Fuelly Forums - DIY decel fuel cutoff

A visitor to MetroMPG.com has been in touch with me about some of the things he's doing to his car ('96 Euro spec Golf - he's in Germany).

Thought some of you might be interested in reading about his plans (posted with permission - he may eventually register here when he has time).

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Since I also want to control some functions (switch engine off and on) I plan on using a microcontroller.

I finally got around to sniffing around my car's wiring (I've got a good shop manual with wiring diagrams) and have added sensing wires for: fuel injector valve control signal (basically a pulse-width modulated injection period), idle switch, engine speed Hall sensor on distributor (with interrupt switch), vehicle speed, coolant temperature, clutch pedal pressed (new switch), transmission in neutral (new reed switch).

When I hook this up to a microcontroller (probably an Atmel AVR ATmega32) I intend to do the following:

1) Fuel economy meter with logging functions. I can then show momentary, trip, tank and long-term data, save data for each trip in a table (to be downloaded to a PC), "strip-chart" recording of continuous vehicle data during a trip or segment. This assumes that I can find a good correlation with and calibrate the pulse-width modulated fuel injector signal.

2) Automatic engine shutoff while coasting & braking. My car has a fuel cutoff, but it only works above 1700 RPM, which I'm almost never at in city and suburban driving; on my route it's maybe 20 seconds of my 32-minute drive to work. If I extend this down to, say, 1000 rpm then I've easily saved at least 2.5 to 4 minutes of coasting-at-idle fuel, which means about 3% fuel savings. I have to leave the clutch engaged during shutoff so that I have brakes and power steering. The engine braking effect in 5th gear is relatively mild (at a suburban cruising speed of 60 km/h the engine's turning 1500 RPM in 5th).