372 Diesel Engine Technology Copyright Goodheart-Willcox Co., Inc. Most hole type injector nozzles are set to an opening pressure of between 2500–3500 psi (17,235–24,129 kPa) although some settings may approach 4400 psi (30,333 kPa). This high pressure reduces the injection period and the danger of gas blowback through the holes. In general, opening pressures increase with the degree of engine turbo- charging since the injector must be capable of overcoming the increased cylinder pressures generated during turbo- charging. Higher opening pressures do increase stress on the nozzle springs and seats. Low speed idling may also be rougher and starting more difficult. Differential Ratio The differential ratio is the ratio between the diameters of the valve guide and seat. When the valve is closed, fuel pressure acts on the annulus located between the guide and valve seat. When the valve is open, pressure acts on the full guide area. This means the pressure required to open the valve is greater than the pressure required to hold the valve open. The increase in pressure area once the valve lifts off its seat results in very rapid opening. When the fuel flow rate is low, a condition known as valve overshoot can occur. This is characterized by nozzle buzz, a sound often heard during nozzle bench testing. Most nozzles operate using a differential pressure between 1.7:1–2.7:1. This gener- ates a closing pressure that is between 65-85% of opening pressure. A large differential ratio decreases the amount of stroke-to-stroke variation between injectors, particularly at idle speeds. However, spring forces and the stress placed on these springs during operation are increased. Needle Mass The greater the mass of the valve needle, the slower it will move. This is important during the closing cycle of fuel injection when it is necessary to reseat the valve before the fuel pressure below the seat has dropped to the level where combustion gases from the cylinder can blow back through the nozzle opening. A nozzle that is experiencing gas blowback will have a very short valve seat life. Valve Guide Diameter The valve guide and nozzle valve can be compared to a cylinder and piston. Like the piston, the nozzle valve moves up and down inside the bore of the guide. This means the valve guide diameter has a direct effect on the pushing power of the nozzle and the distribution of the pressures generated. During opening, the rapid expansion of space around the valve seat causes a drop in fuel pressure at the nozzle opening. The opposite effect occurs when the valve closes to cut off injection. A large diameter guide enables the valve to better control this drop in pressure and reduce the danger of combustion gas blowback into the nozzle. Needle Lift The valve needle must lift off the seat high enough to provide room for sufficient fuel flow through the seat opening. Too much lift will cause excessive fuel flow and pressure drop. Once again, the danger of gas blowback is increased. Fuel Sac Volume There is a direct relationship between the fuel sac volume under the seat and the level of hydrocarbon emis- sions the engine produces. The sac must be as small as practical to reduce emission levels. Fuel remaining in the sac after injection has ended may drip into the combustion chamber as cylinder pressure falls. The fuel then mixes with and dilutes the engine’s lubricating oil. Reducing the volume of the injector nozzle fuel sac requires an increase in needle lift setting to obtain the same volume of fuel flow. Pintle Nozzles Pintle injector nozzles are used in small-bore, high speed engines. Pintle nozzles are also used in engines having precombustion, divided, air cell or energy-cell combustion chambers. This type of nozzle has a throttling characteristic whereby only a small amount of fuel is dis- charged at the beginning of injection. The fuel flow rate then increases progressively as the valve lifts higher off its seat. Pintle Nozzle Construction The valve of a pintle nozzle is equipped with an exten- sion protruding through the hole in the bottom of the nozzle body, Figure 18-5. This protruding extension creates a hollow cone-shaped spray pattern. The angle of the spray cone is usually between 0–60°, depending on the exact type of com- bustion chamber used. A pintle nozzle generally opens at a lower fuel pressure than a hole nozzle because fuel flows more readily from the larger hole of the pintle nozzle. Pressure pin Feed passage Pintle Body Needle Pressure chamber Spray hole Figure 18-5. Pintle nozzle design and components.