How Does a Fuel Injector Work Inside the Engine?

The way you take care of your car is a direct reflection of how well you understand the various components … Continued

The way you take care of your car is a direct reflection of how well you understand the various components that make it a modern-day marvel. Unfortunately, one of the most often confusing aspects of modern cars is in the way fuel is delivered to the engine. We all know how important this is because the power generated by a car’s engine is directly proportional to the correct amount of fuel delivered to it. While in the past cars relied on not-so-perfect carbureted mechanisms to deliver fuel to the engine, today is different. Modern cars now come with fuel injectors to perform the same basic task. Understanding how fuel injectors work is thus crucial to helping you better care for your car.

Fundamental Issues in Engine Fuel Delivery

We all know the purpose of an engine. Everyone is also cognizant of the fact that air and fuel must be combined or mixed in the combustion chamber to generate controlled explosions and bring the engine to life. As such, it is crucial that fuel be delivered to the combustion chamber in very precise amounts. Too much (rich fuel mixture) and you risk choking the engine, making it difficult to start, or even stall. Too little (lean) and you won’t be able to start your engine as well. That is why it is important to provide the combustion chamber with the correct amount of fuel to be mixed with the correct amount of air.

Unfortunately, this is no easy feat because there are a lot of factors that can impact the delivery of both air and fuel. This has always been an issue in the past especially among carbureted engines. The main problem was that a single carburetor had to supply a certain number of cylinders with fuel. This usually meant the cylinder farthest from the carburetor will be receiving slightly less fuel than the cylinder closer to the carburetor. That is why some older systems had twin carburetors to better supply fuel to the engine. Sadly, these were a lot more difficult to tune or synchronize and worst, they made fuel mileage poor.

With these issues, a more efficient mechanism for the delivery of more precise measurements of fuel had to be designed. This is where fuel injection systems come in.

fuel injector inside engine

The Fuel Injection System

A modern fuel injection system technically includes some sort of a sensing mechanism to determine the correct amounts of fuel that need to be sprayed into the engine’s intake manifold. Another mechanism is needed to deliver or spray the ‘computed’ amount of fuel into each cylinder. This is the function of fuel injectors which we will be discussing in greater detail in the following section.

There are two types of fuel injection systems that generally correspond to the two fundamental types of engines we have in the market today.


The way in which some engines are designed require the fuel to be delivered or sprayed directly into the combustion chamber of the engine. Each cylinder is already filled with compressed air. When atomized fuel is injected into each cylinder it self-ignites. This is true for most diesel engines. We said ‘most’ because there are certain diesel engine designs that move fuel to a pre-combustion chamber first before reaching the cylinder.


Cars that run on petrol have indirect fuel injection systems. Pressurized fuel is delivered to the engine bay from the car’s fuel tank. The pressurized fuel is delivered to the intake port or the intake manifold, depending on the design of the engine. This allows the fuel to be mixed first with air that passes through the inlet port or manifold before the mixture is pushed into the combustion chamber.

The latest cars of today come with multi-point injection. In this system, each cylinder receives fuel from one specific fuel injector. So, if you have 6 cylinders you can also expect 6 fuel injectors. It is this 1:1 configuration that makes this system very powerful and efficient, albeit complex and expensive to fix. Most cars, however, have a single-point fuel injection system or even an injector for every two cylinders.

What are Fuel Injectors?

Fuel injectors are parts of modern car engines that deliver fuel to the engine’s combustion chamber, directly or indirectly. These small electro-mechanical devices are typically positioned at a certain angle to make sure the fuel is sprayed towards the engine’s inlet valve or directly into the cylinder.

How Does a Mechanical Fuel Injector Work?

A lot of folks confuse a mechanical fuel injection system with a carburetor. While the principle is fundamentally similar, there is a major difference in the type of fuel delivered to the engine. Whereas carburetor systems deliver low pressure fuel from the gas tank, a mechanical fuel injector delivers high pressure fuel to an accumulator. You can think of this as a temporary storage area for your fuel. The fuel then passes through a distributor which is typically considered as the metering control unit of the system. From here, the fuel is then ‘distributed’ into each cylinder in the correct amount and at the correct time.

fuel injectors

The flow of fuel that is injected into the inlet port is managed by the flap valve which is located in the air intake of the engine since air and fuel must be mixed first prior to entry into the cylinder. As you accelerate, the flap valve opens to increase the amount of air flowing through it. This also stimulates the fuel distributor to increase the amount of fuel pushed through the injector to maintain the right balance of air and fuel.

If fuel is not squirted into the inlet port, the valve inside the fuel injector remains closed by virtue of the tension on its spring mechanism. When fuel is sent to mix with air at the air inlet, the pressure of the fuel opens this valve to allow fuel to be squirted. This is the reason why they call mechanical fuel injectors to be spring-loaded injectors.

During cold starting, a microprocessor activates a specialized injector to add extra fuel into the mixture to facilitate smoother starts. Once the engine has warmed up, fuel from the cold start fuel injector is automatically cut off. This is different from a carburetor since you only need to block the airflow to create a richer mixture.

How do Electronic Fuel Injectors Work?

Many modern cars come with electronic injection systems. These are often denoted by the acronym EFI. In essence, these are almost the same as mechanical fuel injection systems except that they are not dependent on the amount of fuel and spring tension to open and close the valve of the injectors. What they do have are highly complex mini computers called Electronic Control Unit or ECU. The ECU performs a variety of functions, including the following.

  • It controls fuel mixture.
  • It controls idle speed.
  • It manages ignition timing.
  • It controls valve timing.

Sensors that measure air pressure, air intake temperature, accelerator position, engine temperature, and engine speed are mounted on the car’s engine. These sensors feed information to the ECU which processes these bits of data to compute the right amounts of fuel to be injected into the engine’s cylinders. The valves on the fuel injectors receive input from the ECU so that it knows exactly when to open to allow fuel to be squirted into the inlet port.

The system is so efficient that all of these complex processes – from the gathering of sensor data to its integration at the ECU level to its processing and its eventual input into the fuel injector valve – occur in a split second.

Following the journey of your fuel from the gas tank, it enters the fuel rail, thanks to an electric fuel pump that draws fuel from the tank. This is where another difference from mechanical fuel injectors comes in, by the way. Since fuel movement is controlled electronically, there is no need to have it at high pressure. The system only needs to maintain it at a constant pressure to deliver the fuel from the tank to the rail.

Fuel injectors are connected to the rail and, as we have already mentioned above, will only open their valves upon receiving input from the ECU. Electronic signals from the ECU travel to one of two pins on the injectors. The other pin is wired to the battery and through an ignition relay. Closing the circuit is accomplished by the sending of a pulsing ground by the ECU to the injector. This activates the injector’s solenoid which draws the magnetic top of the plunger, opening the valves. Since fuel pressure inside the rail is already high, this helps send the fuel through the injector spray tip at high velocity. Here, it enters the intake manifold or directly into the cylinder, depending on the type of fuel injection system your car has.

Fuel injectors are very innovative devices that helps ensure your engine gets the right amounts of fuel at the right time. While there are still systems that use a mechanical type of fuel injection, many modern cars now use electronic fuel injection systems. These allow for greater fuel efficiency and economy as different factors are taken into consideration to determine the correct amount of fuel to put into each cylinder.

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  1. How Does Fuel Injection Work? – ThoughtCo
  2. How Fuel Injection Systems Work – howstuffworks