By Definition The Engine Management System is a collection of Computers, Input Devices (Sensors), Output Devices (Actuators), and A Set of instructions (programming) that define the operations of actuators based on sensor information.
The goal of the Engine Management System is to manage the operation of your cars engine. So That it operates reliably in all driving conditions providing maximum fuel economy and power with minimal exhaust emissions.
There are two (2) Basic types of engine management strategies related to controlling the air to fuel ratio. The first is known as speed density, the second is known as mass air flow. These terms refer to the way in which the amount of air used by the engine is calculated. In both strategies the value describing the amount of air used by the engine is used to calculate the amount of fuel that will be supplied to the engine for proper combustion. Once combustion occurs the computer then checks to see whether or not the amount of fuel supplied was correct. This is known as feedback and is accomplished by looking at input from the exhaust oxygen sensor. If the feedback shows that proper combustion did not take place a percent change to the amount of fuel supplied is factored into the next fuel distribution event. This percent change is either a positive or negative value. A positive value is used if the exhaust oxygen content indicates a lean condition (Not enough Fuel). A negative value is used if the exhaust oxygen content indicates a rich condition (Too much Fuel). After several cycles a learned value called fuel trim is stored and applied to the base fuel calculation. Fuel Trim values are learned for all operating conditions and are stored in a lookup table. I will discuss fuel trim in greater detail later.
Speed Density
The speed density method of measuring air is accomplished by performing a calculation using known design values and variable measured values. The known or fixed values are related to physical characteristics of the engine that have an affect on the amount of air the engine is able ingest. The Variable or Measured values are related conditions that affect the amount of air the engine is able to ingest. I will go into more detail on these values later.
Design Values
Engine Displacement
Camshaft Profile
Camshaft Timing
Intake Air Flow
Exhaust Air Flow
Variable Measured Values
Throttle Position
Manifold Absolute Pressure
Barometric Pressure
Engine Revolutions Per Minute
Intake Air Temperature
Coolant Temperature
Engine Displacement
This is the amount of Air that the Engine can theoretically displace during a complete cycle. A cycle represents the intake, compression, power and exhaust events of the Engine. In a 2-Stroke engine this happens once every crankshaft revolution for each cylinder. In a 4-Stroke engine this happens once every 2 crankshaft revolutions for each cylinder. The mathematical representation is:
Where Nc is the Number of Cylinders, Rc is the Radius of the Cylinder and Sl is the Stroke Length (The distance the piston moves from the top of the cylinder to the bottom of the cylinder). In other words this is the volume (amount) of air that can be passed through the engine, assuming 100% volumetric efficiency, during one revolution of the crankshaft.
Mass Air Flow
The mass airflow method actually measures the amount of air entering the engine.
