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There has been a great deal of
interest on the Team FC3S list in a cheaper (and if
possible, superior) alternative to the expensive FCD's
offered commercially. This is understandable given that
no modification to the Turbo II that raises the possible
boost pressures can be attempted without an FCD. This
article presents a simple FCD circuit that can be built
by anyone with moderate electronics assembly expertise.
In addition, the article covers the reasons why an FCD
is needed, why this FCD is superior to other FCD
designs, and the theory of operation. By the time you
are done reading this article, you will hopefully
understand a little more about how your TII works, and a
little electronics as well.
The FCD (acronym for Fuel Cut Defencer)
is any device that prevents the stock ECU from cutting
fuel to the rear rotor when the boost exceeds a preset
boost level. This maximum safe boost level and behavior
is coded into the firm ware in the engine management
unit and can not be changed without reprogramming the
ECU. The maximum allowed boost pressure is about 8.6 psi.
The way an FCD defeats the fuel cut is by lying to the
ECU about what the boost pressure is. The FCD is placed
between the boost sensor and the ECU where it modifies
the boost pressure signal by some amount so that as the
boost pressure rises above the preset "safe limit", the
ECU continues to see a signal that is below the limit.
There are a couple of immediate
consequences to this fooling around. As the boost rises,
the ECU must increase the amount of fuel being delivered
to the engine in order to maintain safe and efficient
operation. As the FCD starts lying to the ECU, the ECU
will begin to under-compensate for the rise in pressure
leading to a gradual leaning of the air-fuel mixture.
The amount of error increases as the boost rises. For
relatively small errors, the only penalty is efficiency.
As the error gets larger, however, detonation becomes
likely, exacerbated by the high boost pressures and
accompanying high intake charge temperatures. Detonation
under these conditions will quickly kill an engine. So,
before we go any further, be forewarned that using an
FCD and increasing boost pressure without also
compensating for the ECU error with fuel enrichment (and
preferably more efficient intercooling) can cause
serious damage to your engine!
What you would like to have is an FCD
that leaves the boost signal alone until it approaches
the cutoff level, and then kicks in, holding the signal
below the critical level. This can be accomplished with
a circuit element called a clamp. The FCD circuit
described in this article utilizes an active clamp which
performs the necessary function very efficiently.
Following is a graph of the boost sensor
signal without an FCD, with two commercially available
FCDs, and with our cheap little DIY FCD.
As you can see from the graph, FCD #1 is
a clamp circuit. The output follows the input until the
clamp voltage of 3.33 V (about 6 psi) is reached. At
this point the output stops rising. FCD #2 works by
reducing the input by a percentage. This creates an
error across the entire range, as opposed to only when
the boost is over the limit. (At a safe 5.5 psi, the
computer is only seeing 4 psi.) This is not the desired
behavior. Our FCD works similarly to FCD #1, except we
have raised the clamping voltage to 3.65V (about 7.9 psi).
A linear regression ran on the data
gathered shows that the equation for the best fit line
is:
Out(V) = 0.169V/psi*P(psi) + 2.318V
Solving for Pressure, we get:
P(psi) = (Out(V) - 2.318V)/0.169V/psi
NOTE: If you look closely at the graph,
you can see that our FCD has an output that is 0.05 V
below the input up to the clamping voltage. I have since
fixed this problem by changing R1 from 2.2k ohms to 680
ohms. The FCD output is now within 0.02V of the input,
right up to the clamping voltage. Sorry, I didn't have
time to rerun the numbers.
The measurements above were obtained
with the FCD under test connected to the TII boost
sensor and TII wiring harness in order to simulate
actual operating conditions as closely as possible.
Pressures were read on a diagnostic pressure/vacuum
gauge. If you are interested in it, I would be happy to
send you the Excel spreadsheet containing the raw data,
the linear regression, and the graph above.
The circuit schematic is shown in the
following figure:
There are a number of requirements on an
FCD that is going to work well and survive the stresses
in your Rx7. Here are the most important ones:
-
Sharp clamping behavior. This is, of
course, the primary requirement. The FCD output must
follow the input voltage until the clamping voltage is
exceeded, at which point it should clamp the output to
the setpoint.
-
High input impedance. The boost pressure
sensor has a very high output impedance. That is, if you
think of it as a battery that produces a voltage that is
proportional to the input pressure, that battery has a
very large resistor in series with it. This is a common
characteristic of strain gauge based pressure sensors.
If the input impedance of the circuit that it is driving
is low, there will be an error caused by the voltage
drop across the internal resistor. For this reason, our FCD must have a high input impedance.
-
Noise suppression. High impedance
devices make good antennas which can pick up everything
from alternator and ignition noise to your favorite
Rock-'n-Roll station. This calls for some filtering to
eliminate these sources of interference.
-
Input protection. Because our FCD will
live in a hostile environment where stray voltage spikes
above and below ground may appear, we need to provide
some input protection for our device.
-
Thermal stability. Temperatures in the
automotive environment can range from 20 degrees below
zero to 200 degrees Fahrenheit. It is necessary that our
circuit be durable enough to operate correctly given
these temperature extremes.
In our circuit, the clamping is
performed by op-amp U1-2 and D3. These two devices form
what is called an active clamp. While the signal at the
(-) input of the op-amp is below the voltage setting at
the (+) input, the output of the op-amp will be high, D3
will be reverse biased, and the output will follow the
input. When the signal at the (-) input of the op-amp
exceeds the voltage setting at the (+) input, D3
conducts closing the feedback loop and causing the
output to follow the (+) input, which is set at the
clamping voltage. This all happens so quickly that it
might as well be instantaneous. Notice that the output
impedance of the clamp is not zero. When clamping is not
occurring, and the output is following the input, the
signal is passing through R1, which makes the output
impedance 680 ohms. Not to worry, this output impedance
is substantially lower than the boost sensor's output
impedance.
The clamping voltage is adjustable and
is set by the trimmer at R3 which is wired as an
adjustable voltage divider between the Vref lead to the
pressure sensor and ground. Vref is supplied by a
voltage regulator in the ECU and is maintained at 5V .
High input impedance is achieved by
buffering the input to the FCD. U1-1 is wired as a
voltage follower, meaning that the output just follows
the input. The input impedance of the buffer is nearly
infinite.
Noise suppression is achieved via the
use of a low pass filter on the input, formed by R2 and
C1. High frequency roll-off is at about 100 Hz, allowing
the circuit to be responsive but effectively suppressing
RF noise.
Input protection is provided by D1 and
D2 which clamp the inputs to +12V and ground,
effectively protecting the op-amp inputs. Stray voltage
spikes above or below ground will be shorted to the
appropriate supply rail.
Thermal stability is potentially a small
problem for our circuit. The resistance of R3 can vary
as a result of extreme temperature changes. For this
reason, I would recommend installing the FCD inside the
car, alongside the ECU. This article will provide
instructions for installation in both places.
For the active component in our FCD, we
have chosen the LM358 dual op-amp. This IC puts two
single supply op-amps in a single package. It's not a
high precision op-amp, but it serves the FCD circuit
very well. Having two op-amps in the package makes it
possible for us to include a buffer with the active
clamp, which improves the input impedance as mentioned
above.
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