PART II. -
At speeds in excess of half maximum engine speed, the performance
of engines developed for racing purposes is not affected markedly by
changes in injection timing.
It is usual to begin injecting fuel at some portion of the induction
stroke when air is moving forward towards the inlet valve. The
beginning of injection is considered to be the point at which the
port hole in the rotor just begins to eclipse the hole in the
sleeve at the injector outlet to the timing cylinder. To establish
this point, remove the injector outlet connection on the metering
distributor. With all drive backlash taken up, rotate the engine by
hand in the direction of rotation until the hole in the rotor is
seen to just start to appear.
This procedure is carried out without any fuel feed to the unit.
It may be found necessary to dry out the hole to prevent optical
distortion due to fuel remaining in it.
Due to the injection delay period, the required beginning of injection
may vary between about 60 deg. and 200 deg. B.T.D.C. on the inlet stroke,
according to the type of engine. A recommended starting point for
injection timing investigations would be of the order of 100 deg. B.T.D.C.
Having determined the optimum timing for maximum power, there will usually
be a tolerance of about + or - 20 deg. without significant loss of power.
Determining Control Characteristics
Variation of fuel quantity with throttle opening is achieved by means of
a control cam linked to the throttle mechanism and attached to the
control stop end of the metering distributor. The cam operates on
the adjustable control stop to vary shuttle stroke and hence fuel
delivery. It is therefore necessary to relate the fuel required
(i.e. shuttle stroke) to throttle opening, and to reproduce this
relationship by suitably contouring the control cam.
The required control characteristic is determined on the engine test
bed in the manner described below.
Micrometer Hand Fuel Control
A Micrometer Hand Fuel Control is required. Drawings of
this can be made
available upon application to the Lucas P.I. Design Office.
When fitted to the metering unit this device regulates fuel
quantities to the
engine by controlling shuttle travel.
The micrometer spindle is machined with a thread pitch of 40 T.P.I.
and the control wheel graduated in 25 divisions registering against a
fixed datum. This gives micrometer readings of 0.001 in. for each
division on the control wheel.