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Old 11-28-2007, 06:48 PM   #1 (permalink)
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Blackfly - '98 Geo Metro
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Coasting experiment: engine on VS engine off on a fixed route = 12.9% gain

(Originally posted March/06)


I used to do a fair amount of engine-off coasting in my 89 accord. but after I got the Firefly and the ScanGauge, I let that habit wane because switching the key off to kill the motor unfortunatly also put the SG to sleep.

And unfortunately for me, my car also uses the ISO obd protocol, which takes longer to initialize (roughly 15 seconds) when switching the key back on than the newer CAN protocol. The result of that was seriously skewed fuel consumption on the SG from engine-off coasting.

So I stopped doing as much engine-off coasting.

Then krousdb alerted me to the possibility that I could install a kill switch to shut off the engine without using the key, and the SG might stay active. I added the switch - and it worked.

So it was time to find out what I had been missing by not coasting the Blackfly with the engine off. And holy smokes, I'd been missing A LOT.

The test:

2 laps (of the same route) in city driving. One I let the engine idle when coasting, the other I switched the engine off (if the coast opportunity is longer than about a block in length).

The route:

- 9.7 km of city driving
- in light/moderate traffic
- stops or turns every 3-5 blocks
- the route avoided traffic lights where possible (to preserve as much predictability between the two loops; 3 traffic lights were encountered)
- several small hills (under 40 feet)
- 3 longer or steeper hills (one, approx 70 feet elev.)
- 4 stop signs
- lots of turns at intersections

The test conditions:

- light drizzle; damp/wet roads

- Conditions at SUPN6 as of (2:00 pm EST) 1900 GMT on 03/10/2006:

Wind Direction (WDIR): SW ( 220 deg true )
Wind Speed (WSPD): 22 kts
Wind Gust (GST): 25 kts
Atmospheric Pressure (PRES): 29.44 in
Air Temperature (ATMP): 45.7 °F

Car was warmed up prior to test with 1 hour EBH; 20 minutes city driving (errand run)

The results:

- 60.3 mpg (US) - lap 1 "engine OFF while coasting"

- 53.4 mpg (US) - lap 2 "engine idling while coasting"

Notes & observations:

- this test is subject to experimenter influence/bias

- the traffic & light patterns slightly favoured the "idling" loop. of the 3 lights i went through, one was green for this lap, but they were all red on the "engine off" lap. some minor traffic flow issues also helped the "idling" loop.

- time to complete: 17:57 sec (engine off lap); 17:44 (idling lap)

- on the "off" loop, i killed the engine 13 times; 3 key starts and 10 clutch starts

- the engine was off for 3.9 km of the 9.7 total (thanks, google earth!), or 40% of the distance travelled.

- max / avg speed (idling loop): 39 mph / 17 mph

- under these circumstances, engine-off = 12.9% better than engine on

- obviously since this is a "new" technique, I haven't yet calibrated the SG accuracy using engine-off coasting at fill-up time.

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