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The Motorola 68HC11 is a microcontroller used extensively by Chrysler EMSs in the 1980s. |
However, fuel injection didn’t really begin to replace carburetors until the computer boom of the 1980s. It was during this time that fuel systems began to be controlled by computer chips called microcontrollers, the first of which were manufactured by Motorola. The capabilities of these microcontrollers allowed the system to gain some sort of “intelligence;” it could now automatically adjust for varying operating conditions through use of feedback sensors.
The most commonly used feedback sensor is the oxygen sensor, or O2 sensor, which was originally used by Volvo in the 1970s. The oxygen sensor is located in the exhaust path of the engine. The original (“narrowband”) oxygen sensors output a voltage signal that ranged from zero to one volts. If the voltage signal was in the upper part of the voltage range (usually around 0.8 volts), the engine was running too rich. If it was in the lower range (around 0.2 volts), it was running too lean. A voltage signal of 0.5 volts was desirable, because this meant the engine was burning the correct amount of fuel. This was known as being “stoichiometric,” which is an air-to-fuel ratio of 14.7:1 by mass.
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The Chrysler turbo motors of the ‘80s and ‘90s are an example of a closed-loop control system that uses narrowband O2 sensors for feedback. |
If the engine is running too lean, the microcontroller compensates by injecting more fuel into the engine. If it is running too rich, it injects less fuel. This allows the engine to operate over a very wide spectrum of operating conditions. For instance, on a hot, humid day, a carbureted engine might run rich because less fuel can be burnt by moist, humid air. However, an electronically-controlled engine can immediately sense the rich running condition and compensate accordingly.
A more recent development is the wideband O2 sensor, which provides more information to the EMS beyond simply rich, lean or stoichiometric. These sensors have a greater voltage output range that covers the spectrum from below 9:1 and all the way up past 20:1 air-fuel ratios.
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| Megasquirt is another example of a standalone EMS. Megasquirt is completely customizable and assembled by the user. It supports nearly all types of engines, is inexpensive, and is a great way to learn a ton about how an EMS works. Photo courtesy of DIYAutoTune.com. |
Fuel injection systems operate in closed-loop most of the time (after the engine has warmed up). This means that the fuel trim is continually adjusted based on feedback from the O2 sensor(s). On older cars with narrowband O2 sensors, this continuous fuel trim adjustment could be seen as the “cycling” of a narrowband air-fuel ratio gauge.
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The Coyote “5.0” V8 in the new Mustangs is equipped with two separate wideband O2 sensors, one for each bank of cylinders. The EMS is able to use this information to tune fuel and spark for individual cylinders. |
Three Basic Types of EMS Tuning Methods: Alpha-N, Speed-Density, and Mass Airflow
There are three main configurations that are most widely used for an EMS.
Alpha-N is the simplest method, but often doesn’t involve feedback. In an Alpha-N configuration, two primary sensor values are used in a lookup table to set the engine’s fuel and spark: the angular position of the throttle (alpha) and the number (N) of revolutions per minute. Feedback from an O2 sensor may or may not be used. Alpha-N is most often used as a “limp mode” in production engines. If one of the feedback sensors stops working, the computer goes into Alpha-N mode and uses a “best guess” of what the fuel and spark values should be. This table is tuned very conservatively to prevent engine damage, which is why most cars feel very sluggish in limp mode.
