What is Fuel Injection? How Does it Work?

What Fuel Injectors Do

an engine throttle valve

The throttle's butterfly valve adjusts airflow to the engine What happens when you step on the gas pedal? The engine revs up and your car goes faster. You might think that it's pretty simple stuff, but it actually takes a lot of sophisticated engineering to get that process to work so seamlessly. A big part of that is getting fuel to the engine where it can be combusted to generate power. Your fuel injectors spray gasoline into the intake or directly into the engine cylinders, so that it can be ignited quickly. There are a lot of steps involved in getting the gas to that point, and a lot of steps that have brought fuel injection technology to that point. We're going to walk you through how the gas gets where it's going, and gets you where you're going, and we're going to learn about the different developments in fuel injection along the way.

How the Fuel Pump Transfers Gas

Before gasoline can get sprayed out of your fuel injectors, it has to get up to them. That's what fuel pump or pumps are for. The fuel starts in the fuel tank until you start your engine. Then a pump starts moving fuel through the fuel lines at very high pressure.

Older models used mechanical pumps that were driven by the crankshaft or the camshaft. The faster the engine ran, the faster the pump pumped to meet the increased fuel demand of the engine. Most gas cars and trucks today use electrical fuel pumps. Diesel engines still use mechanical pumps, though. Electric fuel pumps are run on electricity and controlled by the ECU. This allows finer control and makes them more efficient. Some are mounted inside your gas tank (where the fuel cools them), while some are mounted outside the tank to the vehicle's frame. In some cases, an internal pump is used to feed fuel to an external pump.

Regardless of exactly where it is and how it works, the job of the fuel pump is to pump fuel through the fuel lines, where it can be passed on to the engine. Feeding gas to the engine has been accomplished through a a number of different means, but the first of these was carburetion.

How the Engine Receives Gas: When Carburetors Roamed the Earth

Carburetion was the simple system for getting fuel to the engine that predated fuel injection. While fuel injection systems rely on electronics, carburetion was purely mechanical. Fuel flow increased in response to airflow in the intake manifold.

When you step on the accelerator pedal it opens a butterfly valve in the air intake called the throttle. The more the throttle is open the more air can flow into the intake. This is why putting the pedal to the metal is known as going "wide open." The intake has a narrowed area called the venture. The narrowing makes the air move faster, which causes an area of low pressure. The carburetor has a fuel outlet called a jet that's open to the venturi. The faster the air moves through the venturi, the lower the pressure, and the more gas gets sucked in. So technically, the gas pedal doesn't give the engine more gas; it gives the engine more air. The increased airflow sucks in more gas. So next time you want someone to drive faster, say "stomp on the air!"

Carburetion is a simple system, but eventually, it became outdated and went the way of the dinosaur. The 1991 Jeep Grand Wagoneer was the last road car offered in the United States with a carburetor. The two biggest problems with the carburetor were its inefficiency and inflexibility. A carburetor can be tuned to produce the perfect air/fuel ratio at a certain engine speed, but the further you deviate from that speed, the further away you can get from the ideal ratio. The simplicity of the carburetor is, in some ways, its downfall, since it has no way to tweak or adjust for slightly different scenarios.

The Development of Fuel Injection

Although fuel injection has only become the norm in the past couple of decades, the technology has been around for a long time. Early fuel injection systems were used on airplane engines early in the twentieth century. Diesel engines have used direct fuel injection since the 1920s (we'll have more to say about types of diesel and direct injection later). Following World War II, hot-rodders started replacing carburetors with fuel injectors to give their cars added power. Mercedes-Benz used direct gasoline injection, modeled on the diesel type, in a Formula 1 racer in the 1950s. It adapted the technology to a production sports car, the 300SL in 1955. The more efficient combustion gave the 300SL great power and speed, propelling it to racing success.

Fuel injection was more complicated and expensive than carburetors, so it tended to be used only in some sports cars from the 1950s through the 1970s. Many of these early fuel injection systems were usually mechanically driven, continuous injection systems. Fuel wasn't pulsed to the engine like in today's electronic systems but flowed continuously at a rate that varied in response to the throttle position or measured airflow in the air intake. Chrysler offered an early analog electronic system in the Chrysler 300D and Plymouth Fury. The system was prone to failure, though, and wasn't used for long. With these complications, the allure of power wasn't enough to bring fuel injection to the forefront.

It would take the increasing engine emissions regulations of the 1970s and 1980s, and the 1970s oil crisis, to bring fuel injection to the fore. As automakers sought to decrease emissions and increase fuel mileage, they realized that fuel injection leads to the engine burning gas more efficiently. That same benefit that could provide power could also make cars friendlier to the environment and to drivers' wallets.

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Types of Fuel Injection

Throttle Body Injection

At first, automakers tried simple throttle body injection systems, with one or two fuel injectors attached to the throttle body. Throttle body injection worked very similarly to carburetion. Fuel was added at the intake manifold. This was not as efficient as later fuel injection systems, but it did have certain advantages over carburetors. Namely, the throttle body fuel injector could adjust better to different situations. As mentioned before, a carburetor may be tuned to give the ideal amount of fuel at a certain engine speed but may be slightly too lean or too rich at different engine speeds. Since the throttle body fuel injector is electronically controlled, it can give a better air/fuel ratio across the whole band of engine speed.

Multi-Port Fuel Injection Systems

There were still more improvements to come, though. Next up were multi-port injection systems. These inject fuel above each intake valve. This leads to more fuel being burned in the combustion chamber and less being wasted than in throttle body injection systems. Port injection requires that there be one injector for each engine cylinder.

A fuel injector GM's famous "Spider" injector

Earlier port injection systems supplied fuel to all the cylinders at the same time. The fuel would collect at each intake valve for a fraction of a second before entering the combustion chamber. General Motors used one such system called Central Port Injection, but sometimes called the "spider" injector because of its resemblance to an arachnid. Fuel would be distributed from a central point down "legs" to poppet valves at each intake valve. The poppet valves would open under pressure and release the fuel at each leg at the same time. The spider was eventually phased out because the poppet valves tended to become clogged with carbon build-up from combustion byproducts.

Electronic Multi-Port Fuel Injection

Eventually, more advanced, sequential port injection systems came to be. In these systems, each injector is signaled to fire separately by the ECU, so that each cylinder receives fuel just as the intake valve opens. This results in a more efficient burn than in older multi-port systems.

In these modern systems, the fuel injectors are electronically controlled valves that spray an extremely fine mist of fuel into the cylinder intake valves at high pressure. They are found mounted into the engine head. The injectors receive fuel either from fuel lines or a fuel rail, which, in turn, get fuel from the fuel pump. The opening and closing of the injectors are controlled by the engine control module (ECU), the vehicle's onboard computer. The ECU uses data from the mass air flow sensor, oxygen sensors, and other sensors to determine the timing of the fuel injectors. Remember that the purpose of the carburetor was to change the fuel flow in response to the airflow. The ECU uses information from the mass airflow sensor to the same effect.

Closeup of a fuel injector rail The fuel rail and injectors

Direct Gasoline Fuel Injection

The most advanced fuel injection system so far is direct gasoline injection. Indirect injection, gas is sprayed not into the intake but into the cylinder directly. The gas doesn't mix with air until it is in the cylinder, which prevents it from condensing. This gives an even more direct burn. Direct injection has long been in use on diesel engines but is becoming increasingly common in gasoline engines. You may recall that this is the system that was used way back on the Mercedes 300SL. While the technology was so expensive then that it was only made available on what was essentially a road-going race car, today direct injection is able to be used in many gas engines. Modern direct injection systems are also electronically controlled, while the earlier versions were mechanically controlled.

Direct injection systems are at the leading edge of fuel injection technology, but indirect sequential systems remain more common. One disadvantage of direct injection is that the injectors must be built to withstand the high forces and temperatures of combustion. Since the parts need to be more durable, they are necessarily more expensive.

Diesel Fuel Injection Systems

Diesel engines work differently than gasoline engines, although the role of the fuel injectors remains basically the same. Diesel engines don't use a throttle. Instead, when you step on the accelerator, more fuel is pumped to the injectors, and that is what speeds the engine up. Diesel engines have used direct injection right from the start. These work basically the same as the direct injection systems described above.

One big difference is the fuel pressure at the fuel injectors. Diesel engines don't ignite the fuel with spark plugs but through compression, and diesel fuel is less volatile (burns less readily) than gasoline. So diesel needs to be sprayed in an even finer mist. Gas fuel injectors usually have a pressure of about 40 to 60 pounds per square inch (PSI), or three to four-bar (that's three to four times the atmospheric pressure at sea level). Diesel injectors have around 14,500 to 29,000 PSI, or 1,000 to 2,000 bar.

Signs the Fuel Injection System Is Failing

Slow starting and acceleration, stalling, misfires, or gasoline odor

Problems with the fuel injection system can take many different forms, but the result is usually the same: not enough fuel getting to the cylinders. This can decrease engine power and efficiency. You may find that the car struggles to start and to accelerate. Stalling and misfires are also possible. Due to the inefficient combustion that results from faulty fuel injection, there might be a strong gasoline odor in the engine compartment after running the vehicle.

What Causes the Fuel Injection System to Go Bad?

Clogged Fuel Injectors

The fuel injectors themselves should be the first suspect when these kinds of problems arise. They can develop electrical problems or, more commonly, they can become clogged. An electrical problem can stop the injector from opening and closing with the correct timing. A clog will, obviously, keep the fuel injector from properly spraying fuel. Clogs can result from debris in the fuel, which can indicate a problem elsewhere in your fuel system. The fuel filter, found in the fuel tank or the fuel line is the most likely culprit and should be checked if you are replacing a fuel injector.

Your local garage may have the equipment to test fuel injectors. With this equipment, it is possible to determine the pressure output of each injector. Any injector that deviates too far from the proper pressure for your vehicle will need to be replaced. Since fuel injectors do generally wear out over time, you may choose to replace all of your fuel injectors as a set.

Worn Fuel Pump or Leaking Fuel Lines

Fuel pumps can also fail. The internal, mechanical parts can wear out, or, in the case of electric fuel pumps, the electric motor can go bad. If the fuel pump isn't pumping, no gas will get to your engine, and you won't be able to start your car at all. Fuel lines, fuel tanks, and the fuel filler neck can all, of course, develop leaks, which will cause you to lose gas, which could be costly over time.

Can I Repair the Fuel Injection System Myself?

You can definitely work on your own fuel injection system, although the difficulty of doing so will vary from one model to another, depending on the exact layout of all the parts. Since the system can be fairly complicated it would be a good idea to take photographs or make drawings before taking anything apart. You can use those images as a reference during the re-installation phase of the repair.

Certain safety precautions need to be taken when working with the fuel system. Fuel's flammability makes it dangerous, and the high pressure in the system poses a potential hazard. Basically, you don't want gas spraying everywhere, and especially not on yourself. Before working on the fuel system, especially before removing the fuel injectors, you'll want to take the pressure out of the system. You can do this by disconnecting power from the fuel pump and then idling the engine. This will drop the pressure in the fuel lines.

With these tips in mind, you should be able to get through your fuel system repair without incident. For more information about particular repairs, you may want to go to the associated part page or to our how-to auto repair videos.

Having Problems with Your Fuel Injection System?

If you are having issues with your fuel injection system, then you have come to the right place. 1A Auto is your source for replacement parts to get your fuel injection system back in working order again! Below is a list of common fuel injection system parts that you may need to replace.

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