® Fuel Injection Simplified
Part 3: How Fuel Injection Works

In Fuel Injection Simplified, Part 2, we took a close look at fuel injection system parts, noting their jobs and how they interacted, and explored their roles in terms of three groups: input, control, and output. This time, we’re examining fuel injection system operation.

Let's observe the whole thing working together. At startup, many fuel injection systems use the engine temperature and air temperature sensor inputs to establish a starting mixture. Intake manifold air pressure is not included yet because it is too unstable during starting. As the crankshaft turns over, inputs from the crankshaft and camshaft sensors combine to communicate piston position, while the coolant temperature sensor tells the ECU to compensate for a cold engine, if necessary. The ECU takes the information supplied by the sensors and either opens an air bypass or swings the secondary throttle shaft, if applicable, to prop the idle cam up slightly, to proactively prepare for a temporary fast idle. The ECU also selects the correct map and develops a timed signal to the fuel injectors, which have been holding back fuel pressurized by the fuel pump since the keyswtch was turned on. When pulsed by the ECU's signal, the fuel injectors open and the fuel escapes in a fog-like spray for a period of time that will deliver the mixture needs of the engine. The engine starts and idles.

At idle, intake air pressure input is added to the mix and temperature sensor input into the ECU modifies the mixture as needed by affecting the injector open period. As the engine warms up, based on information from the coolant temperature sensor, the ECU gradually turns down the fast idle by adjusting that system.

Once the vehicle is underway, rpm information from the crankshaft position sensor is added and the system goes into the D-J operational mode. Centered on rpm and intake air pressure, this low speed mode is air density (D) based and has roots in Bosch Jetronic (J) technology. When rpm and/or loads increase, the system shifts to the Alpha-N mode, which relies more on a combination of throttle position (Alpha) and engine rpm (N).

Should a fault occur, many systems will default back to the D-J mode until the problem is found and corrected. Some also revert back to the old-school 360° injection modality, resulting in normal engine operation but excessive emissions. Similarly, the crankshaft position sensors, whose input is vital and thus can't be done without, are in many cases able to still allow the engine to run even if one half of their input goes out (i.e, one of two cylinders or two of four giving signals) as the ECU will mathematically compute the missing signal's position. But when the key is turned off, this fault recovery feature is not able to permit the engine to restart, unfortunately.

There are actually two forms of internal control for fuel injection systems. Thus far we have spoken in terms of a Speed-Density system, one relying heavily on the ECU's maps. This system is also known as "open loop." However, the most common system in use today uses a special component called an oxygen (or O2) sensor. The O2 sensor, mounted in the exhaust, reacts electrolytically in the presence of oxygen, producing a tiny voltage much as if it were a battery. The O2 supported fuel injection system compares the amount of oxygen in the exhaust with the amount in the outside air, and adjusts the fuel mixture accordingly. This "closed loop" system relies more heavily on the O2 sensor than on the ECU's maps and has all but replaced the less sophisticated open loop system.

Pursuant to a "gentleman's agreement" among powersports manufacturers, high performance fuel injected vehicles have been speed limited since 2001 -- road bikes to 300 kph (186 mph) and personal watercraft to 67 mph (US Coast Guard agreement specification). Current machines carry this off by monitoring the fuel injection system's speed sensor.

Fuel injection system diagnostcs via laptop or handheld computers includes the ability to test-operate the various mechanical components such as injectors, evap valves, air injection solenoids, exhaust pipe butterflies, secondary throttles and more. Graphing, trends, real time sensor values, and part failure history are all accessible, as is in many cases stored operational data such as rpm and vehicle speed. Many manufacturers also allow reflashing of their fuel injection system ECUs for various purposes including ECU updates and performance enhancements, but presently none of the Big Four Japanese facilitate this. Various piggyback style EFI boxes connect to the OEM ECUs to either intercept or reinterpret sensor inputs so as to result in the same thing as if the ECU were reflashed. This brings tuneability to the beforementioned Japanese vehicles, and is useful on other brands as well, though none of the major brands endorse their use, partly because some issues with vehicle electronics have been experienced.

Go on to Part 4: Fuel Injection's Future

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