Sylvie Willis
Modern diesel engines are technological marvels, boasting impressive outputs, fuel efficiency, and low emissions. To achieve these feats, most modern diesel engines require two fuel pumps: an electric low-pressure pump within the fuel tank and a high-pressure fuel pump driven mechanically by the engine. The high-pressure pump pressurises diesel fuel to around 2,000 PSI, enabling fuel injectors to pulse fuel into the engine with precision. This intricate process is controlled by an ECM, which monitors and responds to engine, machine, and operator inputs. The high-pressure pump is a critical component, with failure symptoms including the inability to restart the engine and poor running, especially under hard acceleration. To prolong the life of the high-pressure diesel fuel pump, it is essential to avoid fuel contamination, maintain proper fuel levels, and ensure the use of high-quality fuel filters.
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What You'll Learn
Two fuel pumps are required
Most modern diesel engines require two fuel pumps. The first is a relatively inexpensive electrical low-pressure pump that lies within the fuel tank. This pump is powered electrically using a dedicated fuel pump relay. It delivers fuel to the second pump, the high-pressure fuel pump, which is driven mechanically by the engine.
The high-pressure pump pressurises the diesel fuel to around 2,000 PSI (for context, a typical car tyre is inflated to 30 PSI). This high pressure permits the fuel injectors to pulse fuel into the engine with great accuracy. This accuracy allows fuel to be injected in precise quantities at more than one point during the combustion cycle, producing more complete and efficient combustion.
The basic principles of diesel engines are the same whether they are mechanical or electronic. In both systems, fuel is injected during the compression stroke before the piston reaches the top dead centre. Only the air is compressed on the compression stroke, which causes it to become hot enough to ignite the fuel.
The injection 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, the nozzle opens, and fuel is injected into the combustion chamber. Any excess fuel is sent out through a return line back to the fuel tank.
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The high-pressure pump is mechanically driven
The injection pump can be a rotary plunger-type unit with an individual piston for each cylinder of the engine, or a single-chamber pump that meters fuel to each cylinder using a sleeve or scroll system. In both cases, 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 limit, the nozzle opens, and fuel is injected into the combustion chamber. The resulting combustion gases then drive the piston down to produce the power stroke.
In a direct-injected engine, an electric pump sends fuel to the high-pressure fuel pump, which is located under the hood. The high-pressure pump then pressurises the fuel to a much higher pressure than the low-pressure pump. This allows for more precise fuel injection, which results in more complete and efficient combustion.
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Fuel contamination is a common cause of failure
Diesel fuel injection systems are precision engineering masterpieces, designed to deliver precise amounts of fuel at extremely high pressures. Due to their intricate design, these systems are sensitive to contamination, which can lead to significant issues and costly repairs.
Foreign object debris, or FOD, is another serious contaminant. This can enter the system during maintenance if tools or other foreign objects are left in the engine bay, or if the fuel is contaminated with dirt, dust, or other particles. FOD can cause physical damage to the precision components of the fuel pump, leading to a loss of pressure, leaks, and potential engine failure. Even small particles can cause significant wear and damage over time, reducing the lifespan of the pump and other fuel system components.
To mitigate the risk of fuel contamination, it is essential to maintain fuel quality and store it in clean, dry conditions. Regular fuel filtering and the use of high-quality, clean fuel are critical to the long-term health of the injection system. Additionally, good housekeeping practices during maintenance can help prevent FOD-related issues, ensuring no foreign objects are left where they could find their way into the fuel system.
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The pump maintains rail pressure
There are several approaches to controlling the pressure in the common rail. One early method was to supply more fuel than necessary to the common rail and use a pressure control valve to spill excess fuel back into the fuel tank. This approach, used in early fuel injection systems, was found to be inefficient and resulted in excessively high fuel return temperatures.
A more modern approach is to meter the fuel at the high-pressure pump to minimize the amount of fuel pressurized to the rail pressure. This method can be combined with the previous one, depending on the engine operating conditions. Common rail fuel systems are equipped with a closed-loop high-pressure control system that stabilizes the rail pressure within a small margin of the nominal value specified by the electronic control unit.
The injection 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 limit, 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.
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Injectors are controlled electronically
Modern diesel engines require two fuel pumps: a low-pressure electrical pump within the fuel tank and a high-pressure fuel pump driven 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.
The principle is the same in electronic systems, but the components are different. Mechanical Electronic Unit Injectors (MEUI) and Hydraulic Electronic Unit Injectors (HEUI) were among the first technologies to successfully replace mechanical components on the high-pressure side of the fuel system. They replace the injection pump, fuel lines, and nozzles used in mechanical engines with an electronically triggered unit injector in each cylinder. Low-pressure fuel is delivered to each injector through a gallery in the cylinder head, eliminating individual fuel lines.
Since the fuel is supplied at injection pressure, the injectors themselves can be much simpler and more responsive. This allows fuel to be injected in precise quantities at more than one point during the combustion cycle, producing more complete and efficient combustion. This process is controlled by a sophisticated ECM that monitors and responds to multiple engine, machine, and operator inputs.
In common rail fuel systems, the fuel injection pump supplies fuel to the manifold and injectors at pressures up to 200 MPa (29,000 psi), making it critical to follow all common rail-specific safety procedures when servicing these systems.
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Frequently asked questions
A high-pressure diesel fuel pump is a component of a diesel engine that pressurises and delivers fuel to the engine's injectors.
The pump pressurises the diesel fuel to around 2,000 PSI, allowing fuel injectors to pulse fuel into the engine with precision. The injectors spray the pressurised fuel into the combustion chamber, where it ignites, driving the piston down and producing power.
High-pressure diesel fuel pumps enable impressive output and fuel efficiency in modern diesel engines. They also contribute to lower overall exhaust emissions.
A regular fuel pump in an older car with a port-injected engine is typically located in the fuel tank and has lower pressure. In contrast, a high-pressure fuel pump is usually found under the bonnet and is mechanically driven by the engine camshaft.
A faulty high-pressure diesel fuel pump can prevent the engine from restarting and cause poor running, especially under hard acceleration. An illuminated dashboard malfunction indicator lamp and fault codes may also suggest a low fuel pressure issue. However, it is important to investigate other potential causes before condemning the pump.
