The turboprop engine is similar to a piston: both have a propeller. But otherwise they are different. Consider what this unit is, how it works, what are its pros and cons.
general characteristics
The turboprop engine belongs to the class of gas turbines, which were developed as universal energy converters and began to be widely used in aviation. They consist of a heat engine, where the expanded gases rotate the turbine and generate torque, and other units are attached to its shaft. The turboprop engine is equipped with a propeller.
It is a cross between piston and turbojet units. First, piston engines were installed in the aircraft , consisting of star-shaped cylinders with a shaft located inside. But due to the fact that they had too large dimensions and weight, as well as a low possibility of speed, they stopped using them, giving preference to the turbojet plants that appeared. But these engines were not without flaws. They could develop supersonic speed, but they consumed a lot of fuel. Therefore, their operation was too expensive for passenger traffic.
The turboprop engine had to deal with a similar flaw. And this problem has been solved. The design and principle of operation were taken from the mechanism of a turbojet engine, and from the piston - propellers. Thus, it became possible to combine small dimensions, efficiency and high efficiency.
Engines were invented and built back in the thirties of the last century under the Soviet Union, and two decades later began their mass production. Power ranged from 1880 to 11000 kW. For a long period they were used in military and civil aviation. However, they were not suitable for supersonic speed. Therefore, with the advent of such capacities in military aviation, they were abandoned. But civilian aircraft are mainly supplied with them.
Turboprop engine design and principle of operation
The motor design is very simple. It includes:
- gearbox;
- air propeller;
- the combustion chamber;
- compressor;
- nozzle.
The turboprop engine diagram is as follows: after injection and compression by the compressor, air enters the combustion chamber. Fuel is injected there. The resulting mixture ignites and creates gases that, when expanded, enter the turbine and rotate it, and it, in turn, rotates the compressor and screw. Unspent energy exits through the nozzle, creating jet propulsion. Since its value is not significant (only ten percent), a turbojet turboprop engine is not considered.
The principle of operation and construction, however, are similar to it, but the energy here does not completely exit through the nozzle, creating jet thrust, but only partially, since the useful energy also rotates the screw.
Working shaft
There are engines with one or two shafts. In a single-shaft version, a compressor, a turbine, and a screw are located on one shaft. In a two-shaft one, a turbine and a compressor are installed on one of them, and a screw through a gearbox is installed on the other. There are also two turbines connected to each other in a gas-dynamic manner. One of them is for the screw, and the other is for the compressor. This option is most common, since energy can be used without starting the screws. And this is especially convenient when the plane is on the ground.
Compressor
This part consists of two to six steps, allowing you to perceive significant changes in temperature and pressure, as well as reduce speed. Thanks to this design, it is possible to reduce weight and dimensions, which is very important for aircraft engines. The compressor includes impellers and a guide van. The latter may or may not provide for regulation.
Air propeller
Due to this detail, traction is formed, but speed is limited. The best indicator is considered to be a level from 750 to 1500 revolutions per minute, since with an increase the efficiency will begin to fall, and instead of acceleration, the screw will turn into a brake. The phenomenon is called the “locking effect”. It is caused by the rotor blades, which at high speeds during rotation exceeding the speed of sound begin to function incorrectly. The same effect will be observed with an increase in their diameter.
Turbine
The turbine is able to reach speeds of up to twenty thousand revolutions per minute, but the screw will not be able to match it, therefore, there is a reduction gear that reduces speed and increases torque. Gearboxes can be different, but their main task, regardless of type, is to reduce speed and increase torque.
It is this characteristic that limits the use of a turboprop engine in military aircraft. However, the development of a supersonic engine does not stop, although so far they have not been successful. To increase traction, a turboprop engine is sometimes supplied with two screws. The principle of operation in this case is realized by rotation in opposite directions, but with the help of a single gearbox.
As an example, consider the D-27 engine (turbofan), which has two screw fans mounted on a free turbine by a gearbox. This is the only model of this design used in civil aviation. But its successful application is considered a great leap in improving the performance of the motor in question.
Advantages and disadvantages
We highlight the minuses and pluses that characterize the operation of a turboprop engine. The advantages are:
- low weight in comparison with piston units;
- efficiency compared to turbojet engines (thanks to the propeller, the efficiency reaches eighty-six percent).
However, despite such indisputable advantages, jet engines in some cases are the more preferred option. The speed limit of a turboprop motor is seven hundred and fifty kilometers per hour. However, for modern aviation this is not enough. In addition, the noise is very high, exceeding the permissible values of the International Civil Aviation Organization.
Therefore, the production of turboprop engines in Russia is limited. Basically, they are installed in aircraft that fly long distances and at low speed. Then the application is justified.
However, in military aviation, where the main characteristics that airplanes should have are high maneuverability and quiet operation, rather than profitability, these engines do not meet the necessary requirements and turbojet units are used here.
At the same time, developments are ongoing to create supersonic screws to overcome the “locking effect” and reach a new level. Perhaps when the invention becomes a reality, jet engines will be abandoned in favor of turboprops and in military aircraft. But at present they can only be called “workhorses”, not the most powerful, but stably functioning.