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Old 05-12-2008, 06:13 PM   #1
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Traffic Jam = P&G opportunity = FE++

The other day I was again caught in a "traffic jam" ("bumper to bumper traffic" going significantly slower than the speed limit) on the way to a conference (for work). And what I realized, was that the traffic (slow and packed as it was) actually had a lot of opportunities for saving fuel. Here's how I started driving through that "traffic jam":

1) Rather than "ride the bumper" of the car in front of me (like most of the traffic was doing), I let myself fall a few car length's back. This gave me a little maneuvering room, while still not falling so far back that people were tempted to dart into the gap opened up.

2) As soon as I got several car lengths back, I would "pulse" briefly (usually in 5th gear), so that I started approaching the car in front of me.

3) I would then "glide" until I either got too close to the car in front of me (and had to brake) or I fell back to my several car length's following distance (in which case I would "pulse" again). Naturally, I had this mostly timed correctly, so that I rarely had to brake.

NOTE: At speeds averaging 30MPH+, I was generally gliding in 5th gear (most of the time with FI cutoff, due to my foot off the throttle). At traffic speeds averaging 15-30MPH I would do the same thing in 3rd gear (again, usually with FI cutoff). And when traffic couldn't even keep up to 15MPH, I would generally "glide" in neutral (no FI cutoff, but a really nice glide for a very short "pulse").

Guess what? I found that I was spending the vast majority of my time "gliding" (often with FI cutoff) in the middle of this "traffic jam". And while I can't tell for sure what my FE was (as I don't yet have a supermid, or other real time FE instrument), I can say that it seemed (based upon comparing the distance traveled to my gas gauge) that I was doing pretty well (and that also seemed to be confirmed at the last fillup). So the technique seemed to be working. And as an added bonus, it even resulted in me having a greater (average) following distance than most of the other drivers (and therefore my driving style was probably not only better for FE, but also probably safer as well)!

So, as the old cliche goes, "Every cloud has a silver lining". And the FE "silver lining" in this case, was that I was able to use the poor traffic to my advantage (to give me a P&G opportunity that didn't otherwise disrupt the traffic around me).
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Old 05-12-2008, 09:45 PM   #2
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Nice! It's good to keep an open mind and look for opporitunity wherever you can.
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Old 05-12-2008, 10:01 PM   #3
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Yeah, I always take advantage of traffic to save fuel. My attitude of late has been the slower the traffic the better. Slow traffic means little p's and BIG G's :-D At 20mph the forces working on the car to slow it down are so small relative to the mass and momentum of the car.

You'd do even better if you turned the engine off to extend the glide.
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Old 05-13-2008, 08:13 AM   #4
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I call that playing the "traffic accordion". At least go into neutral for the glide, or even better, engine-off. Any time you have to use brakes, it hurts your mileage, so smaller-still pulses are best.
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Old 05-13-2008, 09:12 AM   #5
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It's true. I've gotten my best trips in heavy traffic. I laugh at the sheep testing their 0-40-0 times while my trip MPG climbs over 50.
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Old 05-13-2008, 09:17 AM   #6
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A prius LOVES these situations. It'll easily go over 100 mpg in stop-and-go traffic. On the highway, it's about equal with a civic hybrid and a TDI, but in town, nothing can touch it.
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Old 05-14-2008, 08:15 AM   #7
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Quote:
Originally Posted by PaleMelanesian View Post
I call that playing the "traffic accordion". At least go into neutral for the glide, or even better, engine-off.
As to neutral coast, I do that for lower speeds (average traffic < 15mph for example). And I also do it when I need just a little more distance to get where I'm going (such as the approaching red light) without the need to do another "pulse".

However, as far as I can tell, my P&G actually does better (from a FE standpoint) staying IN GEAR at higher speeds, because of the "fuel injector cutoff" (zero fuel used, not even idle) that occurs when I'm in gear with my RPMs at least 300 over my current (ECU controlled) idle RPMs.

Granted, even my 5th gear "glide" is shorter than my neutral glide, but the additional fuel savings from the FI cutoff seems to more than make up for the lost glide distance. And for 5th gear with the engine warm, I can usually maintain the FI cutoff (on my glide) down to around 30mph in my car. And if/when my speed goes below 30mph, I'll sometimes switch to 3rd gear (slightly higher engine drag, but also much higher RPMs) to get a 2nd (FI cutoff) glide down to around 15mph. Of course, below around 15mph, I do almost all my glides in neutral (or just press in the clutch, if I might need to make a quick emergency maneuver), as the engine drag in 1st or 2nd gear is high enough that the glide is just too short "in gear" (so I might as well go for a long neutral glide)...

Now, I could do even better (FE wise) if I combined neutral with engine off. But I wouldn't consider that safe, in such "bumper to bumper" traffic, where you might have to make an emergency maneuver at any time. And even in cases where engine off coast might be totally "safe" (final coast pulling into a parking space, for example), I still run into that "your key is still in the ignition" alarm in my car (the one that does a VERY LOUD "beep beep" when you turn the engine off, and only stops after removing your key and reinserting it). That (very distracting) alarm alone is reason enough for me to not to try "engine off" with my car (even in cases where it would likely be very safe to do).
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Old 05-14-2008, 08:31 AM   #8
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Draco, I have been experimenting with in-gear glides, but on my car there's not a high enough gear for a decent glide.

You shouldn't get the "your key is still in the ignition" alarm if you turn it back to "On" after killing the engine. I've been EOCing into my parking space every morning lately.
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Old 05-14-2008, 09:57 AM   #9
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If you have enough speed to be actually using the fuel-cut, you pulsed too high earlier. Try a lighter pulse and then neutral-coast.

Are you sure you're actually in fuel-cut? Most cars have an rpm threshold and only cut fuel above that point. Someone said that it's above 1,000 rpm on Hondas, but I haven't confirmed that.
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Old 05-14-2008, 12:17 PM   #10
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Quote:
Originally Posted by PaleMelanesian View Post
If you have enough speed to be actually using the fuel-cut, you pulsed too high earlier. Try a lighter pulse and then neutral-coast.
That's what I do for really low speeds. However, I find that staying in gear usually works better (for my car) at medium to high speeds.

Quote:
Originally Posted by PaleMelanesian View Post
Are you sure you're actually in fuel-cut? Most cars have an rpm threshold and only cut fuel above that point. Someone said that it's above 1,000 rpm on Hondas, but I haven't confirmed that.
I don't have instrumentation that proves it, but the symptoms are correct. I can feel the slight extra drag that kicks on when the fuel cuts out. And likewise, I can feel the reasonably gentle "bump start" that the ECU automatically does when the fuel cuts back on when the RPMs drop low enough.

And FWIW I used to think the fuel cutoff threshold was around 1000 RPMs in my car. Then, by watching the car closely, I noticed that the threshold kept changing. And finally, I realized that the threshold changed with my current idle speed (which the ECU adjusts for current engine condition). As far as I can tell, my fuel cutoff threshold is almost exactly 300 RPMs faster than whatever the current idle RPM speed as set by the ECU. So when my current idle RPMs is currently set (by the ECU) to around 700 (a fairly common idle speed for my ECU to pick on this car), my fuel cutoff is indeed around 1000 RPMs. However, when the car is cold and has an idle set at 1300 RPMs, the fuel cutoff is around 1600 RPMs! And conversely, when the car is very warm and has the idle around 450 RPMs, the fuel cutoff threshold is in the 750-800 RPM range.

In any event, I can often get fuel cutoff down to around 30mph in 5th gear, and 10-15mph slower than that in 3rd gear. And even in gear (like that), I still get a pretty long glide, given how well my car is lubed up.
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