Because the VID was designed to expect a certain type of measurable injector pulse width, it is therefore assured to work with so-called "saturate and hold" type fuel injectors. Further investigation is needed before I can say for sure how the device will function with "peak and hold" type fuel injectors. See NOTES.
The other important constant needed by the VID is the number of counts the Vehicle Speed Sensor (VSS) sends for each mile the vehicle is driven. In my case, the number turned out to be about 4000 counts/mile. However, once the VID is installed, the user can easily determine the correct number for his/her type of vehicle by simply driving it for one measured mile and using the host PC software to note the number of actual counts the device received.
The picture above shows the calibration dialog box of the VID monitor program. After the program establishes a connection with the VID, the user invokes this dialog box. The user then enters the pulses per mile for distance and cc/minute for fuel, number of cylinders and pushes the corresponding button to convert the values. Then the user pushes send button. The user can then push "get" to confirm the VIM values.
Once the user knows what these two numbers are, it is a simple matter to start the program and connect to the VID. Then, open up the calibration dialog box in the Windows program and enter the three numbers. Pushing the send button transmits the numbers down to the VID where they are stored in flash memory.Once the unit is powered off and on, it should be ready to make use of the new constants.
And that is about all there is to calibrating the VID. For additional precision, the user should now drive about 10 miles and check the amount of error in the distance traveled. He should recompute the pulses per mile count and enter that number. In this fashion, the device will eventually know the exact numbers for this vehicle. If after driving the vehicle for some distance and computing MPG by hand, the user determines that the MPG calculation is off by some percentage, he can then recalibrate the fuel constant accordingly using the empirical data--by adding to, or deducting the percent error to/from from the cc/min constant.
The final version of the device will allow the user to enter these calibration constants using the UP/DOWN/ACTION switches and the BCD display.
NOTES:
Peak and Hold verses Saturated:
Peak and hold type injectors usually are of lower coil resistance than saturated-- around 2 ohms. P&H type injectors require the ECM to first turn on the injector using one current level, and then switch to another current level to maintain the injector on-state. This implies that the energy required to energize the fuel injector coil is higher than that required to keep it turned on. The , the maker is therefore able to reduce power dissipation across the fuel injector while improving performance and dynamic response time by overcoming the inductive reactance of the coil with higher current.
Saturated types usually feature higher coil resistance than P&H. The ECM turns them on for the full duration of the fuel injector pulse. In my case, the coil resistance is about 4 ohms with a series ballast resister of about 7 ohmes resistance. The inductive reactance (X sub L) of the coil, dictates the rise/fall time that can be expected of the fuel injector. My measurements shows that the typical rise time is around 500-1000 microseconds for this particular coil. The only outstanding question I had was exactly when the fuel starts to flow during the transition from fully off to fully on. Currently, the VID currently assumes that some fuel starts to flow once the voltage across coil reaches the 1/3 point. This is the reason the comparator reference voltage is set to 9.1 volts. Of course, the value of the zener diode can be changed very easily. In any case, any error generated would be swamped by other factors--i.e. the user simply adjusting the device until his/her reading agreed with empirical measurements.
Fuel Injection Resources:
http://www.robietherobot.com/Storm/fuelinjectorguide.htm
http://users.erols.com/srweiss/tableifc.htm
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