Fuel-Injection Systems: How They Work

Your vehicle’s engine must run efficiently and smoothly, which means it needs to have the right amount of fuel and air. Typical fuel-injection systems are indirect whereby the fuel pump sends the gasoline to the engine which is injected into your inlet manifold using an injector. Many older vehicles use a carburetor to control the fuel and air mixture. However, a single carburetor must supply your four-cylinder engine and can’t give each cylinder the same amount of fuel and air because of their placement. Therefore, some choose to use twin carburetors, though they can be hard to tune. Therefore, the carburetor system is mostly done away with, and newer vehicles use other fuel-injection systems.


While most personal vehicles don’t run on diesel, you can find ways to change the fuel system so that your truck or car runs diesel. Likewise, many commercial vehicles and large trucks use diesel, so it’s helpful to understand how their fuel-injection system works.

All diesel fuel injection systems are indirect where the fuel is injected into your inlet manifold or port rather than into the combustion chamber itself. Therefore, you’re sure the fuel mixes well with air before it enters the chamber.

Many newer diesel engines use direct injection whereby the diesel fuel is injected into the cylinder (filled with compressed air) directly. The goal here is for the cylinder to draw air into it, allowing it to heat up by compression so that the atomized fuel is injected at the right time so that it self-ignites.

Basic Injection

Modern injection systems still use indirect injection. They use a special pump that sends pressurized fuel from the tank to the engine bay. Throughout the process, it stays under pressure and gets distributed to each cylinder at the right time.

Of course, different systems can produce various differences. For example, the fuel can be fired into an inlet port or manifold through the injector. An injector is like a spray nozzle, allowing the fuel to spray in a fine mist so that it quickly mixes with the air and can enter the combustion chamber.

You can also find multi-point fuel injection systems where each cylinder gets fed by its personal injector. These systems are much more complex and expensive, so they are usually reserved for luxury vehicles.


The injectors allow fuel to be sprayed out of them and they are screwed to the cylinder head or inlet manifold at an angle, so the fuel spray is fired toward the inlet valve.

Depending on the system you have, injectors can be continuous or timed. Continuous injections allow the fuel to be squirted into your inlet port all the time as long as the engine runs. It acts as the spray nozzle to break fuel up into a mist but doesn’t control flow. The fuel amount sprayed can be decreased or increased by electrical or mechanical control units.

The other system is the timed injection system, where fuel is only delivered in bursts that coincide with the cylinder’s induction stroke. However, it can also be controlled both electronically and mechanically.

Earlier systems were controlled mechanically and required a regulator assembly. These systems have many drawbacks because they are mechanically complex and can have a poor response when backing off the throttle.

Most mechanical systems in vehicles have been taken over by electronic fuel injection (EFI) because they’re more reliable and less expensive to create and use.

Types of Injectors

You can find two primary types of injectors, though it depends on whether your system is electronically or mechanically controlled. Mechanical systems have a spring-loaded injector that always sits in the closed position until fuel pressure causes it to open. The electronic system also stays closed thanks to a spring but is opened using an electromagnet instead of the fuel. The control unit decides how long it stays open.

Mechanical Fuel Injection

Mechanical fuel injectors were primarily used in the 60s and 70s by manufacturers of high-performance sports vehicles. They used high-pressure and electric fuel pumps that were mounted nearer to the fuel tanks to pump fuel at higher pressures. They pumped the fuel to a fuel accumulator, which is a small reservoir that keeps the pressure constant and irons out pulses of fuel that come up from the pump.

Once it passed through the accumulator, the fuel passed through an element filter made of paper and was fed into the fuel-metering unit, also called a fuel distributor. It drives up from the camshaft and distributes fuel to all the cylinders in the right amounts and at the right time.

The amount of fuel required is injected and controlled by flap valves located in the air intake valve of the engine. The flat sat beneath the control unit, rising and falling in response to the airflow. When the throttle is opened, the cylinders suck in more air to raise the flap. Therefore, the metering control unit allows more fuel to be administered to the cylinders.

However, this option isn’t used much anymore because it’s harder to cold-start the car and you required a throttle that must be pushed in or out to the right degree.

Electronic Fuel Injection

Electronic fuel injection systems use a complex ECU or microprocessor control unit, which is a mini computer.

The computer gets information from various sensors throughout the engine. These sensors measure all the factors, such as temperature in the intake valve, air pressure, engine temperature, engine speed, and accelerator positions. Therefore, the mini computer can meter out the right amount of fuel more accurately than mechanical systems.

These systems can also help raise fuel economy because it is much more accurate, though not 100 percent efficient.

The computer can then compare the signals from all the sensors and determine how much fuel must be delivered and where. While fuel economy is improved, it also makes the engine quieter because electronic systems operate at lower pressures than mechanical ones. Therefore, you experience a quieter ride and make fewer trips to the gas station.

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Fuel-Injection Systems: How They Work

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