Propulsion Systems - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 124

fly-by-wire control systems offer the potential for 1-3% improvement in overall
fuel efficiency including improved pollutant and noise emissions [ 17 ].
8.3.2 Retrofitting Old Engines
The incorporation of a new staged combustion system into an existing engine
requires changing several parts of the engine's high pressure section including the
combustor, the compressor and the outlet diffuser. In most cases, major changes in
the fuel control and in the fuel supply systems are also required. Additional
modifications of the turbine may be needed to accommodate changing tempera-
tures during the retrofitted operation. The centre and the shaft sections of the old
engine may be significantly different from the same engine with current combustor
technology. These measures can unfortunately increase weight, maintenance costs
and fuel consumption [ 18 ].
Retrofitting an older engine with one of new or advanced combustors is tech-
nically feasible, but it can involve not only the replacement of the existing com-
bustor but also the replacement of almost all other elements of the engine core.
The retrofitting could cost about 30-40% of the price of a new engine, even if it
were to be done during a standard hot section overhaul. The type of the combustor
chosen can affect the choice of aircraft systems and components such as the
cockpit indicators, the auto throttle, the flight management computer and the
interfaces [ 19 ].
In some cases, improvements of the combustion chamber and of its components
should be done when building new engines.
8.3.3 Thermal Efficiency
Higher thermal efficiency can be grouped into improvements to current, simple
bypass designs and to new, more complex engine constructions. The approaches
include:
• Further increases of the pressure ratio of the compression system;
• Wide ranged use of improved hot sections with reduced or eliminated cooling
requirements; and
• Application of components with higher thermal resistance.
Making all the improvements would require substantial investments in many
research and development fields, including aerodynamics, cooling technology,
light weight materials, new mechanical design and optimized engine control.
These common options could improve thermal efficiency by 10-20% [ 20 ].