Sylvie Willis
The primary function of a diesel fuel injection system is to deliver fuel into the engine cylinders, while precisely controlling the injection timing, fuel atomization, and other parameters. 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 injectors are responsible for providing precise and metered quantities of atomized fuel at the right time. The nozzle, a tiny part of the injector, atomizes the fuel into a fine mist of tiny droplets before it is injected into the combustion chamber. Modern diesel fuel injectors, such as those found in common rail fuel systems, use piezoelectricity to handle the high pressures in common-rail applications.
| Characteristics | Values |
|---|---|
| Basic principle of fuel injection | The right amount of fuel must be injected at the right time to satisfy engine horsepower demands. |
| Fuel injection process | Fuel is injected during the compression stroke before the piston reaches top dead centre (TDC). |
| Injector nozzle | When the fuel pressure exceeds the injector nozzle's pre-set pressure limitation, it opens, and fuel is injected into the combustion chamber. |
| Fuel return | Any excess fuel is sent out through a return line back to the fuel tank. |
| Fuel system types | Pump-line-nozzle, unit injector, and common rail. |
| Fuel injection pressure | Up to 200 MPa (29,000 psi) in common rail fuel systems. |
| Fuel injection types | Individual pump and injector for each cylinder, combined pump and injector for each cylinder (unit injector type), one pump serving injectors for several cylinders (distributor type), and pumps in a common housing with injectors for each cylinder (common rail system). |
| Fuel atomization | Fuel is atomized into a fine mist of tiny droplets before injection for rapid and efficient combustion. |
| Injector pressure | Up to 36,000 PSIG. |
| Injector cycle firing | Completed in 1.5 to 5 milliseconds. |
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What You'll Learn
Injector nozzle opens when fuel pressure exceeds its pre-set pressure limit
The injector nozzle is a tiny but integral part of a vehicle's fuel injection system. The nozzle atomises the fuel, turning it into a fine mist of tiny droplets, before it is injected into the combustion chamber. The nozzle is precision-engineered to ensure a consistent fuel spray pattern for even and complete combustion.
The injector nozzle opens when fuel pressure exceeds its pre-set pressure limit. This is known as the injector nozzle's "pop-off" pressure. When the pressure limit is exceeded, the nozzle opens, and fuel is injected into the combustion chamber. This process is essential for the engine to function, as it allows the fuel to be ignited and burned, powering the engine.
The amount of pressure required to open the injector nozzle varies depending on the specific engine and its fuel injection system. In some cases, injector pressure can reach extremely high levels, up to 36,000 PSIG (Pounds per Square Inch Gauge). This is significantly higher than atmospheric pressure, which is around 1 PSIG.
The high pressure in the fuel injection system is generated by a high-pressure pump, which pressurises the fuel so that it can overcome the high pressure inside the combustion chamber. This pump is driven by the engine and supplies fuel automatically to the diesel injection system. Modern injection pumps often use the plunger and cam method of fuel injection.
The timing of the fuel injection is also critical. The fuel must be injected at the right time during the compression stroke, so that it burns completely. This timing is controlled by the Engine Control Module (ECM), which determines how long the injectors need to be grounded to inject the exact amount of fuel required by the engine. This entire process, from opening the injector nozzle to dispensing the fuel, happens extremely quickly, often in just milliseconds.
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Fuel atomization ensures fuel particles are small enough to vaporize
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. The main purpose of the fuel injection system is to deliver fuel into the cylinders of an engine. The injectors deliver a precise amount of atomized and pressurized fuel into each cylinder.
Fuel atomization is a primary design objective for diesel fuel injection systems. Fuel atomization ensures that the fuel breaks down into very small particles or droplets. This is critical to the injection system's performance because small droplets ensure that all the fuel has a chance to vaporize and take part in the combustion process.
The atomized and pressurized fuel is distributed evenly throughout the cylinder, resulting in increased power and fuel economy, decreased engine noise, and smoother operation. The fuel must be injected at the proper time, i.e., during the compression stroke before the piston reaches top dead centre (TDC).
Modern injection systems reach very high injection pressures and utilize sophisticated electronic control methods. The injection system design heavily influences the performance of diesel engines. While the main purpose is to deliver fuel to the cylinders, it is the method of delivery that makes the difference in engine performance, emissions, and noise characteristics.
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Injector pressure can reach up to 36,000 PSIG
The basic principle of fuel injection is that the right amount of fuel must be injected at the right time to satisfy engine horsepower demands. Fuel injection systems deliver fuel under extremely high injection pressures, and the pressure must be sufficiently large to deliver the fuel to the cylinder in a short time period. The higher the pressure, the greater the fuel injection rate.
Modern injection systems reach very high injection pressures, and common rail fuel systems supply fuel to the manifold and injectors at pressures up to 200 MPa (29,000 psi). This is significantly higher than the pressure in the hydraulic lines of between 1000 and 5000 psi, which translates to an injection pressure between 10,000 and 20,000 psi.
The injectors must be able to handle these very high pressures, and piezoelectric injectors are extremely precise and can manage the high pressures found in common-rail applications. Mechanical Electronic Unit Injectors (MEUI) and Hydraulic Electronic Unit Injectors (HEUI) replace the mechanical components on the high-pressure side of the fuel system, including the injection pump, fuel lines, and nozzles.
The injector pressure can reach up to an incredible 36,000 PSIG, which is far higher than the pressures found in most common rail fuel systems. This high pressure results in increased power and fuel economy, decreased engine noise, and smoother operation.
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Mechanical and electronic approaches to fuel injection
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. The fuel injection system is responsible for delivering fuel into the engine cylinders while precisely controlling the injection timing, fuel atomization, and other parameters.
Mechanical Approaches to Fuel Injection
Mechanical fuel injection systems use a variety of methods to inject fuel into the engine. Some early mechanical injection systems used helix-controlled injection pumps to meter fuel and create injection pressure. Other mechanical systems use a combination of pumps and injectors to deliver fuel to each cylinder. There are four primary types of mechanical fuel injection systems:
- Individual pump and injector for each cylinder
- Combined pump and injector for each cylinder (unit injector type)
- One pump serving injectors for several cylinders (distributor type)
- Pumps in a common housing with injectors for each cylinder (common rail system)
Electronic Approaches to Fuel Injection
Electronic fuel injection (EFI) systems use an engine control unit to control the injection of fuel into the engine. The first EFI system was introduced in the 1950s by Bosch and used in small automotive two-stroke petrol engines. Over time, EFI systems became more widespread and were adopted by major car manufacturers such as Volkswagen, General Motors, and Chrysler. Modern EFI systems use sophisticated electronic control methods to achieve very high injection pressures and improve engine performance.
Comparison of Mechanical and Electronic Approaches
Both mechanical and electronic fuel injection systems aim to deliver the right amount of fuel at the right time to the engine cylinders. Mechanical systems rely on mechanical components such as pumps and injectors to control the fuel injection process, while electronic systems use an engine control unit to electronically control the injection of fuel. Electronic systems offer more flexibility and improved performance compared to mechanical systems, especially in terms of lowering emissions and improving fuel efficiency.
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Injectors are powered when the key is turned on
The primary purpose of the fuel injection system in diesel engines is to deliver fuel into the engine cylinders, while precisely controlling the injection timing, fuel atomization, and other parameters. The main types of injection systems include pump-line-nozzle, unit injector, and common rail. Modern injection systems reach very high injection pressures, and utilize sophisticated electronic control methods.
The basic principle of fuel injection is that the right amount of fuel must be injected at the right time to satisfy engine horsepower demands. Since fuel requires time to burn, the right amount of fuel must be injected at the appropriate point in the compression stroke, so that it burns completely. Over the years, a number of different systems have been developed to implement and control the fuel injection process. These can be broadly categorized into mechanical and electronic approaches, although the basic principles of diesel fuel systems are consistent regardless of the technology used to implement them.
In diesel engines, fuel is injected during the compression stroke before the piston reaches top dead center (TDC). The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine. The injector is located in the combustion chamber, inlet manifold, or, less commonly, the throttle body. Fuel injectors that also control the metering are called injection valves, while injectors that perform all three functions are called unit injectors. Direct injection means that the fuel is injected into the main combustion chamber of each cylinder.
Modern diesel fuel injectors, such as those found in common rail fuel systems, use piezoelectricity. Piezoelectric injectors are extremely precise and can handle the very high pressures found in common-rail applications. Third-generation common-rail diesels use piezoelectric injectors for increased precision, with fuel pressures up to 300 MPa or 44,000 psi.
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