What is the difference between a turbojet engine and a turbofan engine?
I know that both of them have a fan and have turbines but how to distinguish them?
Turbojets and turbofans are very similar indeed:
$\ $ Junkers Jumo 109-004
So what is the difference? There are four types of turbine engines, named after their main means of generating thrust:
The four types incrementally use bypass airflow to generate thrust. Increasing bypass air allows for accelerating a larger mass of air at a lower speed, generating the required thrust at a higher efficiency. But bypass air volume is inversely proportional to airspeed: the faster the aircraft, the lower the amount of bypass air that can be utilised. At supersonic speeds, bypass air is out for thrust generation.
The picture below shows the Olympus engine used for propelling Concorde, a pure turbojet with 2 axes and 7 compressor stages per axis.
The GE CF6 depicted below was used for B747, A300 and DC10, and is a turbofan: the first stage of the compressor sticks out over the remaining stages, and bypasses some of the air inflow outside of the main turbojet engine. This bypass air is the main distinguishing feature between turbojet and turbofan. Visually, the large fan is very distinguishable of course.
In a turbojet, all the air goes through the engine proper, through the combustion chamber and all the stages of compressor and post-combustion turbine blades.
In a turbofan, some of the air is just pushed by a fan around the rest of the engine. This is the "bypass". As Harper points out, it's not fundamentally different from a turboprop or extracting other mechanical work from a turbine engine by having the exhaust do more work turning a shaft.
Low vs. high bypass turbofan is about how much of the air goes around the combustion chamber.
In a high-bypass turbofan, almost all the thrust comes from the fan, with turbo-shafts extracting almost all the work from the jet exhaust to power the fan. The thrust from the hot combustion products coming out the back is minor.
In a low-bypass turbofan, a good fraction of the thrust still comes from the jet part so it's part way between a pure turbojet and a modern high-bypass turbofan. Better for higher speeds, and faster throttle response without a huge fan to spool up.
A very low bypass turbofan like 0.3:1 is used in the Gripen supersonic fighter as "an efficiency tweak" according to @J... This is technically a turbofan but performance-wise it's very close to a turbojet, just more fuel efficient at lower speeds and thrusts. The F-16 uses a turbofan with a 0.71:1 bypass. These engines can use an afterburner to augment the jet part of their thrust even more.
Early jet fighters often used pure turbojets, but F-14, F-15, F-16, and other fighters in those eras and newer use very-low-bypass turbofans.
Other answers go into more details and comparisons, but remember the key difference is that a turbojet has zero bypass.
I know that both of them have a fan
No, as comments on the question point out, a turbojet doesn't have a fan.
They are both turbine engines, and that is where the similarity ends.
In a turbojet, the compressor-burner-turbine package is optimized to make thrust.
A turbofan engine is a type of turboshaft engine. These use a compressor-burner-turbine core, but use a secondary set of turbine blades to convert its thrust into shaft rotation. Typically this is on a separate shaft that spins at its own speed. The shaft rotation is used in a huge variety of applications, largely to replace piston engines:
But these are all the same thing, really: a turboshaft engine rotationally driving some sort of thrust maker. (in the top instance, electron thrust, but let's not quibble).
The mere fact that the shaft rotation comes from a compressor-burner-turbine engine is mere coincidence. It would be perfectly possible to have a diesel engine spin a ducted fan, if you could get a diesel compact and powerful enough.
Get it? The ducted fan may have a cosmetic resemblance to the intake of a turbojet engine, but it's simply used because it's the best "prop/fan" for the job. If turboprops or actual propfans were more efficient, they'd be used instead.
Of course, the secondary turbine stage could be intentionally made less efficient so it fails to convert all the thrust to shaft rotation. In that case, some thrust would still occur in turbojet fashion. That's actually a "knob" the engine designer can turn. In helicopters, they peg it all the way to "shaft", and you see where many helicopters put a 60 degree bend on their turbine exhaust. A stationary generator also has zero use for thrust except to get the exhaust plume to move along. For most civil turbofans, it's inconsequential; the thrust reversers don't even bother reversing jet thrust and only reverse fan thrust.
A burning turbine consists of compressor stage, burning stage and gas turbine stage. Both compressor and turbine stages consist of sets of stator and rotor blades and the rotors are connected via shaft so part of the work the turbine generates can be used to compress the intake air. The output from the turbine stage is a high-velocity jet of hot air-fuel-burning products mixture (resulting in exhaust thrust) and a revolving shaft (carrying torque). All turbine motors share this base.
The differences between engines is their optimisations and thus designs. Turbojet engines are optimised to maximum exhaust thrust and minimum torque securing optimal work of the engine.
Turboprop, Turbofan and other Turbo- engines are optimised to produce maximum torque on the shaft and the exhaust thrust is of lesser interest here. The propeller then produces the main thrust of the whole engine. Turbofan engines may be considered advanced -prop engines because they optimise the airflow through the bypass resulting lower losses caused by air escaping the propeller crossection. And the propeller is called fan because we need to distinguish the designs, right?
As you can see in Kyovis' images, when looking at the front of the engine, the thing you can see on the Turbojet engine are the blades of the first compressor stage. On the Turbofan engine you can see the blades of the propeller - here called fan -, the compressor blades are much smaller and are hiden behind the large propeller.
In other words, the engines consists of blades, shaft(s) and casings (neglecting fuel supply and controllers). The blades are:
After reading all the answers I felt that none of them really explained the answer in a way understandable to a layperson, so I will attempt to do so.
First of all, both types of engines will burn fuel to generate energy, which is ultimately used to accelerate a stream of air towards the rear of the aircraft to create thrust. They differ in the method by which they accelerate that stream of air. Newton's third law (roughly) states that for every action there is an equal and opposite reaction, i.e. if the engine pushes the air backward, the engine (and the aircraft it is attached to) must accelerate forward. This is what is meant by the word thrust. Note also that fuel needs an oxidizer to burn, and that oxygen comes from the air.
A turbofan is akin to the classic and intuitive propeller engine. The fan blades push the air just as a fan does in your house, and the engine housing directs that airstream backwards. Some of the air must also enter the engine core to oxidize the fuel in order to generate the energy which drives the propeller shaft, but most of it goes around the core and out the back. In essence, the fan blades are "pushing off" of the air, as you might push yourself off the wall of a swimming pool to accelerate your body through the water (or better, just as paddling in water pushes you forward while pushing the water backward). Note that the engine exhaust here does not play a direct role in generating thrust, the thrust comes from the large fan blades pushing air backward.
As an analogy, consider a car: Fuel is burned in the engine by mixing it with oxygen from the air (often using a turbocharger which compresses air in order to burn the fuel faster, which the turbofan engine also uses); the burning of fuel pushes the pistons which then turn the drive (propeller) shaft; the drive (propeller) shaft then turns the wheels (fan blades) which push off of the pavement (air around the fan blades) in order to push the car (plane) forward. Note again that the exhaust from your car doesn't play a direct role in pushing the car forward, it's just the byproduct of the burning of fuel.
A turbojet, on the other hand, is exactly as its name implies, a jet. It directs all of the air into the engine core, mixes it with fuel and compresses it to achieve high temperatures and a large pressure gradient, rapidly accelerating the air and shooting it out the back like a rocket. The physics is akin to covering the end of a garden hose with your thumb to increase the speed of the stream of water. In this case, however, you are also mixing it with fuel to raise its temperature and increase the speed even further. Thrust is then created by ejecting the high-temperature, high-speed exhaust of the engine backward, which is akin to the way that a rocket works. The difference is that a turbojet uses compressed air as the oxidizer while a rocket has to carry its own oxidizer (since there is no air in space!). Note the engine still has fan blades that are needed to push the air back and compress it. Contrast this to a ramjet, which is an engine that operates on the same principle but only works at extremely high speed where the sheer pressure of the incoming air is enough to run the engine without the assistance of fans. In any case, the word "jet" implies that the thrust is generated by a stream of high-speed exhaust directed out the back of the engine, i.e. the exhaust plays the primary role of what is being ejected to generate thrust.
In case it isn't clear, the prefix "turbo" is referring to the compression stage. Compressed air has a higher density of oxygen and will burn the fuel faster and more completely than uncompressed air. At high altitude, compression is necessary since the density of the air is so low there.
One more condensed explanation of all the above using the analogy of the internal combustion engine with a turbocharger. In the internal combustion engine, the turbocharger compresses the air, which is then mixed with fuel and injected into the combustion chamber. The piston compresses the mixture further and it is then ignited. The ignition of the fuel causes the gas to rapidly heat up and expand, and that pressure is used to push the piston back which turns the crankshaft and thereby turns the wheel.
Now to the turbofan and turbojet. Both engines have a core that compresses incoming air, mixes it with fuel, and combusts it. The difference is that a turbofan uses the resulting pressure of the expanding gas to rotate a crankshaft which turns the large fan blades to generate thrust. A turbojet, instead of using the pressure to turn a crankshaft, simply allows it to escape out the back end of the engine thereby generating forward thrust.
Hopefully this clarifies some of the other answers.
100% fans like most big passenger transport turn as much heat as possible into mechanical power to turn the fan that pushes the plane along. Lowest noise best energy conversion. Turbo jets convert heat into high pressure exhaust to produce reaction thrust like a rocket to enable fighter jets to break the sound barrier. Not so good thermal efficiency and very noisy. Late 1960's engines started bypassing some of the intake air past the power producing part of the engine to mix with the hot gas in the tailpipe come from the core power generator to create expansion of the cooler by passing air and hence raise thermal efficiency. So that's really the 3 types of jet engine. Hot high pressure for pure light weight supersonic thrust. (20:1 pressure change),High bypass jet for some reaction but with an efficient cruze option (8:1) pressure difference = virtually all modern multirole combat planes. Total mechanical conversion (1.75:1 pressure change) = virtually all modern passenger aircraft.
I'll make it really simple
Turbofan engines have a core and a bypass duct. 83% of the thrust is produced by the bypass air whereas the other 17% is produced by the core. The bypass air is simply cold air which is propelled rearward by the huge fans on the front of the engine. The rest of the air enters the core where it goes through the compressors, combustion chamber and exits out of the exhaust with high velocity- producing thrust.
Turbojet is basically a turbofan with NO BYPASS DUCT. The only source of thrust is the core. That's it! I'll provide you with 2 images below so that you can understand better.