Katerina Swanson
The diesel engine was invented in 1894 by Rudolf Diesel, who copied and improved George Bailey Brayton's air-blast injection system. The diesel engine uses fuel injection to introduce fuel into the engine cylinders, with the main types of injection systems being pump-line-nozzle, unit injector, and common rail. 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 injector in a diesel engine is crucial, as it must handle high pressure and heat while distributing fuel evenly in a fine mist. The fuel injection system can be divided into low-pressure and high-pressure sides, with the low-pressure components including the fuel tank, fuel supply pump, and fuel filter, and the high-pressure side components including a high-pressure pump, accumulator, fuel injector, and fuel injector nozzle.
| 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 timing | During the compression stroke before the piston reaches top dead centre (TDC) |
| Fuel injection pressure | In common rail fuel systems, the fuel injection pump supplies fuel at pressures up to 200 MPa (29,000 psi) |
| Fuel type | Diesel |
| Fuel properties | Derived from heavier residues of crude oil; most common grade is 2-D or ultra-low sulfur diesel (ULSD) |
| Fuel system components | Fuel tank, fuel lines, injection pump, injectors, fuel filters |
| Fuel tank characteristics | Capable of storing sufficient fuel; closed to prevent contamination; vented to allow air in and fuel out |
| Fuel line types | Heavyweight, medium weight, lightweight |
| Fuel filter systems | Series filters, parallel filters |
| Fuel pump | Driven by the engine; often includes a hand primer lever for bleeding air |
| Fuel injection types | Individual pump and injector per cylinder, combined pump and injector per cylinder, one pump serving multiple cylinders, common rail system |
| Fuel injector function | Deliver a precise amount of atomized and pressurized fuel into each cylinder |
| Fuel injector technology | Piezoelectric injectors, Mechanical Electronic Unit Injectors (MEUI), Hydraulic Electronic Unit Injectors (HEUI) |
| Fuel issues | Water contamination, gelling at low temperatures |
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What You'll Learn
Injecting the right amount of fuel at the right time
The primary purpose 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. The function of the diesel fuel system is to inject a precise amount of atomized and pressurised fuel into each engine cylinder at the proper time.
The fuel injection system is made up of several components, including the fuel tank, fuel lines, injection pump, injectors, and, in some cases, a common header or accumulator. The fuel tank stores the diesel fuel and is designed to prevent contamination by foreign objects and water, which is a common enemy of diesel fuel systems. The tank has openings for filling, discharging, and draining fuel, and it is vented to allow air to enter as fuel is drawn out.
Fuel lines connect the tank to the injection pump, and there are different types of fuel lines depending on the pressure levels in different parts of the system. The injection pump, driven by the engine, supplies fuel to the injectors, which can be individual or combined with the pump for each cylinder. Modern injection pumps often use the plunger and cam method of fuel injection, and common rail systems are becoming increasingly popular for on-road applications.
The injectors deliver a precise amount of atomized and pressurised fuel into each cylinder. Modern injectors, such as piezoelectric injectors, offer extremely high precision and can handle the high pressures found in common-rail applications. The injectors spray the fuel into the combustion chambers, where it mixes with compressed air and combusts, driving the piston down and producing power.
To ensure the engine makes full use of the fuel injection process, it is crucial to control both the injection timing and metering. Advancing the timing can increase power when operating at faster speeds or towing heavier weights. It can also prevent premature combustion, reduce smoke, and repair lag. On the other hand, retarding, or delaying the fuel release, can address performance and fuel economy issues, especially in marine engines.
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Injector nozzle's pressure limitation
The design of the diesel fuel injector nozzle is critical to the performance and emissions of diesel engines. Injector nozzles are responsible for delivering a precise amount of atomized and pressurized fuel into each cylinder. The nozzle hole size and geometry, injector seat, and injector sac are important parameters that affect the combustion characteristics of the diesel engine, as well as the stability of emissions and performance over the engine's lifetime.
The internal nozzle flow has a significant influence on fuel injection and spray characteristics. During the choking flow period, when back pressure declines, the liquid velocity increases, and super cavitation induces higher turbulence kinetic energy. This results in an increased outlet velocity, which maintains the total mass flow even as back pressure decreases. The combustion process and emission characteristics of diesel engines are governed by the fuel injection process and fuel atomization, with evidence of cavitation in the diesel fuel nozzle.
The pressure limitation of injector nozzles is a critical aspect of their design. The nozzle opening pressure can be set using the injector spring or by determining the seat diameter. However, achieving the required accuracy in pressure settings can be challenging due to various factors, including friction, seal leakage, and pressure pulses. The geometry of the injector needle seat also plays a role in pressure limitation, with non-cylindrical geometries like double-cone shapes offering greater accuracy.
To overcome these limitations and achieve the desired pressure accuracy, methods such as optical or geometric measurements of the seat region, vibration testing, and modifications to address friction interference have been proposed. These techniques aim to enhance the precision of nozzle opening pressure settings, ensuring optimal performance and emissions in diesel engines.
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Filtering diesel fuel
A typical system might have three stages of progressive filters: a filter screen at the tank or transfer pump, a primary fuel filter, and a secondary fuel filter. The primary fuel filter removes particles ranging from 10 to 15 microns, while the secondary fuel filter removes smaller particles ranging from 3 to 5 microns. These filters work to prevent tiny particles from reaching the engine, where they could cause damage and affect performance.
It is important to note that the fuel itself is often what plugs the filter, rather than the particles that are being filtered out. This is because diesel fuel also contains wax, which can form crystals as the temperature of the fuel decreases. These wax crystals can build up and plug the filter, so it is important to take steps to prevent this, such as using cold flow improvers that keep wax crystals in solution.
Some people have suggested using coffee filters or paper to filter diesel fuel, but it is important to use the correct type of fuel filter to ensure that all harmful particles are removed and that the filter is compatible with your diesel engine.
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Piezoelectric injectors
Due to concerns regarding diesel engine emissions, piezoelectric injectors have been developed to reduce emissions by making the combustion of fuel within the cylinder more efficient. This technology makes use of the piezoelectric effect, named after the phenomenon discovered in 1880 by Pierre Curie and his brother Jacques.
The piezoelectric effect occurs when mechanical stress is exerted along certain crystal axes, generating an electrical charge on the crystal surface. This electrical polarisation is caused by shifting positive and negative ions in the crystal, and an electrical field forms between the ends of the crystal. The inverse piezoelectric effect is when an electrical voltage is applied to the crystal, causing it to expand or contract.
In piezoelectric injectors, hundreds of piezoelectric wafers are stacked over one another. When an electrical voltage is applied, the piezo crystals rapidly alter their structures by expanding. This expansion results in a linear movement that transmits directly to an injector needle, allowing fuel to be injected into the combustion chambers.
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Mechanical and electronic approaches
The basic principle of diesel fuel systems remains the same regardless of the technology used to implement them. The function of the diesel fuel system is to inject a precise amount of atomized and pressurized fuel into each engine cylinder at the correct time. The fuel is mixed with hot, compressed air, and no electrical spark is used.
A mechanical fuel system contains a low-pressure circuit and a high-pressure circuit. The transfer or lift pump draws fuel from the fuel tank through the inlet port of the primary fuel filter, which may contain a water separator to remove water from the fuel. The fuel is then pressurised before flowing to the secondary fuel filter and then the high-pressure fuel injection pump. The transfer pump is typically a simple diaphragm type actuated by a cam lobe acting on a lever that depresses and releases the spring-loaded diaphragm to move the fuel. Mechanical systems use a governor to balance flyweight force against spring force, which determines the correct engine rpm for the applied load and operator input.
Electronic systems have been developed in response to increasingly stringent global emission standards. In electronic systems, the components are different, but the principle is the same. Mechanical Electronic Unit Injectors (MEUI) and Hydraulic Electronic Unit Injectors (HEUI) were among the first technologies to replace mechanical components on the high-pressure side of the fuel system. These systems replace the injection pump, fuel lines, and nozzles with an electronically triggered unit injector in each cylinder. Low-pressure fuel is delivered to each injector through a gallery in the cylinder head, and the fuel is then pressurised inside each injector with a mechanically or hydraulically powered plunger. The fuel is released into the cylinder by a solenoid responding to an electronic signal from the Engine Control Module (ECM), which eliminates the need for a mechanical governor.
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Frequently asked questions
The basic principle is that the right amount of fuel must be injected at the right time to satisfy engine horsepower demands. The fuel is injected directly into the cylinder, unlike gas engines where fuel is injected outside the cylinder.
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.
In a common rail system, fuel from the fuel tank is supplied to a common header (accumulator) and then sent through tubing to the injectors, which inject it into the combustion chambers. The accumulator has a high-pressure relief valve to maintain pressure and return excess fuel to the fuel tank.
