fcd fuel cut defencer

Posted by Author makecircuits

FCD (Fuel Cut Defencer)

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.

What is an FCD?

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 betw1een 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 tw1o 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

The circuit schematic is shown in the following figure:

Theory of Operation

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 tw1o 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 betw1een 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 tw1o 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 tw1o 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.