Lesson 2/Learning Event 1
Unit Injection System
This system consists of injectors, fuel supply pump, fuel oil filter, and the fuel oil manifold.
Fuel is drawn from the supply tank through the primary filter by the fuel supply pump. From the pump, fuel
is forced through the secondary filter and to the fuel intake manifold which supplies the injectors. Surplus
fuel, flowing through the injectors, is returned through the fuel outlet manifold to the supply tank.
Unit injectors combine the injection pump, the fuel valve, and the nozzle in a single housing to eliminate the
These units provide a complete and independent injection system for each cylinder and are mounted in the
cylinder head, with their spray tips slightly below the top of the inside surface of the combustion chambers.
U-clamps, bolted to the cylinder head and fitting into a machined recess in each side of the injector body,
hold the injector in place in a water-cooled copper tube which passes through the cylinder head. The tapered
lower end of the injector seats in the copper tube, forming a tight seal to withstand the high pressures inside
Fuel oil is supplied to the injector at about 20 PSI and enters the body at the top through the filter cap.
After passing through the fine-grained filter element in the inlet passage, the fuel oil fills the annular (ring-
shaped) supply chamber between the bushing and split deflector. The plunger operates up and down in the
bushing, the bore of which is connected to the fuel supply in the annular chamber by two funnel-shaped
ports, one on each side at different heights.
Injector rocker arms
The injector arms are actuated through pushrods from the engine camshaft. The motion of the injector
rocker arm is transmitted to the plunger by the follower, which bears against the return spring. In addition
to this reciprocating motion, the plunger can be rotated in operation around its axis by the gear, which is in
mesh with the control rack.
Each injector control rack is connected by a joint to a lever on a common control tube which, in turn, is
linked to the governor and throttle.