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Fuel Injection 101

The days of carburetors are long gone. Carburetors have been replaced by fuel injection systems. Fuel injection provides cleaner emissions, better fuel economy, better performance and improved reliability.

Although fuel injection was a great leap forward in technology, the system still has its quirks. The requirements for the fuel injector are astounding. It must fire millions of times, precisely metering the fuel for each combustion event. With requirements like that, there is a lot that can go wrong.

Read on for a discussion about:

Types of fuel injection systems

Throttle body injection system diagram.
Figure 1: Throttle Body Injection System
Multi-port injection system diagram.
Figure 2: Multi-Port Injection System

Several types of fuel injection systems exist on the market.

The two most prevalent systems are the Throttle Body Injection (TBI) system (see Figure 1) and Multi-Port Fuel Injection (MPI) system (see Figure 2).

The TBI system was a first generation injection system used by manufacturers to replace carburetors. It consisted of one or two fuel injectors that sprayed fuel over the throttle body. The incoming air would draw the fuel and air into the cylinders through the intake manifold.

Although TBI was an improvement over carburetors, it was soon replaced by the MPI system which provides far better fuel control. MPI systems use one injector per cylinder, mounted in the intake runner positioned above each intake valve. Fuel is injected onto the intake valve where it is mixed with the incoming air and is drawn into the combustion chamber.

Since the injector is positioned closer to the combustion chamber fuel delivery is more immediate and precise.

Although the TBI and MPI injectors are physically different their internal operation is very similar. We'll cover the operation of the Multi-Port fuel injector.

How fuel injectors work

The fuel injector is an electro-mechanical component which is controlled by the vehicle's on-board computer.

The operation is as follows (see Figure 3).

Multi-Port Fuel Injector diagram.
Figure 3: Multi-Port Fuel Injector

Pressurized fuel enters the top of the fuel injector through the fuel filter inside the injector. The inside of the injector is pressurized with fuel. At this point fuel does not flow out of the injector because the spring is pushing against the pintle armature, closing the opening at the bottom of the injector. Electrical power is supplied to one terminal of the fuel injector connector through the ignition switch. The other terminal is connected to the vehicle's on-board computer.

When the computer has determined the proper time and amount of fuel to inject it grounds the injector, completing the electrical circuit. When this occurs, the coil inside the injector is energized, creating a strong magnetic field. The magnetic field acts upon the pintle armature and pulls it up off of its seat, overcoming the spring pressure. When this occurs, it exposes the hole at the bottom of the injector allowing fuel to exit the injector.

When the computer turns off the ground, the magnetic field collapses and the spring returns the pintle to the closed position. The pintle is micro-machined to provide the proper spray pattern when the injector is opened.

The amount of time the injector is opened determines how much fuel is injected. The open-time is measured in milliseconds (1,000ths of a second) and referred to as pulse-width. The computer measures numerous parameters to determine the amount of fuel to inject. This includes; engine RPM, manifold pressure, air temperature, air mass, throttle position and barometric pressure, to name a few. Depending on the manufacturer and system, the injector may be fired once every crankshaft revolution or every-other crankshaft revolution. For instance, at 5,000 RPM the injector could be firing more than 80 times a second! The computer monitors the crankshaft and camshaft position sensors to precisely time the injection.

What's next for fuel injection technology?

Although MPI is used on almost every current production, vehicle manufacturers haven’t stopped evolving how fuel is delivered to the engine.

To explore the next generation of fuel injection, check out our article on Gasoline Direct Injection (GDI).


Curious to learn more? Visit our Knowledge Center, packed with fuel injection-related articles and topics to assist you in better understanding fuel injection systems and technologies.


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