Compensation Tables
While the base injection and ignition tables provide a good starting point for the ECU they are only ever going to be accurate for one precise set of conditions. Changes in temperature, humidity, and air pressure will all affect how much air and fuel is needed, and when it needs igniting.To cater for this an ECU will also feature many compensation tables, which will tweak the values on the base injection and ignition tables to compensate for changes in conditions.
Cold start — temperature sensors
The simplest of which is the cold start function. When the engine is cold more fuel is needed, so the ECU requires an input from a coolant temperature sensor to know how warm the engine is.The ECU will initially refer to the base injection map, but will then look at the cold start table and trim or increase the fuelling as necessary. This is usually done on a percentage basis. As an example let’s say the base value is 10milliseconds, and the cold start table says that value needs to be increased by 10%, the ECU will actually open the injector for 11 milliseconds therefore delivering more fuel.
Lambda sensors
Many engines now run lambda control too, and nearly all aftermarket ECUs have the ability to run this. An oxygen (or lambda) sensor is mounted within the exhaust system and monitors the air/ fuel ratios. Narrowband sensors only measure around lambda 1 (14.7 AFR) and are not really much use on tuned engines. Wideband lambda sensors however measure AFRs over a greater range, and can therefore be used to aid engine tuning.
The ECU will once again look at the base injection map first, but will then look at the lambda reading. As with cold start it works on a percentage basis, and depending on the AFR reading the ECU will then increase or decrease the injector duration, controlling the fuelling so the target AFRs are met.
Air density compensation
As air temperature and density will affect an engine’s performance this is something else the ECU needs to monitor and compensate for. Therefore an air temperature sensor will be mounted somewhere in the intake system, and allows the ECU to compensate both the fuelling and ignition dependant upon air density. Again it works in exactly the same way as other compensation tables by referring to the base figure and then tweaking the fuelling and ignition depending on the compensation value.
“CHANGES IN TEMPERATURE, HUMIDITY AND AIR PRESSURE WILL AFFECT HOW MUCH AIR AND FUEL IS NEEDED”