Flex Fuel doubt

[Ricardo_Penteado]

I noticed that for Flex Fuel usage it is necessary to use a special sensor.

Is it really necessary? As we use alcohol a lot, nowadays, here in Brazil something between 80% of the 0km cars are Flex Fuel originally (I myself have one), and they really don´t have any special sensor; all the "magic" is made only by the EGO sensor (a narrow band one).

[jakobsladderz]

If you only run on one type of fuel then you won't need the sensor. If you run on only two types and run the tank down completely between changes you may be able to use a table switch/change the inj cal value manually. The sensor allows the car to seemlessly compensate for changes in the fuel between alcohol and petrol. It would be possible to use an O2 or wideband for the same task but even so, changing the fuel type changes the sensitivity of the O2 sensor, so it's not optimal, although a narrowband will be correct about the stoichiometric point..

There are no hard-and-fast rules, if you can think of a way to do something with one less sensor (take ion knock sensing vs seperate knock sensor) and make it work, all the better. From a coding/logical simplicity point of view, using a flex fuel sensor allows the same VE map to be used and just changing the inj cal in relation to the fuel blend (note, this doesn't take ignition into account). The O2 approach may take a little longer to stabilise and require a bit more 'guesswork' on behalf of the control system, but may otherwise work just as well.