Like it or not, engines need to
operate over a range of temperatures, both ambient and the
temperature of the engine itself. The engine has to be above
the ambient temperature because it generates heat, and the
only way to get rid of the heat is to have it above the
ambient temperature. The higher the temperature, the faster
the heat rejection by Newton’s Law of Cooling, so most engines
are designed from the factory to run a bit under the boiling
point of water. However we want the air temperature going into the
engine to be as cold as possible to maximise its density,
subject to other considerations such as fuel condensing out of
suspension and so on. Fuel can also change in temperature, although that’s
discussed in the injector model video, not in this one. Because of all these factors, what ECUs have
traditionally done is to have a set of coolant temperature
based fuel trims and air temperature based fuel trims. Often
these tend to interact and produce unexpected results. So this
article describes the different ways in which the Modular ECU
can handle changes in temperature. The first way is our preferred method which is
called Charge Temperature Estimation. This is selected in the
main tuning modes page. Rather than having separate
corrections for air temperature and coolant temperature, the
ECU estimates the charge temperature and uses this single
variable instead to calculate the air density. The charge temperature is estimated using the heat
soak percentage map, which can be found under fuel tuning
-> temperature corrections. This percentage relates to how severe the heat soak
effect is. Normally the intake manifold will heat up to the
engine’s operating temperature, so the temperature of the air
going into the engine will be some where in between the
temperature of the incoming air and the coolant temperature. A
value of 100% in this map means that the ECU only looks at the
coolant temperature – so this would be an extreme case where
the temperature of the incoming air, measured by the air
temperature sensor, is irrelevant because the heat soak effect
is so strong that the air is at engine temperature when it
goes into the cylinder. A value of 0% would mean that there’s
no heatsoak at all, and that the air temperature sensor gives
you the actual temperature of the air going into the cylinder.
The closest case to this I can think of would be a naturally
aspirated, individual throttle engine with an extremely short
inlet tract and no plenum, but even that would have some heat
soak effect. In practice the heat soak effect is greater at low
engine speeds, and at low manifold pressures. So at idle it
might be as high as 70%, but at higher engine speeds on boost
it might be as low as 20%. These are the starting points that
I use and then adjust them manually if I notice a temperature
compensation problem. If the mixture gets richer as the air temperature
increases but coolant temperature remains the same, that means
that the heat soak value is too low and the ECU needs to
weight the value from the air temperature sensor more heavily.
If the mixture gets leaner as the air temperature increases
but coolant temperature remains the same, then that means the
ECU is giving too much weight to the air temperature sensor
and the heat soak percentage should be reduced. Because the charge temperature is used as part of
the air mass calculation, there should be no need for any
additional correction, and therefore the charge temperature
fuel trim table should be left at zero. However if you do want
to many any changes manually, this is where it is done. You
can trim the value differently at different loads as well if
you only want to make the difference on vacuum or on boost. If you want to do the temperature trims manually
instead, then you can disable the manifold heat soak system in
the tuning modes setup page. Then the ECU will use the air temperature as the
variable in the air mass calculation, and any trims that you
want to apply on top of this must be placed in the separate
air temp and coolant temp trim tables. Setting up engines so they behave correctly over a
wide range of temperatures can be fiddly if you have to do it
all manually, but we find that with the charge temperature
model, the theory actually gets the fuel calculation pretty
close and very little adjustment is required on top of this. Thank you very much!