Good news, everyone...
I have actually started the project.
If you speak German, here's a run-down of the project specifics so far:
http://mgboard.de/viewtopic.php?f=3&t=11758
I am in the process of developing a modular architecture for this car computer, with a module in the back of the car measuring injection cycles right at the injectors, as well as the vehicle speed off a transmission hall sensor, and the fuel tank level. There will be a display module in the front of the car, which will receive data from the measuring module via I2C data bus, evaluate it, do calculations with it, and then display your fuel consumption data on a little 2.2'' TFT display, probably this one here:
https://www.adafruit.com/product/1480.
Due to this modular setup, it's thinkable in the future to add functions like a light bulb monitor, or displaying things like outside and inside air temperature... but that's in the future.
So far, I've measured the injection signal with a scope:
Basically, as you can see, the injectors are switched to GND by the engine control unit to open. When the injection interval is up (usually around 2 milliseconds), the injectors are switched back to 13 volts.
By counting the number of milliseconds that an injector was open during a given time interval, I can work out how much fuel was injected, because the capacity of these Bosch made injectors is known and is about 200 ml/min for MPI injectors and 210 ml/min for VVC injectors. For example, if I add up injection intervals once every 3 seconds and during those three seconds the injectors were open 450 milliseconds, then I just have to multiply that by the injector's flow capacity per millisecond, and I can then extrapolate that to litres per hour.
Also, I have looked at the speed signal, which can be tapped into at the panel under the steering wheel... this could be used to calculate miles per gallon or litres per 100 kilometres:
Unfortunately, being that I've got an MkI, the speed signal is, erm, underwhelmingly accurate. The above screenshot was taken at a deliberately constant 20 mph. As we know, the speed signal on the MkI is conveyed from the transmission using an old fashioned speedo wire. The turns of the wire drive a magnetic inductor inside the speedometer which produces this square wave, but the same way that the speedometer needle tends to "twitch" on MkI cars, the signal that the speedometer produces is kind of "all over the place" and it's close to impossible to get a tidy, steady square wave at a given constant speed. It gets better from about 50 to 60 mph upwards, but what good is a fuel consumption metre if it's only accurate above 50 mph...
I could just upgrade to a MkII/TF instrument cluster with the electronic speed transducer... I've even got a working TF instrument cluster collecting dust here on the top of my cupboard. But I would very much like to keep my analog cream coloured dials
![Wink ;)](./images/smilies/icon_e_wink.gif)
So I've bought an electrical hall sensor on eBay today which goes right in between the transmission and the speedometer wire, and it should produce a more reliable speed signal.
Moreover, I've built a test rig here at home to develop the circuits:
The breadboard on the left serves the function of what's called a bit banger... it simulates the square waves that are present in the car and on which my fuel consumption measurements will be based. The breadboard in the middle is home to the actual measuring module (still under development), and the board on the right is my first attempt at building the display module (the finished version will have the above mentioned TFT colour screen).
As you can see, the project is based on the Arduino IDE framework.
Once these "proofs of concept" work and have all the bells and whistles of the finished product, I will then carefully solder all the components neatly onto (probably) perfboards, put them in module boxes and wire them up in the car. I've made good progress with this project in a relatively short amount of time, but I don't expect the finished project to be installed before next spring. There is still a whole heap of things that I need to think through and come up with solutions for. But hey, even the longest journey begins with the first steps...
![Wink ;)](./images/smilies/icon_e_wink.gif)