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Unread 09-24-2006, 02:10 PM   #1
BFH
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Default Engine Controls

Basic Engine and Engine Control Operation

An old saying used to be that there was three things needed to get an engine started, "compression, fuel, and spark". Well, that saying still holds true today but with todayís cars we should add another to that list, "Computer Controls". So now it should be said it takes four things to get an engine started, "compression, fuel, spark and computer controls". Why do we need these four things for an engine to run? First, an engine needs proper compression to compress the gas in the cylinder in order to ignite the gas. Second is fuel - whether itís carbureted or fuel injected - an engine needs good clean fuel and good fuel pressures. Third is spark, without the proper spark at the proper time an engine will not start. These first three things have to take place for an engine to start and run properly. There is a lot more that goes on during this process than I have mentioned, but these are three of the basic things an engine has to have in order to start and they are the first things a good mechanic will check if a car doesn't start. (For more info on basic engine operation, see SpookedJungList article on Performance Tuning Methods, Performance Tuning Methods and definitions Guide. ). Now for the fourth thing, Computer Controls.

Why should Computer Controls be added to the list of "compression, fuel and spark"? Because with todayís cars, the Engine Controls regulate the running of the compression (i.e.. variable valve timing), fuel and spark through a computer. If the computer or any of its parts fail, then it could affect one or all of the above and could cause the car not to start. So thatís why this important fourth variable needs to be added to the list. So what and how do the Engine Controls work? The purpose of this article is to give you a basic idea on how the Engine Controls work. There is much more about Controls than I will describe here, mostly because I am not a technical writer and it would be more typing than I would care to do.

To start, letís break it down, like compression, fuel and spark, Engine Controls can also be broke down into three categories; sensors, actuators and controls. What are sensors? Sensors monitor what takes place in the engine. What are actuators? Actuators are the workhorses, or in other words actuators do the work required to make the engine run. What are controls? Controls take the information received from the sensors and tell the actuators to work. All of this works together to form a "closed loop" of information received (input), information computed (control) and work done (output). When an engine is in the "closed loop" mode it is running in its optimum mode for fuel efficiency. Next, I'll describe some of the parts and how they operate.

Sensors

MAF Mass Air Flow Sensor. This sensor uses two small wire resistors to measure incoming air volume. One resistor has a fixed voltage/current value and the other resistor has changing voltage/current dependent on incoming air volume. With this information, the computer delivers the correct amount of fuel and timing the engine needs.

IAT Inlet Air Temp. This sensor measures the temperature of the incoming air. The computer uses this to help determine rich and lean (fuel trim) conditions.

ECT Engine Coolant Temp. This sensor measures the temperature of the engine coolant. The computer uses this to determine cold start mode and also fuel trim.

TP Throttle Position. This sensor measures the opening of the throttle plate (gas pedal). The computer uses this to determine fuel injector opening time or the amount of fuel through the injectors.

EGR Exhaust Gas Recirculation. This sensor measures EGR valve position (see EGR actuators). The computer uses this for EGR position and to help control fuel trim and timing.

EGO Exhaust Gas Oxygen. This sensor measures the amount of oxygen in the exhaust. As the sensor detects low oxygen (lean) it sends a low voltage back to the computer and when it detects a high amount of oxygen (rich) in the exhaust it sends a high voltage back to the computer. The computer uses this for fuel trim.

VSS Vehicle Speed Sensor. This sensor measures the speed of the trans output shaft. The computer uses this to determine speed and load of the vehicle and also this is what makes your speedometer work.



Actuators

IAC Idle Air Control valve. This actuator controls the idle speed for cold start, normal idle and increase for idle when any load is detected (i.e., turning on AC ).

EGR Exhaust Gas Recirculation. This actuator controls the flow of exhaust gas that is sent back into the engine to help cool the cylinders down, this helps reduce engine ping or detonation.

VMV Evaporative Emissions Control Valve. This actuator controls the purging of gas tank fumes back to the engine to help reduce emissions.

Fuel Injectors Delivers fuel to the cylinders.

Ignition coils Delivers spark to the spark plugs and then to the combustion chamber.


Controls

PCM. Powertrain Control Module. Putting it all together - this is the brain of the system it takes the information received from the sensors and then uses internal "look up tables" within the computer to make its calculations. It then uses the actuators to deliver the correct amounts of fuel, timing and spark to the engine.

To put it another way, imagine a big loop. In that loop there is a sensor that tells a computer what just happened within the loop, the computer calculates that then tells the actuator to deliver the computers request. The sensor then sees what the actuator just did and tells the computer what just happened. And the process continues. This is just a real basic explanation of engines and Engine Controls. This is for anyone who needs a basic primer as to what goes on in a late model vehicle. Any comments, questions or suggestions are welcome.

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