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Old 07-04-2010, 02:26 AM   #1 (permalink)
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Question ? about Pulse and neutral coasting

I've been doing what i call pulse and neutral coast down hills or slight declines. Say i'm going 40mph i will see a slight decline/hill coming up and i will "pulse" (50-70% throttle) to about 50mph then coast in neutral with the engine on until i get back down to 35-40mph. I also do this on the highway I will be going about 60mph see a hill and "pulse" to 70mph and then coast in neutral down to 55-60mph. I will not pulse if i dont think that the coast will be long enough, that is why i pretty much always do it downhill. I've been doing this since i started hypermiling. I dont see how there could be much benefit from pulsing and then coasting on flat road, or am i wrong? I drive mostly highway and i am consistently seeing 40mpg. My question is am i wasting fuel by pulsing and then neutral coasting? Should i just coast down from the speed i am already going? Thanks in advance

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Old 07-04-2010, 08:09 AM   #2 (permalink)
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Most of my driving is in flat terrain and P&G has helped me use less fuel. I haven't done an A-B-A-B test to compare P&G vs steady speed, but it is in my plans. Aeromods and higher tire pressure definitely help your glide in flat terrain, they also allow you to keep a steady coast down slight inclines where previously you would lose speed. Win-win.

I'm not sure if I remember the exact reason for this correctly, but here is what's floating around in my head:
My engine consumes 0.5 liters of fuel per hour (8.3 milliliters per minute) when idling. Coasting down from 70 km/h to 50 km/h takes about 10 seconds. Accelerating from 50 km/h to 70 km/h registers as roughly 8.5 lph (142 mlpm) and takes about 5 seconds. So, one pulse/glide cycle takes 15 seconds and uses 11.83+1.38=13.21 ml of fuel, or 52.84 mlpm (3.17 lph). Driving at a steady 60 km/h registers at about 3.5 lph.

I used semi-random numbers just for illustration, I don't have the actual data, but those numbers look right. The longer you coast compared to the pulse, the better the results. Downhill and/or lower speeds is where P&G is best, flat terrain is still good, unless you're driving a brick on the highway. Groar once remarked that P&G-ing at highway speeds (above 90 km/h, 55mph) is pointless, same goes for hill climbing.
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Old 07-04-2010, 08:54 AM   #3 (permalink)
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As the load on your engine increases the efficiency increases, as well as the fuel consumption.

If 20 HP takes 1 unit of fuel, 50 HP takes 1.5 units of fuel, so the extra 30 HP cost you only half again as much fuel as the first 20. Therefore the cost per HP goes from 10 HP per half unit to 16.66 HP per half unit.

Its because you are getting higher effective compression with less throttle restriction, and your "lever" is stronger.

I would actually let the speed drop slightly uphill then recover it downhill if the grade is steep enough, but I think that is not your case.

In my Insight I accelerate very gradually uphill, and the mileage stays above 55 MPG. Then I semi coast downhill and the mileage jumps to 125+ MPG.

These are very slight grade hills, probably 1% grade, maybe even less.

I have seen readings of 95 MPG on that section of road, no traffic lights, and very little traffic since the road runs parallel to the Interstate.

I doubt you will see much gain if you have to pulse to 70 MPH. If you are trying to average over 50 MPH, the peak speeds you must reach can double the aero drag of your average speed and negate and increase in BSFC in your engine.

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Old 07-15-2010, 05:25 PM   #4 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
I doubt you will see much gain if you have to pulse to 70 MPH. If you are trying to average over 50 MPH, the peak speeds you must reach can double the aero drag of your average speed and negate and increase in BSFC in your engine.

regards
Mech
I'm forever being amazed by someone putting a simple reason behind why I'm hitting a glass ceiling with any new technique, and this bit is the nuggett which explains why my P&G is not working.

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Old 07-16-2010, 05:04 PM   #5 (permalink)
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I do much the same thing on a hill that I travel frequently. I'll accelerate to 60, put it in neutral, and can coast from 1 mile to 1.5 miles depending on traffic; when traffic is heavy, that last .5 mile puts me going too slow. Were I to try coasting at 50, I wouldn't have the speed to coast over the rolling bits of the hill without putting it back into gear.

According to info from my SG higher speeds while coasting gives ridiculous high mpg readings compared to even 10 mph slower. And if I do it right I can generally accelerate to the 60 mph without making the slushbox downshift. I expiremented a bit with using the coasting fuel cutoff built into the computer to get "free" miles, but coasting seems to use less fuel in the long run.

Maybe it's just the design of the engine, or an unusually slippery body, but I've gotten my best mpg figures when traveling north of 60, even compared to being ridiculously light on the throttle and trying to stay in top gear at slower speeds. It honestly makes no sense.
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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 07-16-2010, 06:03 PM   #6 (permalink)
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Maybe your car is geared so that the engine is in its efficiency sweet spot at a higher speed, and the sweet spot gains enough to overcome the aero penalty.
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Old 07-17-2010, 12:03 AM   #7 (permalink)
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I drive in the foothils and the Rockies so I have some great down hill runs in neutral with the engine on. My scanguage tells me that I am getting 0.4L/100km [587.5mpg US] and at the top of the hill I am going 90km/hr [55mph] and at the bottom I'm going 115km/hr [72mph] this goes on for miles at a time on occasion. We also get some high winds and this leads to some amazing fe when going down hill with a tail wind all the way to Calgary [50miles]. under these conditions I have got as low as 62mpg with the cruse control set at 92km/hr [56mph] if only I did not have to go back the other way and pay big time...
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Old 07-17-2010, 01:58 AM   #8 (permalink)
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Quote:
Originally Posted by comptiger5000 View Post
Maybe your car is geared so that the engine is in its efficiency sweet spot at a higher speed, and the sweet spot gains enough to overcome the aero penalty.
That is the only thing i can figure. I've even tried downshifting to 3rd when going 50 to try to find that sweet spot at a "normal" speed but no dice.

I'm at a loss as to why the mpg have plummeted lately. I repaired a vaccuum leak which made the car run much smoother (it wouldn't idle, it would lope) but it seems that now my o2 sensors have faulted because the mpg dropped like a rock afterwards.
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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 07-17-2010, 11:47 AM   #9 (permalink)
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Pull the ECU fuse on the car, let it sit for 10 mins, put it back, fire it up and take it for a drive. After having a vacuum leak, re-learning the fuel trims may restore your MPG.

Regarding the high speed sweet spot, consider yourself lucky. You can get the best highway mpg the car wants to give (at a steady speed) without having to dawdle along like the rest of us.
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Old 07-18-2010, 04:01 AM   #10 (permalink)
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Quote:
Originally Posted by comptiger5000 View Post
Pull the ECU fuse on the car, let it sit for 10 mins, put it back, fire it up and take it for a drive. After having a vacuum leak, re-learning the fuel trims may restore your MPG.
I think I may have done that......or maybe not, I can't remember? Bad thing is I know disconnecting the battery also resets my tripometer, which is annoying at best, so I tend to avoid such things. Not sure if pulling the fuse will reset it or not. I suppose I could write down the odometer reading....oh wait, the SG will store the miles for me. Problem solved!


Quote:
Originally Posted by comptiger5000 View Post
Regarding the high speed sweet spot, consider yourself lucky. You can get the best highway mpg the car wants to give (at a steady speed) without having to dawdle along like the rest of us.
LOL, I'm thankful in that part of the reason for buying the car was to have a good highway cruiser, and it certainly comes through on that aspect. But I rarely can cruise at anywhere near 80+ in my area, which is when I got that ridiculous 30+ mpg figure for a full tank. My brother and I were convinced for about an hour the gas guage must have been stuck, because it wouldn't hardly move! And we were flat getting with it!

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Quote:
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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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