Explained: Carburetion VS Fuel Injection

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Explained: Carburetion VS Fuel Injection

Carburetion

Carburetor is the most basic, and until recent times the most prevalent fuelling system used in two-wheelers, especially in India. To explain the basic working of a carburetor, think of it as a tube that feeds an air-fuel mixture into the cylinder from one end, with an air filter attached to another. Now, somewhere in the middle of this tube the air passage area is restricted to increase the velocity of the air passing through. This little area, or part of the carburetor system is known as venturi. By increasing the velocity of the air through the narrow area, a low pressure pocket is created, which in turn facilitates drawing fuel from a jet placed near the venturi through suction. This phenomenon is in line with Bernaulli’s principle which states that the velocity of a fluid (or air) passing through a tube is inversely proportional to the pressure created by it.

The amount of air sucked into the carburetor is decided by a valve at the end of the tube connected to the cylinder. This valve is termed as the throttle valve and is connected to the accelerator grip of your two-wheeler and controls the air-fuel flow through the throttle inputs provided by the rider. When you wring the throttle, the throttle valve opens – allowing a generous flow of air through the carburetor. Conversely, it’s closed when the throttle on the handlebar is fully rolled back.

The fuel jet, which is placed near the venturi draws its fuel directly from the fuel tank via a float chamber, which is a small reservoiur for fuel, with a float valve which cuts fuel supply when it’s full  and resumes it when the jet is drawing fuel from it. The resultant air-fuel mixture is then supplied to the cylinder where combustion takes place.

This is a very basic explanation of how a carburetor works, though modern carburetors including constant velocity or CV carburetors are generally more complex in design. These carburetors make use of components like a diaphragm, needle valve and pilot jet to control the air-fuel mixture. The important point to note here, though, is that this entire setup is rather simple and is fully mechanical with no electronics or sensors coming into play.

Fuel Injection

Unlike carburetors, a Fuel Injection sys

Automotive fuel injection in general measures the airflow going into an engine and then supplies the fuel that mass of air flowing every second requires. Motorcycle fuel injection, however, uses what is called an "N Alpha" system, which is mapped injection.

Manufacturers take an example of the engine, run it on a dyno at all different throttle levels and rpm, work out the correct fuel mixture for every condition, and then they make a map. They have all the N variables and the map connects those variables by supplying a particular amount of fuel and telling the injectors how long to stay open.

Air is controlled by a butterfly valve. If you have a steel throttle cable, when you turn the throttle the cable physically pulls the throttles open. If you have a throttle-by-wire system, like those found on more modern motorcycles, turning the grip tells the computer what you want and it opens the throttles a certain amount.

The throttle-position sensor reports the throttle angle, called Alpha. The teeth on the wheel located on the end of the crankshaft whiz by another sensor, which determines engine rpm. With those two pieces of data, the ECU looks up in the map the correct amount of fuel for that condition.

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When you go to the races and see a technician with a laptop plug into a bike and start doing what appear to be mysterious things, they are not mysterious at all. The rider has said, “Could you do something about that hiccup coming off turn 8?” And the technician fixes the problem with a few keystrokes. It’s a different way of doing things from the old carburetor days, but the end result is the same: trying to get to the correct fuel mixture.

Fuel injection compensates automatically for altitude, temperature, and barometric pressure because it has sensors that measure air pressure. If you ride up Pikes Peak, for example, you will have the same mixture at the top that you had at the bottom. You won’t have the same power because air density is lower. On a cold, high-pressure day, your engine will make more power but it also won’t go lean like a carbureted engine does because it has automatic mixture compensation.

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