Power Gearbox for Future Ducted Geared Engine Configurations

24 April 2015

ENOVAL main objective is to develop technologies for low pressure system focused on ultra-high by-pass ratio (12<BPR<20) and ultra-high pressure ratio (50<OPR<70) propulsion systems in order to reduce both the specific fuel consumption and the engine noise. To obtain the best performance of the overall engine, a mechanical transmission connecting Fan and low pressure turbine (LPT) has been considered in order to decouple the speeds of the two modules connected with the advantage to design them in a more efficient way.Power gearbox architecture

The feasibility of the power gearbox module for each of the three different engine platforms studied is one of the Avio Aero main contribution to ENOVAL.

Gearbox definition has to observe the main geometrical and functional requirements coming from the engine platforms and in order obtain the best gearbox integration, an iterative approach with the engine manufactures will be followed.

The power gearbox architecture selected for these feasibility analysis is an epicyclic geartrain that is able to achieve the reduction ratio between Fan and LPT speeds by fixing either the carrier rotation (“star” architecture) or the ring gear rotation (“planetary” architecture). The power flows from LPT to Fan involving all the gearbox components starting from the sun gear directly connected to the LPT; the exit shaft of the gearbox, instead, is directly connected to the Fan and could be directly driven by the ring gear (“star” architecture) or by the planet carrier (“planetary” architecture).

Epicyclic gearbox is the preferred architecture, with respect to others geartrain configurations, due to the high power density and for its basic geometrical configuration that permits to have both the inlet and the outlet drive shafts on the same axis.

The following picture highlights the preliminary gearbox definition for each of the three different engine platforms assessed:

  • Medium Turbofan Engine – Medium Range Mission
  • Large Turbofan Engine – Long Range Mission
  • Very Large Turbofan Engine – Very Long Range Mission

Preliminary gearbox definition

In addition to the technical challenges encountered to comply with the geometrical and functional constraints, the gearbox itself is a module that requires particular attention in terms of heat generation. Generally an epicyclic geartrain has a mechanical efficiency value greater than 99%. If the power transmitted is small the temperature increase due to loss effects is also small but if the power to transfer reaches values of 20-30MW and more, also small variations in the mechanical transmission efficiency lead to power losses values that could reach hundreds of kilowatts. This significant amount of energy dissipation needs to be managed by the oil system and dedicated heat exchangers.

In the end also the dynamic behavior of the gearbox needs to be evaluated due to the high power density and the significant mass / inertia values of the entire driveline modules (from LPT to Fan).

Avio Aero will advance technology state of the art on both the gearbox losses and mechanical transmission dynamic behavior topics in dedicated tasks planned in ENOVAL.