LOSSES IN AVAILABLE ENERGY

LOSSES IN AVAILABLE ENERGY

The term losses describes the difference between the energy ideally
needed to propel an airplane a given distance and the latent energy of the
fuel actually consumed. The use of efficiencies has implied the existence of
losses without identifying their cause. Since minimizing losses is a major
function of engine design, it is now appropriate to review the sources of
various losses in engines. This is the purpose of this section.

Note first that the thermodynamic cycle used in propulsion engines has
an inherent loss mechanism that is not included under aerodynamics--the
fuel energy rendered unavailable because adiabatic combustion is necessary.
In comparison to a reversible isothermal process, this type of combustion
wastes about half of the potential energy of the fuel. This loss is not
recognized in Eq. (1.3).

The losses implied by the "ideal cycle efficiency," the quantity (1 - to/T3)
in Eq. (1.3), are not aerodynamic either. Neither are they included as part
of the compression or expansion efficiencies. It is the losses causing the
component efficiencies to be less than unity that are the subject of the rest
of this section, which first examines the requirements for a consistent
definition of losses and then discusses their sources.