For good tuning results we need a constant baseline of Temperature,
Engine revolutions and load for each part of the map. This will result in a certain
power and economy value.
Before we can start tuning we need to understand the role of temperature
on the AFR. Warm air is less dense and therefore has a lower mass per volume
than cold air.
If you start tuning a engine at say 50 degrees Celsius at 2000rpm the engine will
draw in ”x” cubic meter of air every minute.
After running for 10 minutes that same engine now has a water temperature of
90 degrees Celsius and still draws in the same “x” cubic meter per minute at
2000rpm , however the mass of the air will be much lower and hence the AFR HAS CHANGED. Therefor start tuning with the engine at normal working temperature and maintain it by using a good cooling fan on the Dynometer.
Our problem with temperatures when tuning is how the original ECU changes the
timing or fuel when the engine gets hotter than normal. Most ECUs will make
the engine richer, the timing slower or both when the water
temperature goes above a certain point. Whenever we map an engine it is very
important that we keep the water temperature in the normal range. Otherwise we
will struggle to obtain the correct AFR .
In closed loop systems on modern engines the ECU triggers the injectors.
The lambda sensor measure the oxygen content of the exhaust gas and sends a signal
BACK TO the ECU. The ECU now modifies the injector pulse width and or timing
to get the desired CO value. Therefor a closed loop system is any system that
measures and corrects itself.
Under normal conditions you should. Never change the mixtures on a engine with lambda sensor where the ECU is in close loop. On engines
with lambda sensors you should normally only change the mixtures for the full
load maps. The part-load timing can be adjusted on these engines without any
problem. Some modern engines are now equipped with wide-band lambda sensors.
these sensors are also accurate at mixtures richer and leaner than lambda-1. They
control the mixtures under all load conditions. They typically have a voltage output between 0 and 5 volts to the sensor.