IGNITION
As we’ve mentioned the ECU controls everything an engine does and when it does it. In terms of ignition this means it tells the spark plug exactly when to fire. It takes a certain amount of time for the air/fuel mixture to burn, meaning the spark needs to fire slightly before the piston reaches TDC to ensure the ignition occurs at the correct time.
However when you start to increase the engine load, the time it takes the mixture to burn differs, so the ignition timing needs to be changed to compensate.
Ignition tables
Therefore ignition tables stored within the ECU are required. These tables contain values for the amount of ignition advance needed for a given engine load and engine speed, and there is also a separate table for engine start-up too.The table will have engine speed sites on one axis, and engine load sites on the other axis.The ECU will then use this table to look up the required value. For example if the engine is running at 2500 rpm the ECU will start by looking at this column of the table. It will then take the info on engine load (for argument’s sake let’s say load site 7) and look at that row of the ignition table until it finds the value where the load site meets the speed site.The value in this cell where
the two intersect is the amount of degrees of ignition advanced required for that point. When the engine is at a point between these values the ECU interpolates to ensure a smooth transition from one to the next. For example, if at 1500 rpm the ignition advance was 20 degrees and at 2000 rpm it was 30 degrees, at 1750 rpm the ECU would interpolate between the two values to give an ignition advance of 25 degrees.
Inputs
For the ECU to be able to calculate this it needs to know both the engine speed and engine load. Measuring engine speed is fairly straightforward, and is usually done via a trigger wheel and sensor mounted on the crankshaft.The most common of which is known as a 36/-1 set-up. This consists simply of a metal trigger wheel with 35 teeth on it (would be 36 but one is missing, hence the name) which creates a signal as it passes the sensor. From this the ECU can accurately calculate the engine speed.The missing tooth also provides a reference point so the ECU can also calculate the crankshaft position, too.
Measuring the engine load can be done using a selection, or on some ECUs a combination, of different sensors.The easiest example to explain is a throttle position sensor (TPS), which is commonly used as the load source when mapping naturally-aspirated engines. Say for example there are 16 load sites on the ignition table, load site 0 would be throttle fully closed, and load site 15 would be throttle fully open.
Wasted spark
Most engines running aftermarket management use a wasted spark ignition set-up. On a wasted spark set-up the spark plugs fire in pairs, on both the compression and exhaust strokes. Firing the spark plug on the exhaust stroke has no effect and thus the spark is wasted, hence the name.The main benefit of running wasted spark is that you don’t need to use a distributor, and can therefore eliminate any potential problems associated with distributor caps, rotor arms and the like.