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Old 02-16-2007, 02:33 PM   #1
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The single most important improvement to FE you can ever do. FREE.

Two days ago I drove 20 miles to the top of a mountain near me. I wanted to do a concrete test of the benefits of Engine Off Coasting in a manner that anyone can understand. I already know that this is the longest coast in my area.

My test would compare Engine Off Coasting in high gear with Engine Off Coasting in neutral. Engine on coasting can only be worse in each case.

Turned out to not be much of a test at first. In gear, car would not even roll downhill on a sizable grade. No problem with that in neutral.

So I decided on a running start from 55 MPH for each scenario. I would travel down the mountain till car speed slowed to 30 MPH, stop and note the miles travelled, immediately turn around and drive back up to the start.

1. Engine Off Coast in 5th gear. Car slowed immediately after engine was shut off. Fastest speed recorded with ScanGauge was 53 MPH. Average speed was 36 MPH. Distance travelled 5.1 miles, much of it near 30 MPH. Even so, no fuel was used. Turned around and drove back up mountain to start. At that point, SG current trip 47.2 MPG for 10.2 miles. This is a climb that would indicate about 25 MPG without the coast being involved.

2. Engine Off Coast in neutral. Car gained speed from the start. Fastest speed recorded 76 MPH with hitting the brakes twice to stay near 75 MPH. Average speed was 56 MPH. Distance travelled 8.7 miles. No fuel used.
Turned around, drove back to start. SG current trip 67.4 MPG for 17.4 miles.

This amounts to 70% more miles travelled and +20 MPG over the 17.4 miles.

I ran each test a second time and results were so close they don't bear repeating.

I attribute much of my FE success to this very thing. I honestly believe this is the single most important ingredient of my FE. And it's all FREE.
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Old 02-16-2007, 03:14 PM   #2
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I agree - in order of economy, the options would be (downhill)
(1) engine off coasting - best
(2) engine on coasting - uses the amount of fuel for idling
(3) engine off in-gear - uses no fuel but high mechanical drag so
'wastes' as much fuel as idling at whatever RPM the engine is at
as you go down the hill.
(4) engine on in-gear - on my car, up to 1200rpm, uses as much fuel
as when it idles.

I drove a 300 mile round trip recently, haven't topped off the tank,
but I I drove 80 miles through mountains, then 70+mph on the motorway
for the rest of the way, and drove back in such high winds that, even on
steep downhills at 30mph, the car wouldn't speed up when coasting. From
the position of the needle I have still got a fairly impressive tank - mainly
due to the first 80 miles of engine-off coasting (for >50% of the drive)
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Old 02-16-2007, 04:05 PM   #3
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Hey CO, good on ya for testing, but something doesn't add up. No argument that you got 3.6 free miles on the second test in one direction. But the MPG differences might be misleading.

Since you used no fuel on the way down, the climb back up MPG is 1/2 the total MPG. Trip 1 = 23.6 mpg on the climb, Trip 2 = 33.7mpg on the climb.

Given this (and the drag of the engine) it seems apparent that the car stopped where the average return trip is significantly steeper in test 1.

I think you would have to cover the same course/distance (i.e. enable the ignition a couple times on test 1) in order to get an accurate comparison.

Had you slammed on the brakes and turned around at 5.1 miles in test 2, the mpg results would have been identical, the question is how much extra fuel does it take to extend test1 to cover the same ground as test2?
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Old 02-16-2007, 05:10 PM   #4
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Your test results agree with my driving style. I coast the downhills with the engine off and only use engine braking on steep downgrades where my speed would rapidly get too high or where there is no straight runout area at the bottom.

An extension of these tests would be to test the uphills. Do you get better FE if you accelerate on the flat at the bottom of the hill to build your speed up before reaching the bottom of the hill so you can get further up the hill before having to drop into a lower gear, or is it better to maintain the lower aero drag of your normal cruising speed and accept having to drop into a higher fuel consuming lower gear earlier on the uphill?
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Old 02-16-2007, 05:40 PM   #5
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ok, I appreciate that the precision isn't terribly justified here but:
return trip 1 = .2161 gallons for 5.1mi
return trip 2 = .2581 gallons for 8.7mi

Therefore the mpg to get back to the spot where test one ended =
(8.7mi - 5.1mi)/(.2581 - .2161) = 85.7mpg for 3.6 miles (uphill)?

I think there's more to those uncovered 3.6 miles in test 1 than implied.
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Old 02-16-2007, 06:01 PM   #6
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The single most important improvement to FE you can ever do. FREE

Walk or bike
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Old 02-16-2007, 06:06 PM   #7
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I agree: bike!

So how's the braking with engine off, on the second or third application? does the car have ABS?
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Old 02-16-2007, 06:35 PM   #8
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Full ABS

With the recent snows, I've performed quite a bit of testing in engine-off coast conditions.

With full vacuum (not having pressed the brake pedal), the ABS system on my car works with the key in the "on" position. There's a particular stretch where I coast for 1/2 mile and brake at stop light. It's often icy there and the ABS kicks-in from time-to-time.

YMMV depending on the model, but if you have enough vacuum and the ignition "on", I don't see why ABS/ESP wouldn't engage.

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Old 02-16-2007, 06:47 PM   #9
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Quote:
Originally Posted by zpiloto View Post
The single most important improvement to FE you can ever do. FREE

Walk or bike
Unfortunately, riding my bike doesn't help me to get on the top-10-list here.
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Old 02-16-2007, 07:05 PM   #10
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Peakster -

Quote:
Originally Posted by Peakster View Post
Unfortunately, riding my bike doesn't help me to get on the top-10-list here.
It will if you use a stationary bike to juice up the car battery .

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