Katerina Swanson
Diesel fuel systems are used to power a variety of machines, from trucks and buses to construction equipment and generators. The fundamental function of a diesel fuel system is to inject a precise amount of atomized and pressurized fuel into each engine cylinder at the right time. This is achieved through fuel injectors, which are considered the most important component of a diesel fuel system. The injectors deliver a precise amount of fuel into the combustion chamber, where it is mixed with hot compressed air to ignite the fuel. This process is critical for the diesel engine's operation as it relies solely on the compression of air to ignite, without the use of an electrical spark as in gasoline engines.
| Characteristics | Values |
|---|---|
| Basic principle of fuel injection | Injecting the right amount of fuel at the right time to satisfy horsepower demands |
| Fuel injection process | Mechanical and electronic approaches |
| Fuel injection timing | Before the piston reaches top dead center (TDC) |
| Injection pump | Rotary plunger-type unit with an individual piston for each cylinder or a single chamber pump |
| Injection pump function | Sends fuel through the high-pressure fuel line to each injector |
| Injector function | Delivers a precise amount of atomized and pressurized fuel into each cylinder |
| Fuel line types | Heavyweight, medium-weight, and lightweight lines for different pressure levels |
| Fuel filtration | Multiple stages of filtration to remove impurities, water, and debris |
| Fuel transfer | Gravity, air pressure, or a fuel transfer pump |
| Fuel type | Derived from heavier residues of crude oil; most common grade is ultra-low sulfur diesel (ULSD) |
| Fuel system maintenance | Regularly changing fuel filters, monitoring fuel quality, and adhering to service intervals |
| Fuel system applications | Trucks, buses, construction equipment, generators, and more |
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What You'll Learn
Injecting the right amount of fuel at the right time
The injection pump is typically a rotary plunger-type unit with a dedicated piston for each cylinder. Alternatively, it could be a single-chamber pump that uses a sleeve or scroll system to meter fuel to each cylinder. The pump delivers fuel through a high-pressure line to each injector in the correct sequence. When the fuel pressure surpasses the injector nozzle's preset pressure limit, the nozzle opens, and fuel is injected into the combustion chamber.
The fuel injection process is controlled by mechanical or electronic methods, depending on the engine's design and age. Modern injection pumps primarily use the plunger and cam method of fuel injection. There are four main injection systems:
- Individual pump and injector: Each cylinder has its own pump and injector.
- Combined pump and injector: A unit injector type where each cylinder has a combined pump and injector.
- Distributor: A single pump supplies fuel to multiple cylinders.
- Common rail: Multiple pumps are housed together, each with an injector for a specific cylinder.
Modern diesel fuel injectors, particularly in common rail fuel systems, use piezoelectricity to achieve extreme precision and handle high pressures. These injectors deliver a precise amount of atomized and pressurized fuel into each cylinder.
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Injector function and fuel combustion
Injectors are arguably the most important component of a diesel fuel system. Their function is to deliver a precise amount of atomized and pressurised fuel into each cylinder at the right moment. This process is critical for the diesel engine's operation because it relies on the compression of air alone to ignite the fuel.
The basic principle of fuel injection is that the right amount of fuel must be injected at the right time to satisfy the engine's horsepower demands. This is because fuel requires time to burn, so the right amount of fuel must be injected at the appropriate point in the compression stroke, so that it burns completely.
The injection pump is often a rotary plunger-type unit with an individual piston for each cylinder of the engine. It may also be a single chamber pump that meters fuel to each cylinder using a sleeve or scroll system. In either case, the pump sends a small amount of fuel through the high-pressure fuel line to each injector in the appropriate sequence. When the fuel pressure exceeds the injector nozzle's pre-set pressure limitation, it opens, and fuel is injected into the combustion chamber.
Any excess fuel is sent out through a return line back to the fuel tank. This is because all of these functions are performed in what is essentially a closed loop that generates positive feedback, so a diesel engine requires a governor to regulate fuel delivery and keep the engine from overspeeding. Diesel engines are not self-speed-limiting because the air (oxygen) entering the engine is always the maximum amount.
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. The common rail system is rapidly gaining popularity for on-road applications.
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Types of fuel pumps
The function of a diesel fuel pump is to pump fuel at high pressure and inject it into the engine chambers via fuel injectors. There are several types of diesel fuel pumps, each serving different needs. Modern diesel engines use the Direct Injection (DI) method, where pressurized fuel is directly injected into the engine chambers. 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.
One type of diesel fuel pump is the individual inline injection pump, which is preferred for heavy-duty diesel engines as they can generate high pressure. Inline pumps have a plunger for each injector and cylinder, and the number of plungers and injectors matches the number of cylinders. The plunger is operated by a camshaft, which derives its power from the engine crankshaft.
Another type is the distributor-type pump, which has one plunger that generates enough force to pump fuel through all the injectors. Due to this, the pressure generated is lower than an individual inline injection pump. This type of pump is ideal for limited-space applications due to its compact size and straightforward design, which also reduces manufacturing and maintenance costs.
The common rail fuel injection pump is another type of diesel fuel pump. It is an electronically controlled diesel fuel supply system that consists of a supply pump, common rail, electronically controlled injectors, sensors, and a computer that controls all these devices. The engine drives the supply pump, which produces high-pressure fuel, and the common rail distributes the fuel to the injectors mounted on each cylinder.
The fourth type of diesel fuel pump is the in-line fuel injection pump, one of two mechanically controlled diesel fuel systems. Similar to the distributor pump, it has one fuel pressure mechanism. The in-line pump matches the engine cylinders in their number of fuel pressure mechanisms, and a camshaft drives the fuel pressure and injection quantity control mechanisms within the pump body.
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Fuel filtration
The filtration process typically begins with a filter screen at the tank or transfer pump. This initial filter helps to remove larger contaminants and water from the fuel. After passing through the first filter, the fuel then moves on to the primary fuel filter. This filter is designed to capture smaller particles and impurities that may be present in the fuel. In some systems, the fuel may then pass through a secondary fuel filter for further purification.
There are two main types of filter configurations: series and parallel. In series filters, all the fuel passes through one filter and then through the other in sequence. This type of configuration ensures that the fuel is progressively filtered and that no unfiltered fuel mixes with the filtered fuel. In parallel filters, on the other hand, the fuel is split, and a portion of it goes through each filter simultaneously. This configuration can help increase the flow rate of the fuel while still providing filtration.
Water is a common contaminant in diesel fuel and can cause significant damage to the injection system. Water can rapidly oxidize ferrous metal components, leading to seizure of injection components and sticky metering parts in both the pump and injector. Therefore, it is crucial to use fuel filters that can effectively remove water from the diesel fuel.
Regular maintenance of the fuel filtration system is essential to ensure the reliability and longevity of the diesel fuel system. This includes changing fuel filters at recommended intervals, monitoring fuel quality, and adhering to service schedules. Proper fuel filtration helps to maintain the performance and efficiency of the diesel engine while protecting its vital components.
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Maintenance
To ensure the reliability and longevity of a diesel fuel system, regular maintenance is crucial. Diesel fuel systems are the lifeblood of many industries, powering everything from trucks and buses to construction equipment and generators.
One of the most important aspects of diesel fuel maintenance is cleanliness. The fuel should not contain more than a trace of foreign substances; otherwise, fuel pump and injector problems will occur. Diesel fuel, because it is more viscous than gasoline, will hold dirt in suspension for longer periods. Therefore, every precaution should be taken to keep the fuel clean. Fuel tanks must be closed to prevent contamination by foreign objects. It is also important to monitor fuel quality and adhere to the recommended service intervals.
Fuel polishing (or cleaning) is critical for modern-day diesel engines. The tolerance for contaminated diesel fuel in these engines is considerably lower than in older engines. Contaminated fuel can potentially cause engine failure. Fuel injectors and pumps can clog easily when not maintained properly. Diesel fuel will deteriorate if stored for more than a year. When high-capacity tanks are used, fuel should be periodically drained and replaced with fresh fuel. If draining the tank is not possible, a fuel polishing system can be used to clean the fuel. The purpose of a fuel polishing system is to remove contamination such as water and dirt from diesel fuel.
Water is a common enemy of diesel fuel systems. Water can enter the system through normal condensation on the tank walls. If the tank is kept water-free and treated with a biocide, the risk of microbiological contamination is minimal. Biocides and stabilizers are recommended for diesel fuel as they prevent degradation of the fuel when stored for long periods. Stabilizers inhibit oxidation of the fuel. When diesel fuel oxidizes, it creates a sticky substance that can clog fuel filters and diesel engine injectors.
Air trapped in diesel fuel systems is one of the main reasons for a hard-starting engine. Air can enter the fuel system at loose joints in the piping or through a spray nozzle that does not close properly. Letting the vehicle run out of fuel will also cause air to enter the system. Bleeding the system, as set forth in the manufacturer's maintenance manual, can remove air from the system.
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