Car engine backfires are a common sight. The explosions that follow these backfires leave us doubting the working conditions and safety of our cars. It is frightening enough to think of car engines backfiring, however, is it possible to have the same experience with airplanes? Can a jet engine backfire?
Although aircraft with piston engines can backfire, due to their design, jet engines do not backfire. Backfires are explosions that occur in the air intake or exhaust systems of cars as a result of the ignition of unburnt fuel in their exhaust systems.
This poorly timed ignition is usually followed by a burning flame, loud pop noise, loss of power, and propulsion.
Light aircraft engines backfire because they have piston engines that mimic the engines of automobiles.
Nevertheless, there have been several accounts of ‘loud banging noise’ heard in the engines of jet engines by passengers. This loud banging noise, which is often confused with an engine backfire is actually due to a phenomenon known as a compressor stall.
In this article, we will explain why jet engines cannot backfire. We will also explain compressor stalls by focusing on the factors that cause them and the effects they have on jet engines.
Table of Contents
Why Jet Engines Cannot Backfire?
Simply put, a jet engine cannot backfire because its design does not create the environment necessary for an engine backfire to occur. In other words, although the jet engine is an internal combustion engine like most car engines, it does not perform like a motorcar piston engine and thus does not possess the elementary parts necessary for a backfire to occur as earlier defined.
Car engines backfire because they adopt a rotary engine model with pistons, which are known for leaking oil. When leaked oil finds itself in the exhaust system of the engine and is ignited, the engine backfires. For an engine backfire to occur, an engine would need a carbureted system of pistons to malfunction in its ignition timing. This, therefore, explains the reason why jet engines cannot backfire.
Compressor Stall
If jet engines cannot backfire, how then do we explain the loud banging noise? The loud banging noise heard from the jet engines of airplanes is not as a result of an engine backfire. It is the product of an exclusive jet engine phenomenon known as a compressor stall.
Basically, jet engine compressors consist of rotating turbine blades arranged in series.
When the air flowing through these blades stops flowing smoothly or even reverses direction, a compressor stall occurs, which is usually followed by a loud bang.
A compressor stall is a disruption in the airflow of the compressor of jet engines resulting from a stall in the aerofoil blades themselves.
This aerodynamic stall occurs when these blades assume an angle of attack that exceeds their critical angle of attack, consequently leading to the inability of the compressor’s rotor blades to pass air smoothly from stage to stage.
This results in a rapid loss of air pressure inside the engine, which means that less oxygen is added, while the amount of fuel remains the same. As such, a compressor stall can cause flames to erupt behind the engine, as that fuel is ignited.
The image below shows a normal airflow into the engine, as well as a distorted airflow, which could lead to a compressor stall.
Although a loud bang noise is also heard when compressors stall, a compressor stall should not be a frightening experience. Unlike in car engines, when the compressor of jet engines stall, they are neither exploding nor going up in flames.
Usually, all a pilot needs to do in the event of a compressor stall is to immediately decrease the thrust on the engine in a steady fashion, in order not to shock the engine even more.
Causes of A Compressor Stall
The underlying condition that is required for a compressor to stall is the inability of the compressor to create a steady and stable flow of air. However, the quality of airflow in a compressor is subject to a particular pressure ratio. If the compressor goes beyond this value, airflow will become unstable as a direct consequence. Thus, the emergence of the surge line.
A surge line represents points on a compressor map, beyond which airflow in a compressor breaks down, which leads to a compressor stall. This is known as the erosion of the compressors’ surge margin. The major factors that lead to the erosion of a compressors’ surge margin and in effect create a compressor stall include:
- The intrusion of foreign objects: The intrusion of foreign objects debris (FOD) like sand and stray birds into the compressor of a jet engine can cause damage and the erosion of its surface, consequently, leading to the erosion of its surge line.
- Worn-out compressor components: When the compressor parts of engines wear, dirt builds up in their compartments, thus exposing the compressor to stalling as a result of the erosion of its surge margin.
- In-flight icing: When supercooled water makes contact with the parts of a compressor, it freezes. This compromises the working state of the compressor and exposes it to stalling.
- Operation outside design envelope: The compressor of a jet engine is likely to stall when it is operated outside its design envelope. This includes flying the airplane in excessive altitudes or undertaking complex maneuvers that are beyond the primary capability of the plane.
- Poor maintenance and handling of the engine: Improper handling of engines and engine operations is a major reason why jet engines experience a compressor stall. When flight engineers and pilots fail to carry out the necessary checks, they most certainly fail to identify factors that could expose the compressor of jet engines to stalling.
- Turbulent airflow into the engine: Turbulent or hot air flowing into the engine intake of airplanes can also lead to a compressor stall when reverse thrust is used at a low forward speed.
Effects of A Compressor Stall
The most common effect of a compressor stall is the obvious loud banging noise heard in the engines when the compressor is stalling. However, a compressor stall could also leave a number of effects in its trail. Other effects of a compressor stall include:
- Exhaust gas overheat: A compressor stall leads to an increase in the temperature of the exhaust gas.
- Increase in rotor speed: An increase in the rotor speed of the compressor occurs as a result of the reduction in the work done when stalling occurs.
- Loss of thrust: As a result of the reduction in work done when stalling occurs, the jet engine loses thrust.
- Yawing: Yawing is likely to occur as a result of the loss of thrust when the compressor of a jet engine stalls. In a multi-engine aircraft, yawing will occur in the direction of the engine that has stalled as it loses thrust and would cause the airplane to abruptly rotate around its vertical axis. As a consequence, this makes the airplane more difficult to pilot.
- Stress: The jet engine begins to experience a lot of stress due to the unstable flow of air into the compressor.
- Flames: Flames can also be identified in an engine when its compressor stalls. This is because there is an excess of fuel relative to the air passing through the engine, as the airflow has been hindered by the compressor stall.
- Engine failure: In extreme cases, a sustained compression stall can lead to engine failure and damage the engine.
Summary
In summary, although jet engines experience a backfire-like sound, they can not backfire because they do not possess the same components as piston engines and therefore, cannot support the environment necessary for a backfire to occur. However, this loud backfire-like sound is produced by a compressor stall and we can rest easy on our flights knowing that its consequences are not as fatal as that of an engine backfire, except in extreme cases.
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