Fueling Your Curiosity: How Modern Fuel Injection Works

 

[This feature is part of an ongoing series of tech explainers. If there are any automotive technologies or cool features you’d like explained, let us know in the comments.]

If you’ve only started driving in the last 20 years, you probably don’t know how much of an improvement there is over cars of just a few years older than that. If you’ve never experienced the manual choke, just assume the ability to start on the first crank at temperatures well below the freezing point, or have never heard the urging from your passenger to “just give it a bit more gas! A bit more!” followed by “oh great, you flooded it,” then you have modern fuel injection to thank for it. So how does electronic fuel injection work?

From the dawn of the car until sometime in the mid-1980s, the carburetor ruled the engine bay. From the single barrel economizer to the dual four-barrel monsters, that’s how gasoline got from the lines into the cylinders. And before you say “there was fuel injection before the 1980s,” you’re right. It was invented in the 1880s and pioneered on a diesel-like engine. Then mechanical injection made random appearances in racing Mercedes, 1957 Chevrolets, and a few other cars. But it was far from common, and those systems weren’t much better than carbs, so please bear with me.

Vintage Solex carburetor

The carburetor works by basically sucking fuel out of a small chamber. The air passing through each barrel pulls out a little bit of fuel as the air is sucked into the engine. The fuel flow is controlled by the airflow and by small holes called jets. You can change the size of the jets on the bench, but not in the vehicle. So you set the amount of fuel that can flow and that’s it. That one setting is expected to work in all weather, temperatures, altitudes, throttle positions, and so on. It works, and cars can make lots of power using them, but it can’t be ideal in every situation. And even though it sounds that simple, it’s full of tiny parts that all need to be maintained an in perfect adjustment for everything to work properly. It should be called the compromiseburetor.

The modern fuel injector is able to control exactly how much or how little fuel comes out, along with when it is injected. And instead of the fuel being pulled out, it pushes the fuel out and sprays the fuel into much finer droplets. Those droplets mix better with the intake air which means better combustion. That means better fuel economy and more power. Sometimes both at the same time. Combine that with an oxygen sensor in the exhaust and things start to get real efficient.

Aftermarket throttle-body injection

The electronic fuel injection system needs to know what’s happening in the engine. So it keeps track of the air coming into the engine in one of two ways. A mass airflow sensor in the intake that actually measures flow, or a speed density system like GM’s TPI or Ford’s 1980s 5.0-HO Mustangs that calculates the airflow using engine speed and vacuum. That second system was never common and has been left by the roadside with time.

The system also measures the air going out, but it measures what is in it, not how much. After all, all the air that goes in has to come out. The oxygen sensor measures the amount of oxygen left in the exhaust to tell the computer if there is too much or too little. That means too much or too little fuel is being burned.

So the fuel injection computer knows what’s going into the engine. A table of values in the computer charts that information plus information about engine load, rpm, throttle position, and if the driver had a sandwich for lunch, and it decides how much fuel to inject.

The fuel injector itself is more of a fuel-pass through. The pressure and flow come from the fuel pump, not the injector. Inside the injector body are a spring, plunger, and an electronic solenoid. The spring and plunger are in the path where the fuel flows. The spring keeps the plunger in a spot that stops fuel from flowing through. When the computer wants fuel, it sends an electric current through the solenoid. That magnetic solenoid pulls the plunger back and allows the gasoline to flow. The gas then comes out the injector tip as a fine mist. Like the mist setting on your garden hose. Unless clogged with debris, it allows very accurate fuel metering. So keep your fuel filter clean.

But even those extensive tables aren’t enough to make things perfect. Maybe the sandwich shop put on too much mustard this morning. Maybe there is an extra cucumber. That’s what the oxygen sensor is for. It tells the computer “hey, too much fuel” and “nope, not enough” hundreds of times a minute. The computer constantly adds and subtracts fuel until the mixture is just right. Then it remembers that for your specific car. Then it changes it some more because it’s a constantly moving target depending on what the car is doing. The computer tells the injector how much fuel to spray by telling it how long to stay open. That’s called the injector pulse width. When an injector is open all the time, it’s a 100 percent pulse width, although most injectors lose control efficiency and flow pattern before they get to 100 percent.

So that’s how it knows how much fuel, and for a time that’s all that it needed to know. That’s because the early fuel injection systems just replaced the carburetor with one or two fuel injectors in the intake manifold inlet along with the throttle plate. They sprayed fuel, and that mix went to every cylinder. That’s what a TBI fuel system is, throttle-body injection.

Port Injection Intake. Note the four black fuel injectors, one for each cylinder on this side of the intake

Automakers wanted something better, so they added more injectors. Called port injection, now cars had one injector for every cylinder mounted in the intake. Using the crankshaft position, each injector sprayed the right amount of gasoline to arrive at just the right time in the cylinder. Power goes up, fuel economy goes up, driveability goes up.

Modern fuel injection needs a few more wires than a carburetor, and a few more sensors. But the magic of computers has improved nearly every aspect of driving. From cold-start to full throttle, that little electronic gizmo is what gives us the no-thought daily driver we all love. So go out, start your car on the first key turn, and go for a drive. Or weigh in in the comments on how your vintage carbed car is the best thing to ever contain internal combustion and how EFI is the devil.

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