Carburetion and fuel-injection are the two main types of fuel delivery systems used in internal combustion engines. So how exactly do they work, and how do they differ from one another?
Carburetors have been around for decades. Their simple mechanism is one of the reasons why many enthusiasts still prefer carburetors in muscle cars and other classics.
Carburetors are entirely mechanical, meaning they don’t rely on computers or other electronics to operate. Instead, they work via cables and vacuum systems.
Sitting on top of the intake manifold and beneath the air filter, the carburetor is responsible for mixing air and fuel before they get to the cylinders.
Carburetion is a straightforward process compared to fuel-injection. Basically, if you step on the gas pedal, the carburetor opens up and delivers the air-fuel mixture to the cylinders.
Sounds simple, right? Not quite.
The carburetor is essentially a tube that has a venturi, a restricted air passage in the middle of the tube that helps increase air velocity.
As the velocity increases, a low-pressure pocket forms and helps draw fuel from a nearby jet.
The throttle valve determines the amount of air that goes in the carburetor, so the harder you step on the gas pedal, the wider the throttle valve opens.
Once the jet is done drawing fuel from the tank, the air-fuel mixture then travels to the cylinders for combustion.
Carbureted engines can have a single or double barrel.
Single-barrel carburetors only have one venturi operating, while double-barrel carburetors have two venturi for additional power.
There are also some vehicles that have four barrels where two of the barrels (primary) are always operating while the other two (secondary) open up at higher speeds.
Mainstream cars began to feature fuel-injected systems during the 1970s, leaving carburetors in the dust not long after.
Compared to carburetors, electronic fuel-injection (EFI) systems are more precise when it comes to producing air-fuel ratios.
EFI systems rely on the powertrain control module (PCM) to pulse the fuel injectors on and off and operate the fuel pump relay circuit.
Most EFI systems have the following components:
There are generally two types of EFI systems: throttle body injection (TBI) and port fuel injection.
In a TBI setup, the PCM controls injector pulses in two ways: synchronized and non-synchronized.
Back in the day, TBI had a throttle body with injectors that fired fuel like fuel-injected vehicles. The only difference is that this fired fuel directly into the throttle body and into the intake manifold and then mixed into all the cylinders.
Back in the day, TBI had a throttle body with injectors that fired fuel like fuel-injected vehicles.
–Anthony Harlin, ASE Certified Master Automobile Technician
Later there were fuel injected vehicles with a fuel injector for each cylinder.
In synchronized mode, the fuel injector pulses once for each distributor reference pulse. In a non-synchronized setup, the injectors are pulsed depending on the calibration. The pulses are also independent of distributor reference pulses.
Unlike port fuel-injection systems, the regulators on throttle-body-injection systems aren’t connected to a vacuum. TBI systems also rely on the nozzle on time to determine the amount of fuel the injectors need to deliver.
In a TBI system, a long pulse width means more fuel while a short pulse means less.
In a port fuel-injection system, fuel distribution is equal for all cylinders because each of them has a separate injector.
In this kind of setup, fuel is also injected almost directly into the combustion chamber, leaving no chance for condensation in a cold intake manifold.
Although port fuel-injection systems have separate injectors for each cylinder, they don’t fire the same way.
Injectors can be triggered in three ways, namely grouped double-fire, simultaneous double-fire, and sequential.
Fuel-injection systems have two ways of measuring how much air the engine is breathing in to calculate the appropriate air-fuel mixture: speed density and mass airflow.
The speed-density method only calculates the amount of fuel the engine needs and doesn’t measure air quantity. The computer is responsible for calculating the amount of air in each cylinder using manifold pressure and engine RPM.
For the speed-density method to be accurate, the computer needs information from the following sensors:
In a mass air flow fuel-injected system, the air flow is divided by the engine RPM to derive the injection base pulse width.
In this setup, the computer relies on the readings given by the following sensors:
There’s a reason why carburetors were eventually booted out of the spotlight, but there’s no denying that the old engine still had some advantages over its successor.
Let’s break down the main differences between carbureted and fuel-injected engines and how one is better than the other.
Maintenance is arguably one of the few parameters where carburetors are better than fuel-injection systems.
The carburetor is a separate component from the engine, making it easier to clean and modify. Meanwhile, fuel-injection systems are a bit more complex and might require a professional to have them cleaned or replaced.
Carburetors can be modified according to altitudes and driving styles. But unlike fuel-injection systems, they’re incompatible with diesel engines.
Fuel-injection systems can deliver a more precise air-fuel ratio than carburetors because of their electronically controlled fuel induction capabilities.
Meanwhile, carburetors struggle with this concept, especially when the air pressure and fuel temperature are constantly changing.
The precision of measuring the correct air-fuel ratio means that fuel-injection systems don’t waste as much fuel as carbureted engines.
Carburetors are relatively inexpensive to maintain. Fuel-injection systems, on the other hand, can be quite costly.
The carburetor was once the simplest form of fuel delivery in an internal combustion engine. However, it eventually fell out of the limelight with the introduction of fuel-injection systems, which are much more efficient in calculating air-fuel ratios. Emission demands and fuel economy also led to the carburetor’s demise.
While carburetors were much easier to maintain, fuel-injection systems were better at doing their job, which makes up for their complexity and costly maintenance.
A bad carburetor may cause engine backfires, stalling, flooding, hesitation on acceleration, and other issues that may affect your vehicle’s drivability. As such, it’s crucial to replace it once the carburetor shows signs of damage or wear. Luckily, CarParts.com has a catalog of high-quality replacement parts at the ready.
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