This technical article discusses architectural design and requirements for high-efficiency engines. Advanced fuel-efficient commercial aircrafts for entry into service in 2035 have quite different configurations than current transports, requiring new architectures for their jet engines. The alternative engine architecture potentially enables a high-overall pressure ratio all-axial compression system at small core size, while maintaining high compressor and turbine efficiencies. The concept engine also makes use of the reverse angled core to address the FAA 1-in-20 rule. It is noted that the reverse angled core engine concept architecture is only feasible at ultra-high bypass ration such that the core engine can be packaged within ducting positioned crosswise to the fan exit flow. Future studies would consider 3D design of the high turning ducts and compressor design technology for reduced exit Mach number to further increase exit annulus height and reduce sensitivity to tip clearances.
Advanced fuel-efficient commercial aircraft for entry into service in 2035 may have quite different configurations than current transports, requiring new architectures for theirjet engines. One such airframe design, shown in Fig. 1, features close-coupled, rear-mounted turbofan jet engines, with the fuselage sculpted to sweep fuselage boundary layers into engine inlets for reduced fuel consumption.
NASA has defined environmental goals for future subsonic transport aircraft as a function of the technology generations beyond today's capability. The goals for long-term technology at three generations beyond today, N+3, include reduction in aircraft fuel consumption of 60% relative to a B737-800. To meet this goal for a 2035 entry into service, a new rear-mounted engine architecture is described here (and in more detail in ), the result of conceptual design studies conducted by an MIT, Aurora Flight Sciences, and Pratt & Whitney team, and based on the original engine configuration developed by Pratt & Whitney ,
The vehicle was designed for a mission that is representative of a B737-size aircraft. It has a twin-aisle “double-bubble” fuselage with two fiush-mounted boundary-layer ingesting engines located at the upper aft fuselage (Fig 1). As a result of the unique D8 aircraft configuration, as well as envisioned advances in aerodynamics, airframe materials, and propulsion, the estimated takeoff thrust size of the engines is 13,000 Ibf (58 kN), half that of the current engine on a B737-800.
Aircraft engine overall efficiency can be considered to be the product of thermal and propulsive efficiency. To meet the aggressive N+3 performance goal, significant increases in both thermal and propulsive efficiency, compared to current engines, are required. Thermal efficiency is associated with the process of burning fuel to produce usable power. High thermal efficiency requires high overall pressure ratio (OPR), high component efficiencies, high turbine inlet temperature, and minimal turbine cooling air. Propulsive efficiency is associated with the process of converting the power output of the core engine to propulsive power. High propulsive efficiency in a subsonic transport aircraft requires low thrust per unit mass flow, implying very high bypass ratio with a large fan at relatively low fan pressure ratio (FPR). Fuselage boundary layer ingestion also provides a propulsive efficiency benefit in the concept engine. The target engine cycle for the N+3 engine had 50 OPR at cruise, FPR in the range 1.3 to 1.4, and BPR of 20 or higher.
The N+3 engine design space of high OPR, low FPR, and small thrust size drives the engine design to a small core size, defined as the compressor exit flow corrected for exit pressure and temperature (lbm/s or kg/s). In these units the core size of our N+3 engine is about 1.5 lbm/s, a factor of 5 smaller than that of current engines for B737/A320 aircraft.
There is a direct correspondence between corrected flow size and the physical scale of the fiowpath at the back end of the compressor. Existing engines in service show a change in architecture at a core size of about 3.5 lbm/s, from all-axial compressors to compressors that have a centrifugal rear stage. At this condition the blade height of the last airfoil inan all-axial machine is about 0.5 inches (1.3 cm). As the blade height goes below 0.5 inches, the efficiency of an all-axial compressor decreases rapidly due to increased sensitivity to tip clearance and airfoil manufacturing tolerances. Current engines with CS < 3.5 and centrifugal rear stage, however, are limited to OPR less than 25 due to thermal-mechanical stresses in the impeller. A major challenge is thus how to (i) achieve 50 OPR and (ii) maintain high efficiency levels at 1.5 core size.
Another major challenge is associated with the D8 engine installation, with its two closely spaced engines located within the pi (n) tail. The FAA “l-in-20 rule”  concerns engine installation design guidelines to prevent loss of aircraft in the unlikely event of an uncontained turbomachinery rotor failure; a turbine disk burst in one engine should not cause shutdown of the other engine or damage the tail surfaces to the extent that there is loss of flight control.
Alternative Engine Architecture
The proposed unconventional engine architecture addresses both of these challenges. The concept engine has a high-OPR two- spool gas generator that is aerodynamically coupled to the power turbine/propulsor in a reverse offset arrangement, as illustrated schematically on the right side of Fig 2.
Conventional architecture for high BPR engines has the low pressure turbine that drives the fan located at the back of the engine, so the drive shaft must pass through the core. In the alternative architecture, the gas generator is not mechanically coupled to the propulsor, and the drive shaft connecting the power turbine to the fan does not pass through the core. This allows the core engine flowpath to be pulled inward to a smaller radius, enabling a larger blade height for a given flow area. Elimination of the power turbine drive shaft constraint allows us to push the design space boundary for an all-axial compressor to smaller core size at the target OPR and to operate within a feasible axial compressor rim speed mechanical design space. The alternative engine architecture also allows the cores to be oriented at an angle that keeps the adjacent engine, and the critical aircraft flight control surfaces, out of the burst zone, providing an effective means to address the l-in-20 rule (Fig 3).
In summary, the alternative engine architecture potentially enables a high-OPR all-axial compression system at small core size, while maintaining high compressor and turbine efficiencies. The concept engine also makes use of the reverse angled core to address the FAA l-in-20 rule. It is noted that the reverse angled core engine concept architecture is only feasible at ultra-high BPR such that the core engine can be packaged within ducting positioned crosswise to the fan exit flow. Future studies would consider 3-D design of the high turning ducts and compressor design technology for reduced exit Mach number to further increase exit annulus height and reduce sensitivity to tip clearances.