bearings.

Why Don’t All Jet Engines Have Visible Exit Cones?

Now some of you might be wondering why do some jet engines have visible exit cones, while others seem to lack them entirely? The answer lies in the specific design considerations of each engine. Factors that influence the presence and shape of the exit cone include:

• Overall size of the engine core: Larger engine cores tend to have more space between the turbine and the outer casing, necessitating a more prominent exit cone to ensure smooth exhaust flow.
• Number of turbine stages: Engines with more turbine stages may have a smaller final stage due to the reasons mentioned earlier. This can necessitate a more pronounced exit cone to manage the exhaust flow.
• Presence of an afterburner assembly: Afterburners are devices used in some jet engines, particularly military aircraft, to inject additional fuel into the exhaust stream for a temporary increase in thrust. The presence of an afterburner can alter the design of the jet pipe and exit cone.

So, I hope I was able to satisfy your curiosity about the exit cones found on jet engines. Always remember that every part of an aircraft has a specific function. Rarely are components included solely for aesthetic purposes, as these could affect the performance of the aircraft.