I have an '01

On our recent trip to Florida, I experimented to see how the new PDK "sailing" feature (which disconnects the power train - just like pushing in the clutch or shifting into neutral) to save fuel when coasting would effect gas mileage.

Two cars we've had - an Audi A4 and current BMW 3 series have an OBC that shows either average fuel consumption or current fuel consumption. Both are metric only, so the readout is in liters used per 100 km. Roughly this translates to how many quarts you use per 60 miles of travel, so the LESS you use, the better.

Rough translation:
11 liters/100 km = 21.4 mpg
10 liters/100 km = 23.5 mpg
9 liters/100 km = 26.1 mpg
8 liters/100 km = 29.4 mpg

So I conducted this experiment along I-77 headed south between Wytheville, VA and Charlotte, NC driving down hill marked at 5% grade at a speed of around 70 mph. On the return trip, I tried it again headed north on I-77 between Wytheville, VA and Beckley, WV. Same result both times.

With clutch pushed in. Result:
Car very gradually sped up and consumption showed 1 liters/100 km (235.2 mpg) consumption. Had to apply brakes to maintain speed.

Let the clutch back out. Result.
Car maintained sped with engine braking and consumption showed 0 liters/100 km (infinite mpg - no fuel being used).

So by "sailing", I was using gas and brakes. Having the transmission engaged, I was using neither.

I've found similar results when coasting to slow down (like approaching a red light or a stop sign) except that consumption is higher at lower speeds. (at idle, the engine uses a constant amount of fuel, so as you slow down, the distance travelled per liter used decreases.

In town with the clutch disengaged and slowing from about 30 mph, consumption will be around the 2 - 3 liters.100 km (117 mpg - 78 mpg) - the amount of fuel you use increases as you slow down.

With the transmission engaged, again, consumption drops to 0 usage. (obviously you have to disengage the clutch before coming to a stop - around 1000 rpm)

I don't understand how disconnecting the powertrain to coast can save fuel when the engine management shuts down fuel delivery when the momentum of the car keeps the engine running.

Am I missing something?

I know I often make references to past cars I've owned, but another case in point is that both my Alfas, with mechanical fuel injection, had fuel cut-off solenoids. Adjusted properly, no fuel was used when decelerating completely off throttle. I'd occasionally set the solenoid a little out so that I could get a pleasant burbling from the exhaust when not completely off throttle. An idling engine is using fuel. It seems that modern fuel injected engines would do well to continue to have fuel cut-off rather than coasting idling. Perhaps eventually there will be auto stop-start while coasting.

An aside: doesn't the Sport Exhaust's off throttle crackling and popping indicate fuel usage?



Edited 1 time(s). Last edit at 04/16/2012 09:28AM by Laz. (view changes)

fuel consumption processing by the DME/on-board is paused.

I have done the same tests -- on a section of east bound I-580 down the grade into Dublin CA (my work to home commute route) -- with my Turbo and its on-board computer, and while I do not like to do it, I sometimes slip the transmission into neutral and coast down the grade with the engine idling.

Coasting in neutral with the engine idling delivers better fuel economy -- the avg. mpg reading improves by a couple of tenths mpg in just that short time coasting -- while if I leave the transmission in gear and coast with the throttle closed -- the grade is steep enough if I have light traffic I can speed up and coast all the way down without losing too much speed and becoming a road hazard -- the avg. mpg reading doesn't improve nearly as much, if at all, in this case.

However, gas consumption under closed throttle has to be less than with the engine idling, so I suspect the difference is an artifact of the DME/on-board computer which I opened with.

If one could measure mpg -- using some other method than relying upon the on-board computer -- over some time and distance using the two different coasting techniques the coasting with closed throttle would probably be found to deliver better mpg numbers.

Sincerely,

MarcW.

Next time you're on that stretch. Try setting your consumption to read l/100km instead of mpg. As you begin to coast (car in gear), reset the fuel consumption. Once you start to get a read out, I bet it comes out at 0.0 l/100km. Once you disengage the clutch or slip it into neutral, it will start to show some consumption.

Zero fuel consumption while coasting with the transmission in gear.

It WILL coast faster (and presumably further) with the transmission in neutral, but does that increased speed/distance offset the fact that fuel, rather than momentum, is being used to keep the engine running?

I don't know the answer...

which is removed when the clutch is disengaged.

And auto stop/start while coasting would maximize the mileage, albeit weird.

Quote
Laz
And auto stop/start while coasting would maximize the mileage, albeit weird.

That won't be offered until the 2014 PDK models.

Guenter
2014 Boxster S
GT Silver, 6 Speed Manual, Bi-Xenons, Sports Suspension (lowers car 20mm), Porsche Sports Exhaust, Porsche Torque Vectoring, Auto Climate control, heated and vented seats, 20" Carrera S Wheels, Pedro's TechNoWind, Sport Design steering wheel, Roll bars in GT Silver
[www.cyberdesignconcepts.com]

from the U-joints.

Sincerely,

MarcW.

on a parts diagram?

and was going home from Vegas to L.A. and there is a stretch on the 15 that has about 5 mile(?) downhill and I would usually average around a 21-22 mpg for highway driving, but this time I put it in neutral going down the hill and it rose my trip avg from 21-22 mpg to 26 mpg for the entire trip. The only explanation I could think of is that since the rpm is about 800 at idle and if I was in gear it would be about 2500-3000 rpm. So the engine was using less gas when the engine does not spin as fast. I was under the assumption that if the car is running some gas is always fired into the cylinders, regardless if there was any load on the engine. This is all anecdotal but I have noticed my mileage increase doing the same sort of driving in my other car as well.

On a steep downhill 2 mile stretch near my house I usually reset the mileage for fun -- I keep it in gear. It usually goes to 99 mpg!
mike

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Lawdevil
On a steep downhill 2 mile stretch near my house I usually reset the mileage for fun -- I keep it in gear. It usually goes to 99 mpg!
mike

Mike it's limited to 3 digits that's why it "only" goes to 99.9 mpg.

The metric conversion measures how much fuel you use over a specific distance (how many liters you use in 100 km) so the lower the number, the less you're using

If you want to have some fun next time, before heading out, set your OBC to show "consumption" as l/100km instead of mpg

Your numbers will come out fairly low - maybe between 8 and 9 on the highway. When you get to that hill, reset your mileage (there's no such thing as kilometerage )

If you reset it once your foot is off the gas, I bet once you get a reading, it comes out at 0.0 l/100 km . In other words, you're using no fuel. (at that rate, you'd be using 0 liters if you went for 100 km (about 60 miles). It doesn't get any better than that.

Now if we could only get them to build all our roads down hill, we could do away with all those pesky gas stations.

I would love to have someone explain what I experienced to me. I had seen the posts saying no gas is used if the car is in gear but then I just don't know how I experienced what I experienced. Anyone have an idea how I got what I got?

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Michael SYS Driver
I would love to have someone explain what I experienced to me. I had seen the posts saying no gas is used if the car is in gear but then I just don't know how I experienced what I experienced. Anyone have an idea how I got what I got?

There are lots of different variables that can have an effect on your mileage.

One variable could be tail wind which can improve gas mileage - the higher the wind, the better the mileage. Opposite is true too, a head wind will decrease your mileage.

Having the top up or down can make a difference. My experience has been top up driving will improve gas mileage about 2 mpg on the highway. (The car is more aerodynamic with the top up, offering lower wind resistance.)

Speed will make a difference. Driving faster, causes more wind resistance and will result in lower gas mileage.

Load in the car has an effect on your mileage. Add a 160 lb passenger and 30 - 40 lb of stuff in the trunk, that 200 lbs will lower your mileage.

Driving more enthusiastically - even on a highway run will use more fuel. Every time you floor the gas pedal, there's lots of fuel being pushed through your injectors to help achieve that extra acceleration.

Those are just some of the things that can effect gas mileage.

I thought of those but if I am driving as I usually would on the whole trip except for the downhill section with the weather being a nice day with nothing out of the ordinary, and I was coming back from a track weekend so the car is a bit loaded more then usual, and I must have had the top up because if it was down then I would have gotten even better mileage. I guess I could have had a tailwind but there was traffic so I would not think it would effect my mileage any appreciable amount. I do understand these things and thought of them but there was nothing out of the ordinary. Going home from a track weekend I would think I was driving more "enthusiastically" but maybe I was driving easier. I guess I need to do another experiment and see what happens.

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Michael SYS Driver
I thought of those but if I am driving as I usually would on the whole trip except for the downhill section with the weather being a nice day with nothing out of the ordinary, and I was coming back from a track weekend so the car is a bit loaded more then usual, and I must have had the top up because if it was down then I would have gotten even better mileage. I guess I could have had a tailwind but there was traffic so I would not think it would effect my mileage any appreciable amount. I do understand these things and thought of them but there was nothing out of the ordinary. Going home from a track weekend I would think I was driving more "enthusiastically" but maybe I was driving easier. I guess I need to do another experiment and see what happens.

You've mentioned another variable - traffic, depending on how heavy and if there were are any slowdowns (coming to a crawl or having to stop) that will hurt mileage. On the other hand, if you're "drafting" a car or truck in front of you, that will increase your mileage. (It's way too dangerous with the way people drive, since you're following way to closely, but that's what NASCAR drivers do.)

You mentioned top up driving, that will give better mileage because the car is more aerodynamic than it is with the top down. (That's why Porsche puts all those plastic panels along the bottom of the car to make the bottom smoother, therefore, more aerodynamic.)

It's an interesting thing to experiment around with.

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Michael SYS Driver
and was going home from Vegas to L.A. and there is a stretch on the 15 that has about 5 mile(?) downhill and I would usually average around a 21-22 mpg for highway driving, but this time I put it in neutral going down the hill and it rose my trip avg from 21-22 mpg to 26 mpg for the entire trip. The only explanation I could think of is that since the rpm is about 800 at idle and if I was in gear it would be about 2500-3000 rpm. So the engine was using less gas when the engine does not spin as fast. I was under the assumption that if the car is running some gas is always fired into the cylinders, regardless if there was any load on the engine. This is all anecdotal but I have noticed my mileage increase doing the same sort of driving in my other car as well.

I think there must be other factors at play for an improvement of 4 or 5 mpg on this trip. LA to LV is roughly a 250 mile trip, so 5 miles would only be 2% of the entire trip.

At 22 mpg for a 250 mile trip, your car would use 11.36 gallons of fuel.
At 26 mpg for a 250 mile trip, your car would use 9.62 gallons of fuel

I don't see how you can save 1.7 gallons in 5 miles. Even at 22 mpg, 1.7 gallons should take you 22 X 1.7 = 37 miles rather than only 5 miles.

Someone on another board found this explanation as to what happens when you come off the gas and leave the car in gear. It explains why the car uses no fuel when decelerating while in gear. If you're on a relatively steep hill (5% grade or more) the momentum and weight of your car will roughly maintain your speed and basically "push" to keep your engine turning without using any fuel.

1. Only with transmission engaged. Must be fuel injected ECU style.
2. Triggered only for certain very strictly controlled scenarios. Most of these seem to work only above 1800-1900 rpm. If you are above this rpm and release the throttle in gear, the DFCO will shut off your fuel injectors. Your car will slow quickly with your engine just being pushed through its strokes by the tranny/tires moving on the road without fuel. When the engine reaches the low limit of say 1850 rpm (different cars are set differently)
the injectors open back up and the engine fires back up as unobtrusively as possible.
3. There are also some references that suggest DFCO is used to maintain better catalytic converter life as well as wear and tear on some other things.

So it still seems to come down to decelerate in gear and you use no fuel.

Decelerate (coast) in neutral, you car will be using some fuel to keep the engine at idle speed.