Over the last few weeks we’ve looked at systems that switch on and off Part 1
, and those that use pulsed signals for control
(Part 2 and
Parts 3). This week we’re going to tackle analog voltage signals of the sort
that are produced by most airflow meters and MAP sensors.
Airflow Meters and Map Sensors
An airflow meter measures the volume (vane meters) or mass (hotwire meters)
of air that passes through it. In the case of the vane type, the ECU also takes
an input from a temperature sensor and with these two bits of information, it
can calculate the mass flow (ie grams per second) of air passing into the
engine. The main task of the fuel side of the ECU is to then add the appropriate
amount of fuel to this air to maintain the required air/fuel ratio. Since the
desired outcome is an air to fuel ratio, and the ECU knows from the
airflow meter signal the mass of air, the calculation of the required amount of fuel is easy.
A MAP sensor system does not work in the same way. Unlike a
hotwire airflow meter, which provides all the ECU needs to know in order to
calculate how much fuel to add, the MAP sensor signal on its own is
useless for this purpose! Why? Well manifold vacuum (in a naturally aspirated
car) will drop to zero whenever the throttle is fully opened – whether the revs
are at 1000 or 6000 rpm. Going on just the MAP sensor information would result
in the full-throttle mixtures being right at 6000 rpm – and hugely over-rich at
1000 rpm.
To calculate the required addition of fuel, the ECU must have internal
logic that also takes into account rpm. The ECU needs to know both the MAP value
and the RPM before it can calculate the required fuel addition.
That’s why it’s not a simple proposition to swap an airflow meter for a MAP
sensor without either also electronically adding an RPM input to the MAP sensor
signal or getting into the internal software of the ECU to make changes. (Or
fitting a whole new ECU designed to work with MAP and rpm.)
It’s also why intercepting and altering the signals coming from an airflow
meter works much better than doing the same for a MAP sensor.
One Dimensional Interceptor
When you think about an airflow meter signal and realise that it’s all the
ECU needs to have to calculate the required addition of fuel, it becomes obvious
that making changes to this input can be used to alter the air/fuel ratio at all
loads. (We’ll come to closed-loop operation in a moment – at this stage, assume
that the system is working always in open loop, as it does in electronically
injected cars released prior to the intro of cat converters.)
To put it another way, in airflow meter cars, there’s simply no need to use
an approach that maps load versus rpm. This is because to get the addition of
fuel right, it doesn’t matter if the engine revs are high or low. Nor does it
matter what the throttle position is. The only thing that does matter is the
amount of air entering the engine. (Sure, on throttle transients – like quickly
opening the throttle – more fuel will be needed. But the ECU has that logic
already built in.) So making changes to the airflow meter output allows the
revision of the air/fuel ratio right through the engine load range.
Since the voltage output of most airflow meters rises with airflow, we could
add a certain voltage all the way through if we wanted the air/fuel ratio to be
richer, or subtract a voltage all the way through if we wanted the air/fuel
ratio to be leaner. However, normally the change in air/fuel ratio is wanted
only at certain loads – like leaning-out the full-power air/fuel ratio, for
example. This is easily achieved because the signal from the airflow meter
already shows load – high voltages show high loads, and low voltages show low
loads.
The Silicon Chip Digital Fuel Adjuster (DFA) kit makes use of the
ideas that we’ve been discussing. It intercepts the signal coming from the
airflow meter and can makes changes to it before sending it onto the ECU. The
DFA splits up the voltage range coming from the airflow meter into 128 ‘load
sites’. So for example, if the airflow meter output peaks at 5 volts at full
power, the 0-5V range is split into 128 increments of 0.04 volts each. The
voltage at each of these load sites can be adjusted up or down; when no changes
are made, the voltage remains as it was coming out of the airflow
meter.
This is the actual map of DFA adjustments to the airflow meter voltages that
were made on a 1985 BMW 735i. The vane airflow meter spring had been tightened,
resulting in mixtures that were a bit lean. This explains why nearly all the
corrections increased the voltage coming from the airflow meter. In fact, it was
only at load sites around 63 that no changes were made – at this load, the
air/fuel ratio was as desired.
The BMW runs in open loop all the time – there’s no oxygen sensor. However,
in cars where an oxygen sensor is used, the system will learn its way around
airflow meter signal changes if these take mixtures away from standard. In these
cars (and they’re by far the majority) the modification to the air/fuel ratios
can be made only at loads high enough that the car is working out of closed loop
– ie it’s ignoring the oxy sensor.
(However, the DFA is often used to make changes at loads where the car is in
closed loop. Confused? We’ll come back to this in a moment.)
This DFA map shows the changes made to the high-load mixtures on a 1998 Lexus
LS400. (This particular map was done with an early DFA with only 64 load sites.)
As can be seen, the top-end airflow meter output voltages were reduced, so
leaning-out the otherwise very rich mixtures.
As we’ve indicated, a MAP sensor is a totally different type of animal on
which to run a one-dimensional interceptor. However, there is one application
where intercepting it will still work well. That’s in a turbo or supercharged
car, with the interception of its signal done just for the load sites when the
car’s on boost. In this situation, the MAP sensor output is much more
proportional to load, and so at high loads (ie on boost) some tweaking of the
mixtures can be carried out with the DFA.
Airflow Meter Swaps
Airflow meter swaps are very popular – but we think that they’re a helluva
lot of work when the same thing can be achieved much more easily and at a lower
cost.
Usually, an airflow meter is changed for a larger one when (a) its flow
performance isn’t good enough and so it’s starting to cause a restriction,
and/or (b) the airflow meter output voltage reaches a ceiling well before the
car reaches maximum power.
When a larger meter is fitted, the voltage output for a given airflow is
lower than it was previously. Therefore, except perhaps at the very top end, the voltage
will need to be lifted throughout the load range. The DFA can do this with
ease.
Note that the signal needs to be lifted in closed loop operation as well as
open loop. If the signal isn’t lifted in closed loop, the car will run badly
because the ECU won’t have enough capability to learn around the lean mixtures
that will be occurring with the lower airflow meter voltage signal. But with the
DFA lifting these levels, the car will happily run in closed loop. So you can
see, it’s important that the interceptor has the resolution to make changes
right across the load range – in closed loop as well as open loop.
But what was that about swapping airflow meters being a case of making things
hard for yourself? Well, using an airflow meter bypass can achieve exactly the
same outcome at a much lower cost and difficulty. See
Airflow Meter Bypass, Part 1
for Part 1 of a 2-part series on doing this, using the DFA to adjust the
mixtures across the whole load range.
Injector Swaps
Injectors are upgraded to those with greater flow when they’re reaching 100
per cent duty cycle (ie they’re fully open all the time) and the mixtures are
still leaner than required. When larger injectors are fitted, the air/fuel ratio
will be too rich (except possibly at max load). Again, working with the airflow
meter, the DFA can be used to correct the mixtures across the full load range by
decreasing the output signal.
Unexpected Changes
When an interceptor is used on the airflow meter signal, the ECU is no longer
correctly measuring load. For example, if you reduce the voltage signal coming
from the airflow meter at high loads, the ECU thinks that the load is lower than
it actually is. The result is that all functions controlled by the ECU on the
basis of load will be altered slightly.
One important example of this is the ignition timing. If the ECU thinks the
load is less than it really is, the ECU will run more advanced timing that it
otherwise would. If you dramatically lean-out the high load mixtures, this could
result in detonation. For this reason, when using an interceptor on the load
input, you should always check for other outcomes – eg listening for
detonation.
Conclusion
There are some important points in this article – things we often see people
misunderstanding.
- Airflow meters and MAP sensors measure completely different things – their
signals are not interchangeable
- Intercepting of just the airflow meter signal can give extremely good
results
- Intercepting of just the MAP sensor signal is more problematic
- Airflow meter swaps are lots of work and cost – it’s cheaper and easier to
use a bypass
- Either airflow meter swaps or an airflow meter bypass can be catered for by
the Digital Fuel Adjuster
- Injector swaps can be catered for by the Digital Fuel Adjuster
- Load interceptors will cause other parameters (eg ignition timing) to
change
- It’s important that the interceptor works across the whole engine load range,
including in closed loop
Whew!
Digital Fuel Adjuster
Hand Controller
High Performance
Electronics for Cars
Next week we’ll look at pull-up and pull-down resistors, knowledge that’s
very important when intercepting digital signals.
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