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National Advisory Committee for Aeronautics, Technical Notes - Alterations and Tests of the ''Farnboro'' Engine Indicator

In the study of engine operation, it is of great importance
to examine the cycle of events occurring in the cylinder in or—
der to determine the effect of variations in engine operating
conditions. A number of indicators have been developed to aid
this study, and from those available the "Farnboro" was select—
ed, because tests indicated that it was inherently a good type
of high—speed indicator (Reference 1). However, the records
obtained at the laboratory were incomplete, which showed that
the instrument required modification in order to make itsfoper—
ation suitable for high—speed engine testing.

The "Farnboro" indicator is classified as an electro—.
pneumatic type of indicator (Reference 2). The principle em?
ployed consists in balancing the cylinder pressure, by means of
gas pressure, on a small disk valve near the cylinder Wells.
This balancing pressure also acts on a piston whose motion is
restrained by springs. The motion of the piston is transmitted
by a link to a pointer which moves parallel to the axis of a
drum rotating in time with the engine. The position of the
pointer indicates the balancing pressure. _The disk acts as a
circuit breaker in the primary circuit of an induction coil, and
induces a high tension Spark at the instant the disk is lifted
from its seat. This spark, in its path from the pointer to
the drum, perforates a sheet of paper which is rotated on the
drum. Two balance points are obtained for each cycle, one on
the up—stroke and one on the down.stroke, and the card is_pro—
gressively built up as the pointer moves parallel to the axis
of the drum (Fig. 1) (Reference 3).

The "Farnboro" indicator was developed originally by the
Royal Aircraft Establishment at Farnborough (Reference 4).
This development has been continued by different institutions
and reports published recommending minor changes in design and
material (Reference 5). The present work covers a redesigniof
the indicating unit and additions and alterations to the record—
ing mechanism.