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Ethanol & Flex Fuel Tuning: Flex Fuel Sensors

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Flex Fuel Sensors

06.17

00:00 - By this point you should have a solid understanding of what ethanol fuel is, and how the ethanol content of the fuel can affect our tuning.
00:08 You should also now understand that the ethanol percentage of the fuel, is critical to ensuring the stability of your tune and the safety of your engine.
00:18 Particularly if you want to develop a flex fuel system, we obviously need some way of telling the ECU what the ethanol content of your fuel currently is.
00:28 Fortunately we have the OE manufacturers to turn to here as they have exactly the same requirements from their flex fuel capable vehicles.
00:38 This has meant that the aftermarket now has ready access to the same fuel composition sensors that are used by mainstream manufacturers such as GM.
00:48 Probably the most common sensor we're seeing in the aftermarket is the Continental fuel composition sensor.
00:54 For convenience these are now sold by many of the aftermarket ECU manufacturers with their own sticker added to the sensor so you can order the sensor at the same time as your ECU.
01:05 The manufacturers often go one step further to make our job easier by supplying the correct calibration data inside the ECU so that the ECU can easily interpret the information from the sensor and all we need to do is select the sensor from a dropdown menu.
01:23 The fuel composition sensor is basically a cylindrical capacitor that comprises two tubes which the fuel flows through as its electrodes.
01:33 The basis of the sensor's operation is that the permittivity of the fuel and hence capacitance changes as the ethanol content varies.
01:43 An AC voltage is applied to the electrodes inside the sensor and as the capacitance increases the oscillation frequency of the system will change, and a microprocessor determines the capacitance from this measured frequency.
01:59 Since the permittivity of the fluid changes with temperature as well as ethanol content, a fuel temperature sensor is also required and hence this must be accounted for in order to ensure accuracy.
02:13 We don't need to do this ourselves though, and the calculation is handled by the sensor internally.
02:19 Once the ethanol content has been determined, this and the fuel temperature are output to the ECU as a digital signal.
02:28 While not strictly necessary for flex fuel operation, fuel temperature affects the density of the fuel, as we saw in the module on fuel characteristics, and hence if your ECU is capable of accounting for this parameter, fuel temperature can be used to maintain more accurate control of the air fuel ratio.
02:47 The output of the sensor is quite unique as both pieces of information are sent via a single square wave digital signal.
02:56 The frequency or number of waves per second, and the pulse width, or how long the signal is in the low state, are used to tell the ECU what's happening with regard to both ethanol content and fuel temp.
03:10 The calibration for the Continental fuel sensor is as follows.
03:14 A frequency of 50 hertz represents an ethanol content of zero.
03:19 While a frequency of 150 hertz represents an ethanol content of 100%.
03:25 At the same time a pulse width of one millisecond represents a fuel temperature of minus 40 degrees centigrade, while a pulse width of five milliseconds represents a temperature of 125 degrees centigrade.
03:39 The connection of the fuel composition sensor is quite straightforward, however there are a couple of traps you can fall into.
03:47 Firstly the sensor is designed to operate with a power supply of nine through to 18 volts, and a common mistake is to use a regulated five volt sensor power supply which isn't high enough to allow the sensor to function.
04:01 Secondly the output from the sensor needs to be connected to a digital input on your ECU that provides a pull up resistor to five volts.
04:11 This may be selectable within the tuning software, but if the pull up resistor isn't enabled, the ECU won't see any signal from the sensor.
04:20 If you're dealing with an ECU that offers no pull up resistor then you can get around this by installing an external 10 kilo ohm resistor between the power supply and voltage output pins of the wiring to the sensor.
04:34 The other issue you may find is a fuel temperature reading that's completely unrealistic.
04:40 While the ethanol content is just the frequency of the square wave output from the sensor, when it comes to the pulse width used to measure temperature, the ECU can either look at the time when the signal is high, or the time when the signal is low.
04:56 In order to read the fuel temperature, it's the low edge of the square wave we're interested in, and if the ECU is looking at the wrong edge, the fuel temperature data will be garbage.
05:07 Fortunately with a lot of standalone ECUs now, the flex fuel sensor configuration is as simple as choosing the sensor from a drop down menu, however it's always an advantage to understand the operation of the sensor.
05:21 If you're dealing with a factory ECU that isn't designed for a flex fuel vehicle, then you may not be able to directly incorporate the flex fuel sensor straight into the ECU.
05:32 That's not to say that you can't use a flex fuel sensor, however you'll often need to use a separate ethanol content analyser that can interface with the flex fuel sensor, read the ethanol content and fuel temperature, and finally output these parameters as two zero to five volt analog outputs, which are simpler for an ECU to read and make sense of.