Katerina Swanson
The basic principle of fuel injection is that the right amount of fuel must be injected at the right time to meet horsepower demands. Fuel injection is the introduction of fuel into an internal combustion engine by means of a fuel injector. Diesel fuel injection systems deliver fuel under extremely high injection pressures directly into the cylinder, unlike gas engines where fuel is injected outside the cylinder. The injector in a diesel engine is crucial as it must handle high pressure and heat while distributing fuel evenly in a fine mist. 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.
| Characteristics | Values |
|---|---|
| Purpose of fuel injection system | To deliver fuel into the engine cylinders, while precisely controlling the injection timing, fuel atomization, and other parameters |
| Main types of injection systems | Pump-line-nozzle, unit injector, and common rail |
| Injection delay | The difference between the start of delivery and SOI |
| End of injection (EOI) | The time in the cycle when fuel injection stops |
| Injected fuel quantity | The amount of fuel delivered to an engine cylinder per power stroke |
| Injection duration | The period of time during which fuel enters the combustion chamber from the injector |
| Fuel injection in diesel engines | Fuel is injected during the compression stroke before the piston reaches top dead centre (TDC) |
| Fuel injection in gas engines | Fuel is injected outside the cylinder before the intake stroke |
| Fuel injectors that control metering | Injection valves |
| Injectors that perform all three functions | Unit injectors |
| Continuous injection system | Fuel flows at all times from the fuel injectors, but at a variable flow rate |
| Intermittent injection system | Injection is timed to coincide with each cylinder's intake stroke |
| Common-rail system | Fuel from the fuel tank is supplied to a common header (called the accumulator), and then sent through tubing to the injectors, which inject it into the combustion chambers |
| P-pump | An inline-injection pump that uses a cam to actuate plungers to pressurize the fuel |
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What You'll Learn
- Injector nozzle opens when fuel pressure exceeds its pre-set pressure limitation
- Injectors deliver fuel at the right time to satisfy horsepower demands
- Fuel is injected during the compression stroke before the piston reaches TDC
- Injectors must handle high pressure and heat while distributing fuel evenly
- Common-rail systems use an accumulator rail to maintain fuel at high pressure
Injector nozzle opens when fuel pressure exceeds its pre-set pressure limitation
The nozzle opening pressure is determined by the dimensions of the pressure stage of the nozzle and the rigidity of the injector spring. The nozzle opening pressure is especially important to ensure the precise injection of a small pilot quantity of fuel. The internal pressure within the nozzle works on the pressure stage of the nozzle, which applies a force that opens the injector needle. The injector spring works against this force to ensure that the injector needle remains closed until the nozzle opening pressure is reached.
The nozzle opening pressure can be adjusted by changing the geometry of the injector needle seat. For example, the geometry can be changed from a cylindrical seat geometry to a non-cylindrical geometry, such as a double-cone shape. The choice of injector spring can also impact the nozzle opening pressure.
In a diesel engine, the fuel injection system delivers fuel into the engine cylinders while controlling the injection timing, fuel atomization, and other parameters. The injection pump pressurises the fuel and passes it to the high-pressure injectors, which release the fuel into the cylinder. The injector nozzle opens when the fuel pressure exceeds its pre-set pressure limitation, allowing the fuel to be sprayed into the combustion chamber.
The start of injection (SOI) or injection timing refers to the time at which fuel injection into the combustion chamber begins. The SOI is influenced by the length of time it takes for a pressure wave to travel between the pump and injector, as well as the length of the line between the high-pressure pump and the injector. The end of injection (EOI) is the time in the cycle when fuel injection stops, and the injected fuel quantity is the amount of fuel delivered to an engine cylinder per power stroke.
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Injectors deliver fuel at the right time to satisfy horsepower demands
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 requirements. The amount of fuel supplied to the engine is determined by the length of time the injector stays open, known as the pulse width. This is controlled by the engine control unit (ECU).
The injector is an electronically controlled valve that is supplied with pressurised fuel by the fuel pump. When the injector is energised, an electromagnet moves a plunger that opens the valve, allowing the pressurised fuel to squirt out through a tiny nozzle. The nozzle is designed to atomise the fuel, creating a fine mist that burns easily.
In diesel engines, fuel is injected during the compression stroke before the piston reaches top dead centre (TDC). Only the air is compressed on the compression stroke, which causes it to become hot enough to ignite the fuel. The timing of the injection is critical, as fuel requires time to burn. Therefore, the right amount of fuel must be injected at the appropriate point in the compression stroke so that it burns completely.
There are several types of fuel injection systems, including pump-line-nozzle, unit injector, and common rail. In a common rail system, a dedicated fuel pump generates high fuel pressure and delivers it to a fuel manifold or rail. The high-pressure fuel then flows through individual steel fuel pipes to electronically controlled injectors. This allows fuel to be injected in precise quantities at more than one point during the combustion cycle, resulting in more complete and efficient combustion.
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Fuel is injected during the compression stroke before the piston reaches TDC
In diesel engines, fuel is injected during the compression stroke before the piston reaches top dead centre (TDC). This is achieved through a fuel injection system, which delivers fuel to the cylinders of a diesel engine. The injection pump sends fuel through a high-pressure fuel line to the injectors, which then inject the fuel into the combustion chamber. This process is known as the start of injection (SOI) or injection timing.
The purpose of the fuel injection system is to control the injection timing, fuel atomization, and other parameters. There are three main types of injection systems: pump-line-nozzle, unit injector, and common rail. Modern injection systems use very high injection pressures and sophisticated electronic control methods. The performance of diesel engines is heavily influenced by their injection system design, and advancements in diesel engine performance have resulted from superior fuel injection system designs.
During the compression stroke, the piston moves back to TDC, pushing the air-fuel mixture into a smaller space. This compression increases the pressure and temperature of the mixture, causing some of the fuel to vaporize and disperse more evenly with the air. This vaporization increases the amount of energy generated during combustion, ensuring a more complete burn.
The timing of the fuel injection is critical, as it must coincide with the compression stroke before the piston reaches TDC. This ensures that the air-fuel mixture is at the right pressure and temperature for combustion to occur. The injection of fuel during the compression stroke is a key aspect of diesel engine operation, and the precise control of this process contributes to the overall performance and efficiency of the engine.
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Injectors must handle high pressure and heat while distributing fuel evenly
Injectors are a critical component of fuel injection systems, responsible for introducing fuel into the engine cylinders of internal combustion engines. To ensure optimal engine performance, injectors must be able to withstand high pressures and temperatures while delivering fuel uniformly.
The injector's ability to handle high pressure is crucial as it directly affects the injection timing, fuel atomization, and overall engine efficiency. High-pressure fuel injection systems, such as those found in diesel engines, utilize pressures of up to 300 MPa or 44,000 psi. This high pressure enables the precise control of fuel delivery, resulting in improved combustion and reduced emissions.
The injector's heat management capabilities are equally important. During the compression stroke, only air is compressed, causing it to become hot enough to ignite the fuel. The injector must be able to withstand these high temperatures without failing. Additionally, the injector's design and materials play a vital role in heat management, ensuring that excess heat is dissipated effectively to maintain the fuel's temperature below its flash point.
To achieve uniform fuel distribution, injectors employ sophisticated nozzle designs and actuation methods. The nozzle, also known as the injector body, interfaces directly with the combustion chamber. Standardized nozzle parameters, such as P-Type, M-Type, and S-Type, ensure precise fuel atomization and injection timing. The nozzle holder, or injector body, securely mounts the nozzle and facilitates the connection to the combustion chamber.
The injector's ability to handle high pressure and heat while distributing fuel evenly is essential for maintaining engine performance, fuel efficiency, and emissions control. By effectively managing these factors, injectors play a critical role in the reliable operation of diesel engines.
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Common-rail systems use an accumulator rail to maintain fuel at high pressure
In a diesel engine, fuel is injected during the compression stroke before the piston reaches top dead centre (TDC). The purpose of the fuel injection system is to deliver fuel into the engine cylinders while precisely controlling the injection timing, fuel atomization, and other parameters.
The first common-rail system for a passenger car diesel engine was the Fiat Multijet straight-four engine, introduced in the 1999 Alfa Romeo 156 1.9 JTD model. Common rail injection is widely used in diesel engines and is also the basis of gasoline direct injection systems used in petrol engines. Delphi Diesel Systems has developed a range of Ultra High-Pressure Common Rail fuel systems, with up to 3000 bar capability, to enable medium and heavy-duty vehicles to meet emissions legislation requirements.
The essential elements of a common rail fuel system are a high-pressure pump, which delivers fuel to an accumulator, or common rail, and injectors that can switch the rail pressure to the injection nozzle on demand. In a common rail system, the high pressure can be accumulated in a common rail high-pressure tube, allowing for arbitrary timing and multiple injections, which is beneficial for NOx reduction.
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Frequently asked questions
The purpose of a diesel fuel injector is to deliver fuel into the engine cylinders while precisely controlling the injection timing, fuel atomization, and other parameters.
In a diesel engine, the injector sprays fuel directly into the cylinder. The injector must be able to handle high pressure and heat while distributing the fuel in a fine, even mist. The fuel is then pressurised inside each injector with a mechanically or hydraulically powered plunger and released into the cylinder by a solenoid responding to an electronic signal from the Engine Control Module (ECM).
In a gas engine, a port injection or carburetor is used to inject fuel prior to the intake stroke (outside the cylinder), where the fuel mixes with air before entering the cylinder. In a diesel engine, the fuel is injected directly into the cylinder, and the injector must be able to withstand much higher pressure and heat.
