Understanding Diesel High-Pressure Fuel Pumps: How They Work

Modern diesel engines are technological marvels, offering impressive outputs and huge fuel efficiency. They require two fuel pumps: a low-pressure electrical pump within the fuel tank, and a high-pressure fuel pump, driven mechanically by the engine. The high-pressure pump pressurises the diesel fuel to around 2,000 PSI, allowing the fuel injectors to pulse fuel into the engine with precision. This article will explain how a diesel high-pressure fuel pump works.

The role of the control solenoid

The control solenoid plays a critical role in the operation of a high-pressure fuel pump in a diesel engine. The solenoid is located on the side of the pump and its function is to control the volume of fuel that is compressed during the compression stroke.

During the suction cycle, the solenoid allows fuel from the low-pressure side of the fuel system to enter the pump. As the piston starts its upward movement, the solenoid remains open, pushing the fuel into the low-pressure side of the fuel system. When the solenoid is closed, the low-pressure and high-pressure sides of the fuel system are isolated from each other.

The solenoid's role is dependent on the engine's demand. When there is low demand, the solenoid stays open for a longer duration, resulting in a smaller volume of fuel being compressed. Conversely, during high engine demand, the solenoid closes sooner, allowing a larger volume of fuel to be compressed. The length of time the solenoid remains open directly determines the amount of fuel that reaches the fuel injectors.

The operation of the solenoid is closely tied to the engine's position. The engine control unit (ECU) and the pump utilise a camshaft position sensor to determine the precise position of the pump's lobe on the camshaft. This engine position information enables accurate timing of the solenoid's events on the high-pressure fuel pump.

How direct injection fuel pumps work

Direct injection fuel pumps are typically found in modern diesel engines, which require two fuel pumps to function. The first is a low-pressure electrical pump located within the fuel tank, which delivers fuel to the second pump, a high-pressure fuel pump driven by the engine.

The high-pressure pump is the focus of this discussion and is responsible for pressurising the diesel fuel to around 2,000 PSI. This high pressure permits the fuel injectors to pulse fuel into the engine with precision. The pump is driven by a camshaft, with a lobe on the camshaft pushing on a follower or roller that moves a piston. The piston has two cycles: suction and compression. During the suction cycle, the solenoid will allow fuel from the low-pressure side of the fuel system to enter the pump. As the piston starts to travel upwards, the solenoid will remain open, pushing fuel into the low-pressure side of the system. When the solenoid is shut, the low and high-pressure sides of the system are isolated.

The solenoid on the side of the pump controls the volume of fuel that is compressed during the compression stroke. The length of time the solenoid stays open will determine the volume of fuel that reaches the fuel injectors. If there is a high demand on the engine, the solenoid will close sooner, and a higher volume of fuel will be compressed. The operation of the solenoid is dependent on the engine position, and the ECU and pump use the camshaft position sensor to know the position of the pump's lobe on the camshaft.

The high-pressure fuel pump is a critical component of modern diesel engines, ensuring the precise delivery of fuel to the engine and enabling the impressive performance and fuel efficiency of these engines.

The difference between low-pressure and high-pressure fuel pumps

The main difference between low-pressure and high-pressure fuel pumps lies in their function within the fuel injection system of an engine.

Low-pressure fuel pumps are typically used in carbureted engines, such as those found in older cars, lawnmowers, and power tools. These pumps operate at a relatively low fuel pressure, usually between 10 and 15 psi (0.7-1.0 bar). They are often mechanical pumps, such as diaphragm or plunger pumps, and their main function is to transfer fuel from the fuel tank to the carburetor.

On the other hand, high-pressure fuel pumps are used in fuel-injected engines, which can be found in modern diesel-powered vehicles. These pumps operate at a much higher pressure, up to 30,000 psi (2,100 bar). High-pressure pumps can be mechanical or electric. Mechanical high-pressure pumps are usually mounted on the engine and driven by the camshaft, while electric high-pressure pumps are typically located inside the fuel tank.

The main function of high-pressure pumps is to deliver fuel at extremely high pressures directly to the fuel injectors. This is necessary for modern diesel engines, which use direct injection. In direct injection systems, the fuel is injected separately from the air, instead of being mixed beforehand in a carburetor. The high pressure is required for the fuel to start burning as soon as it leaves the injector nozzle, creating power.

In some fuel injection systems, there may be both a low-pressure and a high-pressure pump working together. The low-pressure pump sends fuel to the high-pressure pump, which then further pressurizes the fuel before it is injected into the engine. This two-stage pumping system ensures that the fuel is delivered at the precise pressure and volume required for optimal engine performance.

The dangers of working on diesel systems

When working on diesel systems, and in particular, the high-pressure fuel pump, there are several dangers and safety hazards that need to be considered. Due to the nature of the fuel system and the extremely high pressures involved, it is crucial to approach any maintenance or repairs with caution and a thorough understanding of the potential risks.

One of the primary dangers is the risk of fire and explosion. Diesel fuel, especially when pressurized, can be highly flammable and explosive. Any work on the fuel system should only be undertaken in a well-ventilated area, and all potential ignition sources must be eliminated. This includes sparks, open flames, and even static electricity, which can be generated by simple movements such as removing a fuel line or cap. It is essential to ground all tools and equipment to prevent static buildup and the potential for an arc or spark.

The high-pressure fuel pump in a diesel system operates at an extremely high pressure, often exceeding 20,000 psi. This means that any breach in the system, such as a broken or damaged fuel line, can result in a high-pressure spray of fuel, which can be extremely dangerous. This fuel spray, even without an ignition source, can cause serious injury, including fuel injection into the skin, which can lead to amputation or long-term health issues. It is vital to always depressurize the system and release the fuel pressure before beginning any work.

In addition to the risk of fire and high-pressure fuel spray, there is also the danger of air embolism when working on diesel systems. Air embolism, or gas embolism, can occur if high-pressure diesel fuel enters the bloodstream through a cut or wound. This can lead to serious health issues, including stroke or heart attack, and requires immediate medical attention. It is essential to always wear appropriate personal protective equipment (PPE), including gloves and eye protection, to minimize the risk of injury and fuel contact with the skin or eyes.

Furthermore, diesel fuel and the by-products of combustion can pose a health risk when inhaled or ingested. Fumes and vapors from diesel fuel can cause dizziness, headaches, and nausea, and long-term exposure can lead to more serious health issues. When working on diesel systems, ensure adequate ventilation and consider using a respirator to minimize the risk of inhaling harmful fumes.

Understanding these dangers and taking the necessary precautions is essential for anyone working on diesel systems. With the right knowledge, safety measures, and respect for the potential hazards, many of these risks can be mitigated, ensuring a safer working environment.

The evolution of injection pumps

The next significant development was the introduction of common-rail fuel injection, which is now found in virtually all new road vehicles produced in the US and Europe. Common-rail injection systems pump high-pressure fuel through a rail that feeds multiple injectors simultaneously, allowing for multiple injection events, variable timing, and precise control. This system also enables higher pressures to be developed, much finer control of injection volumes, and multiple injection stages compared to mechanical systems.

The latest type of diesel engine injection pump is the continuous pump, which is small and concise due to its limited functions. It is responsible for stably increasing fuel pressure to high levels (30,000-40,000 PSI) and regulating the timing and volume of sprayed fuel. This type of pump is widely used in direct injection systems and is valued for its durability and efficiency.

Frequently asked questions