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HOW THE SYSTEM WORKS The car's computer is expecting to see an oscillating signal from the oxygen sensor which goes from zero volts to plus one volt approximately. The fuel flow is adjusted to maintain the average voltage at close to 0.5 volts The signal from the sensor isn't a square wave, but more like a smooth triangular wave form. The computer doesn't care about the exact shape but simply tries to maintain the average. The Electronic Mixture Controller is installed in the system between the oxygen sensor and the car's computer. What this device does is convert the wave form into a square wave, but more importantly it sets up a threshold voltage that is lower than 0.5 volts. When the sensor output is above the threshold, which is set quite low, say 0.1 volts, then the device will send a high signal to the computer. When the sensor signal drops below the threshold the device signal out will be low. The computer adjusts the fuel flow accordingly and now is actually maintaining the average voltage from the sensor at 0.1 Volts (100mV) instead of 0.5 volts. (500mV) From the Sensor Output graph you can see that the mixture
is now slightly leaner than it was. The operating range is shifted to the
right.
If your oxygen sensor is old and sluggish this device
will also improve the reaction time. Because it instantly tells the computer
when the sensor output is below or above the threshold, there is less overshoot.
Smaller, quicker corrections to the mixture occur rather than long slow
corrections.
CIRCUIT DESCRIPTION The heart of the circuit is the LM3914 linear LED
dot/bar Driver IC, which we operate in bar mode. This is the same IC as
is in the Mixture Display circuit. We set the sensitivity to 500mV full
scale for this controller.
Electronic Mixture Controller Circuit Diagram
Handle it carefully and always use an IC socket to mount it. Don't directly solder the IC into the printed circuit board. Install this component last. We use this IC to sample the sensor voltage and provide outputs at various thresholds that we can select from. The trim pot R1 sets the sensitivity and we adjust this for 500 mV full scale. Each LED output then is 50 mV apart. We don't actually install LED's on each output, and any unused outputs are left open circuit. The front panel rotary switch selects which ever output we choose. We only need 2 or 3 to choose from. You can leave out this rotary switch and simply select one of the outputs to connect if you prefer. The front panel on/off switch is a DPDT toggle switch. All the capacitors are electrolytic type of about 16 volt rating. All resistors are 1/4 watt. The input resistor/capacitor circuit provides filtering of the sensor signal. Because the entire circuit comprises high impedance components, including the sensor and IC input, the input line is susceptible to induced noise. Ignition noise in particular will affect the circuit and cause incorrect operation. If you install the LED Mixture Display as recommended, you will see that until the sensor heats up all the LED's will be dimly lit. This is showing that there is a lot of noise on the line. When the sensor heats up, the signal becomes cleaner and then only the appropriate LED will be lit. We also include a delay circuit so that after start up, the output is held low for a few minutes to simulate a cold sensor. The sensor must be operating correctly before we send signals to the computer. The most common problem, if we don't have this delay, is that the output will be high simply from the noise on the signal line. The computer will think the sensor is working, because it is high, and will cut back the fuel to make the signal go low. When this happens we end up with a very lean condition and very poor acceleration. The front panel switch is very important. It doesn't switch the power to the device. What it does is allow the sensor signal to bypass the device altogether. This is an essential feature. You can switch your vehicle back to it's unmodified state instantly if you suspect that there may be a problem with the device or if the vehicle simply isn't performing as it should. Remember, only you know what you have done to your car and other family members that drive the car may not be able to fix any problems that may arise. Just show them the switch. The front panel LED is not just to show that the device is operating, but forms a simple voltage regulator for the output signal to the computer. In operation the LED is lit when the output is high. So the correct state for the LED to be in is flashing. BEFORE BEGINNING This is a simple test you should perform first. The oxygen sensor earth connection is the exhaust system, which is firmly bolted to the engine. The computer earth is the vehicle body. We have seen that 0.5 volts can make a large difference to the mixture. If the engine is not well and truelly earthed to the body then a voltage difference can exist between the two, and 0.5 volts would normally go unnoticed. We can't afford to have that sort of voltage difference when trying to accurately control the mixture. Start the engine, switch the headlights on high beam, then measure the voltage between the engine and the body. Use an accurate digital volt meter. Any more than 50 mV will mean you have a bad earth connection which will need cleaning and tightening. Modern cars usually have more than one connection so look around. If you have trouble achieving this then use an engine earth connection for you circuit rather than a body connection. What is most important is the signal voltage from the sensor, since we are operating at such low voltages. PARTS LIST
CONSTRUCTION Read this through completely before beginning. All the parts needed should be available from your local Radio Shack store. They will also be able to show you the component orientation and which legs are which etc. You will require a soldering iron, a 12 volt power supply such as a small power pack and an accurate digital volt meter for this project. No other test equipment will be needed. The 12 volt supply should be well filtered. You want proper DC, not simple rectified AC, which contains too much ripple. Lastly you will require a variable voltage source that can go from 0 to 1 volt to simulate a sensor input. It's simple enough to make this using a resistor and a variable resistor. The transistors are nothing special, just general purpose devices so it should be OK to substitute where necessary. The darlington transistor (MPSA14) is a special high gain device needed for the delay circuit. Again it is just a general purpose darlington transistor. The printed circuit board can be any general experimenters board approximately 2 x 3 inches. Try to plan ahead and think where you are going to mount the device, either behind the dash or in a small case mounted somewhere. The printed circuit board has to fit and after the components are mounted it will be more difficult to fit in a tight location. Start with the IC socket, and mount it slightly in from
one end. The circuit diagram can give an indication of the general layout
of the components. It makes it easier to follow the circuit if the components
are in the same position as on the diagram.
The IC legs are numbered 1 to 9, left to right across the bottom as seen on the diagram, and 10 to 18, right to left across the top. The notch shows the left end, this is standard for all IC's. Try to plan the component positions so that you require the least amount of additional wire to make all the connections on the board. Don't connect the wires to the front panel rotary switch just yet, except for one which connects to pin 10 on the IC. This is the full scale output and will be connected to the rotary switch in the position of FULL RICH, whichever you prefer, fully clockwise or anti-clockwise position. You are going to test you device first on the bench, then decide which outputs you will use for the other switch positions. Don't install the delay capacitor C3 yet. Don't
install the IC yet.
With the IC not installed the output transistor should be off, and the output LED off. The darlington transistor should be off because the capacitor is not installed. ADJUSTING ON THE BENCH Disconnect the power before installing the IC.
Set up the test voltage source to 0.5 volts and apply
to the input.
Now adjust the sensitivity control trimpot VR1 so that
the output LED is just lit. Leave the trimpot alone and now adjust the
test voltage lower then higher to test the adjustment. The LED should come
on at 0.5 volts, and go off just below 0.5 volts.
All the outputs should be about 50mV apart in their threshold points. With the output high, (LED lit) adjust now the output voltage to the computer by adjusting the trimpot VR2. You want to set the output to 1.0 volts. Adjust the test voltage to below the threshold to turn off the LED. The output voltage should be zero volts. If all the above happens as it should then your circuit is working correctly. Next install the delay capacitor C3. Set the test voltage above 0.5 volts and turn the power on. It should take about 30 - 120 seconds before the LED comes on. You can adjust the delay by changing the value of the 3.9M timing resistor and/or 2.2uF capacitor. If you find the oxygen sensor heats up quickly then set the timer to a lesser value. Having too long a delay is bad, since the computer could be adding extra fuel to try and make the mixture rich. The next task is to select which other outputs you want
to use, and connect these to the front panel rotary switch. We recommend
you use 100mV or 150mV as your lowest output, depending on what other high
mileage devices you use.
Thoroughly test the device on the bench to be certain
it functions as it should.
TESTING IN THE CAR You can now test the device in the car. Don't install
it yet though. Lift the hood and locate the oxygen sensor. Don't cut the
sensor wire. Find a convenient place along the wire where you can strip
back some of the insulation. You are going to cut it here later, but not
yet. Connect this point to the input of your mixture controller and attach
the power leads to the battery.
Now comes the exciting bit, cutting the oxygen sensor
wire and inserting the controller. Cut the wire in a convenient place.
You are going to use crimp connectors to finish the installation. Use a
matching set on the wire you just cut, in case you need to reconnect it
back together.
INSTALL THE CONTROLLER Fit the controller to the vehicle and finish hooking up
the wiring. For the 12 volt supply find a connection which is switched
with the vehicle ignition. You don't want to have to turn it off every
time you stop.
IMPORTANT Only connect the display input to the raw sensor output, not the controller output. The display is independant of the controller, and is not switched off when the controller is switched off. We can at all times see on the display what the sensor is putting out. The controller doesn't directly change the sensor output, it fools the computer into cutting back the fuel. It is up to you to decide which setting you will use for normal driving. If you have not installed any other high mileage device or water injection then you should be conservative in your adjustment. We have installed water injection only and are driving on a setting around 240mV. We believe it is close to ideal at this setting. Mileage Gain Since installing this device and the steam injection our mileage has improved approximately 18%. This is in a vehicle that has always been serviced regularly and has driven over 150000 miles.(250000 kilometers) Good luck with your project and safe motoring.
Coming soon will be sections on alternative fuel, biodiesel, hydrogen, alcohol etc.
Free Electronic Water Injection
Controller Plans.
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reproduced or sold.
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We
have cut the fuel quantity by no more than few percent, perhaps 5 %. This
by itself will produce some mileage improvement but not a lot. The greatest
benefit occurs when applying the device in support of some other high mileage
system. Especially cold vapor systems and water injection. The computer
will normally fight these systems to compensate for the added exhaust oxygen.
This device fools the computer and enables the maximum possible mileage
improvements.
