28/05/2014
The Heart of a Diesel: Understanding the Fuel Supply System
The fuel supply system is undeniably the heart of a diesel engine. Its primary role is to ensure that the precise amount of fuel is delivered at the correct time, in the right state, for efficient combustion. Without a properly functioning fuel supply system, a diesel engine simply cannot perform. It's responsible for supplying, metering, injecting, and atomising the diesel fuel. Due to the high precision required, fuel injection systems are often complex and can be more costly to manufacture than their petrol counterparts.
Fuel movement within the system can be initiated either by gravity, in simpler setups, or more commonly, by a fuel feed pump. This pump draws fuel from the storage tank, pushes it through filters to remove impurities, and then delivers it to the injection pump. The injection pump then pressurises the fuel and delivers it to the injectors, which are strategically placed within the cylinder heads to spray the fuel directly into the combustion chambers.
Types of Diesel Fuel Injection Systems
Historically, diesel engines have employed different methods for injecting fuel. These can be broadly categorised into two main types:
1. Air Injection System
In this older system, fuel was injected into the combustion chamber using high-pressure compressed air. This required large, multi-stage air compressors, making the system bulky, inefficient, and expensive. Consequently, air injection systems are rarely found in modern diesel engines.
2. Solid Injection System
This is the system used in virtually all modern diesel engines. Here, diesel fuel is injected directly into the combustion chamber at very high pressure by a fuel pump, such as the well-known Bosch pump. Solid injection systems can be further classified based on their architecture:
A. Individual Pump System
This system, often seen in larger, slower-speed engines, features a separate metering and pressure pump for each cylinder. Fuel flows from the tank, through filters, to a low-pressure pump. This low-pressure pump then supplies fuel to four individual metering and pressure pumps. Each of these pumps is directly linked to an injector in a specific cylinder head, ensuring precise fuel delivery for that cylinder.
B. Distributor System
More common in small and medium-sized engines, the distributor system uses a single metering and pressure pump. Fuel travels from the tank, through filters, to this central pump. The pump then delivers the pressurised fuel to a distributor unit. This distributor cleverly routes the correct amount of fuel to each injector, synchronised with the engine's cycle. This design is more compact and cost-effective than the individual pump system.
C. Common Rail System
The common rail system represents a significant advancement in diesel fuel injection, widely adopted by manufacturers like Cummins and in many multi-cylinder engines. In this setup, fuel from the tank goes through filters and a low-pressure pump, which then feeds a high-pressure pump. This high-pressure pump generates extremely high fuel pressure, which is stored in a central 'common rail'. From this rail, fuel is delivered to electronically controlled injectors. The amount of fuel injected is precisely metered by the injectors themselves, allowing for exceptional control over combustion, leading to improved performance, fuel economy, and reduced emissions. This system offers superior flexibility and precision.
How a Mechanical Fuel Pump Works (Simplified)
To understand the delivery of fuel, let's look at the basic operation of a mechanical injection pump, often found in older systems:
- Rack and Quadrant Gear: A rack, connected to the accelerator pedal or governor, moves in and out based on driver input or engine load. This rack engages with a quadrant gear.
- Plunger Rotation: The quadrant gear has a cylindrical bottom part with a cross slot. The plunger, which reciprocates within the pump barrel, has a cross-shaped bottom part that fits into this slot. As the rack moves, the quadrant gear rotates, causing the plunger to rotate.
- Helical Groove: The plunger has a precisely machined helical groove along its length.
- Inlet and Overflow Ports: The pump barrel (cylinder) has inlet and overflow ports.
- Fuel Delivery: During the plunger's upward stroke, it covers the ports. As it continues to move up, the fuel trapped is pressurised. When the pressure is high enough to overcome the spring force on the injector valve, fuel is forced through the delivery pipe to the injector.
- Injection Control: The helical groove on the plunger plays a crucial role in metering the fuel. As the plunger rotates, the effective length of the groove that covers the ports changes. When the edge of the helical groove uncovers the overflow port, the pressure is released, and injection stops. The position of the rack dictates the plunger's rotation, thereby controlling the amount of fuel injected.
The Role of the Injector
The injector is the final component in the fuel delivery chain, responsible for atomising the fuel and spraying it into the combustion chamber. Injectors can be categorised by their operating mechanism and nozzle type:
Injector Operating Mechanisms
- Air Blast Injectors: As mentioned earlier, these were used with air injection systems and are now obsolete.
- Mechanically Operated Injectors: These injectors are activated by mechanical linkages, similar to how engine valves are operated – using camshafts, pushrods, and rocker arms. The camshaft directly operates a plunger within the injector.
- Automatic Fuel Injectors: These are the standard in modern automotive diesel engines. They feature a needle valve that is lifted off its seat by the fuel pressure generated by the fuel pump. This pressure is carefully controlled to ensure precise injection timing and duration.
Types of Diesel Nozzles
The design of the nozzle significantly impacts the spray pattern and the efficiency of fuel-air mixing:
| Nozzle Type | Description | Spray Angle | Applications | Advantages | Disadvantages |
|---|---|---|---|---|---|
| Single Hole Type | Features a single, small hole (around 0.2 mm). | Approximately 15°. | Open combustion chambers. | Requires high pressure for atomisation. | Poor air mixing, tendency to dribble. |
| Multiple Hole Type | Has multiple holes (4 to 18), ranging from 0.35 to 1.5 mm in diameter. | Varies. | Most common type for efficient combustion. | Excellent air-fuel mixing, better atomisation. | More complex to manufacture. |
| Pintle Type | A spindle with a projection (pintle) protrudes through the nozzle opening. | Varies. | Precombustion chambers, air cells, swirl chambers. | Avoids weak injection and dribbling. | Can be less efficient in open chambers. |
| Pintaux | A variation of the pintle nozzle with an auxiliary hole. | Varies. | Aids in cold starting. | Improves cold starting performance. | Auxiliary hole can be prone to choking; requires better filtration. |
The Rise of Electronic Fuel Injection (EFI)
The integration of electronics into automobiles, starting significantly around the 1960s, revolutionised engine management. EFI systems use a network of sensors to monitor various engine parameters, such as air temperature, intake air pressure, throttle position, and engine speed. This data is fed into an Electronic Control Unit (ECU), essentially a sophisticated computer. The ECU processes this information and precisely controls the fuel injectors, optimising fuel delivery for maximum power, best fuel economy, and minimal emissions. This level of control was a game-changer for diesel engine efficiency and environmental impact.
Multipoint Fuel Injection (MPFI)
MPFI systems are designed to deliver a precisely controlled air-fuel mixture to each cylinder of a multi-cylinder engine, regardless of engine speed or load. This ensures optimal combustion under all operating conditions.
MPFI System Arrangements:
- Port Injection: In this configuration, injectors are located in the intake manifold, close to the inlet valve of each cylinder. The injector sprays fuel into the incoming air stream, creating a homogenous air-fuel mixture that then enters the cylinder. This method offers advantages like uniform fuel distribution to each cylinder, leading to increased power output and a more precise control over the air-fuel ratio.
- Throttle Body Injection: A simpler variation where a single injector (or sometimes two) is positioned at a single point, usually within the throttle body. The throttle valve, controlled by the accelerator pedal, regulates the amount of air entering the intake manifold, and the injector meters fuel into this airflow. While less precise than port injection, it was an improvement over older carbureted systems.
Frequently Asked Questions
Q1: What is the main function of the fuel supply system in a diesel engine?
A1: Its main function is to supply, meter, inject, and atomise diesel fuel into the combustion chamber at the precise moment and in the correct quantity for efficient combustion.
Q2: What are the main types of diesel fuel injection systems?
A2: The two main types are Air Injection Systems (older, less common) and Solid Injection Systems (modern, widely used), which include Individual Pump, Distributor, and Common Rail systems.
Q3: What is the advantage of a Common Rail system?
A3: Common Rail systems offer superior fuel pressure control, precise injection timing, and better atomisation, leading to improved engine performance, fuel economy, and reduced emissions.
Q4: How does the helical groove on a mechanical pump plunger control fuel delivery?
A4: The helical groove determines when the overflow port is uncovered during the plunger's stroke. By rotating the plunger (controlled by the rack), the effective length of the groove changes, altering the point at which fuel pressure is released and thus controlling the amount of fuel injected.
Q5: Why are electronic fuel injection systems preferred in modern diesel engines?
A5: EFI systems use sensors and an ECU to precisely control fuel delivery, optimising power, fuel efficiency, and emissions under all operating conditions, something purely mechanical systems cannot achieve.
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