The maximum pressure used in systems with DFI 1.1-1.4 type injectors reaches up to 1800 bar, and a large force is also used to lift the needle. This means that it is impossible to control the injector needle using an electromagnetic valve because it would require a very strong current. The time for such current buildup is quite long, while the needle must rise and fall in a much shorter time. Moreover, such a strong current can cause the injector to overheat. Therefore, the needle inside the injector is controlled by a valve responsible for the pressure directly above the chamber located over the needle. At the beginning of injection, when the needle must rise and uncover the hole in the lower part of the nozzle, the valve opens, and the contents of the chamber flow back into the return line. To lower the needle, the valve inside the chamber creates pressure that pushes the needle down. The valve’s purpose in the injector is to consume as little energy as possible during operation. For this reason, it weighs little and moves with maximum force. The closed valve must be in hydraulic balance. This balance is achieved by the geometric identity of the chamber, ensuring that the entire valve surface is subjected to equal force. Thus, a very soft spring, easy to compress under valve load, can be used to hold the valve in place and lift the needle.

Problems related to contaminated fuel became a reason to change the injector design – to control temperature, the internal surfaces of the valve were coated with diamond-like carbon (DLC). An adaptive bushing is mounted at the valve fixing point. It connects the control chamber with three ports – injection feed, return fuel line with control chamber, and a port for fuel supply to the chamber.

The pressure distribution inside the injector can be divided into several stages:

Adaptive bushing DELPHI DFI 1 – before filling the adaptive bushing, high-pressure fuel enters the injector, first filling the control chamber channel, then the injector fuel gallery channel, and then supplied to the valve chamber channel;

High-pressure fuel fills the control chamber, adaptive bushing, and spiral grooves in the needle.

At this stage, the fuel inside the injector is balanced, and the injector remains closed. The fuel pressure on both sides of the valve inside the injector, in a resting state, is equal. When the DCU activates the relay, the valve opens. After the valve opens, fuel flows into the return line, pressure decreases in the valve chamber, then in the fuel gallery channel, and finally in the control chamber. However, the needle itself remains still because the pressure in the control chamber does not decrease. The needle starts moving when the pressure drop reaches the control chamber and a pressure imbalance appears on both ends of the valve. Since the pressure on the needle’s rear side is higher than in the control chamber, the needle moves up, opening the path for fuel through the gallery to enter the combustion chamber. Pressure in the gallery drops after passing through the orifice and becomes lower than the pressure in the rail. At maximum rail pressure, the pressure in the gallery is 100 bar lower. When the DCU stops supplying current to the valve spool, the force acting on it becomes weaker than the spring force, and the spring pushes the valve back, closing it. Pressure inside the injector increases, but the needle does not close the injector because it requires different pressure at the needle ends. Different pressure appears when the pressure in the fuel gallery becomes lower than in the control chamber, which equals rail pressure. Once the pressure in the control chamber becomes higher than the needle’s rear pressure, the needle moves down and closes.

Operation of solenoid DELPHI injector

The return fuel line is connected to the injector either by attaching a rubber fitting with a metal pipe or via a special plastic adapter. This type of injectors can perform from one to five individual openings per cycle, divided into: separate pilot, close pilot, preliminary, main, close subsequent injection, post-injection, additional post-injection.

Additionally, these injectors have a feature of fuel release into the return line in case of failure. This is necessary when the accelerator is quickly released or the ECU detects a fault that requires rapid pressure reduction in the rail. In such case, the injector coil receives a pulse from the DCU sufficient to open the valve and connect the fuel in the rail with the return line but insufficient to lift the needle and open the fuel path to the engine. Such control is possible only if the valve opening and needle lifting times are known precisely. These times depend on the physical properties of each injector, influenced by its natural wear. Therefore, the ECU program needs to know the physical condition of each injector precisely. This is achieved by calibrating injectors at the factory and assigning each an individual code. Delphi uses two injector calibration methods:

C2I (Correction Individual Injector) – 16-character code;

C3I (Improved Individual Injector Correction) – more accurate calibration and 20-character code.

The code is written to the DCU memory when replacing injectors with new ones or when entering the old injector code into a new DCU using a scanner. Based on calibration data from the code, the control unit can perform correction for each injector.

Injectors DFI1.5/1.5.2

The DELPHI DFI 1.5 fuel injector was designed to meet Euro 5 standards, increase injection efficiency, and maintain up to 7 openings during injection. Compared to predecessors, they better protect against dirt and ensure a more stable fuel flow during injection.

DFI 1.5 injectors consist of a nozzle with needle, injector body with feed hole with filter and outlet hole, electrical connector on the upper part of the injector, adaptive plate (CVA) with calibrated needle control ports, and combined valve.

Depending on the generation, the injector can operate at pressures up to 2000 bar. At such pressure, the needle cannot be directly controlled by an electromagnetic actuator because the force would be too high, which would overheat not only the control unit but also the injector, and the reaction time would be too long. Therefore, needle movement is controlled by a control chamber where fuel for needle lifting flows to the return line, and to lower the needle, pressure in the chamber is restored.

Compared to the first generation, the second generation injectors have needles and seats with an angle changed to 60 degrees and are coated with lacquer, the angle between nozzles is reduced, the outlet valve diameter is increased, an adaptive plate with valve is installed, the return spring force is increased, and a new type of connector is used with diameter increased from 17 mm to 19 mm. Two connector types are used – old generation and type V. The return system connection is similar to DFI 1.1. Calibration uses the C31 method.

DELPHI DFI 1.5.2 injectors were developed for Euro 6 standard and can operate at pressures up to 2200 bar. This generation uses more efficient spools, stronger valve return springs, improved CVA block design, and the injector inside can maintain a force of up to 3000 Newtons when the cover is tightened. The return system uses an adapter, and the injector is calibrated with the C31 method.

Injectors DFI1.20

DELPHI DFI 1.2 series injectors were created to meet Euro 6 ecological standard and can operate at pressures up to 2200 bar. The injector design elements are identical to previous generation injectors. The main differences are the use of AK type connector, which allows the return system to use up to 6 bar pressure, new improved spools, narrower injector needles with redesigned internal channels, reinforced springs, and a modified CVA module design.

Since in this type of injector fuel is supplied to the return line at 6 bar pressure, the back pressure port tip is metal with a rubber sealing ring. The injector operates similarly to previous generations. For more precise calibration of this type, the C31 method is used. Also, a 20-digit code is used, so the injector’s calibration cannot be visually determined and can only be identified by the part catalog number.

Injectors DFI2.3

DELPHI DFI 2.3 injectors were developed as version 1.3 but adapted for large fuel quantities and designed for commercial transport and high displacement engines. The injector consists of a nozzle with needle, main body with fuel feed chamber and filter, outlet to the return line, electrical connector integrated into the coil, adaptive bushing with control chamber and calibration chamber for needle control, valve, and gaskets.

Depending on generation, the injector can operate up to 1600 bar pressure. Since this is relatively high pressure, the needle cannot be directly controlled by the solenoid, as this would require a very strong current and would not allow synchronization of several fast openings. Therefore, these injectors use a hydraulic control method that previous generations with control chambers also had.

These injectors are widely used in engines of trucks, heavy vehicles, and construction equipment meeting Euro 4 and higher standards. The injector’s return channel has a special LP type connector. Calibration of these injectors can be done by both C21 and C31 methods.