Unified Thermodynamic Evaluation of Radical Aero Engine Cycles

Abstract

The Joule-/Brayton thermodynamic cycle is the base cycle of all major contemporary aero engines. Over the decades, the achievement of further significant improvements has become progressively challenging, and the increase of efficiency approaches physical limitations. In order to meet the ambitious long-term emission reduction targets, the introduction of radical new propulsion system concepts is indispensable. Various cycles promising significant efficiency improvements over the conventional Joule-/Brayton-cycle are being examined by the engine community. However, as no clear favorite has emerged from these potential technical solutions, a transparent methodological approach for the consistent evaluation of the concepts is necessary. Consistent thermodynamic description and performance metrics for three engine cycles are presented in this paper: The turbofan as reference and two radical engine cycles, namely the composite cycle and the cycle-integrated parallel hybrid. Laws for the estimation of component performance for large parametric variations are introduced. A method for the estimation of power plant system mass for the investigated engine cycles is proposed to evaluate fuel burn reduction. The studies substantiated that the turbofan improvement potential is saturating. The composite cycle engine offers a tremendous potential for fuel burn improvement of 24.5% over state of the art turbofan engines, which allows meeting the emission reduction targets in 2035. The cycle-integrated parallel hybrid engine improves the turbofan moderately with year 2035 technology, but is not capable of meeting the corresponding emission reduction targets on a short-to-medium range aircraft platform.

Investigations of the synergy of Composite Cycle and intercooled recuperation

“…The piston system is represented as a Seiliger cycle with adapted performance parameters. The detailed modelling is extensively described in [5]. The method set represents the most important design parameters by incorporating compressor sizing effects, cooling air demand and preliminarily component weights.…”

“…It applied the concept to a regional aircraft platform with a regional turbofan of year 2025 technology standard with a crankshaft connected piston system, and showed a fuel burn advantage of 15.2% for the CCE. Further studies showed a consistent advantage of about 18% for both state of the art technology and year 2035 technology standard [5]. Challenges associated with the concept are engine weight increase of 17-32%, mainly driven by the piston components, and demanding operating conditions for the piston components at high temperatures and pressures.…”

“…From today's perspective, it appears highly unlikely that such radical improvements can be achieved with the conventional Joule-/Braytoncycle [5]. Under realistic assumptions, even the target of achieving a 30% emission reduction by a turbofan architecture appears to be very challenging [5]. For example, for a large turbofan with entry into service year 2025, an improvement of 23.4% was forecasted [6], and the Rolls-Royce UltraFan™ has a projected efficiency improvement of 25% with year 2025+ technology readiness [7].…”

“…This may reduce the emissions by 7%, still requiring further 40% reduction by propulsion system and aircraft (4) . From today's perspective, it appears highly unlikely that such radical improvements can be achieved with the conventional Joule-/Brayton cycle (5) . Under realistic assumptions, even the target of achieving a 30% emission reduction by a turbofan architecture appears to be very challenging (5) .…”

Sizing implications of a regional aircraft for inner-city operations

“…To evaluate the applicability and the benefits of such a novel architecture, a reference geared turbofan has been set up first with assumed year 2035 technology level. Detailed modelling of turbofan and CCE are described in (Kaiser, 2016).…”