EFIE: Electronic Fuel Injection Enhancer, Described
As you can see from the title, EFIE stands for Electronic Fuel Injection Enhancer. It was developed for fuel injected vehicles, and was found to be necessary in order for other fuel efficiency devices to work on them. This includes virtually all cars and trucks today.
Photos of EFIE Devices
One fundamental point about an EFIE, is that it is not a fuel efficiency device on it's own. If all you did was add an EFIE to your car, with no other fuel efficiency system, you might gain a few mpg, but you also might not. The reason is that you are fooling the car's computer and making it run out of spec, or differently than it was designed for. The EFIE was designed to make the car run according to spec after another fuel efficiency device has been installed.
The purpose of the EFIE is not to provide fuel efficiency. It's purpose is to make it possible for other fuel efficiency devices to work. Basically a fuel efficiency device makes the engine think something is wrong, and makes it do things to adjust for this "wrongness". The actions it takes based on the oxygen sensor data, makes it negate the efficiency increase that would have been realized by the efficiency device.
The mechanics of this are that any time you add a technology to your vehicle that improves the efficiency of combustion of the petroleum fuel you are burning, one of the results of this process is that there will be more oxygen appearing in the exhaust. When the sensors "see" this additional oxygen the computer interprets this as a "lean" condition, and incorrectly adds more fuel. This can severely reduce the mileage increase you will get from your combustion enhancement technology.
The EFIE solves this by adjusting the oxygen sensor's signal to the computer so the computer is happy with the readings it's getting and it's making the correct adjustments for the various conditions of the engine. A more detailed description of how the EFIE does this can be found in Oxygen Sensor Adjustment - General Information and in Wide Band Oxygen Sensors. Basically, all we are trying to do is compensate for these changes brought about by the improved combustion so the computer can do its job correctly.
There are a number of different types of types of oxygen sensor and EFIEs. You can find out about the oxygen sensors by reading Oxygen Sensors, Types of. It can seem a little confusing at first, but basically there are only 2 types of oxygen sensors. They are the narrow band and wide band sensors. As of this writing, we make the only EFIE for wide band oxygen sensors. And since it works with all types of wide band sensor, its not too complicated. All other EFIEs are made for narrow band oxygen sensors. If you don't know if your vehicle has narrow or wide band sensors, then you should read What Do I Need To Know About My Oxygen Sensor?