Sylvie Willis
Diesel fuel injectors are a critical component of a diesel engine's fuel system. The purpose of the fuel injection system is to deliver fuel into the engine cylinders while precisely controlling the timing, fuel atomization, and other parameters. The basic principle of fuel injection is that the right amount of fuel must be injected at the right time to meet the engine's horsepower demands. Diesel fuel injectors are small electrical components that deliver fuel via a spray directly into the intake manifold in front of the intake valve in a diesel engine. The injector in a diesel engine is crucial, as it must handle high pressure and heat while distributing fuel evenly in a fine mist.
| Characteristics | Values |
|---|---|
| Purpose | To deliver fuel into the engine cylinders |
| Main types of injection systems | Pump-line-nozzle, unit injector, and common rail |
| Fuel injection principle | The right amount of fuel must be injected at the right time to satisfy horsepower demands |
| Fuel injection timing | During the compression stroke before the piston reaches top dead center (TDC) |
| Fuel injection process | Fuel is metered and pressurized by the fuel injection pump and passed to the high-pressure injectors via pipes before being sprayed into the combustion chamber |
| Injector types | Throttle body injection, individual port type fuel injector |
| Injector cycle firing | On average completed in 1.5 to 5 milliseconds |
| Fuel injector shapes and sizes | Vary depending on the engine make and model as well as power demand |
| Fuel injector components | Filter, guide ring, core spring, seat spring, seat, pole piece, stop, solenoid coil, solenoid body, core ring, core, spray tip housing, director, and spray tip |
| Fuel tank characteristics | Capable of storing enough fuel, closed to prevent contamination, vented to allow air to enter |
| Fuel line types | Heavyweight, medium weight, and lightweight |
| Fuel filtration | Filtered multiple times in most systems |
| Fuel system enemies | Water, which can cause injection component seizure and governor/metering component failure |
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What You'll Learn
Injector types
There are several types of diesel fuel injectors, including mechanical, common rail, and unit injector systems. The main types of injection systems include pump-line-nozzle, unit injector, and common rail.
Mechanical fuel injectors use a mechanical linkage to open and close the injector valve. They contain a low-pressure circuit and a high-pressure circuit. The fuel is accurately metered and pressurised by the fuel injection pump and passed to the high-pressure injectors via pipes before being sprayed into the combustion chamber. The injection pump can be a rotary plunger-type unit with an individual piston for each cylinder of the engine, or a single chamber pump that meters fuel to each cylinder using a sleeve or scroll system.
Common rail systems use high-pressure fuel pumps to deliver fuel to a central rail, which is then distributed to the individual injectors. The fuel injection pump supplies fuel to the manifold and injectors at pressures up to 200 MPa (29,000 psi). This type of system allows for more precise control and efficiency.
Unit injector systems combine the fuel pump and injector into a single unit.
The injector in a diesel engine is crucial as it must handle high pressure and heat while distributing fuel evenly in a fine mist. It is the most complex part of the engine. The latest injector technology helps to reduce carbon and other emissions by delivering accurate fueling with optimised air mixing.
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Injection timing
The start of injection (SOI) is the time at which injection of fuel into the combustion chamber begins. In some fuel systems, fuel injection is coordinated with the generation of high pressure. In such systems, the start of delivery is the time when the high-pressure pump starts to deliver fuel to the injector. The difference between the start of delivery and SOI can be referred to as injection delay. The end of injection (EOI) is the time in the cycle when fuel injection stops. The injected fuel quantity is the amount of fuel delivered to an engine cylinder per power stroke. It is often expressed in mm3/stroke or mg/stroke. Injection duration is the period of time during which fuel enters the combustion chamber from the injector. It is the difference between EOI and SOI and is related to the injected fuel quantity. The rate of injection of fuel often varies during the injection duration period.
Advancing or retarding an engine's timing is possible. Advancing an engine's timing causes the injection process to occur earlier than the manufacturer's settings. On the other hand, retarding involves making changes so that the fuel releases after the recommended time. Although retarding is less common than advancing, it can repair a lag or smoking problem within a marine engine and support performance and fuel economy issues.
The injection timing and pressure play a crucial role in determining engine performance, especially in pollutant emissions. Early injection provides lower soot and higher NOx emissions than late injection.
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Fuel atomization
The atomized fuel, in the form of a fine spray, ensures that each particle comes into contact with oxygen and has the opportunity to vaporize and participate in the combustion process. This atomization enhances the mixing of fuel and air, promoting more efficient and complete combustion. Any remaining larger droplets may burn poorly or be exhausted from the engine without contributing optimally to the power output.
The fuel atomizer, also known as the fuel injector, is a crucial component in a diesel engine. It must withstand high pressure and heat while atomizing and distributing the fuel evenly. The injector may be designed with a swirl or pre-combustion chamber to facilitate air swirling for even fuel distribution. Additionally, the injector's nozzle may be a pintle nozzle, creating a hollow conical spray, or a delay nozzle, which increases the fuel injection rate towards the end of the delivery.
The injection pump, which can be a rotary plunger-type unit or a single-chamber pump, plays a vital role in fuel atomization. It meters and pressurizes the fuel before sending it through the high-pressure fuel line to the injector. When the fuel pressure exceeds the nozzle's pre-set limitation, the nozzle opens, allowing the atomized fuel to be injected into the combustion chamber. The precise control of fuel atomization, along with injection timing and metering, ensures the engine's effective utilization of fuel.
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Injector maintenance
Before beginning any maintenance work, always ensure you have taken the necessary safety precautions to avoid injury. Start by shutting off the engine and allowing it to cool down completely. This depressurizes the fuel injection system, protecting you from dangerous sprays of fuel when removing parts.
Next, clean the engine thoroughly with rags and a household spray cleaner to remove any dirt. Use a vacuum to reach any remaining dirt, paying close attention to the injector wells. Turn off the fuel supply to prevent fuel from reaching the injectors. Remove the fuel lines and block the feed lines and leak-off lines with plastic and tape to prevent dirt from entering.
Now you can remove the injectors. This is generally a straightforward process, but it requires a fastidiously clean work environment to avoid any debris entering the engine. Once removed, take the injectors to a diesel shop, injection shop, or diesel service center for specialized servicing. These facilities will disassemble, clean, and replace any necessary parts, such as seals, springs, and washers. They will also test and adjust the injectors to ensure they are functioning optimally.
After receiving your serviced injectors, it's time to reinstall them. Replace all return lines and any other components removed during the process. Consult your engine manufacturer's recommendations for bleeding the fuel system. Once this is done, start the engine and closely inspect it during the first few minutes of operation. Listen for any loud hissing noises, which could indicate leaks between the injectors and cylinder head. Also, check for fuel leaks at all unions and tighten fasteners if necessary.
To maintain your fuel injection system effectively, regular servicing is essential. Clean the system and perform maintenance at least every 36 months or 45,000 miles, or more frequently for heavy-use machines. Keep the injector systems and fuel itself clean to prevent clogging, which can cause engine issues. Additionally, follow the manufacturer's recommendations for servicing intervals and procedures to ensure your diesel engine remains in peak condition.
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Fuel injection history
Fuel injection systems have been around for over a century, with the first fuel-injected compression-ignition engine constructed in 1902 and entering production in 1906. This was the French Antoinette 8V engine, designed by aviation pioneer Léon Levavasseur. However, the idea behind fuel injection was conceived even earlier than this. In 1885, British engineer Herbert Akroyd Stuart discovered that hot kerosene vapours ignite far more readily than liquid kerosene, leading to the construction of the hot-bulb engine in 1891. Rudolf Diesel improved upon this design with his air-blast injection system in 1894, increasing the air blast pressure to 6,400 kPa (920 psi).
The first direct-injected compression-ignition engine used in road-going vehicles was the Hesselman engine, invented by Swedish engineer Jonass Hesselman in 1925. This engine burned petrol until it warmed up, then switched to a heavier fuel such as diesel or kerosene. It was used in buses and trucks from the late 1920s to the early 1940s. In the 1920s, German engineer Robert Bosch developed the in-line injection pump, which is still in use today. The 1930s and 40s saw the first mass-produced diesel engines for passenger cars, such as the Mercedes-Benz OM 138. During World War II, Germany equipped some fighter aircraft with direct fuel injection to prevent engine stall-out during high-speed aerial maneuvers.
In the 1950s and 60s, diesel engines became popular for use in taxis and vans, and in the 1970s, the VW diesel-powered Golf marked the arrival of the first 'user-friendly' diesel car. The 1970s also saw the introduction of the Bosch K-Jetronic system, a continuous injection system used until the mid-1990s. From the 1960s to the 1980s, some diesel engines used the M-System, which sprayed fuel onto the walls of the combustion chamber, as opposed to the middle. From the 1980s to the mid-1990s, single-point injection was used extensively in American-made passenger cars and light trucks, and in some European cars. In the 21st century, direct injection in petrol engines has become increasingly common.
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