Chapter 18 Injection Nozzles 381 Copyright Goodheart-Willcox Co., Inc. Once the nozzle has been installed, inspect and clean the ends of each injection tubing line before connecting it to the nozzle. The line nut should be left loose to allow air to be bled from the fuel line. Bleeding the Fuel Injector Bleed the system by placing the fuel control to the full fuel position and cranking the engine until fuel is visible at all nozzle connections. When bleeding, do not crank the starter motor for more than 30 seconds without a 2 minute cooldown period. After bleeding, tighten all line con- nections and start the engine. Check for evidence of fuel leakage at all line connections. If a line leak is present, try loosening the connection slightly and allowing fuel to flow from the connection. This may flush out any dirt that is pre- venting a tight fit. Retighten and recheck. Pencil Nozzles The pencil fuel injection nozzle is quite different than those discussed to this point. It is a multi-hole, dif- ferential pressure type nozzle that is much smaller than other nozzle types, Figure 18-26. The spray-tip body and inlet fitting are permanently attached to the nozzle body. The needle valve and the spindle are also one unit. The valve is guided by the spray tip body and by the closely fitted valve guide. The lower spring seat rests against the needle valve while the upper spring seat rests against the pressure-adjusting screw. The lift adjustment screw threads into the pressure adjusting screw. Inlet fittings vary in length and design from engine to engine. Two seals are used: a Teflon® seal to prevent carbon buildup in the cylinder head bore and a nylon seal to prevent loss of cylinder compression and prevent debris from entering the nozzle bore. The nozzle body between the seals is also Teflon coated to prevent carbon buildup and allow for smooth installation and removal. Pencil Nozzle Operation The pencil nozzle operates in the same way as the nozzles covered earlier except that its needle valve opens and closes more rapidly. When tested, a good pencil injector will give off a distinct chatter. Metered fuel under pressure from the injection pump flows through the inlet, the edge filter, and around the valve, filling the nozzle body. When the pressure acting on the dif- ferential area overcomes the force of the pressure adjusting spring, the nozzle valve lifts off its seat. As the valve raises to its predetermined lift height, high pressure fuel is allowed to flow through the spray orifices in the tip. When delivery to the nozzle ends and injection pressure drops below the preset nozzle opening pressure, the spring returns the valve to its seat. Between injections, positive sealing is maintained by the interference angle, which results in the line contact between the valve and its seat, Figure 18-27. Leak-off fuel that passes through the clearance between the valve and valve guide lubricates the upper half of the injector and returns to the fuel tank through the leak-off connection. The needle valve lift is limited by the lift adjusting screw extension. Line contact Figure 18-27. Positive seal is maintained by the contact between the valve and seat. (Stanadyne Diesel Systems) Lift adjusting screw Pressure adjusting screw Locknuts Valve guide Inlet fitting Edge filter Nozzle body Compression seal Valve Carbon dam Tip Spray orifices Figure 18-26. Components of a pencil nozzle. (Stanadyne Diesel Systems)