Last week in
The Digital Fuel Adjuster, Part 1 we covered the basics of the Digital Fuel Adjuster. A
kit designed and developed by Silicon
Chip
magazine, it intercepts the airflow meter
output voltage and allows the adjustment of air/fuel ratios at all loads. It is
controlled by a digital Hand Controller and both real time and non-real-time
mapping modes are available. In a typical application there will be about 80
load sites between idle and full load able to be modified in air/fuel ratio.
Where To Buy The Kits
The kit costs just AUD$79.95 while you can also buy the fully built and tested
version from the AutoSpeed shop for just AUD$299 Please Note: The Handcontroller - Basic and Pre-assembled are also available here AutoSpeed Shop
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As mentioned, the Hand Controller has three modes – RUN (real time tuning),
VIEW (non-real-time tuning) and LOCK (no tuning but map viewing available).
However, before getting to the tuning stage, the Digital Fuel Adjuster (DFA)
needs to be configured for your particular application. These steps are covered
in detail in the kit instructions, so the following is just an overview so that
you have an idea of what’s involved.
Bench Set-Up
The DFA can work with 0-1, 0-5 and 0-12V signals. Most airflow meters have a
0-5V signal output (although some stretch to 0-5.5V). Setting the DFA’s input
range involves applying a known maximum signal voltage, making some measurements
with a multimeter (at the input, output and test point) and then adjusting some
pots. It’s a 10 minute job at max. (The input voltage is provided by wiring a
pot across the supply voltage – the pot is included in the kit).
To find out what input range you need, measure the output of the airflow
meter across all loads. This is easily done if you mount the multimeter in the
cabin and then fang the beegeezus out of the car. A peak-hold multimeter makes
it even easier – otherwise, you should use an assistant in the car to watch the
readings.
The DFA can be configured to work in either fine or coarse tuning modes. Once
set in a particular mode, all tuning is carried out in that mode. Coarse mode
allows a plus/minus 50 per cent adjustment to the airflow meter output signal
voltage – which is an enormous amount. Using Coarse mode means that just small up/down adjustments on the
Hand Controller will make noticeable changes to the mixtures.
Fine mode reduces the strength of the up/down changes by a factor of 5. This
means that a tuning input that in Coarse mode required a -5 adjustment will need
a -25 adjustment in fine mode. Even Fine mode has enough power to do big airflow
meter swaps, etc, but because of the range of numbers available, Fine mode
tuning takes longer.
Fine and Coarse modes are selected by a moveable link on the circuit
board.
Testing of prototype DFAs found that if the DFA is intercepting on start-up,
in some cars the Check Engine light is triggered. Apparently in these cars the
ECU is watching the resistance of the airflow meter to check on wiring
integrity, etc, and when it sees the DFA in the way, it doesn’t like it.
To overcome this, the DFA can be configured so that it switches in the
interception only after the car has started.
The DFA works this out by watching battery voltage. If you set the DFA so
that it will click in only at 13.8V battery voltage, the car will start without
the DFA operating and then once the engine is running (and the so battery
voltage has risen) the changeover will occur. This changeover is very quick –
there aren’t any glitches in how the car runs when this occurs.
The changeover voltage can be set with a pot – if you set it to 11V the DFA
will start intercepting as soon as you turn on the ignition, but if you set it
to (say) 13.8V the DFA will start to intercept only when the car is running. A
red LED on the board comes on when the DFA is actually intercepting.
Installation
Installing the DFA in a car is a doddle – all that you need to do is access
the airflow meter output signal wire, 12V and ground. Normally, these
connections should all be made at the ECU, however, during prototype testing
these connections were often made at the airflow meter, as shown in the
following box. (If making the connections at the airflow meter, you should use
twin-core shielded cable with the shield
[ie the braid]
earthed at the ECU.)
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Access the airflow meter wiring at the back of the plug.
Earth the negative lead of the multimeter and with the meter set to read
Volts DC, backprobe the airflow meter pug until you find a connection that has a
varying voltage output that varies with load (eg when the engine is
free-revved).
Cut this wire and bare the ends of insulation.
Tin the bare copper with solder.
Solder the INPUT wire from the DFA to the wire leading to the airflow meter. Solder the OUTPUT wire from the DFA to the wire that leads off to the ECU.
Insulate the connections. Connect ignition-switched power and earth to the DPA.
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Testing
So, the DFA is installed in the car. What next?
Use a pointy tool to press the RESET button on the Hand Controller for at
least 4 seconds – RESET will flash on the screen. Then start the car, making
sure after you have done so that the DFA has actually switched-in its
interception (ie red LED on board has come on). The car should start and drive
just as it did before fitting the DFA. If it doesn’t, check with a multimeter
that the input and output voltages are the same (if not, redo the bench
calibration), check your wiring connections to the car, and check that the DFA
is switching-in its interception appropriately.
If everything is working as it should, press the VIEW/RUN mode button to get
into RUN mode. An INPUT number should appear on the screen – eg an input of 35.
This is the Load Site number that the DFA is currently on. By blipping the
throttle, you should be able to change the Load Site.
While in RUN mode and holding a constant Load Site (eg at idle), press the
white ‘Up’ key. The upwards adjustment that you are making to the mixtures will
show on the OUTPUT line of the LCD, and as you keep pressing ‘Up’ the engine
should start to stagger as it runs over-rich. (If you are working in Fine mode,
use the higher speed black ‘Up’ key.) Now do the same the other way – making the
mixtures leaner by pressing the ‘Down’ button. Again the engine should start to
stagger when it runs too lean.
Easy to use, huh?
Tuning
The tuning process that you follow depends on what you’re trying to achieve.
For example, if you’ve fitted a bigger airflow meter, you’ll need to firstly
disconnect the oxygen sensor (unless doing so puts the car in limp-home mode) and then tune across the full load range. On the
other hand, if you’re just leaning out the top-end mixtures a little, you’ll be
tuning only at high load sites which usually aren’t in closed loop. (See ‘Closed
Loop?’ breakout below.)
This graph shows the tuning changes made to the high load (ie out of closed
loop) mixtures on a 1998 Lexus LS400. The tuning was being carried out in Coarse
Mode and used a prototype DFA which had 64 load points rather than the current
unit’s 128 load points. Air/fuel ratio monitoring was by an Autronic
professional air/fuel ratio meter. The mixtures were altered from being in the
high Tens / low Elevens to about 12.5:1 AFR at full load. Performance improved
by about two-tenths of a second to 100 km/h. Driveability was perfect.
When tuning, you should always move in small increments, getting a ‘feel’ for
how much adjustment results in what alteration to the air/fuel ratio. Making random changes will almost certainly
result in a blown engine! However, having said that, if you are careful, it’s possible to make major tuning
changes on the road, using only the output of a standard heated oxygen sensor
placed in the tail-pipe. That’s just what was done with a DFA in Real World Air/Fuel Ratio Tuning.
Conclusion
The DFA allows the alteration of the air/fuel ratio across the full range of
loads in the engine’s operating range. This allows the running of larger
injectors or a larger airflow meter. When making less radical tuning changes,
the air/fuel ratio can be altered at high loads. Tuning is easy and effective
and driveability is seamlessly excellent.
Next week – we make a radical change
to the way in which an airflow meter works and then use the DFA to achieve
perfect mixtures. Airflow intake restriction was halved....
Closed Loop
In ‘closed loop’ operation the ECU constantly watches the output of the
oxygen sensor, modifying the injector flow to keep the air/fuel ratios around
14.7:1. (This is the ratio at which the cat converter works best.) Closed loop
operation in most cars occurs at idle, light loads and cruise. At full load most
cars drop out of closed loop, enrichening the air/fuel ratios and setting them
on the basis of information mapped in the ECU. When the ECU isn’t using the
oxygen sensor to help determine air/fuel ratios, the car is said to be in ‘open
loop’.
Tuning changes made with the DFA at loads where the car is in closed loop
will usually be learned around by the ECU, unless those changes are so great
that the ECU reaches the maximum amount it can learn. This means that when
modifying the air/fuel ratio, it is usual to make these changes only at loads
where the system is in open loop.
The loads at which the ECU changes from closed to open loop can be easily
assessed using a Mixture Meter (see
Cheaply Monitoring Air/Fuel Ratios)
or any form of air/fuel ratio monitoring. The Closed Loop Monitor
(see The Closed Loop Monitor)
can also be used.
But if you’ve made a really major change to the system (eg upsized injectors
or the airflow meter) you will need to DFA-tune the mixtures right through the
load range. You won’t be able to alter them away from 14.7:1 in closed loop, but
without the DFA helping, the ECU wouldn’t have been able to learn enough to work
with the large injectors and airflow meter, anyway.
In this sort of situation it’s usual to disconnect the oxy signal input
(unless that causes the car to enter limp-mode) and do all the tuning with the
DFA, plugging the oxy sensor back in only when you’re finished. In this way, the
oxy sensor with do the ‘fine tuning’ for you.
So what air/fuel ratios do you want to run at different loads? See – Tuning
Air/Fuel Ratios
Tuning Air/Fuel Ratios
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