Diesel injectors (ON)

Genesis and application.

The history of diesel engine injectors development spans more than a century of innovation and technological progress. Initially, fuel injection was carried out using high-pressure air, but the first mechanical injectors without air assistance appeared in the 1920s. During this period, Bosch played a key role with the introduction of high-pressure injectors, which found their way into truck and agricultural machinery engines.

In the 1930s, Mercedes-Benz introduced the first mass-produced diesel passenger car, indicating the growing popularity of the technology. World War II accelerated the development of injectors, particularly in a military context. The 1950s and 1960s saw further breakthrough innovations, such as injectors with a back-spring system and the first electronically controlled injection pumps.

In the 1960s and 1970s, dual-spring injectors became standard in many applications, from trucks to agricultural machinery. The introduction of these injectors allowed for better control of the combustion process, which had a direct impact on improving engine performance and reducing fuel consumption.

In the 1990s, Bosch initiated a revolution with its unit injector system, followed by the Common Rail system, which allows precise control of fuel injection at very high pressure. 

The unit injector system, introduced in cars and vans by the Volkswagen Group, was a significant step in the development of diesel injector technology. This system integrated the injection pump directly into the injector, enabling more precise control of fuel injection. Each engine cylinder had a separate unit injector piece, which provided better control over the injection process and fuel combustion. Unit injectors offered greater power and efficiency than traditional injection systems, especially in terms of responsiveness to changes in engine load and operating conditions. The technology was particularly popular in Volkswagen vehicles and other VW group brands for many years. It has also found applications in truck engines and machinery and commercial vehicles.

However, as it develops and seeks to further reduce emissions and improve efficiency, the unit injector system was gradually replaced by more advanced common rail systems with electromagnetically or piezo-controlled injectors. In this type of system, the fuel compression and injection functions are separated. This enables precise control of fuel injection, resulting in improved engine performance and lower exhaust emissions.

Since the 2000s, diesel injector technology has continued to evolve, with a focus on reducing emissions, improving fuel delivery accuracy and integration with hybrid systems.

Construction and principle of operation.

Mechanical injectors.

Mechanical fuel injectors were commonly used in diesel engines because of their simplicity and reliability. 

The injector body is usually made of high-quality steel, which is resistant to high pressure and temperature. The body contains all the other components of the injector and is mounted directly into the engine head. The needle is the key component that controls fuel flow. It is a precision-made piston that moves inside the nozzle. The needle is pressed against the seat by a spring, which prevents fuel leakage when no pressure is applied. The injection nozzle is located at the end of the injector and has very small diameter holes through which fuel is sprayed into the combustion chamber. The fine atomisation of the fuel ensures better mixing with the air and more efficient combustion. A spring presses the needle against the nozzle seat. The spring force must be adjusted to ensure that the nozzle opens at the specified fuel pressure. By adjusting the spring force, the injector's opening pressure can be set precisely. More advanced mechanical injector designs (so-called dual-spring injectors) have two springs with different strengths. This allows the injector to open gradually and perform pre-injection. The shim is used to adjust the strength of the spring and thus the pressure at which the needle starts to rise. By changing the thickness of the shim, the opening point of the injector can be precisely adjusted. A fuel channel located in the body guides the high-pressure fuel from the injection pump to the inside of the injector. The fuel in the fuel channel causes pressure to build up on the needle. When the pressure exceeds the set spring value, the needle rises, opening the injection nozzle. At the end of the injection phase, the fuel pressure drops, causing the spring to press the needle against the seat, closing the nozzle and stopping the fuel flow.

Unit injectors.

The unit injector combines a pump and an injector in its body. The pump section is mechanically driven from the engine, usually by a dedicated shaft. A piston moving in the housing compresses the fuel to a high pressure. Each of the unit injectors is equipped with a control solenoid valve, allowing precise adjustment of the fuel dose supplied to the engine cylinders. The nozzle of the unit injector has a number of small holes to ensure that the fuel is atomised just finely enough for better mixing with the air and complete combustion.

Electromagnetic and piezo injectors (common rail).

The body is made of durable materials, usually steel, which can withstand high pressure and temperature. The body contains all the components of the injector and is mounted directly in the engine head.

The solenoid is the key control element of the injector. Powered by an electric current, it generates a magnetic field, which allows the control (bypass) valve to open. When the solenoid is activated the valve opens, allowing some of the fuel to return to the overflow and thus the start of fuel injection. The nozzle is located at the end of the injector and contains very small holes through which fuel is fed into the combustion chamber. These holes are precisely made to ensure that the fuel is properly atomised. The return spring is responsible for closing the needle valve when the solenoid stops working. It presses the valve needle against the seat, which closes the injector and ends fuel injection. The fuel channels feed high-pressure fuel from the rail to the inside of the injector. The electrical plug is where the injector connects to the engine control unit (ECU). A current flows through it, which activates the solenoid.

Piezo injectors lack a coil, with a piezo crystal stack taking its place. Control of the opening and closing of the injector is carried out using the piezocrystal effect, which involves changing the length of the piezo stack when a voltage is applied to it. The change in piezo stack length activates the injector control valve allowing injection to start and stop. The advantage of this design over electromagnetic injectors is the speed of operation, which translates into precise fuel dosage to the cylinder. 

Remanufacturing process.

Unit injectors.

Tests of Bosch unit injectors both before and after remanufacturing are carried out on a Bosch EPS 815 test bench equipped with a CAMBOX. Only such a set allows the precise measurement of the unit injector performance parameters, including the important value known as BIP (Beginning of Injection Period). Remanufacturing primarily involves replacing the nozzle, replacing the seals, inspecting and cleaning the remaining components. If necessary, other components such as the piston and the spring which is part of the drive system of the unit injector or the control valve components are replaced.

In the case of Delphi's unit injectors, testing and calibration takes place on the Hartridge AVM2-PC test bench. Remanufacturing primarily involves replacing the nozzle and control valve, replacing the seals, inspecting and cleaning the remaining components. If necessary, other components such as the spring, electrical connector and others are replaced. If the manufacturer has provided for such a need, the remanufactured unit injector receives a new correction code, which is generated on the basis of the parameters intended during the calibration test carried out on the test bench.

Electromagnetic and piezo injectors (common rail).

At the customer's request, injector tests are carried out prior to repair. The tests take place on test benches authorised by the individual manufacturers of the injection systems. These are machines manufactured by Bosch, Hartridge and Carbon Zapp. The remanufacturing includes all the steps provided for in the instructions prepared by the individual manufacturers. This is usually the replacement of the nozzle, seals and control valve and, if necessary, the coil. The remaining components are thoroughly cleaned, verified and reused. Importantly, based on the dosage measurements of each injector, the devices generate individual classification or correction codes (IMA/C2i/C3i/C4i). Entering these into the engine control unit guarantees the correct operation of the injectors installed in the vehicle.

Causes and types of damage

Symptoms that may indicate faulty injectors:

1. Uneven engine operation.
2. Loud engine operation.
3. Decrease in engine power.
4. Increased fuel consumption.
5. Engine starting problems.
6. Excessive smoke from the exhaust system.
7. Luminous "check engine" light.

The most common injector failures are:

1. Nozzle damage.
2. Failure of the control valve.
3. Damage to the coil or piezo stack.
4. Internal or external fuel leakage.