The single most important improvement to FE you can ever do. FREE. - Gas Savers

CO ZX2 · 02-16-2007, 03:33 PM

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.

landspeed · 02-16-2007, 04:14 PM

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)

skewbe · 02-16-2007, 05:05 PM

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?

basjoos · 02-16-2007, 06:10 PM

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?

skewbe · 02-16-2007, 06:40 PM

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.

zpiloto · 02-16-2007, 07:01 PM

The single most important improvement to FE you can ever do. FREE

Walk or bike

Mike T · 02-16-2007, 07:06 PM

I agree: bike!

So how's the braking with engine off, on the second or third application? does the car have ABS?

rh77 · 02-16-2007, 07:35 PM

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.

RH77

Peakster · 02-16-2007, 07:47 PM

Quote:

Originally Posted by zpiloto

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.

cfg83 · 02-16-2007, 08:05 PM

Peakster -

Quote:

Originally Posted by Peakster

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

.

CarloSW2

skewbe · 02-16-2007, 08:26 PM

Quote:

Originally Posted by Mike T

...
So how's the braking with engine off, on the second or third application? ..

Mine is stiff but useable. One *could* increase the vacuum capacity of the system simply and lightly enough, and even add a pressure (vacuum) switch to sound a warning when it's time to recharge the brake vacuum.

CO ZX2 · 02-16-2007, 09:36 PM

Quote:

Originally Posted by skewbe

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.

Cheez!! I thought this to be a very simple test with obvious advantages. A free gift. You invented the question, what is your answer?

It seems that if I tried to pass out free $100 to each passerby, some could not decide if they wanted cash, check or money order.

skewbe · 02-16-2007, 10:04 PM

Quote:

Originally Posted by CO ZX2

Cheez!! I thought this to be a very simple test with obvious advantages...

Like I said, I appreciate anyone who is willing to test and post their results.

You coasted 3.6 miles farther in test 2, that's the only specific that one can conclude from the test though. Saying +20mpg is not a conclusion that can be reasonably drawn from this test since the vehicles did not cover the same course, the first one was substantially shorter.

My corrections are gratis also

BeeUU · 02-16-2007, 10:46 PM

Quote:

Originally Posted by Mike T

I agree: bike!

So how's the braking with engine off, on the second or third application? does the car have ABS?

Lately when I have been coasting with the engine off, I have been using the emergency hand brake to "adjust" my speed, saving the "real" vacuum assisted brakes for an "emergency" stop, if I need them.

DRW · 02-16-2007, 11:07 PM

Quote:

Originally Posted by basjoos

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?

I like to look at what the fuel is used for. When you drive fast a larger portion of fuel usage is used to overcome aero drag, which you'll never get back. When you climb a hill the fuel is being used to create potential energy (mass of car raised up to a height) which you have a chance to recoup when you coast down the hill. Hopefully you won't have to brake on the downhill and waste the fuel. I don't think it's bad to have to downshift on a climb. To me this means you're still converting fuel into potential energy as quickly as the car is capable of. Just make sure you don't go into open loop or rev the engine higher than needed.

CO ZX2 · 02-17-2007, 12:32 AM

Quote:

Originally Posted by zpiloto

The single most important improvement to FE you can ever do. FREE

Walk or bike

As long as we're just being silly, wonder if anyone has thought of a car rack for a bike?

cfg83 · 02-17-2007, 01:30 AM

skewbe -

Quote:

Originally Posted by skewbe

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?

I agree that covering the same distance would be a better comparative test, but I don't think the reported MPG would necessarily be the same, at least not for my Saturn.

On the ScanGauge, with my engine off, coasting in Neutral, I still see 0.1 GPH (Gallons Per Hour) consumption. This must be a fudge number to avoid divide-by-zero errors for when the fuel injectors are off. This fudge number leads to *another* silly value I see in the ScanGauge. When the engine is off, at X MPH, with the key in the run position, I see X*10 instantaneous MPG, i.e. 39 MPH yields 390 MPG. In this context it should be the "infinity limit" of the SG, or something like 9999 MPG. But 390 MPG now makes sense to me because :

39 MPH / 0.1 GPH = 390 MPG!!!!!

If CO ZX2 covers the same ground (5.1) miles in less time and his ZX2 talks to the ScanGauge in a similar manner (I can test this, my Dad has a ZX2), the 0.1 GPH will be divided into a smaller portion of time when the downhill MPH is higher and result in greater total MPG for the Engine-Off-Neutral test. Let's plug in the example numbers, assuming that the 56 MPH average would also apply for the first 5.1 miles (this example would apply for my Saturn in relation to the ScanGauge) :

Engine Off in Gear
36 Miles/Hour for 5.1 Miles => 5.1 / 36 = 0.142 Hours
0.142 Hours * 0.1 GPH => 0.014 Pseudo-Gallons

Engine Off in Neutral
56 Miles/Hour for 5.1 Miles => 5.1 / 56 = 0.091 Hours
0.091 Hours * 0.1 GPH => 0.009 Pseudo-Gallons

The second test over 5.1 miles will yield a higher MPG for the round trip because the Scangauge is fudging the engine off numbers. The ScanGauge, when confronted with the infinite (MPG), goes a little loopy,

.

This is a defect in the ScanGauge software, *or* a legitimate compromise in a context where the ScanGauge is not designed to operate, depending on your POV.

Tentative Conclusion : If CO ZX2's car is reporting 0.1 GPH in Engine-Off Coasting mode, he is actually getting even better downhill MPG than he is reporting.

When the engine is off, all bets are off regarding ScanGauge accuracy. Different ECU/PCMs are saying different things to the ScanGauge.

The ScanGauge is good for comparing apples to apples, Saturns to Saturns, Geos to Geos, etc. ad-nauseum ...... It's also good for comparing relative gains of your own car, but the gaslog tells the real truth.

YMW(ill)V!

Orrrrrrrrr, how 'bout them apples!

CarloSW2

95_corolla · 02-17-2007, 02:02 AM

hand brake to slow down that is a great idea I did not think of that becaust it seems so unnatural. I do alot of engine off coasting I'm going to start trying that to save my power brakes for when I really need to stop fast.

diamondlarry · 02-17-2007, 07:30 AM

Quote:

Originally Posted by cfg83

Tentative Conclusion : If CO ZX2's car is reporting 0.1 GPH in Engine-Off Coasting mode, he is actually getting even better downhill MPG than he is reporting.

When the engine is off, all bets are off regarding ScanGauge accuracy. Different ECU/PCMs are saying different things to the ScanGauge.

The ScanGauge is good for comparing apples to apples, Saturns to Saturns, Geos to Geos, etc. ad-nauseum ...... It's also good for comparing relative gains of your own car, but the gaslog tells the real truth.

YMW(ill)V!

Orrrrrrrrr, how 'bout them apples!

CarloSW2

This is the conclusion I came to soon after I got my first SG. The SG is a great tool but, as great as it is, there are so many variables involved that you can't get an absolutely 100% accurate reading ALL the time. You can get it dialed in pretty close but you'll never always be dead on. I use the SG for measuring improvement over different segments and the hand calcs when topping off the tank as the final word. As for those who were criticising CO XX2 recently, Carlos is right, when you analyze things as carefully as Carlos has, CO ZX2 is doing even better than he was reporting. It's all about the commute conditions.

skewbe · 02-17-2007, 07:30 AM

Quote:

Originally Posted by cfg83

skewbe -

I agree that covering the same distance would be a better comparative test, but I don't think the reported MPG would necessarily be the same, at least not for my Saturn...

Yah, on some cars (i.e. mine, apples if you like) the scangauge reports base fuel consumption during overrun/EOC in gear, even there isn't any, which is not a big deal IMHO.

CO reported zero fuel used at the end of his first test leg so I assume his doesn't do that.

Just a thought, A carbureted car very likely would still spew fuel into the engine when coasting with in-gear coasting with the ignition off , and make a nice backfire when you turned it back on

diamondlarry · 02-17-2007, 07:32 AM

Quote:

Originally Posted by skewbe

Just a thought, A carbureted car very likely would still spew fuel into the engine when coasting with in-gear coasting with the ignition off , and make a nice backfire when you turned it back on

My dad said he used to blow up mufflers when he was a kid by doing this.

ELF · 02-17-2007, 07:45 AM

I blew up my share of mufflers back in the day. I used to do that all the time. I found out that a regular glass pack would not blow up the way a regular mufflers did. I was pretty disappointed when I got my first FI vehicle, no more BANG!

korax123 · 02-17-2007, 08:10 AM

Not to be mean but it's obvious that you would get better FE when coasting down a hill with the engine off. Thats just common sense.

The only test that would have been almost worth while would be coasting in Neutral with engine on, and coasting in gear with engine on.

Rather then that if you thought you wouldn't get better milieage with coasting in N with the engine off there is something wrong.

LxMike · 02-17-2007, 09:01 AM

Quote:

I use the SG for measuring improvement over different segments and the hand calcs when topping off the tank as the final word

Thats all i've been using it for. just to compare diff situations. not for absolute numbers. thats why i don't post in the daily updates thread any longer. my last tank was showing just under 30 on the SG but when i refuled was almost 32.

skewbe · 02-17-2007, 09:25 AM

Quote:

Originally Posted by korax123

Not to be mean but it's obvious that you would get better FE when coasting down a hill with the engine off. Thats just common sense.

The only test that would have been almost worth while would be coasting in Neutral with engine on, and coasting in gear with engine on.

Rather then that if you thought you wouldn't get better milieage with coasting in N with the engine off there is something wrong.

The thing is there's some controversy about weather or not it is WISE to coast down a hill in neutral and/or with the engine off. So some measure of exactly what the benefits of each permutation are can be helpful in aiding people to make their own decisions.

Yes it is obvious that you will coast farther down hill in neutral vs with engine braking, and that not burning gas while moving is infinately better than burning gas.

BluEyes · 02-17-2007, 10:22 AM

Quote:

Originally Posted by skewbe

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)?

If his car was slowing down to 30mph while rolling in neutral along these 3.6 miles, it would have to be a very very shallow hill at that point. Perhaps virtually level or even a slight incline? So the FE on the first part of that return trip would be pretty good. I agree that 85 seems odd but with his degree of aero mods, the mileage under these conditions should be wuite good.

I think the best way to run this test would be to do the neutral coast first. Mark where the car falls below 30mph and then on the engine-off, in-gear test, restart the engine and continue driving to the point where the neutral coast ended (but at what speed???).
I would be interested in seeing fuel used numbers at the turnaround point and back at the start, as well as distances to both the turnaround point and where the in-gear test needed the engine restarted.

Regarding fuel use when coasting in gear with the engine running, I think alot of mileage computers aren't going to calculate that right. Especially on newer cars that shut off injection during coasting and you would get a divide by zero error. If the programming is done 'right' and the computer just logs miles traveled divided by gallons used based on injector pulse width then the average mpg should stay right, but instant mpg would be limited by however the computer gets around a potential divide by zero error. Kinda disappointing that the scangauge defaults to a 0.1gal/hr fuel use reading.

cfg83 · 02-17-2007, 10:52 AM

skewbe -

Quote:

Originally Posted by skewbe

The thing is there's some controversy about weather or not it is WISE to coast down a hill in neutral and/or with the engine off. So some measure of exactly what the benefits of each permutation are can be helpful in aiding people to make their own decisions.

Yes it is obvious that you will coast farther down hill in neutral vs with engine braking, and that not burning gas while moving is infinately better than burning gas.

For me, the only question is whether I can get a "free brake" + hydraulics with the engine off/in gear/downhill. I know it will slow me down, but my traffic density is so dangerous that I don't have many spots where I feel safe enough to coast downhill/engine off/in neutral. I need my brakes and steering on the 710 going South because the semis are taking crap to and from the Long Beach Harbor (literally by the boatload).

Any time the tire rolls forward with no gas to the engine is an MPG benefit, period.

CarloSW2

cfg83 · 02-17-2007, 10:57 AM

skewbe -

Quote:

Originally Posted by skewbe

Yah, on some cars (i.e. mine, apples if you like) the scangauge reports base fuel consumption during overrun/EOC in gear, even there isn't any, which is not a big deal IMHO.

CO reported zero fuel used at the end of his first test leg so I assume his doesn't do that.

Just a thought, A carbureted car very likely would still spew fuel into the engine when coasting with in-gear coasting with the ignition off , and make a nice backfire when you turned it back on

In my second ill-fated Karmann-Ghia (the one I don't like to talk about), the engine was so bad that when I floored it, the 4 CONICAL shaped exhaust tips on my muffler would make a righteous explosion from unburned gas. Looked great at night, just like the 4th of July.

I keep looking for that muffler in the Hot VWs mag, but I can't find it. Must have gone out of business. Best sounding VW muffler EVER!

CarloSW2

JanGeo · 02-17-2007, 11:19 AM

Quote:

Originally Posted by Mike T

I agree: bike!

So how's the braking with engine off, on the second or third application? does the car have ABS?

Food energy to travel the same distance on a bike will cost more than gas going the same distance.

JanGeo · 02-17-2007, 11:28 AM

The test is correct giving EOC in neutral higher mileage for more distance since the EOC in gear added drag and reduced the distance traveled. It does not matter that the total distance for each test was different but in fact IS the point with coasting in neutral . . . you go further and thus increase the average MPG. HOWEVER, if you wanted to use less gas, then traveling the same distance would be a factor or at least turning around at high speed to go back up the hill coasting with engine off would still result in less gas used also. Braking is pointless and would make the results the same as you would simply be going up a hill the same distance.