Okay Fuel-Injection, But how does it Really Work

Apparently some people are clueless on the technology behind Fuel-Injection. They read articles about fuel-injection, but they never see anything about how fuel-injection actually works. It get even worse if they start talking about remapping and reprogramming...

Although fuel-injection can be quite a complex system at the design and engineering level, in concept, it's actually pretty simple. Its primary benefit is its ability to control the fuel-air mixture more precisely under more varied operating conditions than a carburetor ever can.

A carburetor's operation is dependent upon intake-tract vacuum, called a “signal,” created by the down-stroke of the piston. The vacuum sucks air in through the intake inlet, and the rapid movement of that air over passages in the carburetor's venturi siphons fuel up out of the float bowl and into the intake stream. If all the jets and passages are properly calibrated for the engine, the fuel mixture should be correct for the prevailing conditions.

But a carburetor doesn't self-adjust very well. If the signal is weak-such as when the throttle is quickly turned wide-open at very low rpm there often is not enough intake vacuum to draw sufficient fuel up out of the float bowl, so the engine sputters and wants to die. At high altitudes, where the air is thinner, the engine runs too rich; at exceptionally low altitudes it runs too lean. Same with changes in temperature: When it's hot, the mixture tends to be rich, and when it's colder, it tends to be lean.
A fuel-injection system, however, does not rely on intake vacuum, and it can adjust itself – within predetermined parameters, at least – to changes in altitude and temperature. It uses relevant data gathered from key points around the engine to determine the proper fuel mixture, regardless of air temperature, air density or the amount of vacuum present in the intake tract.

Let's start at the beginning. The fuel system is pressurized at a constant psi by an electric pump, usually located inside the fuel tank. The system involves at least one injector nozzle in each cylinder's intake port, close to the intake valve. Each nozzle incorporates an electromagnetically controlled on-off valve that determines whether or not any pressurized fuel in the system can pass through the nozzle. The amount of fuel allowed to squirt into the intake port at any given time is determined by how long the nozzle valve remains open, which is called the “duty cycle.” The length of the duty cycle is determined by input delivered to the system's computer - called the ECU (engine control unit) or ECM (engine control module) – which is sent data from various sensors located on or near the engine. The sensors monitor engine rpm, throttle position, engine temperature, ambient air pressure and/or volume of air entering the intake, and exhaust leaving the combustion chamber to annalise the combustion efficiency. Some even send data regarding the motorcycle's road speed and selected transmission gear.

The computer is preprogrammed with a “fuel map,” which is kind of like an electronic cross-reference chart, that plots the desired fuel mixture for all the possible normal operating conditions. The map is developed at the factory through extensive performance and emission testing of that particular model, and it allows for enough variation in mixture to compensate for changes in air temperature, air density and so on.

So, the sensors send the necessary data to the computer, the computer looks at the fuel map and decides the best mixture for that set of conditions, then, knowing that the fuel-line pressure is always constant, tells the nozzle valve how long to stay open between each intake cycle. The end result is a very precisely metered fuel mixture.

Miraculously, all this takes place in a mere microsecond. At 15,000 rpm on a 600cc four-cylinder sportbike, for example, the computer must make about 500 such decisions every second.

Tag: Fuel-Injection EFI Technology Throttle-Body Fuel-Pump Venturi-Siphons Carburetor Development
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