Carburetors and fuel-injection systems describe different technologies for delivering the mixture of fuel and air that internal combustion engines require. For decades now, fuel-injection systems have become more and more standard in automobiles, edging out as they do the old-school carburetor. (As the folks at Jalopnik note, the last American mass-produced vehicle that utilized a carburetor was the 1991 Jeep Grand Wagoneer.)
Nonetheless, the “carb”–still the setup you’ll find on you typical small engine–is not extinct in vehicles: For vintage car fanatics and race car drivers, for example, they’re often still the fuel-delivery mechanism of choice.
So, what’s the difference? You don’t need to be an engineer or auto mechanic to understand the basic distinction between a carbureted and fuel-injected engine, as you’ll see this in this casual primer.
The Carburetor
In simplest terms, envision the carburetor as a tube. At one end is the throttle valve, controlled by the accelerator. When the throttle is opened, air is drawn into the carburetor–in greater and greater quantities as the pedal is further depressed and the throttle’s butterfly valve opens wider.
The airway of the carburetor constricts in a chamber called the venturi. The purpose of this passage has to do with a law of fluid dynamics called Bernoulli’s principle. Now, don’t run and hide: We needn’t parse out the equation in detail here. What’s important is overall idea of the Bernoulli effect: that an increase in the velocity of a fluid corresponds to a decrease in pressure. When air flows through the narrows of the carburetor’s venturi, it speeds up, and as the speed goes up, pressure in the airway goes down.
There’s another component to the carburetor besides the main airway: a float chamber acting as a reservoir for gasoline. The chamber’s connected to the venturi by a jet. When air speeds up in the bottleneck and the pressure in the tube diminishes, atmospheric pressure on the float in the fuel reservoir forces gasoline out of the jet into the main body of the carburetor. This fuel-air mixture then feeds into the intake manifold to power the engine.
A choke at the head of the carburetor controls the amount of air coming into the tube. This facilitates the starting of a cold engine, which requires a richer fuel-air mixture to operate. Activating the choke reduces the amount of incoming air and delivers a more fuel-heavy mixture to the engine.
Even though carburetors have given way to fuel-injected engines in modern mainstream cars, they still have plenty of enthusiasts. You can explore modernized carb-conversion setups through specialized dealers such as JAM Engineering.
Fuel-Injection System
An electronic fuel-injection system is a more complicated beast than a carburetor. The fundamental idea is to improve the precision of the fuel-air combination to maximize engine efficiency and limit emissions. Whereas in a carburetor the amount of airflow controlled the release of gasoline, a fuel-injection system uses a computer and various sensors to constantly regulate the fuel-air mixture.
In a fuel-injected engine, gasoline delivered from the fuel tank via a fuel pump is sprayed into or above the intake manifold or into the cylinder itself in an exact amount dictated by the powertrain control module, or PCM. (It’s also sometimes referred to as the electronic control unit, ECU.) Various sensors communicate factors such as the quantity of airflow to the PCM, which then sets the amount of fuel to be released by the injector nozzle.
Throttle-body injections, in which fuel is sprayed just upstream of a throttle valve that connects to the intake manifold, have mostly ceded the stage to multipoint or direct fuel injection systems. In a multiport configuration, each cylinder is fed by an injector situated on the other side of an intake valve. In direct injection, fuel is misted into the cylinder itself.
There you have it: the basics of carburetors and fuel-injection systems. Both are the linchpins of an internal combustion engine, delivering as they do the raw ingredients for that combustion.
Additional Sources
(1) Jalopnik: How Electronic Fuel Injection Systems Work
(2) Haefner, Ron. The Car Care Book. Clifton Park, NY: Delmar Learning, 2004.