Modern engine management systems (EMS) do a fine job of ensuring that engines run cleanly and efficiently in a wide variety of conditions. They are for the most part reliable and require little or no maintenance. However they seem from the outside to be fearsomely complicated systems which defy all attempts at understanding. Amidst all this apparent hokum it is easy to lose sight of the two basic functions performed by an EMS.
To meter fuel to the engine in the right quantity
To provide a spark at the right time
What is an engine management system?
An EMS is a self-contained custom-built computer which controls the running of an engine by monitoring the engine speed, load and temperatures and providing the ignition spark at the right time for the prevailing conditions and metering the fuel to the engine in the exact quantity required.
There are two discrete subsystems in operation within the EMS - the fuel or injection system and the ignition system. It is possible to run an engine management system which just provides one of these subsystems, for example just the ignition system. It is much more common to use the mapped ignition within an EMS in isolation than it is to use just the injection.
What is a ‘map’ ?
Most of us have heard the term ‘mapped ignition’ and programmed or mapped injection, but may not understand what this actually is. While the engine is running, its requirements for fuel and ignition timing will vary according to certain engine conditions, the main two being engine speed and engine load. A ‘map’ is no more than a lookup table by various engine parameters – for example, engine speed and load, which gives the appropriate fuel or timing setting for each possible speed and load condition. There will normally be a map for the injector timings (fuel map) and a separate map for the ignition timing settings (ignition map) within the EMS.
Each map has entries for a predetermined range of engine speeds (called speed sites) and a predetermined range of engine load conditions (called load sites) which generally indicate how far open the throttle is. The EMS knows the engine speed (derived from a crank sensor or distributor pickup) and the engine load (from the Throttle Position Sensor, MAP or MAF sensor) and will use these two values to ‘look-up’ the appropriate fuel and timing settings in each map.
If the current engine telemetry falls between the sites in the map then the value is interpolated between the nearest two sites. Normally there will be speed sites every 500 or so rpm and 8-16 load sites between closed and open throttle. In the example below speed sites are spaced every 500rpm and the 16 load sites are numbered 0 to 15.
In this example the engine load increases as the load site numbers in the left column increase. If the engine were running at 4000rpm - load site 7, then the value looked up would be 26, i.e. 26 degrees of advance. If the engine were running at 3750rpm - load site 7 then the EMS would interpolate between the value for 3500rpm (24) and the value for 4000rpm (26) and calculate a value of 25 degrees.
Note how ignition advance falls as load increases. This is because cylinder filling is much better when load increases and therefore the mixture burns faster, requiring less advance.