Why aren’t low budget airlines, for example Ryanair using turboprop in short haul flights from Dublin to Liverpool (Just an example, it’s about 200km) not using Turboprop but optin in using a Boeing 737?
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A jet engine uses hot exhaust gases to spin a turbine. The turbine is connected to the compressor which sucks in air and squeezes it into a small space. This causes the air to heat up and start expanding. The expanding air pushes on the blades of the turbine and makes it spin.
Turbojet compresses air to then ignite it and get thrust out of the exiting expanded gas.
Turboprop uses that gas to spin a secondary turbine, that is geared to drive a propeller.
Turbojets are faster overall when cruising, can fly higher, And don’t have the complexity of a gearbox added to it.
Modern airliners use Turbofans, where the passive turbine is used to spin a ducted fan. The ducted fan produces thrust more efficiently than propellers, and the cool air it blows is used around the engine to keep it cool, increasing efficiency as well. In most airliners, the fan part actually produces most of the total thrust over the turbojet.
In the case of airlines like Ryanair, it has almost nothing to do with benefits of any individual technology, and everything to do with economics of running a fleet. Ryanair aims for minimising any cost, up yo an including instructing it’s crew to steal pens from hotels, so they don’t have to provide them.
They run 737s because they were the cheapest second hand option. They have structured their whole business around the 737. All maintenance, pilot training, parts logistics, everything. And because the 737 is so ubiquitous, those are all easily available (and therefore costing less). They do not operate any other plane, because even if an individual plane was marginally more efficient on any particular route, even a single airplane of a different class requires a second set of training and logistics. And because planes, well, move around, you have to have that second set everywhere you make it go. And unlike the 737, turboprops aren’t ubiquitous. So it would be on them to organise it. That’s on top of the fact that people with more skills and certifications demand more pay. And then there’s also the issue that if you have a whole fleet of identical aircraft, you have a whole fleet of “spare” aircraft. You can redirect them to different routes as necessary. If you have a specialised aircraft for a route, and it goes out of commission… You’re screwed. Flight cancelled. And a spare aircraft that’s not flying is only cheaper than a flying one by the fuel and crew manhours. Ao that’s not an option for Ryanair.
Basically economies of scale. If you own one aircraft, you choose the best one for the route. If you have a fleet, you choose the best one for the fleet.
Turboprop engines *are* jet engines, just a specific kind.
Any internal combustion engine needs to go through certain steps. One of the most important steps is to compress the air in the engine. In order for fuel to burn, the molecules of fuel have to physically come into contact with molecules of oxygen in the air. If you compress the air, you greatly increase the odds of fuel and oxygen meeting, and you get a much more efficient burn.
All jet engines use turbines at the intake to suck air in and squeeze it to compress it. When the fuel burns, it expands and has to go somewhere, and the engine forces it out the back. Along the way, it passes through fan blades and turns them. Those blades are attached to the same shaft as the turbine on the front – thus, energy from the burning fuel is what turns the turbine that compresses the incoming air.
With a turbo**jet** engine, almost all of the power from the burning fuel is directed out the back of the engine. Because of equal-and-opposite reactions, the fast, violent movement of the exhaust *back* forces the engine (and the plane attached to it) forward. Turbojet engines move (relatively) a little bit of air *really* fast, which makes the engine good at going *really* fast, but that also means they use a lot of fuel to do it.
In a turbo**fan** engine, a lot of the power is directed backwards, but more of it is siphoned off by the turbine. The turbine on the front of the engine doesn’t only suck air into the engine for compression, it also has a *bypass* that accelerates air around the engine. This accelerated air mixes with the exhaust from the back, slowing the exhaust down but speeding the bypass air up. The result is that the turbofan engine moves a lot of air *pretty* fast. Turbofan engines are good at going pretty fast, and doing it very efficiently. The amount of air used in the bypass can be higher (slower plane but more efficient) or lower (faster plane but less efficient). Commercial airliners are high-bypass engines and fighter jets are low-bypass engines.
Turbo**prop* engines siphon all most all of the power from the exhaust. None, if any, of the power from the exhaust is directly pushing the engine forward. Instead, all of that power is used to rotate the shaft, which is connected via gearbox to a propeller. The propeller moves all the air, which propels the engine (and plane). Propellers move (relatively) a *lot* of air, but not very fast. That makes them *very* efficient at lower speeds.
For smaller planes that aren’t going far, it’s much more efficient to use turboprop engines. The planes aren’t going fast or high, so there’s no reason to use a turbojet or turbofan.
There are also turbo**shaft** engines which are like turboprop engines, except the power is delivered to, for example, the rotors of a helicopter or even the wheels of a car, truck, or tank.
A jet engine produces thrust from the jet of exhaust it expels. A turbo fan uses energy from jet’s a turbine wheel to drive an additional fan wheel up front that drives large volumes of cold air around the engine, which produces thrust more efficiently. A turboprop uses energy from a jet’s turbine wheel to drive a grearbox and propeller, which produces lots of thrust at low speed.
Jet and turbofan engines are most efficient at high subsonic speeds (0.8 to 0.95 Mach), but are very inefficient at lower speeds.
Turbo props cannot travel nearly as fast as jets or turbofans, but offer lots of efficient thrust at lower speeds, which is ideal for commuter aircraft that make shorter trips at comparatively lower altitudes than airliners.
Having said that, there is significant overlap in economic efficiency between specific aircraft models. Despite being powered by turbofans, the 737 has proven itself quite efficient at economically moving high passenger volumes over short/medium distances.
There are specific limits for each powerplant.
A turboprop can use a relatively very wide propeller. This allows to “push you forward” by taking a big mass of air, and pushing it back. Like paddling a raft with a pretty big row.
A turbofan has an enclosed “propeller” called fan and this limits the airmass you can work with. So, you paddle the raft with a smaller row. It’s less efficient. Your row will tend to “slip” through the water.
The advantage of a turbofan, is that you can shape the duct in front of the fan and after the fan. Fan and propellers need to work with air that is below the speed of sound. You can take almost speed of sound air, send it through a divergent duct, this slows the air down, you work that air with the fan, then a convergent duct accelerates the air to a speed close to speed of sound before ejecting it. This allows a turbofan to stay efficient while propelling a plane at very high speed, including speeds well beyond speed of sound. Again, the logic is: take air, transform speed into pressure, add energy, transform the worked air into very fast low pressure air, shoot it out.
When to chose one or the other: short flight won’t benefit much from very high speeds. You can use a slower plane, which means consuming less fuel due to airplane speed; for a slow target speed, having the “big row on the raft” is surely beneficial in terms of getting good thrust from each pound of fuel. So double profit, cheaper cruise speed and a powerplant very efficient for that task.
Then if the flight is longer, you prefer to sell a quicker mean of transport, a faster plane. A faster plane can only work efficiently with a turbofan, which is in the end a glorified propeller in a properly shaped duct.
Note: to understand the efficiency given by working a bigger air mass: do this experiment. You sit on an office chair, facing a wall, push with all your strenght the wall, by reaction, you will be propelled the other direction. Now do the same but instead of pushing the wall, push a colleague that is sitting on another office chair. Yes you propel yourself the other direction, but less, and part of your force is lost in shooting your colleague the opposite direction. The heavier your colleague, the more reaction you get. The lighter the colleague, the less you get. If you push an empty chair you won’t even move you will just shoot the empty chair away. That’s basically the difference in pushing a bigger or smaller mass with the goal of getting a reaction force.
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