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Old 08-28-2016, 03:17 PM   #16 (permalink)
EcoModding Lurker
Join Date: Jul 2016
Location: Near Newcastle, UK
Posts: 19

330i Rag Top - '05 BMW 330i M Sport
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Still getting decent economy from the 330 on longer journeys, with a small contribution from the grill blocks hopefully.

I logged the fuel economy on a 25 mile stretch yesterday, cruising at 60mph. The road in question has a series of gentle uphill and downhill sections, and is never particularly flat. I had about 10 jaunts up to 65-70 while overtaking (I hate stretching out overtakes), and spent roughly 5-10% of the time coasting in neutral downhill. For comparison, at 60mph on a flat road, the economy "BMW swingometer" will show roughly early 40s mpg (UK), late 30s mpg (US) (and it is quite accurate I believe).

The result on this 25 mile trip = 44mpg (UK), ~37mpg (US), perhaps 3mpg better than a journey using cruise control all the time. I used cruise on the flat and uphills, and knocked it off on downhills to avoid a light loaded portion of the BSFC map.

The car was nicely waxed which might have given me a tiny amount of economy

Next time on that journey, I am going to try coasting in gear downhill at that speed of 60mph, as the car is only turning about 2000rpm at that speed. I was mulling over how to increase economy after the trip, and the car uses ~0.35 gallons/hr (US) at idle, so this is the equivalent of about 170mpg (US) when coasting in neutral, and while I have power on (using cruise mainly), it will vary from about 20-50mpg.

So I have my mind the span of fuel usage is 20mpg-170mpg(US), with that data above, using neutral gear coasting.

Next time when I try using in gear coasting, the span of fuel economy will be 20mpg to infinite mpg, with the coasting in gear, and the longer coasting time in gear, the better for economy, obviously.

If I am losing too much speed downhill I will give the throttle a quick stab at about 2/3 throttle to efficiently speed up about 5mph using a good BSFC condition for the car.

I guess the deciding factor is whether the use of drivetrain friction to lose speed (with infinite mpg) and then a high burst of power (a bit like a pulse of P&G), to add a bit of speed, when travelling down slight downhill sections, is more efficient overall than coasting downhill in neutral using a bit of fuel, and having a longer coast phase.

The 330 does feel like a low drag engine, it does not decelerate very much on the overrun, has low tension piston rings which help, 5W-30 oil, and a relatively low compression ratio (think it's 10.2:1). I have a suspicion the idle control valve is held open during DFCO to reduce drag, certainly it feels too free spinning to me for a conventional engine with closed air intakes, I will see if I can get my Delphi diagnostic on it to keep an eye on what the idle control valve is doing on overrun, and check if Torque can display the ICV value (don't think so but I'll check again). The ICV does get used at up to 1500rpm when driving under load, I believe, so it would be good to see if BMW utilise it on the overrun too at higher revs.

I have no idea what the outcome will be, but I hope it might surprise me. If I could get late 40s mpg (UK), early 40s mpg (US), while maintaining an average of 60mph, I would be over the the moon.
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