Engines Exposed: How Carburetors Differ From Fuel Injection
In our post on the five maintenance actions you can’t afford to skip, we referenced fuel injectors and stated that they are “precision pieces of equipment.” While this is absolutely true, the statement might be easier to understand with some context. With that in mind, we’ll explain fuel injection systems, carburetors, and how the two differ. The reason that both of these types of equipment exist is to introduce controlled amounts of fuel into the engine. As we will see, they go about it in very different ways.
Mechanically, carburetors are more complicated than fuel injectors. The simplest way to describe a carburetor is to think about a vertical air tube that leads down to the intake manifold. Inserted into the side of this air tube is a smaller fuel tube, which is connected to a float bowl. The air tube gets narrower at the point where the fuel tube enters it, based on something called the Venturi Effect, which is a derivation of the Bernoulli Principle — the speed of matter (air, in this case) will increase when it encounters a narrower section of a given pipe. This causes the pressure to drop inside the restricted portion of the pipe. Think of this like when there is a lane closed on a road and cars have to merge: If the people in the lanes that aren’t closing accelerate and create space for people to merge, everyone stays happy and the pressure reduces.
The reduced pressure in the air tube creates a vacuum, which draws fuel out of the float bowl through the jet and into the air stream. The float bowl functions like the tank on your toilet, and it will automatically refill when the level is drawn down enough. Its function is to provide a fuel source closer to the engine that is free from some of the perils of being stored in a large tank, such as starving the fuel tube due to sloshing. The fuel pump moves fuel from the fuel tank to the float bowl. At the top and bottom of the air tube, there are two valves that are used to control airflow: the choke plate at the top (before the Venturi) and the throttle plate below (after the Venturi).
Carburetors were originally based around a single air tube. As the size of engines increased, it became apparent that having multiple “barrels” (or even multiple carburetors) was going to be critical for providing all of the fuel demanded by the big-block engines of the ’50s and ’60s. Carburetors with multiple barrels would often have a set of smaller barrels and a set of larger barrels, allowing for more fuel flow when the engine was trying to produce maximum power by allowing all four barrels to open. The Holley carburetor shown above is a four-barrel setup. The two larger barrels are covered by the plate that you can see at the left-center of the image. This is a very similar concept to modern engines that shutdown some cylinders when cruising to improve fuel economy. Carburetors were modified, improved, and redesigned in an attempt to provide precise control of the air-fuel mixture.
Unlike carburetors, which use a lower pressure to draw fuel into the airflow, fuel injection uses elevated pressure to inject it. In one form or another, fuel injection has existed since the late 1800s — the systems have become quite a bit more intricate, but the fundamental concept remains the same. Early fuel injection systems used air injection to atomize the fuel. This involved having compressed air sprayed into liquid fuel, which caused it to vaporize. If you have ever dried something using a compressed air nozzle, you have replicated early fuel injection systems.
While there are many different styles of fuel injectors, one style of injector that was popular for many years was the inwardly opening valve. This triangular valve could be unseated by pressurized fuel, which then allowed for the fuel to be injected. Once part of the fuel had been released, the valve would reseat thanks to pressure from a spring. This allowed for a passive unit that could be adjusted based on the strength of the spring, the size of the valve, and the pressure in the fuel system. Fuel injectors do not open very wide, and they are only open for a short time, which is what requires clean fuel to preserve functionality.
Most modern fuel injection systems use a solenoid based injector that is controlled by the Powertrain Control Module (PCM). This allows the fuels system to respond to a variety of changing needs and vehicle conditions. When the solenoid is activated, it unseats the valve and allows fuel to be injected. Some modern cars use what is known as a fuel rail system: This involves having a tube that connects all of the injectors to the line from the fuel pump. Fuel pressure is maintained in the fuel rail, sometimes at several thousand PSI, and then pushed through the injectors when they are opened. This arrangement simplifies the job of the fuel pump by reducing the number of points where pressure must be measured.
Whereas carburetors were almost universally mounted to introduce fuel at the beginning of the intake manifold (as seen above), fuel injection systems have a few different setups. There are injection systems that inject into the intake manifold near the throttle body (these are called single-point injection setups), but the majority of modern engines are either multi-port injection or direct injection.
Multi-port has injectors mounted in each intake runner, just above the intake valve for the cylinder. Unlike single-point injection, multi-port injection allows the injection for each cylinder to be modified. The same is true for direct injection. Direct injection mounts the injectors so that they are injecting directly into the cylinder. This is how many diesel engines operate, but it is less common for gasoline engines. Direct injection systems require significantly higher pressure in their fuel rails to overcome the pressure that is created during the compression stroke in the cylinder.
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