In Fuel Injection Simplified, Part 1, we explored the purpose and benefits of modern fuel injection. In this article, we'll identify fuel injection system parts.

The heart of the modern fuel injection system is the electronic control unit (sometimes called engine control module), a computer better known as the ECU (or ECM). The ECU is essentially a box of stored maps, that is, programmed-in, three-dimensional records of the engine's fuel and ignition requirements for several sets of anticipated conditions. The ECU has the job of taking inputs from around the vehicle and matching up the resulting overall picture with one of its built-in maps.

The ECU gets the necessary inputs from sensors. Sensors are the most plentiful parts in the fuel injection system; they are its nerve endings, so to speak. All sensors convert the physical conditions of pressures, temperatures, throttle opening and gear position into electrical references. The pressure and temperature sensors do this by accepting a small voltage from the ECU, modifying it according to their perceived conditions, then sending the altered voltage back to the ECU for comparison with the original value. Thus the ECU determines engine pressures and temperatures ectronically. The remainder perform their communication chores a little differently. There are several different kinds of fuel injection system sensors. Some sensors look like bolts with wires attached. These are mostly thermistors, i.e. temperature sensors. A few sensors, such as the atmospheric and intake air pressure sensors, instead use diaphragms to alter the source voltage relative to their pressures. Another sensor type is the variable resistor based throttle position sensor (TPS), which converts angle into the same type of returned voltage. A fourth sensor type is the simple switch function of the vehicle down (or angle) sensor and the gear position switch, which connect and disconnect circuits to signal the ECU with their information. Still other nsors generate signals, either mechanically or chemically. This includes the counting duty of the crankshaft position and vehicle speed sensors, and the chemical reference chore of the O2 sensor. More about that last one later.

One sensor, the camshaft position sensor, provides a very interesting and important chore. Since the four-stroke engine's crankshaft rotates twice per power cycle, the ECU has no way to tell, from the crankshaft sensor's information alone, the engine's compression stroke from its exhaust stroke. -- they're both upward strokes and to the cranshaft sensor, there is no difference. The camshaft position sensor adds half-speed information to the mix to divide the crankshaft's data by two, and thus provides the necessary distinction. Some fuel injected vehicles use the intake air pressure sensor instead of a cam position sensor, with the ECU being programmed to count the number of degrees between the intake and compression stroke. A simpler, less expensive system.

Fuel injection systems also include a throttle body. Looking the most like the old-school carburetor, the throttle body is that cast aluminum part that physically takes the carburetor's place. The throttle grip is connected to the throttle body, just as it was to the carburetor. Although smaller and lighter than the carburetor, the throttle body's appearance is very similar, and its main role of controlling airflow is exactly the same, as that of the carburetor. The throttle body departs vastly from the carburetor in one respect however: it holds no reservoir of fuel underneath.

Instead, fuel injectors, merely electromagnetic valves, are mounted to the throttle body to deliver the fuel from the fuel tank. Pressurized fuel is constantly fed into them, as long as the keyswitch is on. Their ECU-controlled opening and closing then determines when and how much fuel enters the engine. Fuel injectors always open completely, never partially. Air/fuel ratio adjustment is provided not by how far the injector opens, but by how long it is open. Engineeers call this injector "pulse width."

Many fuel injection systems contain additional parts such as secondary fuel injectors on some sport models. These injectors permit very closely balanced air/fuel ratios at part throttle by adding their secondary discharge at large throttle openings. Much like the power jet circuits on vintage racing carburetors. Another optional part is the sub throttle, an extra throttle butterfly upstream of the main throttle that captures air pressure between the two. Maintaining a high pressure air pocket permits superior throttle response at low rpm, and also softens the sudden power hit of high performance sport bike engines. Fuel injection systems also have a means of adjusting the engine's idle speed as it warms up. The idle speed controller works in one of two ways. It either permits air to bypass the throttle butterfly -- an air bleed, in other words -- which raises the idle, or it electro-mechanically props open the throttle butterflies to serve the same purpose. Both systems are in use concurrently.

Let's not leave out the fuel injection system support parts: the fuel tank, fuel pressure regulator, and fuel pump. The fuel pump's job is to provide the push that gets the fuel through the fuel injector. Don't forget how important this is. Some performance issues can be traced to fuel pump problems. The pressure regulator, for its part, ensures that this push is constant despite changes in manifold pressure, to avoid fuel richness when the throttle is closed and the manifold pressure drops suddenly.

A lot of parts, huh? Sure, but we can connect all these parts together into just three groups called input, control, and output. All of the sensors do their jobs on the input side of the fuel injection system. They input information about engine conditions into the electronic control unit (ECU). Once all the data is in, the ECU matches the received information with the map in its collection that most closely meets those conditions. It then fires off signals to the output components, which consist of the fuel injectors but can also include secondary injectors, the idle speed control, and sub throttles.

Go on to Part 3: How Fuel Injection Works