The EMS needs to know a number of things about the engine's condition in order for the fuelling to be metered correctly.  During normal running these boil down to the engine speed and the throttle or load position.  Generally this information is relayed to the EMS by sensors or triggers on the engine.  The engine speed is determined by either a crank position sensor (which gives crank position from which speed can be derived) or a trigger of some kind in the distributor (if fitted).  Engine load can be determined using a number of different methods.  Engine speed and position is normally monitored by one of the following two methods:

Crank Sensor

Typical Crank Trigger wheel and sensorThis is now the most common method of determining engine speed on a modern engine.  It is comprised of a disk mounted on or machined into the flywheel/front pulley that turns with the engine.  The disk has a certain number of teeth around its circumference and a fixed closely mounted induction sensor that pulses when it encounters a tooth.  There is generally a pattern of missing teeth so that the EMS can tell exactly the crank position as well as speed.  Although the EMS knows the engine's crank position from this sensor, it does not know the engine's cycle position.  In a four-stroke engine the engine cycle involves two complete revolutions of the engine with the piston at TDC twice during the cycle.  One of these times the cylinder is ready to fire; the other time is at the end of the exhaust stroke.  A crank sensor alone can only indicate that the piston is at TDC, it cannot know which of the two cycles positions the engine is at.

Distributor pickup

Some older systems use, and many after-market systems can use a distributor pickup to determine engine speed. The type of distributor used is normally Hall effect, magnetic reluctor or Optronic and has no in-built advance mechanism.  A distributor-based system has the advantage of mechanical awareness of the engine's cycle position as well as the crank position.

Throttle Position Sensor

Typical Throttle Position SensorThis is the most common engine load sensor especially on after market systems.  A TPS is a small potentiometer (or ‘throttle pot’) which is connected directly to the throttle shaft and turns with it.  It returns a value to the EMS depending on the throttle position.  TPS sensors are normally used on performance engines where airflow sensors might become confused because of pulses in the inlet tract.  Because they do not measure airflow but simply give a throttle position, airflow is assumed to be constant for any given engine speed and throttle position.  If the engine is further modified, the airflow characteristics may change and the engine may need re-mapping.  EMS systems that use direct airflow measurement can often cope with changes more effectively and can alter the fuelling to suit without a re-mapping session.

Air metering flap

Another way of determining the engine load is to measure the airflow into the engine and this can be done using a flap which is deflected by incoming air.  This is commonly known as an airflow meter.  These are common on older injection systems, but can be confused by reverse pulses in the inlet tract when more extreme cams are used and can be restrictive to the inlet airflow.

Manifold Air Pressure sensor

Typical MAP SensorThese sensors measure the vacuum or air pressure in the inlet manifold which in turn gives an indication of load.  They are more commonly used on turbocharged engines to give an indication of boost level.  This is often referred to as a MAP sensor, although not to be confused with a 'map'.  They are simple, reliable little sensors and more information can be found on them here -

Hot wire - MAF Sensor

Typical MAF meter and MAF DuctThis approach uses a heated platinum wire and measures the current required to keep it at a particular temperature.  As air passes over the wire it cools it down, so the more air that passes, the greater the cooling effect and therefore the greater the current.  The hot wire system can be also be confused by reverse pulses when more extreme cams are used.