ECU (Engine Control Unit) - This is the brain of the car. It uses the input from all of the sensors on the engine to determine optimum spark advance and fuel delivery for the engine. Most modern ECUs can also control other items such as the air conditioner, alternator, and choke. The reason all modern cars have ECUs instead of mechanical carbuerators is that ECUs can better respond to the many variables that make up an engine's performance curve. Remember the days of tuning jets on a carb, and playing with chokes? An ECU can do equivalent control of those things all by itself and compensate for (tune) the engine as it ages. Computers were initially introduced into automobiles to bring their smog levels down. Now they are used to eke out the last bit of performance out of relatively tiny four-cylinder engines.
EPROM - Let's start with the last three letters. ROM means Read-Only Memory. This is a permanent storage place for computer programs. Computers can read their operating program from this device. It cannot be written over (as opposed to RAM, which we are sure you have heard of). It is made with the computer program already etched in right at the factory. PROM means Programmable ROM. A PROM is a device like a ROM, except that it leaves the factory blank and can be filled with a program once, and only once. An EPROM is an Erasable PROM. It can be erased after being written, but only with an intense ultra-violet light source. That is typically why you see thick labels on them - so they won't accidentally get erased due to prolonged exposure to sunlight or flourescent lights. Why don't auto manufacturers use ROMs or PROMs instead of EPROMS? We suspect that at the beginning of a model year, many things in the ECU computer program are still in flux. After all, the smog testing for the entire model line may not have been completed yet, and the smog rules themselves might have changed since they designed the car. In order to accomodate any last minute quick fixes, the manufacturers use EPROMs so they won't get stuck with a bunch of ECUs that have the wrong program in them.
MAF (Mass Airflow Sensor) or MAS (Mass Air Sensor) - Most commonly found on Mitsubishi vehicles, this sensor is used by the ECU to determine the amount of air entering the engine. Notice that "amount of air" is different from "volume of air" depending on the air temperature and pressure. The MAF actually uses three different sensors to determine the amount of air. The most important sensor is the airflow sensor. For this task, Mitsubishi employs a Karmaan Vortex sensor. Long story short, this sensor works by disturbing a laminar flow of air, causing vortices which spin in alternate directions. An ultrasonic transducer detects these vortices, and sends a pulse to the ECU every time one passes by. The MAF also has a barometric pressure sensor and temperature sensor. If you remember PV=nRT from freshman chemistry, you'll see that we have P, V, and T - since R is a constant, we now have n, which is the amount of air. Although a bit more restrictive than other methods due to the need for a laminar flow, the Mitsubishi MAF has the advantage of not depending on the volumetric efficiency of the engine. Thus, when you upgrade various engine, intake, or exhaust components, you do not neccessarily have to reconfigure the ECU's fuel and spark program.
September 29th, 1997