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How Does An EFI Really Work?
Table 2 - Sample Fuel Map
Table 3 - Sample Engine Temperature Adjustments
Table 4 - Sample Intake Air Temperature Adjustments
In is time to try to explain what is happening in the ECU now that we have confused you with a lot of rhetoric, diagrams and tables. While the real details of what is happening is very complex, we are going to present a greatly simplified version of what the computer happens.
- The computer determines the current RPM of the engine based on input signals from the Crank Position Sensor.
- The computer determines the load on the engine by checking the Throttle Position Sensor or MAP sensor.
- The computer takes the RPM and Engine Load information to look in the Fuel Map table located in the ECU. While an actual fuel map has more values in it than the sample map in Table 2, a real table looks very similar to the simplified map shown. In our example, the number in the table represents the duty cycle of the injector, or what percentage of the time is the injector actually open and allowing fuel to flow. If the computer can not find exact matches to the RPMs and Engine Load, it is able to estimate the required value by looking at the values just higher and lower to calculate the needed value. For example if the engine is running at Wide-Open Throttle at 3900 RPMs, it calculates the fuel value to be 44 that is half way between 33 and 55.
- Now that the computer has the main fuel value, it needs to determine if any "adjustments" to the fuel map value are needed.
- The computer determines the engine temperature. If the engine is cold, then the fuel number is adjusted up by a pre-determined percentage. For example when the engine temperature is a 30 degrees F, the computer knows the engine is cold and needs more fuel, just like a choke on a carburetor. The computer looks up an engine temperature adjustment percentage that could be 20% (see table 3). If our previously looked up fuel value was 44, the computer now adds 20% to that number resulting in a new fuel value of 53. As the engine warms up, these adjustment percentages become smaller and at normal operating temperature the value is zero.
- The computer determines the ambient temperature of the air entering the intake by reading the Intake Air Temperature sensor. As with the engine temperature adjustment, the computer will look up the percentage adjustment. Lets go back to our full throttle acceleration at 3900 RPMs and assume our engine has warmed up to normal operating temperature, but the outside air temperature is 90 degrees F. The computer looks up an adjustment percentage of -2% (see table 4). The computer actually understands that at high air temperatures, the engine requires less fuel so it leans the fuel value down from 44 to 43. If the air temperature had been cold, the fuel mixture would have been made slightly richer.
- The computer has now determined that at Wide Open Throttle, at 3900 RPMs, normal engine temperature and a 90 degree air entering the intake that the engine needs to keep each fuel injector open 43% of the time.
- The computer now checks to see if it is time for the fuel injector to be activated. When it is time for the fuel injector to open, the computer allows current to flow through the injector for the required amount of time. The opening time can be controlled very precisely, down to the millisecond.
As you can see, the computer in a fuel injection system is doing a lot of things very quickly. While some EFI systems do more than listed above, the description above is a rough idea of the minimum amount of work the system actually has to do. The real activities are much more complex than explained, but you get the idea that your EFI ECU is pretty good at knowing how much fuel your engine really needs.
Why is EFI better than a carburetor?
Choosing a carburetor and tuning it is always a series of compromises. A carburetor is a demand device dependent upon the velocity of the air entering the venturi to create the air/fuel mixture that feeds the engine. A carburetor maintains a series of fuel circuits to help maintain the best possible fuel mixture. The idle circuit, intermediate and main jetting circuits are used to tune the mixture across the operating RPM range of the engine. These different fuel circuits can interact with each other creating rich and lean spots in the fuel curve. Some riders will go as far as to add one or more additional fuel circuits (Thunderjet) in an effort to improve the fuel curve. The interactions of these additional circuits further complicate the tuning efforts. A change in jetting to one fuel circuit can impact the required jetting in another circuit. Sound complicated? It certainly can be.
Lets simplify matters and assume a carburetor with nothing more than an idle circuit and main circuit. To optimize low RPM performance a small diameter carburetor provides the best performance, but at high RPMs a large volume of air is required to feed the engine. This requires a much larger diameter carburetor. The ability of a carburetor to provide a good air/fuel mixture is very dependent upon the velocity of the air going through the carburetor. This "signal" must be present to maintain good throttle response. If the diameter of the carburetor is too large for the engine, low RPM performance can be very poor. Most HD mechanics and riders are aware that the "large carburetor" syndrome creates a poor running engine. This situation does not occur with an EFI engine.
With an electronic fuel injection system, the required fuel amount for each RPM and engine load condition is located in the fuel map located in the ECU. Once this primary fuel amount is known, then the ECU further adjusts the fuel mixture for the engine and air intake temperatures. In many cases, the mixture will even be adjusted for the barometric pressure and altitude. Based on the various sensor inputs to EFI ECU, there is only one fuel value that is generated. The correct fuel amount is fed into the engine at all times. Because airflow does not have to pass through a venturi to provide an air/fuel mixture, the throttle plate diameter can be quite large. This allows engine is allowed to draw all the air it wants. Since more air equates to higher horsepower potential, all the EFI has to do is provide the correct amount of fuel for the increase in airflow.
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