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Mixing the Fuel

    Now we have fuel and air inside our engine, let's see how it's all mixed together in the proper amounts.

This time around we'll talk about how the air and fuel systems interrelate and talk about the function of the main EFI computer. As always, since all manufacturers do things in a different way, we'll look at these systems in general.

The EFI computer is the heart of the system; in fact, most Techs refer to it as the 'brain.' Just as your brain takes in information from your 5 senses and processes it, so does the EFI computer. It receives information from the sensors in the fuel, air and exhaust system and from that information, it determines how much fuel the engine needs to operate at optimum levels.

From the air intake system it gathers information on how much air is entering the engine, the temperature of that air and at what position the throttle is. It takes that information and calculates how much fuel the engine needs for the ideal mixture. Once it figures it out, it will open the fuel injector for a specified amount of time and fuel will spray into the intake.

The longer the injector is open, the more fuel is injected. At idle the injector is open for about 3 or 4 milliseconds (ms). At medium throttle it'll be open for about 6 to 7 ms. It's interesting to note here that the injector only injects half of the fuel at this point. The fuel sits at the intake valve and in the next cycle the remaining fuel is injected and is drawn into the cylinder. This allows the computer to make any fine adjustments in the mixture.

Another factor affecting the amount of fuel going into the engine is the fuel pressure. Most EFI systems operate at about 36 psi going up to about 45 psi at full throttle. The higher the fuel pressure, the more fuel is injected into the intake in the same amount of time. Some EFI systems will even control the speed of the fuel pump to control fuel pressure. Slowing it down at idle and speeding it up as engine speed increases.

Most fuel pressure regulators now have a fuel temperature sensor built in. This takes into account that warm fuel is less dense than cold fuel. There is a vacuum sensor in or near the intake manifold, usually called the MAP sensor, which tells the computer what the intake manifold vacuum is. From that the computer knows how much load is on the engine and adjusts the fuel mixture accordingly.

Now you have an idea how the EFI computer uses the information it gathers from the various sensors and determines the optimum fuel mixture. But, how do we know if the brain is doing this right? Wouldn't it be nice if there were a way to check and see if the brain is doing all this correctly?

Well, there is. In the exhaust system, mounted before the catalytic converter is a device known as the Oxygen Sensor, referred to as the O2 sensor. What this does is measure the amount of oxygen in the exhaust and relays that information back to the brain. If there is too much oxygen in the exhaust, then the engine is running lean and needs more fuel. The brain will richen the mixture to compensate.

If there is too little oxygen, the engine is running too rich and is using too much fuel. The brain will lean out the mixture to compensate for this. Since the O2 sensor must be hot to operate, most modern O2 sensors have a heater built into them to bring them up to operating temperature quickly.

Okay, so now you have an understanding of how the fuel and air system relate to each other through the main EFI computer. We now have the ideal fuel mixture under any engine operating condition. Now all we need is a way to ignite this mixture. Next time we'll talk about the Ignition system and see how the spark is made. Until then, Happy Motoring!!!!

Additional Information provided courtesy of AllDATA and Warranty Direct

© 2000-2006 Vincent T. Ciulla


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