For many among us, the terms “carburetor” and “fuel injector” are familiar but ultimately meaningless despite the fact that tens of millions of Americans make direct use of one or the other almost every day of their lives. So, you may ask, what is the exact function of each, and how do they differ at their core?
Naturally, it’s important to know a little about the technical and practical advantages and disadvantages of either option, along with an elementary understanding of how engines burn fuel. We’ll tackle the latter as a primer, and then move on to the unique construction of both carburetors and fuel injection systems.
How Engines Burn Fuel
Engines, as most know, function on the principle of internal combustion. That is to say that they burn a predetermined mixture of fuel and air, resulting in the release of thermal energy, along with two primary byproducts: carbon dioxide and water. As with other more visible examples of combustion, such as burning a candle or bonfire, for instance, a firmly positive ratio of air to fuel is required.
There is, of course, such a thing as too much air, and in the case of internal combustion in motor vehicles the sweet spot for the air-fuel ratio – called the stoichiometric ratio – varies among fuels, being 14.7:1 for gasoline and 14.5:1 for diesel.
When fuel burns at its stoichiometric ration, it burns at peak efficiency. This also means that the engine is operating optimally. An imperfect ratio can substantially decrease the efficiency and increase the output of polluting waste products.
A mixture can be too rich or lean, depending on the ratio of air to fuel.
A mixture that is too “rich” has a greater proportion of fuel to air than is optimal, resulting in lower combustion temperature. On the other hand, a mixture that is too “lean” has too much air, and thus the combustion temperature is higher than required. This is a problem that is easily solved with carburetors and fuel injectors. Their function is to ensure that the ideal air to fuel ratio is consistently maintained throughout the vehicle’s operation.
How Carburetors Work
Carburetors, an invention of automotive innovator Karl Benz in their modern form, first found their way into motor vehicles in the nineteenth century, and were a near-constant fixture for almost a century. While there are multiple variations in the particular design of each carburetor, they all operate on the same core principle. A carburetor, in short, is a tube whose function is to feed the correct proportions of air and fuel into the engine, doing so via a valve system that regulates the ratio depending upon driving conditions at that moment.
A carburetor is basically a tube. The most important parts of the carburetor include:
- The Venturi
- The Jet(s)
- The Float chamber with float valve and arm
- The Choke valve
- The Throttle valve
The Venturi is a narrowing of the tube where the air passes. The narrowing causes the air to accelerate and lose pressure. The jet, which leads to the float chamber with fuel, is a hole located in the narrowest part of the Venturi. As the air passes through the Venturi and loses pressure, it creates a vacuum. The vacuum draws the fuel out of the float chamber and introduces it to the air stream via the jet. This is called the “Venturi effect.”
The two swiveling valves of the carburetor help regulate airflow.
- The choke – located above the Venturi – regulates how much air can flow into the carburetor. When the choke if closed, there’s not much air that flows into the pipe, so more fuel is sucked in. This creates a fuel rich mixture, which is perfect for starting the car or warming the engine.
- The throttle – located below the Venturi – regulates how much air can flow through the carburetor. Opening the throttle fully (by pressing the accelerator pedal) results in a bigger vacuum, which then introduces more fuel to the engine. This results in increased power output and vehicle acceleration.
In short: The main difference between the two valves is that the choke regulates how much air flows INTO the carburetor, while the throttle controls how much air flows THROUGH the carburetor.
The float feed chamber, which is a part of the carburetor’s fuel inlet, is essentially a small fuel tank containing a float and a valve. This is the chamber that feeds fuel to the carburetor. When the float drops below a certain level due to fuel expenditure, it will open a valve that refills the chamber with fuel.
How Fuel Injection Works
Due to stricter emission requirements, cars eventually needed to be equipped with catalytic converters. Their main function is to break down serious pollutants into less serious ones before they exit the car via the exhaust system. In order for the catalytic converter to function properly, however, the air to fuel ration had to be very exact, which was something that couldn’t be achieved with carburetors.
Fuel injectors, the relative new kid on the block, have existed since the 1950s, though it wasn’t until the 1980s that electronic fuel injection became commonplace. Nowadays, all cars sold in the United States rely on fuel injection systems.
The key components of a fuel injection system are:
- the computer (ECU – engine control unit)
- the oxygen sensor
- the injectors
- the fuel pressure regulator
- electric fuel pump
The role of the oxygen sensor is to read the oxygen levels in the exhaust, and provide a voltage reading ranging from 0-1000 millivolts. The correct air to fuel mixture will produce a reading 500 millivolts. The computer’s job is to ensure that the voltage remains in the range in which the fuel’s stoichiometric ratio is achieved. When this is achieved, the vehicle’s catalytic converter performs at full efficiency.
When this is not the case, the computer makes the necessary corrections by lengthening or shortening the amount of time the fuel injectors are open.
- In the event of a reading below 500 millivolts – that is, when the mix is “lean,” lacking fuel – the injectors will remain open longer.
- A “rich” mix that is lacking air will result in a reading over 500 millivolts. This will be corrected by decreasing the amount of time the injectors are open.
- In the event that the system is unable to correct for the imbalanced ratio, the dreaded “Check Engine” light will be illuminated.
Carburetors versus Fuel Injectors: Which is Better and Why?
The near-universal conclusion is that fuel injectors are the superior option. Here are a few reasons:
- Carburetors have no means of making fine corrections for an improper air to fuel ratio, and are prone to air leaks. Such leaks can result in excess heat melting spark plugs or burning valves and piston rings, all of which leads to a significant decrease in the effective lifetime of an engine.
- Fuel injectors, however, solved such problems with their closed-loop control: By monitoring the amount of oxygen in the exhaust, the engine control unit is able to make multiple fine adjustments to the ratio on the fly.
- The only real disadvantage of fuel injection systems is that they are much more expensive, but given the advantages you get, they are well worth the price difference.
Overall, there’s a clear winner in this debate. Fuel injection systems are fundamentally more efficient due to the manner in which they deliver fuel and adjust the proper combustion ratio. Likewise, this renders engines with fuel injection systems simultaneously more powerful and more economical, and with the added benefit of a more environmentally-friendly waste output.