The PD-8 engine was created not as a simple replacement for SaM146 with other parts. During refinement, ODK engineers had to separately solve the problem of rapid transition to increased mode without losing the gas-dynamic stability of the compressor.
In the PD-8, the gas-dynamic stability margin of the compressor was increased, and the geometry of the high-pressure turbine nozzle guide vane was changed.
Let's figure out how exactly the idea of Rybinsk engine builders affects the behavior of the aircraft in the air and why, for the sake of momentary responsiveness, engineers had to rethink the hottest part of the engine.
28 vs 22.8 and different gas dynamics
The PD-8 has an overall pressure ratio of up to 28, while the SaM146 has this indicator at about 22.8. This means a higher degree of air compression in the gas generator of the Russian engine. The official characteristic of the PD-8 confirms the value of 28.
At the same time, the PD-8 has 7 stages of high-pressure compressor against 6 for the SaM146. If you count the entire compressor together with the fan and booster stages, you get 11 stages against 10.
It is the HPC that forms the main part of the pressure ratio, and its operation is directly related to the stability margin in transient modes.
The PD-8, meanwhile, develops 8056 kgf at maximum takeoff thrust and 7477 kgf at normal takeoff thrust. The maximum thrust of the SaM146 depends on the modification: for version 1S17 it is 7740 kgf, and for the more powerful 1S18 - up to 8180 kgf. Therefore, it is impossible to speak of a multiple superiority here: in terms of thrust, the engines are in the same class.
New high-pressure turbine
The main design modification concerns the hot section. The PD-8 uses a new supersonic high-pressure turbine stage. A corresponding nozzle guide vane was developed for it, and the pressure ratio across the stage reaches 4:1.
That is, the pressure of the gas flow when passing through this stage decreases by about four times. Such a solution allows for more efficient extraction of hot gas energy to drive the compressor.
Changing the geometry of the nozzle guide vane helps to form the required flow parameters before the impeller and ensure stable operation of the hot section in different modes.
Simply put, the new geometry was given to the turbine so that it could instantly utilize and accelerate the sharply increased volume of gases. This eliminates a dangerous air plug behind the combustion chamber and protects the compressor from surge.
In total, 25 critical technologies were introduced during the creation of the PD-8, 17 of which were developed for the first time for the enterprise. The new supersonic high-pressure turbine stage became one of the most complex solutions of the project.
Pedal to the metal — thrust increases
When the crew sharply switches the engine to an increased mode, the fuel supply increases faster than the steady-state parameters of the airflow have time to change. If the compressor does not have a sufficient stability margin, there is a risk of normal flow disruption - surge.
Therefore, the developers of the PD-8 had to not just configure the control automation, but change the gas-dynamic scheme itself.
ODK directly indicates that to improve the engine's ability to “follow the thrust lever,” an increase in the gas-dynamic stability margin of the compressor and a new geometry of the high-pressure turbine nozzle guide vane were required.
The difference between PD-8 and SaM146 in this part is not just new automation. The Russian engine received a different gas-dynamic tuning of the hot section and an increased compressor stability margin, and one of the key elements of the refinement was the new geometry of the high-pressure turbine nozzle guide vane.