Jaccards Similarity Coefficient

Admin | January 12, 2012 | Comments Off

Jaccards Similarity Coefficient

Jaccards Similarity Coefficient

The Air/Fuel Mixture...

The purpose of the fuel injection system is to precisely maintain the proper air/fuel ratio of 14.7 parts air to 1 part of fuel, or 14.7:1. Before the system can deliver the right amount of fuel, it needs to know how much air there is coming into the engine. In the old days when engines used carburetors we could determine the proper fuel mixture with the use of an emissions analyzer. I'd use a four gas, CO (carbon monoxide), HC (hydrocarbons), CO2 (carbon dioxide) and O2 (oxygen). Then I would turn the mixture screw(s) on the carburetor until the mixture was correct. When CO and HC were near 0 and CO2 was as high as possible I knew the A/F ratio was right. Now this was possible because the incoming air was constant. But the problem is the A/F ratio was only correct at idle. When the engine went off idle the A/F mixture changed because the amount of air entering the engine changed. What we needed back then was a mechanic under the hood with an emissions analyzer all the time adjusting the A/F mixture so it was correct all the time.

Air...

Nowadays we have a computer to take the place of the mechanic under the hood (thank goodness, it was getting awful warm under there) to accurately maintain the A/F mixture. The computer gets inputs from various sensors but today we are just going to talk about the Mass Air Flow Sensor (MAF). Now what is 'Air'? Air is a colorless, odorless, tasteless, gaseous mixture, mainly nitrogen (approximately 78 percent) and oxygen (approximately 21 percent) with lesser amounts of argon, carbon dioxide, hydrogen, neon, helium, and other gases. Air has weight, mass and holds water and these are the properties that must be considered when the computer adjusts the A/F mixture.

Enter The Mass Air Flow Sensor...

The early MAF sensor was a vane type sensor. The incoming air pushed a flap that was connected to a potentiometer. The more air coming into the engine, the further the flap was pushed. By reading the potentiometer the computer knew how much air was coming in and how much fuel should be injected. Modern MAF sensors use a "Hotwire" type sensor. A constant voltage is applied to a heated film or heated wire. This film or wire is positioned in the air stream or in an airflow-sampling channel and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated wire or film is a positive temperature coefficient (PTC) resistor. This means that it's resistance drops when it's temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency or a voltage that is sent to the computer and interpreted as airflow. Adjustments for air temperature and humidity are taken into consideration since they also affect the temperature of the heated wire or film.