Chevrolet 70 Series 1970 Service Manual page 518

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1 C am shaft
2 Cam Follow er A ssy.
3 P ush Rod
4 Hold-Down Clam p
Figure 15—Fuel Injector Installed (Typical " V " Engine Shown)
OPERATION
Fuel, under p re s s u re , en ters the in jecto r at
the in let side through a filte r cap and filte r e le ­
m ent (fig. 11). F rom the filte r elem ent, the fuel
p a ss e s through a d rilled passage into the supply
cham ber, that a re a between the plunger bushing
and the spill deflector, in addition to that a re a
under the in jecto r plunger within the bushing. The
plunger operates up and down in the bushing, the
b o re of which is open to the fuel supply in the an­
n u lar cham ber by two funnel-shaped p o rts in the
plunger bushing.
The motion of the injector ro ck er a rm is
tra n sm itte d to the plunger by the follower which
b e a rs against the follow er spring (fig. 15). In addi­
tion to the recip rocatin g motion, the plunger can
be rotated, during operation, around its axis by the
g ea r which m eshes with the rack . F o r m etering
the fuel, an upper helix and a low er helix a re
m achined in the low er p a rt of the plunger. The
relatio n of the helixes to the two p o rts changes
with the rotation of the plunger (figs. 12 and 13).
As the plunger moves downward, under fo rce
of the injector ro c k e r a rm , a portion of that fuel
trap p ed under the plunger is displaced into the
supply cham ber through the low er p o rt until the
p o rt is closed off by the low er end of the plunger.
A portion of the fuel trap ped below the plunger
is then forced up through a cen tral p assag e in the
plunger into the re c e s s and into the supply cham ­
b e r through the upper p o rt until that p o rt is closed
off by the upper helix of the plunger. With the up­
p e r and low er p o rts both closed off, the rem aining
fuel under the plunger is subjected to in creased
5 R ocker A rm
6 In jecto r A ssy.
7 C ylinder Head
8 Follow er Spring
T-6689
p re s s u r e by the continued downward movement of
the plunger.
When sufficient p re s s u re is built up, it opens
the flat, n o n -retu rn , check valve. The fuel in the
check valve cage and spring cage p assa g es, tip
p assag es and tip fuel cavity is com p ressed until
the fo rce acting upw ard on the needle valve is suf­
ficient to open the valve against the downward
fo rce of the valve spring. As soon as the needle
valve lifts off its seat, the fuel is forced through
the sm all o rifice s in the spray tip and is atom ized
in the com bustion cham ber.
When the low er land of the plunger uncovers
the low er p o rt in the bushing, the fuel p re s s u re
below the plunger is reduced, and the valve spring
clo ses the needle valve, ending injection.
A p re s s u r e re lie f p assag e has been provided
in the sp ring cage to p e rm it bleed off of fuel leak­
ing p a st the needle pilot in the tip assem bly.
A check valve, d irectly below the bushing,
p rev en ts leakage from the com bustion cham ber
into the fuel in jecto r in case the valve is accident­
ally held open by a sm all p a rtic le of d irt. The in­
je c to r plunger is then retu rn ed to its o riginal p o si­
tion by the in jecto r follow er spring. F igure 13
shows the v arious ph ases of in jecto r operation by
the v ertic al tra v e l of the in jecto r plunger.
On the re tu rn upw ard movement of the plunger,
the high p re s s u re cylinder within the bushing is
again filled with fuel oil through the p o rts . The
constant circulation of fre sh cool fuel through the
in jecto r renew s the fuel supply in the cham ber,
helps cool the in jecto r and, also, effectively r e ­
m oves a ll tra c e s of a ir which might otherw ise
accum ulate in the system and in te rfe re with a c ­
cu rate m etering of the fuel.
The fuel in jecto r outlet opening, through which
the excess fuel oil re tu rn s to the fuel re tu rn p a s ­
sage and then back to the fuel tank, is d irectly ad­
jacent to the inlet opening and contains a filte r
elem ent exactly the sam e a s the one on the fuel
inlet side.
Changing the position of the helixes, by ro ta t­
ing the plunger, re ta rd s o r advances the closing of
the p o rts and the beginning and ending of the in­
jection p erio d . At the sam e tim e, it in c re a se s o r
d ec rea ses the amount of fuel injected into the cyl­
in d er. F igure 12 shows the v ario u s plunger p o si­
tions from NO-LOAD to FULL-LOAD. With the
control ra c k pulled out all the way (no injection),
the upper p o rt is not closed by the helix until afte r
the low er p o rt is uncovered. Consequently, with
the ra ck in this position, a ll of the fuel is forced
back into the supply cham ber and no injection of
fuel takes place. With the control rack pushed in
(full injection), the upper p o rt is closed shortly
a fte r the low er p o rt has been covered, th u s p ro ­
ducing a m axim um effective stro k e and maxim um
injection. F rom th is no injection position to full
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL
ENGINE FUEL SYSTEM 6M-25