Mercedes Mullins
Mechanical fuel injection systems, which were first introduced in the early 1900s, use a fuel pump to deliver pressurised fuel to the injectors, which spray it into the combustion chamber or intake. The pressure of the fuel causes the injector to automatically open, and the amount of fuel sprayed is controlled by a mechanical regulator assembly. Mechanical fuel injection was commonly used in diesel engines and high-performance sports cars in the 1960s and 1970s, but they have been largely replaced by electronic fuel injection systems due to their high cost and limited range of adjustments.
| Characteristics | Values |
|---|---|
| Types of fuel injection | Single point, multi-port, sequential, and direct injection |
| Fuel injectors' role | Getting gasoline into the engine cylinder so it can combust and the car can be driven |
| Fuel injector systems | Throttle body fuel injection, multi-port fuel injection, continuous injection, timed injection |
| Mechanical fuel injection | Injector is spring-loaded into the closed position and is opened by fuel pressure |
| Electronic fuel injection | Injector is held closed by a spring and is opened by an electromagnet |
| Mechanical fuel injection usage | Used in automotive racing, naturally-aspirated or forced-induction engines, and handles most types of fuel |
| Mechanical fuel injection components | Mechanical linkage, butterflies, barrel valve, injector nozzles, throttle valve hat assembly |
| Fuel/air mixture | Needs to be correctly balanced for the engine to run smoothly and efficiently |
| Fuel injector function | Injector acts as a spray nozzle to break up the fuel into a fine spray |
| Fuel flow control | Controlled by a mechanical or electrical control unit |
| Fuel pump | Mounted near the fuel tank and pumps fuel at high pressure |
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What You'll Learn
Fuel injection systems
A simple fuel injection system consists of mechanical linkages that connect the butterflies to the barrel valve. When the butterflies are opened, more air is provided to the engine, and the barrel valve opens to supply more fuel. Additional components can be added for air and throttle control, such as multiple injector nozzles to provide fuel to each cylinder. The barrel valve, or metering valve, controls the appropriate amount of fuel for different driving conditions, such as starting, part-throttle, driving, and stopping.
There are two main types of injectors: mechanically and electronically controlled. In a mechanical system, the injector is spring-loaded into the closed position and opened by fuel pressure. The amount of fuel delivered can be controlled mechanically or electrically. The earliest systems used a mechanical regulator assembly to control fuel flow, but these systems were complex and had poor throttle response. Mechanical fuel injection systems can also include an idle control circuit and multiple nozzles feeding a throttle valve hat assembly to modulate the air-fuel mixture.
Electronic fuel injection (EFI) systems, on the other hand, use an electric fuel pump to provide constant fuel pressure, and the duty cycle is modulated electronically based on the throttle position and other factors. Electronic systems offer advantages such as sensors that monitor various parameters to optimize fuel delivery. While mechanical fuel injection systems are being phased out in favour of electronic injection, they continue to play an important role in automotive racing and high-performance applications.
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Continuous vs timed injection
Fuel injection is the introduction of fuel into an internal combustion engine, usually an automotive engine, via a fuel injector. Modern fuel injection includes four basic types: single point, multi-port, sequential, and direct injection. Mechanical fuel injection (MFI) has been used in various formats of racing, including drag racing, circle track racing, and boat racing. MFI works well for naturally aspirated or forced-induction engines and can handle various fuel types, including gas, ethanol blends, methanol, and nitro blends.
Continuous fuel injection systems use a constant flow of fuel from the injectors, while timed injection systems use a pulsed flow. Timed injection systems are a necessity for diesel engines and are used on gasoline engines when more precise fuel metering is required. They tend to be more expensive than continuous systems.
The Bosch K-Jetronic system, introduced in 1974, is an example of a continuous flow system, using a plunger actuated by the intake manifold pressure to control the fuel flow to the injectors. On the other hand, the Bosch L-Jetronic system, also introduced in 1974, is a pulsed flow system that uses an air flow meter to calculate the amount of fuel required. This system was widely adopted in European cars during the 1970s and 1980s.
Intermittent injection systems can be sequential, batched, simultaneous, or cylinder-individual. Sequential injection times the fuel injection to coincide with each cylinder's intake stroke, while batched injection injects fuel to the cylinders in groups without precise synchronization. Simultaneous injection delivers fuel to all cylinders at the same time, and cylinder-individual injection allows the engine control unit to adjust the injection for each cylinder individually.
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Mechanical vs electronic control
Mechanical fuel injection (MFI) has been used in automotive racing since its early days and is still in use today. MFI works well for naturally aspirated or forced-induction engines and can handle most types of fuel. In a mechanical fuel injection system, a mechanical linkage connects the butterflies to the barrel valve. When the butterflies are opened, providing more air to the engine, the barrel valve opens and provides more fuel to the engine. Additional components can be added for air and throttle control and power modulation.
Electronic fuel injection (EFI), on the other hand, uses an electronic control unit (ECU) to monitor the engine's speed and load and alter the fuel injection rate accordingly. This allows for very accurate amounts of fuel to be injected and for multiple fuel injections to be made into the cylinders during the compression cycle, efficiently controlling the engine's speed and power output. Electronic control systems are often required to meet emissions standards, as they help ensure that fuel is burnt effectively, reducing the level of NOx and particulates produced. They also assist with the management of other emissions-reducing technologies.
One of the key differences between mechanical and electronic fuel injection is the level of precision and control they offer. While MFI relies on mechanical linkages to control fuel delivery, EFI uses electronic sensors and controls to fine-tune fuel injection rates. This makes EFI more responsive and adaptable to changing engine conditions.
Another difference lies in their suitability for different applications. MFI is often used in racing and high-performance engines due to its ability to handle various fuels and deliver a constant flow of fuel. In contrast, EFI is commonly found in modern passenger cars and is preferred by OEMs and their customers. EFI is particularly advantageous in heavy-load machines, where its ability to precisely control fuel injection and engine performance is highly valued.
While electronic control systems offer many benefits, they may not be necessary in all cases. Mechanically controlled engines can be sufficient for certain applications, such as light loaders and static equipment with a fairly constant load on the engine. In these cases, a mechanical system's direct approach to fuel injection can be effective and may be more cost-effective than implementing electronic controls.
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Early mechanical fuel systems
Early mechanical fuel injection (MFI) systems were port-injection systems that did not use electronic control. Mechanical fuel injection works well for naturally aspirated or forced-induction engines and can handle most types of fuel, including gas, ethanol blends, methanol, and nitro blends.
The first mechanical fuel-injection system appeared in the 1950s on the C1 Corvette and full-size Chevy sedans. The Rochester Ramjet FI, developed by Zora Arkus-Duntov and John Dolza, was a breakthrough feat of engineering. It featured a fuel meter, an air meter, and an intake manifold. The air metering unit measured the airflow into the engine, and the fuel was delivered directly to the nozzles located in the intake manifold. The pressure of the fuel caused the fuel injectors to open.
Early gasoline fuel-injection systems had mechanical pumps, and the amount of fuel was determined by vacuum signals or mechanical linkage between the air meter and the fuel distribution meter. These systems had minimal or no electronics involved. In contrast, early diesel fuel-injection systems were purely mechanical but required extremely high fuel pressure to inject fuel directly into the cylinder.
In the 1970s and 1980s, the L-Jetronic system, introduced by Bosch in 1974, was widely adopted in European cars. This system used electronically-controlled fuel injectors and an airflow meter to calculate the amount of fuel required.
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Modern automotive diesels
The primary purpose of a fuel injection system is to deliver fuel into the engine cylinders while precisely controlling the injection timing, fuel atomization, and other parameters. 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. The performance of diesel engines is heavily influenced by their injection system design, and the latest innovations have resulted in superior engine performance, lower emissions, and improved noise characteristics.
In a common-rail system, fuel from the 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 maintains pressure and returns excess fuel to the tank via a high-pressure relief valve. The fuel is sprayed with a nozzle that is opened and closed with a solenoid-operated needle valve. Third-generation common-rail diesels use piezoelectric injectors, which offer increased precision and fuel pressures up to 300 MPa or 44,000 psi.
The pinnacle of modern fuel injector technology is the piezoelectric crystal. When electrically energized, this crystal changes shape minutely, switching a small fuel chamber on and off. This on/off flow triggers the top of the fuel injector to open and close, spraying fuel into the engine. This technology allows for precise injection control to the millisecond, enabling modern diesels to receive several injections at precisely timed intervals. As a result, more power is produced, and the knocking sound commonly associated with diesel engines is virtually eliminated.
To prolong the life of modern diesel injectors, regular maintenance is essential. This includes changing the engine oil and filter and replacing fuel filters at or before the prescribed intervals. While they come at an additional cost, certain additives can restore lubricants in modern ultra-low sulfur diesel and prolong injector life. These additives can also boost the cetane rating of the fuel, leading to improved fuel mileage and performance.
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Frequently asked questions
A mechanical fuel injector is a predecessor to the electronic fuel injector. It is a system that injects fuel directly into the cylinders, via a fuel pump, and a fuel distributor, which respectively pressurise and direct the fuel to where it goes.
Early mechanical fuel systems typically relied on a complicated fuel pump that would provide pulses of high-pressure fuel to the injector feeding each cylinder. The pressure of the fuel arriving at the injector would cause it to automatically open, spraying fuel into the combustion chamber or intake.
Mechanical fuel injectors allow for more predictable starting and running behaviour, and are more efficient. They also provide more control over how much fuel is used and can more easily be adjusted with a computer based on varying conditions.
