Future Transportation Systems - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 273

fuel demands continuous development. New fuels based on sugarcane have been
tested in several laboratories of the U.S. Air Force and the Royal Air Force.
Manufacturers and legislators have predicted that biogenic fuels in aviation can be
widely marketed [ 31 ].
17.2.1.4 Improved Propulsion Technology
The most spectacular step in the history of commercial flight was the transition
from the piston engine to the gas turbine. There will not be similar revolutionary
innovations in foreseeable future but a series of small improvements [ 32 ].
Greater bypass and higher compression ratios will further lower the fuel con-
sumption. Enlargement will be limited by the increasing cross-section area and the
higher aerodynamic resistance of larger jet engines with more and more increased
bypass ratios. Currently, the upper limit seems to be a bypass relationship of
approximately 10:1.
Future jet engines will work with even higher compressor pressure ratios which
are over 50:1 but the turbine inlet temperature does not allow a ratio higher than
about 45:1. If the ratio went above this then the maintenance costs would become
too high. Emissions and fuel consumption can be decreased by 10% with improved
combustion chambers. However, the technology will be developed slowly and
research will become increasingly expensive.
New propfan engines will increasingly combine the advantages of turboprops
and turbofans. The modified propeller drive of the fan will reach an efficiency of
80%. However, future improvements require better noise insulation of the fuse-
lage. Passive methods such as optimal insulation walls and active sound reduction
technologies, such as the use of fast regulated loudspeakers with different fre-
quencies and amplitudes will gain a decisive role also in civil aviation. Currently,
this technology is widely used in military transportation. Improvements in the
construction of the transmission and in the design of the propeller blades can
further decrease noise emissions and increase durability.
17.2.1.5 Fuel Cell-Driven Electric Motors for Taxiing
The use of advanced fuel cell technology could make taxiing near terminals more
environmentally friendly. New type of electric motor could be built into the nose
wheel of airplanes. Taxiing a mid-range single aisle airplane requires about 50 kW
(67.1 HP) of energy with high torque [ 33 ].
The starting inertia of an airplane can be optimally overcome through the use of
an electric engine in the first seconds of taxiing because they have relatively high
performance with low inertia. A fuel cell can be installed in the fuselage near the
wheel. The way to practical introduction of this technology is the development of
light weight fuel cells with high efficiency because a large weight of current fuel
cells could result in increased fuel consumption of the airplane in flight.