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MetroMPG 11-29-2007 02:04 PM

DIY decel fuel cutoff; alternator regulation; auto start/stop
A visitor to 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).


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). The algorithm would be:

> Engine warm
> Throttle released (idle)
> Clutch pedal not pressed
> Speed > 25 km/h (to avoid shutoff when slowly rolling in a left-turn lane, etc.)
> RPM > 1000 (to avoid restart problems).

I now have a momentary switch on my dash (as you did, interrupting the distributor's RPM sensor) with which I can stop and restart the engine at will. So I can manually simulate this algorithm. A side effect is that the oil pressure light comes on during shutoff, the ECU is probably throwing codes. Seems to work great, as long as I don't stab the clutch so quickly that the engine doesn't have a chance to restart before being disengaged from the transmission. At low engine speeds when I press the gas pedal the engine jerks very slightly when the engine restarts. This is probably because the engine's idle switch responds first when I want power from the engine. If I switch it on a fraction of second sooner (would require a gas pedal switch that responds as soon as I touch the pedal and before the slack in the throttle cable is taken up) the restart isn't noticeable. All in all, very little effect on driving feel, totally automatic, and it rewards efficient driving style (long coasting, etc.). Pulse & glide (albeit with engine braking) should be a snap.

3) Auto start/stop at stoplights.

Here I intend to use a simplified version of the BMW algorithm (which is semi-automatic; i.e. you decide whether the stoplight is long enough to warrant shutdown):

> Engine warm

> Speed < ? 8 km/h
> Transmission in neutral
> Clutch released
> Restart when clutch pressed.

4) "Microhybrid":

shut off alternator while accelerating (as mentioned above).

5) The above techniques all require a warm engine.

That will be expedited with a partial grill block. As soon as I have my fuel economy meter I can do tests with trash bags, etc. Perhaps I can do a "stealth" version with a plastic plate behind the standard grill.

6) And before winter breaks in again I'd like to work on my Thermos-bottle coolant latent heat device, time permitting.

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