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Old 08-11-2012, 06:56 PM   #1 (permalink)
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Phantom Blot (Spökplumpen in swedish) - '75 Saab 96 V4
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DIY fuel computer w. ATmega & 1970´s LED display

I´m planning to build my own fuel economy computer för my 1975 Saab 96. I´m a big friend of vintage electronics so my design will have a four-digit seven segment LED display, as if the FEC is of the same age as the car. Compared to a standard LCD module I will get big digits from a small package. The display will be a compact unit with it´s own microcontroller to allow a thin three-wire connection.

The FEC main unit will be placed below the dashboard where I can reach it comfortably and the display unit will be placed next to the speedometer. Both units will have an Atmel ATmega88 microcontroller that I program i C.

I will try to keep this project simple. A small flow-sensor for liquids (like a small turbine/paddle-wheel) gives about 9000 pulses per litre fuel. (may be about 2000 per gallon) The FEC will also have input from the ignition system to read engine speed. The third input will be pulses from one of the wheels to meassure distance.

The Saab 96 have manual transmission with a unique feature: A freewheel that allows the engine to idle when the throttle is released in gear and to change gear without using the clutch. This freewheel makes it easier to drive by "pulse & glide" and save a lot of fuel. The freewheel is the reason for meassuring the engine speed. -By comparing that with the road speed the FEC will see if the engine is burning fuel on proper work or wasting fuel on idling.

Since I live in Europe (Sweden) I will meassure fuel economy in litres per 100 kilometers (inverted value compared to mpg). If I drive like most people do the car consumes about 8 L/100 km but my way of driving mostly results in figures around 6,7 ("L100").

I plan to meassure and count a lot of different parameters based on time, distance, road speed and engine rev. The most important is offcourse the L100, litre per 100 kilometers. I think it may be interesting to present the L100 as average during last 30 sec, last three minutes and last 30 minutes.
The FEC will be able to display traveled distance, total ammount of used fuel and also calculate what my savings is or would have been if I hade shut off the engine instead of idling.

There will also be two similar "register pages" with several parameters to use as "trip meters" that I can reset. The last function is to meassure the time it takes the car to coast down from highway speed down to almost zero. By dividing this meassurement into four or five equal intervals I hope to have a way to see if an aerodynamic modification is a success or not.

I think most meassurements need only two or three digits of value, there´s no need to tell fuel consumption, traveled distance or traveled time with a precision of ppm. An automatic decimal point will allow the meassurements to lose precicion as they grow.

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Now my headache:

In total there may be about 20-30 different figures/parameters/numbers that can be displayed one at a time on a simple four digit 7-segment LED-display.
But how to arrange and manage such a lot while driving? A standard alphanumeric LCD module could make the job a lot easier by the help of text labels and a good menu system but I´m quite sure that I´ll be safer in traffic with a clever way to find the right data without browsing a menu system.

My idea is to group the parameters into "pages" containing 5-8 parameters each depending on their use and purpose. I then intend to choose parameter within one "page" by turning a knob with distinct position. To choose "page" I will use another knob or two push-buttons. If I use buttons, one will increase the page number, the other button will return me to the first page.

I hope such a system will be quite easy to learn. The LED display will have a very limited ability to show text labels to guide me in the beginning. Each time i change parameter/page a text label will be displayed (one second if I choose to switch on the label function). The decimal point will help me to recognise different types of L100-values.

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But is this idéa with 3-5 "pages" of 5-8 parameters each a good solution or is there a more clever way to find what I need while driving safely?

Is there any valuable information or calculations I have forgot? My FEC will have an accurate timebase and pulse inputs from consumed fuel, travelled distance and engine rpm.

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Old 08-11-2012, 08:25 PM   #2 (permalink)
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You can still have the vintage look with an LED display, but use the alphanumeric displays. Way retro looking!
Embedded Adventures - Displays - 8 digit 14 segment 0.56 inch alphanumeric display - red
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- http://www.dschmidt.com/MPGuinoJBD.html
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Old 08-12-2012, 09:07 AM   #3 (permalink)
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Phantom Blot (Spökplumpen in swedish) - '75 Saab 96 V4
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Thank´s JellyBeanDriver, that´s an absolutely great product! I´d love to have it in my car and the way I´m designing my FEC it will be quite easy to fit such a display later. The price isn´t too bad but I will build from what I have in my junkbox to start with. If I find my 4-digit display to hard to read I now know exactly what to purchase instead of being tempted to take the easy way out with a standard HD44780-controlled LCD.
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Old 08-12-2012, 09:18 AM   #4 (permalink)
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Phantom Blot (Spökplumpen in swedish) - '75 Saab 96 V4
90 day: 52.77 mpg (US)
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Thanked 55 Times in 30 Posts
As the vintage freak I am, I was first thinking about a row of small nixie-tubes but they are bulkier, trickier to drive and about one decade older than the car. 7-segment VFD´s may be a more suitable alternative to the red LED´s. I have a bunch of high-res. bar-graph VFD´s somewhere too and a desktop calculator with unusual 8- or 9-segment VFD digits. Nixies would be perfectly right for my 1964 two-stroke Saab 96 when it´s finaly ready. To my 1959 Heinkel three-wheeler a row of dekatron tubes would be just right. :-)

My idea to have a separate display unit for my FEC makes it easy to choose and change between any display types I want. I´ll start with the 7-segment LED´s.

It´s funny how easy it is to get carried away by visual gimmics instead of the way more important problem with how to select and present all data...
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Old 08-20-2012, 08:46 AM   #5 (permalink)
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Phantom Blot (Spökplumpen in swedish) - '75 Saab 96 V4
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The project is developing slowly. I´ve decided to change the physical concept to a flat box with LED integrated display on top of the dashboard after finding a perfect box. The display will have six (2+4) 7-segment digits that is seen just under the top of the steeringwheel. Push-buttons and knobs will be placed on the upper side of the box and can then be reached with my hands still on the wheel. -PERFECT!

Soon I may start to build the electronics. I have to decide if my CPU have time to update a multiplexed display often enough to avoid annoying flickering or if I have to go for parallel drive on all 48 display segments to get a steady light. Some moderns cars with LED rear lights are realy annoying in nighttime traffic because the flicker-rate is too low. -Rapid eye movements (such as normal when driving) leave an impression of bright red dots everywhere and makes me confused. I hate those rear lights! Volvo and VW are among the worst in swedish traffic and I don´t want to introduce that s--t inside my car!

When the display is finally set I can start the real work of designing the rest of the electronics and start programming. (Imagecraft C-compiler)

One big advantage when going from four to six digits is that I can have two digits contiously indicating my average fuel consumptopm while the other four digits show something else.

Here I´d like som help from others who use MPGuino or other FEC:s! -Which are the most important functions/data/parameters to have easily accessible while driving? And what functions are these systems missing?
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Old 08-21-2012, 02:27 AM   #6 (permalink)
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The most useful MPGuino displays for me are:

-Tank average & trip average. If you can see them both at the same time, you can do your best to improve them during your each trip.
Tank average will be reset during fillup
And Trip average will automatically reset some minutes after the engine has been stopped.

-Fuel used (litres) since fillup or since starting the car.
I started collecting data on my work route. I always drive the same route to work and take notes of the litres always in the same places. I take notes after first 3,6km of city driving, then after a 6,5 km of highway and then after 3,6 of smaller roads. This is a good way to see if your mods will have any effects and you don't need to go for a separate test drive. The more you collect the data, the more it gets accurate.

-Average speed. This is a function I would like to have. Specially when arriving to work after collecting data. The speed has a big effect on your consumption. And when average speed is known then you can see other effects easier.

-Litres/hour. This is sometimes a nice function,as you can see how much your idle consumes fuel.

-Then of course the tank distance and trip distance are good to to have, but I don't need them all the time. And sometimes the Instant L/100km is used, but it will vary every second so much according to your driving, that I would prefer maybe a 10s. average.

As you can see, I try to keep things simple. I have tried to select the most important displays and keep everything else aside.

I like your idea of a vintage FEC. I have a Zemco fuel computer on MyoldFord. It has only one display with 4 digits and it can display speed, distance since last reset, clock, fuel used since last reset and the instant L/100km and a average of L/100km since last reset. I'm quite happy with it. but I have had some problems with the fuel flow measurement accuracy.
The fuel flow sensor has an ball circulating in a track and the fuel flow is calculated from the ball passing through an optical eye
I had a very small leak in my fuel pipe and air was entering to the fuel line. And the mechanical fuel pump created bubbles when hot. The optical eye counted also the bubbles and then the L/100km reading went crazy.
Also I had and fuel filter between the flow sensor and the carburettor. There was an air pocket in the filter and it was acting like a cushion allowing the fuel flow go back and forth with the pumping. Again the L/100km reading went crazy.
Now I have electrical fuel pump and the leak is fixed and the flow sensor is mounted as near the carburettor as possible. Hopefully I can get accurate readings finally. I just need to calibrate the system once again..
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Old 08-21-2012, 07:42 AM   #7 (permalink)
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Phantom Blot (Spökplumpen in swedish) - '75 Saab 96 V4
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Thank´s for your input, Superturnier! Don´t remember how detailed I was in the first description of my FEC but l/h (fuel per time), average speed, three different average L100, different trip data and more are on my list. But TOO MUCH data can also be bad... What I realy need is a clever system for how to easily reach the information I need most while driving.

Superturnier, do you mean that you rarely read your L100 while driving, simply because it´s averaging is too short in time? I plan to have four different ways of displaying/calculating the L100: 30 seconds, three minutes, 30 minutes and trip average. I guess 3 min. is the one I will prefer to have at one part on the display most of the time while the other four digits show something else. If 3min. is the best delay/filtering of data I don´t know. I also guess that shorter than 30 sec. will give nonsense readings, just like Superturnier wrote.

I plan to have more than one "page" of data logging, each containing 5-9 different meassurements/calculations. Each page can be cleared separately. I can then use one such "page" from one tank filling to next, another page for all driving during a day and a third page that I reset several times during a trip to see how different strategies or inclination of the road will affect the consumption.

Each such "page" of data will contain average L100, total litres of fuel, total time, total distance, average speed, % of fuel wasted on idling and % of distance traveled without help from the engine.

Meassurements of time and distance will have a floating resolution. -Time will start as seconds and tenths of seconds, then change to MM:SS and then to HH:MM as there is no need to keep track of seconds if I have been driving for an hour. Same about distance and even fuel. It´s better to get rid of trailing figures that are worth nothing. Any precision should be meassured in %, not in absolute figures! However, when calculating and comparing averages or repeated meassurements it´s offcourse wise to not get rid of precision too early in the process. What´s realy important to remember is that precision can not be created within a process, it can only be a product of the input.

I try to think a lot about how much precision that is realy needed. Most absolute readings don´t need more than two significant figures. I also find it silly to have more than three significant figures in any comparative data even if the data is collected during long time or distance or is an average of multiple tests. Trailing zeros are just about appearance and readability just like the decimal point.
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Old 08-21-2012, 07:57 AM   #8 (permalink)
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Phantom Blot (Spökplumpen in swedish) - '75 Saab 96 V4
90 day: 52.77 mpg (US)
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Average speed, both during a fixed number of last minutes and during a trip should be very valuable. I guess most people of today´s crazy world are too concerned about saving time all the time. What I hope to see clearly is that lowering the highway speed with say 10% will make no significant difference at all in time but definitely in spended money on fuel.

Since I have a 37 year old car I like a lot I don´t want to ruin it by speeding. My experience tells med that I often catch up with the over-stressed losers when I get into town even though I drive about 20% slower than most people on the highway. By "reading" the traffic and planning my driving I may even pass the poor losers eventually! -And that while saving a lot of fuel! Now I just want to se this proven by figures too! -And to improve my driving even more.
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Old 08-07-2014, 01:59 PM   #9 (permalink)
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Phantom Blot (Spökplumpen in swedish) - '75 Saab 96 V4
90 day: 52.77 mpg (US)
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Without catching up with what has been written before about this topic I feel It's time for an update:

The DIY fuel computer project has been sleeping for a long time. Meanwhile I have improved my eco driving skills a lot and now feels like i'm participating in a true high perferomance sport where every little detail counts. With a fuel consumption of 4.5 L100 (52 mpg) at a speed where most people get 6-7 l/100 km (30-40 mpg) I think it's definitely time to get a more detailed look at the details!

I'll skip the vintage looks and aim for a simple design with a standard alphanumeric LCD module. To get started as quick as possible I will try to keep everything at a minimum, both functions and hardware. The computer will be a little box ontop of the dashboard with a D-sub connector and a magnetic bracket to make it realy easy to remove for software upgrades.

The system will have inputs from the fuel flow-meter (40 year old carburetted engine), wheel rotation and most likely also sense engine speed. The most important to display is average fuel consumption (L100) during the last 5-10 minutes, average speed during the same period and total travelled distance. If I manage to get these things working I can later improve the software with a lot of fantastic features.

The physical box is ready. Many details about computer hardware and software have been decided so I guess it may be time to stop talking and actually start building the thing now!

I will have an AVR ATmega88 on perfboard and use C as the programming language. Very few external components will be used. -LCD, two push-buttons, 5V voltage regulator, some transient filtering, two LED's and perhaps something more. If I succeed making something useful I will share everything needed to know so other electronics DIY'ers can copy and/or improve my design.

Last edited by JohnAh; 08-07-2014 at 02:05 PM..
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Old 09-09-2014, 07:38 PM   #10 (permalink)
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HI I can't say I'm not disappointed about losing the old school LED look!

Here's my reason


This in in my old subaru and is a joke as far as accuracy goes. I would love to make it work as it should

Good luck with your project!

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