How Fuel Injection Systems Work

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A fuel injector is nothing but an electronically controlled valve. It is supplied with pressurized fuel by the fuel pump in your car, and it is capable of opening and closing many times per second.

When the injector is energized, an electromagnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to atomize the fuel -- to make as fine a mist as possible so that it can burn easily.

The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU.

The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe called the fuel rail supplies pressurized fuel to all of the injectors.

In order to provide the right amount of fuel, the engine control unit is equipped with a whole lot of sensors. Let's take a look at some of them.

Engine Sensors

In order to provide the correct amount of fuel for every operating condition, the engine control unit (ECU) has to monitor a huge number of input sensors. Here are just a few:

· Mass airflow sensor - Tells the ECU the mass of air entering the engine

· Oxygen sensor(s) - Monitors the amount of oxygen in the exhaust so the ECU can determine how rich or lean the fuel mixture is and make adjustments accordingly

· Throttle position sensor - Monitors the throttle valve position (which determines how much air goes into the engine) so the ECU can respond quickly to changes, increasing or decreasing the fuel rate as necessary

· Coolant temperature sensor - Allows the ECU to determine when the engine has reached its proper operating temperature

· Voltage sensor - Monitors the system voltage in the car so the ECU can raise the idle speed if voltage is dropping (which would indicate a high electrical load)

· Manifold absolute pressure sensor - Monitors the pressure of the air in the intake manifold , the amount of air being drawn into the engine is a good indication of how much power it is producing; and the more air that goes into the engine, the lower the manifold pressure, so this reading is used to gauge how much power is being produced.

· Engine speed sensor - Monitors engine speed, which is one of the factors used to calculate the pulse width

There are two main types of control for multi-port systems: The fuel injectors can all open at the same time, or each one can open just before the intake valve for its cylinder opens (this is called sequential multi-port fuel injection).

The advantage of sequential fuel injection is that if the driver makes a sudden change, the system can respond more quickly because from the time the change is made, it only has to wait only until the next intake valve opens, instead of for the next complete revolution of the engine.

Engine Controls and Performance Chips

The algorithms that control the engine are quite complicated. The software has to allow the car to satisfy emissions requirements for 100,000 miles, meet EPA fuel economy requirements and protect engines against abuse. And there are dozens of other requirements to meet as well.

The engine control unit uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions. The equation will be a series of many factors multiplied by each other. Many of these factors will come from lookup tables. We'll go through a simplified calculation of the fuel injector pulse width. In this example, our equation will only have three factors, whereas a real control system might have a hundred or more.

Pulse width = (Base pulse width) x (Factor A) x (Factor B)

In order to calculate the pulse width, the ECU first looks up the base pulse width in a lookup table. Base pulse width is a function of engine speed (RPM) and load (which can be calculated from manifold absolute pressure). Let's say the engine speed is 2,000 RPM and load is 4. We find the number at the intersection of 2,000 and 4, which is 8 milliseconds.

In the next examples, A and B are parameters that come from sensors. Let's say that A is coolant temperature and B is oxygen level. If coolant temperature equals 100 and oxygen level equals 3, the look-up tables tell us that Factor A = 0.8 and Factor B = 1.0.

So, since we know that base pulse width is a function of load and RPM, and that pulse width = (base pulse width) x (factor A) x (factor B), the overall pulse width in our example equals:

8 x 0.8 x 1.0 = 6.4 milliseconds

From this example, you can see how the control system makes adjustments. With parameter B as the level of oxygen in the exhaust, the look-up table for B is the point at which there is (according to engine designers) too much oxygen in the exhaust; and accordingly, the ECU cuts back on the fuel.

Real control systems may have more than 100 parameters, each with its own look-up table. Some of the parameters even change over time in order to compensate for changes in the performance of engine components like the catalytic converter. And depending on the engine speed, the ECU may have to do these calculations over a hundred times per second.

Performance Chips
This leads us to our discussion of performance chips. Now that we understand a little bit about how the control algorithms in the ECU work, we can understand what performance-chip makers do to get more power out of the engine.

Performance chips are made by aftermarket companies, and are used to boost engine power. There is a chip in the ECU that holds all of the lookup tables; the performance chip replaces this chip. The tables in the performance chip will contain values that result in higher fuel rates during certain driving conditions. For instance, they may supply more fuel at full throttle at every engine speed. They may also change the spark timing (there are look up tables for that, too). Since the performance-chip makers are not as concerned with issues like reliability, mileage and emissions controls as the car makers are, they use more aggressive settings in the fuel maps of their performance chips.