Diesel Engine Troubleshooting

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Caterpillar EMS Hydraulic/electronic unit injector

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HEUI evolved from the EUI, and since mid-1994 has been used on Caterpillar, International T444E, DT466E, 1530E, certain Perkins models, and, most famously, on the Ford (International) Power Stroke. Unlike conventional camshaft-driven unit injectors, HEUIs are actuated by high-pressure crankcase oil. Since injection is no longer tied to camshaft motion, fuel can be injected at any crank angle. At higher-than-idle speeds, injector pressure is independent of engine rpm and can attain values of 30,000 psi.

Figure 6-18 illustrates the general layout of the Cat system that differs only in detail from Ford and other HEUI applications. Figure 6-19 shows the arrangement of the piping for the 3126B truck engine, which is the focus of this discussion. The high-pressure oil pump delivers crankcase oil to the injectors at cranking pressures of 500 psi that increase to as much as 3300 psi under speed and load. The computer-controlled injector actuation pressure control valve (IAPCV) regulates oil pressure by shunting pump output to the crankcase. Fuel pressure is regulated to 55 psi minimum.

HEUI schematic Caterpillar EMS Hydraulic/electronic unit injector

HEUI plumbing Caterpillar EMS Hydraulic/electronic unit injector

HEUI consists of five major components—a solenoid, poppet valve, mushroom shaped intensifier piston, barrel, and a seven-hole nozzle assembly (Fig. 6-20). Approximately 4A is required to energize the solenoid and 1.5A holds it open.

Upon command, the solenoid opens the poppet valve to admit high-pressure lube oil to the upper end of the intensifier piston (6 in Fig. 6-20). The oil forces the piston and plunger (7) down against fuel trapped in the injector body by a ball check valve. The fuel end of the plunger is smaller in diameter than the upper, or oil, end. The difference in cross-sectional area multiplies the hydraulic force acting on the fuel 7.5 times, to produce injection pressures of 3000–30,000 psi. Pressurized fuel lifts a second check valve, enters the cavity on the lower right of the drawing, and reacts against the needle shoulder to overcome spring tension. Nozzle-opening pressure (NOP) varies with the application. Some Cat injectors have an NOP of 4500 psi. Rebuilders of Power Stroke HEUIs look for 2750 +/_ 75 psi.

HEUI injector Caterpillar EMS Hydraulic/electronic unit injector

Note that both band width (the time the injector is open) and actuator-oil pressure determine the amount of fuel delivery.

HEUI injectors have undergone constant refinement. The most significant change is the split-shot PRIME (preinjection metering) version that first appeared on 1994 California Ford F series trucks and has since become almost universal.

These injectors work like other HEUIs, except that the plunger incorporates a radial groove that receives fuel through six bleed ports drilled in the face of the plunger. As the piston moves downward, the groove aligns with a spill port in the barrel, shunting pressure. The output-side check ball seats and injection stops. Further piston movement masks the port and injection resumes.

Written by Ed

February 14th, 2011 at 2:38 am

Caterpillar EMS Electronic Unit Injector

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Both the 3100-3500 and C series engines employ EUIs that combine mechanical actuation with electronic control over fuel volume. Figure 6-15 illustrates an EUI in cross-section. A roller-tipped rocker arm, acting on in the injector plunger, provides the force necessary to generate the 28,000-psi injection pressure. The solenoid valve, shown on the left of the drawing, opens the spill/fill port to admit fuel into the injector barrel during the period of plunger retraction. A spring-loaded check valve, located near the injector tip and set to open at 5000 psi, remains closed during the fill process.

EUI cross section Caterpillar EMS Electronic Unit Injector

Injection can be initiated any time after the plunger starts its downward travel. But until the ECM signals the solenoid valve to close the spill/fill port, fuel merely cycles through the EUIs as a coolant and as a purge to remove any entrapped air.

Upon signal, the solenoid valve closes the port, trapping fuel in the injector barrel. Further downward movement of the plunger raises fuel pressure sufficiently to overcome spring tension acting on the check valve. The check valve opens and injection begins.

Injection continues until the ECM signals the solenoid to open the spill/fill port. Responding to the sudden loss of pressure at the injector tip, the check valve snaps shut. Injection ceases. The plunger continues its downward stroke, displacing fuel through the open spill/fill port and into the fuel-return gallery.

Although the software has primary responsibility for injector timing, plunger movement is affected by rocker-arm lash. The onset of pressure rise depends upon the clearance between the rocker arm and the plunger. A Caterpillar PN J 35637 height gauge is used to establish this critical variable (Fig. 6-16).

EUI position Caterpillar EMS Electronic Unit Injector

Written by Ed

February 14th, 2011 at 2:28 am