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Though both turboprop and turbofan engines are turbine engines, they are not the same thing. Let’s explore turboprop vs turbofan engines to see the differences.
Turboprop and turbofan engines are both types of turbine engines that operate the same thermodynamically. But turboprops use the exhaust air to turn a propeller which generates its thrust. Turbofans use a combination of combusted air and bypass air to produce thrust using the exhaust air.
Two of the most common types of turbine engines are turboprop and turbofan engines. Although they operate the exact same way thermodynamically, they are very different engines in how they handle the exhaust they produce. In this article, we’ll take a look at what each type of engine is, how it works, and how these two turbine engines differ from one another.
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What Is A Turboprop Engine?
A turboprop engine is a type of turbine engine that has been modified to use the exhaust and associated shaft work to power a propeller, hence the name turboprop. As you may already know, most turbine engines use the exhaust from the combustion process to generate thrust — one of the four components of flight — and propel the airplane through the air.
But inside a turboprop engine, the post-combustion process spins the turbine which is then connected through a propeller through a series of gears. The air comes into the engine and goes through the compressor(s). Turboprop engines are one of the few engines out there that utilize at least one stage of centrifugal compression; most other engines use more simple axial-flow compressors instead.
After being compressed and entering the combustion chamber, the resultant reaction spins the engine’s turbine. Coupled directly to the propellor through a gearbox, this spinning of the turbine turns the propellor which produces the necessary thrust to fly. The exhaust that exits the engine produces negligible energy and is discarded.
Due to the design of the engine and the way that propellers work, these engines tend to lose efficiency at high speeds. This is why turboprop engines are typically only used for smaller, subsonic aircraft that don’t travel as fast as other types of planes do.
What Is A Turbofan Engine?
Similar to a turboprop engine, a turbofan engine is a type of turbine engine. But unlike a turboprop, a turbofan engine doesn’t use the exhaust gasses from the combustion process to turn a propeller. Instead, these engines use the shaft work to power a fan at the front of the engine which can bring in larger amounts of air that is compressed in order to produce thrust. Let’s take a bit of a deeper look.
With a turbofan, air enters the engine through the fan at the front and passes through a series of compressors. This is where the genius of the turbofan engine design comes into play, because some of the air is directed to the jet engine in the center of the engine whereas some of it is passed through more compressors and then directly into the nozzle at the rear of the engine. This air that goes to the nozzle does not have to undergo compression and is converted directly to thrust.
This design leads to quieter engines that produce more thrust while simultaneously being able to be more energy efficient. The differentiation between turbofan engines comes from the ratio of the amount of air that bypasses the combustion chamber and does not undergo combustion to the amount of air that does undergo combustion in order to produce thrust. This ratio is known as the bypass ratio.
A plane with a low bypass ratio is one that produces more thrust because less air is bypassing the combustion chamber, so there is more air being burned to produce thrust. This is good for military aircraft that require more power per pound than other planes. On the other hand, high bypass turbofan engines are ideal when planes that are quieter and more fuel efficient are desired, such as with most commercial airplanes.
Turboprop Vs Turbofan: What Are The Differences?
Now that you know that both turboprop and turbofan engines are just two of the different types of turbine engines out there, let’s highlight some of the key differences.
The biggest differences between turboprop and turbofan engines are the ways that they produce thrust and what each engine does with the exhaust air following combustion. With a turboprop engine, the combustion process is used to spin the turbine and then turn the propeller at the front of the engine. The propeller generates nearly all of the thrust produced by the engine since the amount of exhaust energy is negligible.
With a turbofan engine, on the other hand, the air enters the engine through a fan at the engine’s inlet. Some of the air enters the combustion chamber followed by the nozzle and produces thrust that way, while the rest of the air bypasses combustion and is exhausted directly to generate thrust. Therefore a turbofan engine uses the exhaust — from both combusted air and non-combusted air — to generate thrust.
Efficiency and Limitations
Just like any other type of vehicle out there, such as your car, the efficiency of the engines used in airplanes is always an area of concern. Due to the ways that turboprop and turbofans operate entirely differently from one another, each one is more efficient than the other at certain speeds and altitudes, with each one being limited by a different factor.
For turboprop engines, the limiting factor with regards to their efficiency is propeller speed. Without going into the nitty-gritty details of how propellers work to generate thrust, just know that the propeller blade area must increase proportionally to the engine’s power output. This can be done by either increasing the number of blades, adding chords to each blade, or increasing the blade length.
Usually, the blade area is increased by increasing the blade lengths. But this can only be done to a certain degree since blade tip speed increases exponentially and will eventually need to be supersonic to generate enough speed. This is typically the limitation of the efficient operating range of propeller engines, including turboprops. For that reason, turboprop engines are most efficient at subsonic speeds.
Turbofan engines are typically not limited by airspeed due to the way that the intake works and the lack of a propeller. With turbofan engines, the intakes are specially designed to control the speed of the intake air. Even at higher speeds, the air intakes are designed to lower the airspeed to something more usable, meaning that these engines can usually operate efficiently at speeds far greater than a turboprop.
For turbofans, the limiting factor when it comes to efficiency is a combination of the engine’s internal temperatures, the compression ratio, and the nozzle design. At some point, the engine can only handle so much air and compression, and the nozzle can only produce so much thrust from the amount of air the engine is taking in.
Overall, a turboprop engine is typically going to be more efficient at subsonic speeds than a turbofan flying in the same conditions. But a turbofan engine is able to continue operating at far higher speeds than a turbofan and is necessary for planes that fly at supersonic speeds. Even at lower speeds, a turbofan engine will usually produce more relative thrust to get the plane to its destination more quickly — it’ll just use more fuel in the process.
When it comes to airplane engines, performance is closely tied to efficiency since one usually dictates the other and they are both limited by the same factors. So a turboprop engine’s performance capabilities are limited by propeller speed, and a turbofan engine’s performance is limited by the operating temperatures, compression ratio, and nozzle design.
That said, I won’t spend time explaining the limitations of each engine type again. Rather, let’s take a look at the actual performance limitations of each. As mentioned in the above section, a turboprop is usually limited to subsonic flight. Not only does its efficiency drastically dwindle at supersonic speeds, the ability of the engine to continue producing thrust tapers just as quickly. So for all intents and purposes, a turboprop engine is almost exclusively used at subsonic speeds.
As you might then imagine, a turbofan’s performance capability is significantly higher than a turboprop’s. Undoubtedly the best example of how well a turbofan engine can perform is by taking a look at the fastest plane ever — the Lockheed Martin SR-71 Blackbird. Using turbofan engines, this incredible plane was able to fly at 2,193 miles per hour and reach astounding altitudes of over 85,000 feet.
All said and done, a turbofan has much higher potential performance capabilities than a turboprop engine. Since it’s able to continue producing thrust at higher speeds, a turbofan engine allows planes to continue accelerating, even past the speed of sound. That said, as mentioned in the previous section, a turboprop will perform more efficiently at the same atmospheric conditions during subsonic flights.
Turboprop Vs Turbofan: Which One Is Safer?
Before you begin to worry about any potential safety pitfalls of either type of engine, I’ll preface this section by saying that both turboprop and turbofan engines are incredibly safe and reliable. That said, there are a few things to keep in mind with each type of engine that has more to do with the way they’re installed rather than the safety and reliability that comes up during normal flying operations.
With turboprops, you can probably guess what the biggest issue is when it comes to safety — the propeller. Propellers spin fast (like really fast) and can create a safety hazard for anyone that gets close to it during operation. Secondly, propellers also need adequate clearance from the engines, fuselage, and any other part of the airplane, which can create aerodynamic issues that are not present with turbofan engines.
And even though airplanes can fly when an engine fails, the mounting location and aerodynamic deficiencies of the turboprop engine will produce excessive drag and resultant torque. When one engine fails on a multi-engine turboprop aircraft, the working engine will produce more torque and cause the plane to naturally roll that way, which can create a much more hectic and stressful landing experience for the pilots involved.
If you recall from above how turboprop motors work, by transferring rotational energy from the turbine to the propeller itself, then you’ll understand that these engines also face more vibrational issues than other engine types. The turbine spins, which then runs through a gearbox where the output shaft must spin the propeller at the necessary slower speeds needed for operation. This creates more avenues for failure.
Comparing these factors to a turbofan engine, the effects are less drastic. First of all, turbofans can be mounted much closer to the fuselage or in other locations that present far fewer issues with aerodynamics. Also without the added resistance of a propeller, if one engine fails in a multi-engine turbofan airplane, there will not be nearly as much excessive force acting on the plane. This makes flying a turbofan-powered aircraft easier if engine failure occurs.
Secondly, since there is no gearbox to transfer the rotational energy from the spinning turbine to the propeller, there is less risk of excessive vibration. And while turboprop engines are designed to account for this vibration, it’s worth noting that the issue won’t be as prevalent in turbofans. However, turbofans typically run hotter than turboprop engines, which could potentially present other issues entirely.
All that said, the actual engines themselves are just about as safe as one another, minus any potential issues from the vibrations. The biggest difference, the aerodynamic disparity, is moreso due to the installation requirements of the turboprop engine, not the actual engine itself. For all intent and purposes, both engines are safe and reliable, but a plane using a turbofan engine might be easier to operate in the event of engine failure.
Turboprop Vs Turbofan: Which One Is Better?
Determining which type of engine is better is typically a subjective discussion that cannot so easily be answered. In the end, like with most things, it will come down almost entirely to what you’re looking to get out of the engine and what your goals are for the aircraft in mind. Both a turboprop and a turbofan engine could be better for a given situation.
The best way to figure out which one is better is to combine everything that we’ve looked at so far — performance, efficiency, limitations, and safety. Since I’ve detailed this above, I will just summarize the conclusions here and try to come up with a definitive answer of which engine is better. But that’s not as easy as you might think!
In terms of performance, efficiency, and limitations, it really just comes down to what you want to do with the airplane. If you know you’ll be flying at lower speeds (i.e. subsonic speeds), then a turboprop engine will be more efficient and can save money on airplane fuel. However, if you’ll be operating at subsonic speeds, then you most certainly want to have an airplane using a turbofan engine to operate efficiently.
For these categories, there is no definitive answer.
When it comes to safety, you know from above that both engine types are reliable and safe. Turboprop engines can create excessive drag and unbalanced torque if one were to fail on a multi-engine plane, whereas turbofans won’t have this problem. At least not to the same extent. But then again, turbofans have higher internal operating temperatures at higher speeds, which can lead to other issues.
Again, there is no definitive answer when it comes to safety.
I know that might seem like a cop-out, but it’s actually true! Neither one of these two engine types are inherently better than the other. In certain conditions, a turboprop will be the better choice. In other conditions, a turbofan engine is the way to go. In the end, it comes down to how the aircraft is used and what you hope to get out of flying it!