Electronic Injector Architecture
The electronic fuel injectors installed in a port-injection architecture—a.k.a., indirect injection—consist of a housing, a fuel supply inlet, and two-wire electrical connector, and a valve assembly that consists of valve itself and a solenoid armature. The housing contains the solenoid winding and electrical connections. A helical spring assisted by fuel pressure forces the valve against its seat in the valve assembly when the injector is not energized, blocking fuel from passing through the injector. When the ECM energizes the solenoid windings in a port-injector, the valve lifts by approximately 0.102 inches, and pressurized fuel is forced through the valve body and sprays from the injector. Direct-injection Piezoelectric injectors are similar in concept to port injectors, but instead of an electromagnetic coil pulling a needle valve off its seat, a 4-inch stack of some 200 0.02-inch thick Piezoelectric sheets in the injector energized by 140 volts of electricity expands a maximum of 0.004 inches to lever a needle valve off its seat to inject fuel—with much greater speed than an electromagnetic injector and against much high pressure. Depending on the voltage applied to the Piezoelectric circuit, the stack can push open the valve less than the maximum, which is useful for increasing the precision of injected fuel mass at idle when the injection interval might otherwise be too short for repeatable results. Direct fuel-injection costs more than port injection systems because the injectors are exposed to more heat and pressure, mandating more costly materials, and because the high pressure and more complex operating algorithms require higher-precision electronic management systems and the addition of an expensive high-pressure mechanical fuel pump.
According to Bosch, injectors are designed with the following goals in mind:
1. Precise fuel metering at all operating points
2. Accurate flow at narrow pulse widths, with deviation from
linearity within specified tolerances
3. Broad dynamic flow range
4. Good fuel distribution and atomization
5. Valve leak tightness
6. Corrosion resistance to water, sour gas, and
8. Low noise
9. Ethanol injection: heated injector technology
Port injectors can be divided into the following metering categories:
1. Annular orifice metering, which uses a pintle to optimize the atomization via a conical-shaped spray pattern and which meters fuel by the size of the gap between the pintle and the valve body.
2. Single-hole metering in which fuel spray is injected directly from the drilled passage in the valve body downstream of the needle valve (atomization is not as good as with a pintle design, at worst case producing a “pencil beam”).
3. Multi-hole metering (C-version) that forces fuel through a stationary drilled plate located at the end of the valve body orifice, downstream from the needle valve. This design,
which normally includes four precisely spaced and aligned holes, results in a good conical spray pattern.
4. Multi-hole metering for multi-valve engines is similar to 3, but aligns the metering holes so a separate spray of fuel hits each intake valve.
5. Disc metering uses a drilled disc that moves off a flat seat with the armature. Claimed benefits include resistance to deposit buildup and clogging so no cleaning is ever required, wider
dynamic range for improved idle, quieter operation, lighter weight that improves response, and operation with alternative fuels like natural gas, propane, methanol, and ethanol. Atomization quality may be similar to single-hole metering. The Bosch pintle-type injector is the earliest design but is still widely used. The valve housing in this type of injector narrows to form a seat upon which a tapered needle comes to rest when the injector is closed, choking off fuel flow. Bosch improves the spray pattern by extending the needle valve assembly into a beveled pintle assembly that protrudes out of the end of the injector through a plastic chimney. An electromagnetic solenoid lifts the spring-loaded pintle off its seat to release fuel. Bosch injector pintles and seats are always the same size. Bosch regulates flow by varying the size of the bore after the seat. Problems with the pintle design reportedly include increased chance of clogging in the small orifice area, slower response time because of the heavier armatures used to lift the pintle, and reduced service life. In some cases, injectors will use a ball valve to seal the metering orifice. This allows the use of a lighter armature to improve response time compared to older pintle types. There is also reportedly less wear and a longer service life. The orifice can be designed with multiple openings, which allows a wider spray pattern and higher fuel delivery. The disc-type injector eliminates the armature so the solenoid acts directly on the flat disc through the core of the injector body. The valve assembly in a disc-type injector consists of the armature attached to a disc with six tiny holes drilled around the outside. In a closed position, the disc rests on a seat, the center of the disc covering a hole in the middle of the seat that prevents any fuel from passing through the injector. When the armature is pulled open as the injector energizes, fuel passes through the six outer holes, through the hole in the seat, and sprays out of the injector. The flow of a Lucas disc injector is determined by the size of the six holes drilled in the disc. This arrangement is even lighter than the ball-type for a faster response time. This disc and seat design also results in less deposit buildup at the orifice and longer service life. Injectors vary in the time it takes to open and close. Larger injectors are built with heavier moving assemblies that are a little more sluggish due to higher inertia. To compensate, they are typically designed with more powerful low-resistance windings that require peak-and-hold circuitry to power them open quickly. The Lucas Disc injectors are lighter, according to fuel system expert Russ Collins, which enables disc-type injectors to operate down to 0.8, 0.9, or 1.0 milliseconds. Most pintle injectors are limited to a minimum 1.3 to 2.0 milliseconds. Some performance experts recommend when upgrading injectors for higher flow to switch from traditional pintle-type injectors to ball or disc-type valves.