Downhill engine braking vs. coasting in neutral

Discussion in 'Fuel Economy' started by RickyKohler, Aug 1, 2007.

  1. RickyKohler

    RickyKohler New Member

    I came across an interesting comment in an article I just found, and I am interested in other people's experiences...

    While it is obvious that coasting with the engine off uses the least fuel (none), this article mentions that keeping the car in gear and letting the wheels keep the engine turning uses less fuel (none?) than idling in neutral, which requires fuel to keep the engine running (look for "throttle-off fuel saving technology" in the article):

    http://carsguide.news.com.au/story/0,20384,21210480-5001701,00.html

    First of all, is this true of newer cars? And second, if this is true (ignoring engine-off coasting as an option in this discussion), would it be better to keep it in gear (thus going a bit slower due to engine braking), or to coast in neutral and use a little fuel, but gain more distance due to the increased speed?

    In my case, during my daily commute there is a long downhill stretch, which I usually take in neutral. This actually causes me to accelerate from ~60MPH to well over 70MPH, thus exceeding the 65MPH speed limit (even if only a little). However, during rush hour, I often have to use my brakes to avoid random lane-changers, or when traffic is moving slower. Could the use of engine braking to control the speed of my descent use less fuel than coasting in neutral and using more brakes? I can always drop to neutral toward the bottom of the hill to coast the rest of the way into town.

    Anyone have an opinion on this one?
     
  2. lightfoot

    lightfoot Reformed speeder

    It would help to know exactly what car you have. I'm assuming it's not a hybrid.

    Yes many modern conventional cars have DFCO (Decel Fuel Cut-Off) so fuel flow stops when you take your foot off the gas in gear. However as you noticed when you do this engine braking slows the car down.

    If the ScanGauge is to be believed, however, fuel flow at idle is quite small, contrary to what this article says.

    So coasting in neutral at idle vs leaving the gear becomes a tradeoff that depends on the situation. On a steep downhill or coming to a stop I'll lift off the gas and leave the car in gear (assuming I am not willing to turn the engine off) because I have excess kinetic energy anyway so why not save a bit of gas. On a gentler downhill or when doing P&G, especially in traffic, I'll coast in neutral (again assuming I'm not going to turn the engine off).

    Two suggestions:
    (1) Approach the top of this long downhill at a somewhat slower speed so you don't accelerate to as high a speed at the bottom, and then coast in neutral at idle all the way down. That way you don't waste as much energy to wind drag near the bottom and you save a bit of gas by going to idle sooner at the top.
    (2) Better yet, shut the engine off. A long downhill is an ideal place to FAS (of course try this first in a safe location to see what the car does and how you feel about it!).
     
  3. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Rick:

    ___Modern day – fuel injected engines include a cute little trick called Engine braking with fuel cut. What this does is when you let off the gas from a speed where you would be coasting along at ~ 30% greater RPM then idle, the fuel injectors shut off and the engine begins to act like a compressor causing some drag on the drivetrain and wheels thus slowing you down but using no fuel. The problem is the drag it causes is no different then lightly touching the brakes even with fuel cut. Built up Energy (gas consumed to get your car up to speed) is being thrown away. ICE-Off in a FAS will save fuel of course but there are instances where Engine braking makes good sense. For instance, there is a long downhill with a stop sign or light at the bottom where you must stop, the traffic situation that you described or if you traverse a mountain (down slope side only of course) on your daily grind. Instead of transitioning into a FAS, you would use Engine braking w/ fuel cut to decelerate your vehicle somewhat instead of the brakes and use no fuel in the process. The problem comes when there is no impediment on the bottom and you are well within the speed limit bands. Engine braking slows you down. Some later hybrid vehicles have all kinds of cute valve tricks to reduce pumping losses and can be placed into a mode where they are using no fuel while ICE-Braking has been diminished altogether (Gliding). In every instance other then a hybrid with HSD/eCVT below 41 mph (even those hybrids are consuming energy via the pack and MGSet(s) during the glide), there is a lot of metal connected to the road throughout the drivetrain spinning around with that engine (now acting as a compressor) going along for the ride. This deceleration via engine braking is a known fuel economy killer although much less so then using the brakes depending on your current route’s hills and traffic circumstances.

    ___Back to the question … If you are going to have to slow anyway, by all means allow engine breaking with fuel cut to do its job. From the scenario you described, coming down a long and steep enough hill to reach terminal above the limits and having to tap the brakes to maintain position periodically, fuel cut is definitely the right nothing to do.

    ___A bit more advanced but I would top the hill at lets say 55 in the far right lane in a RR and let the down slope bring me up to terminal in a FAS. I have one of these lengthy sections on my daily grind where I can maintain a high velocity (not terminal however :() and move lane to lane to pass those that may be holding up a single of 3 lanes. Very advanced so YMMV and proceed with caution as usual.

    ___I hope that helped?

    ___Lightfoot, the SG-II cannot deduce fuel cut unfortunately and with air still passing through the engine, it reports this activity as a very low fuel use state at speed even when none is being used. A modern day diesel does report to the SG that you are indeed in fuel cut from much higher speeds (Rich and I saw this in a Jeep Cherokee CRD in Detroit last month) but that is the first and only time I have ever seen the SG report fuel cut as it is. No fuel use while still at speed and the ICE turning over.

    ___Good Luck

    ___Wayne
     
  4. lightfoot

    lightfoot Reformed speeder

    Thanks xcel, I need to keep reminding myself that the Scangage can't pick up DFCO. It reads air flow and then calculates gph from that by assuming a stoichiometric air/fuel mix, correct? But the 0.2gph that it reports when I am coasting at idle in neutral should be correct? So if I am coasting in neutral at say 30mph I would be getting 30 miles/0.2 gal or 150mpg?

    I forgot another reason to avoid using DFCO (lifting off the gas while the car is in gear and the engine is on) too much: it's so much like "typical" driving that one backslides into using it all the time. Most drivers use the accelerator to regulate speed, lifting off when they get going faster than they want to and then pushing down to pick up speed. So they never coast, and never get the benefit of moving along with the engine at idle or stopped. I try to use DFCO only when I make a conscious decision that the situation favors doing that, which at least for me isn't very often.
     
  5. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Mine usually reads 0.3gph at idle, so that sounds right. You can use the Open Loop / Closed Loop to read DFCO on the scangauge. It reads open on fuel cutoff.
     
  6. RickyKohler

    RickyKohler New Member

    Thanks for the replies. Maybe I should do one whole tank of gas one way, and another tank the other way and see if there is a major difference. Of course that would assume that all of my other driving around town, etc. was equal. I have seen a steady increase in FE as my car breaks in (and as I refine my techniques) from ~26MPG to just over 30MPG (average for the entire tank). It is rated at 29MPG for highway driving, so I think there is still room for improvement.

    My car: 2007 Subaru Impreza 2.5I Wagon.

    The other factor in the downhill stretch that I mentioned is that there are only two lanes, and most of the rush hour traffic begins to back up in the right lane for an exit at the bottom of the hill. This leaves the left lane wide open most of the time, except for the occasional driver who flies past everyone on the left, slams on his/her brakes when there is an opening, and merges right into the line of cars. Getting behind one of these people can ruin a good downhill run. And because of the way the traffic backs up, I usually start to coast in the left lane at just below the speed limit, which often causes an annoyed tailgater. Of course, then as I gain 10MPH+ on the way down the hill, I usually leave them behind.
     
  7. johnf514

    johnf514 Zoom? Try Glide!

    I've posted some of Wayne's information on www.mazda3forums.com, as we are having a similar discussion concerning SG-II readings of FE and when injectors shut down.

    Wayne - if I'm not mistaken, your information is from Ford engineers? Or the direct source, IIRC.

    One of the members sent an email to ScanGauge for more information, and another is emailing Mazda to see what they can find out.
     
  8. -mr. bill

    -mr. bill Senior Member

    Minor correction - it can report MAJOR fuel use. The worst case for the ScanGauge is a downshift to near red line. At 8,000 RPM it reports about 2 milliliters per second on my little 2 liter engine. It isn't using any fuel. Even at just over 1,000 RPM (where fuel cutoff ends) it reports above idle fuel use. The error is proportional to RPM.

    -mr. bill
     

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