Fuel Injection Systems
A Beginner's Guide to Fuel Injection
Fuel injection is a system used to deliver a correctly metered amount of fuel into the air entering an engine so as to create an explosive compound. Fuel injected engines are generally computer controlled allowing for unparalleled precision when compared to the carburated engines of yesterday.
Certainly there are too many variations of fuel injection systems than can be described in this short article, therefore, we will focus on the most common passenger car system used today. A computer controlled gasoline injection system. Let's follow one drop of gasoline through the system to see how it is used to power the engine and let the drop itself describe the process.
FUEL TANK. Hi. I am a drop of gasoline that just entered your fuel tank from the gas station. I am stored here in this tank until being pumped to the engine.
FUEL PUMP. The fuel pump is located near or in the gas tank and does just what the name implies. It pumps fuel. Whenever the engine is running, the fuel pump is maintaining fuel flow under pressure to the fuel injectors. These fist size dynamos are capable of pumping thousands of gallons of fuel in their lifetime at a pressure of fifty to ninety PSI (pounds per square inch). As you pull away from the gas station I entered the fuel pump and begin my journey to the engine.
HIGH PRESSURE FUEL LINE. Wow. I'm under a lot of pressure as I pass through the fuel line toward the fuel injectors.
FUEL INJECTORS. The fuel injectors are electrically operated valves that can be opened and closed by the computer. These valves are normally closed. However, when the computer determines that it is time for fuel to be injected, It applies a voltage to a wire coil in the injector causing it to open and allow fuel to spray through the nozzle into the air that is moving toward the engine. Notice that the injector does not pump or pressurize the fuel, it merely opens to allow fuel to flow through it. As I arrive at the injector, it opens and I am forced through its nozzle where I am atomized and start mixing with the air being drawn into the engine.
I pass along the intake manifold as vapor, am drawn past the intake valve into the cylinder. The intake valve closes and the piston comes up compressing me to about nine times atmospheric pressure. The spark plug fires and I explode with great force, pushing the piston down and turning the crankshaft. The crankshaft is connected mechanically to the wheels, propelling the vehicle down the road. As the piston moves up again, the exhaust valve opens and I am forced out of the engine into the exhaust system.
OXYGEN (O2) SENSOR. The O2 sensor in the exhaust manifold senses how lean (too little fuel in the air) or rich (too much fuel in the air) the burned mixture is and sends a signal to the computer.
COMPUTER. The computer monitors the O2 and several other sensors and adjusts how long the injectors stay open thereby keeping the mixture at the perfect ratio. As I pass by the O2 sensor, it senses the fuel mixture and sends the appropriate signal to the computer, after which I pass through the rest of the exhaust system and am expelled out of the vehicle.
FUEL RETURN LINE. Not all fuel that moves through the fuel line is injected into the engine. If the injector is closed, the fuel bypasses the injector, moves through a pressure regulator and returns to the fuel tank where it begins the entire journey again.
I know I'm just one drop of gasoline, but I hope my explanation gives you some idea of how fuel injection works. It is a complex system. The engines of today are truly a technological marvel.
There is one more fact I should mention. When there is a malfunction of a sensor, the computer falls back on a preset program to control the engine. Although not as efficient, it allows you to continue driving until you can service the vehicle. This is called "open loop" or "limp home" mode and is usually accompanied by an illuminated "check engine" warning light.
