Airplane Engines - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 146

HC: 4 %
CO: 11.8 %
NO : 0.2 % X
particle: 0.2 %

1 kg k erosene burn rest

SO : 0.026 % 2
H O: 27.6 % 2
CO : 72 % 2

O : 16.3 % 2
N : 75.2 % 2

15 kg air product 8.5 % product 0.4 %

Fig. 10.1 Exhaust gas components of a turbofan engine in flight

Table 10.1 Characteristics of turbojet, turbofan, and turboshaft engines
Characteristics

Description

Turbojets Turbojets consist of an air inlet, an air compressor, a combustion chamber, a gas

turbine and a nozzle [ 4 ]. The air is compressed into the chamber, and heated
and expanded by the combustion. Turbojets can be made more fuel efficient
by raising the BPR which is the combustor inlet pressure divided by the
intake pressure modified by the turbine temperature

Turbofans Turbofan engines are designed to produce additional thrust by diverting a

secondary airflow around the combustion chamber [ 5 ]. The secondary airflow
bypasses the engine core and mixes with the faster stream from the core. BPR
in turbofan engines means a factor higher than 2.5. The bypass air generates
increased thrust, cools the engine and aids in suppressing exhaust noise.
Turbofans have a higher exhaust gas speed than turbojets and are more
efficient at subsonic speeds up to roughly Mach 1.6

Turboshaft The turboshaft is similarly designed to the turboprop engine [ 6 ]. The shaft is

connected to a transmission system that drives helicopter rotor blades,
electrical generators, compressors and pumps

Combinations of gas turbine and propeller have a high efficiency [ 7 ]. Thermal
efficiency can be increased by minimizing heat losses. Consequently, net thrust
will increase, while SFC, i.e., fuel flow per net thrust decreases. Currently,
improved combustion chamber design, highly resistant turbine materials, and an
optimal vane and blade cooling are required to cope with higher temperatures in
the combustion chamber and in the turbine inlet. The best SFC values are reached
by contra-rotating, unducted turbofan engines [ 8 ].
Turbofans optimally operate in the range from 400 to 2,000 km h -1 (from 250
to 1,300 mph) depending on the net thrust and the intake air flow. They are
classified into two categories:

• Low bypass ratio; and
• High bypass ratio.

In a low bypass turbofan, only a small amount of air passes through the fan
ducts. The fan has a small diameter. The low bypass turbofan is always constructed
in a very compact form. In high bypass turbofans the fan is larger to force a higher
volume of air through the ducts; the thrust is greater, and the thrust specific
consumption is lower than in low bypass turbofans [ 9 ].