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Coasting to a stop


DaveloMA
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A little bit off in the number.  Mythbusters obtained a 20% gain in FE at a distance of 50 feet and a 11% gain at 100 feet  at 55 mph behind a semi-truck.   But I agree with the safety concerns of drafting - at 100 feet back, it takes about 1 1/4 seconds to close the 100 feet at 55 mph (80 feet/second).

 

This has been discussed in other threads and IIRC most believe that the two second rule is a safe driving distance behind another vehicle. So, two seconds at 55 mph is 160 feet which would be very little if any FE improvement from drafting.  Now having said that, driving the freeways around Phoenix, it's virtually impossible to maintain a two second rule.  At 70 mph in rush hour, I'd say the traffic is generally less than 100 feet apart.   At 100 feet cars will be filling the 100 foot gap.

Cool, thanks for the correction. That's a semi though, and most vehicles on the highway are not semis. How effectave is drafing a car? I suspect not very effective at all unless you are right behind them.

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Cool, thanks for the correction. That's a semi though, and most vehicles on the highway are not semis. How effectave is drafing a car? I suspect not very effective at all unless you are right behind them.

I don't know what the effect might be at 6 feet, but I have experimented with drafting behind a semi and a car in my TDI in the past and saw the instantaneous FE go up by about 50% behind a semi at around 25 feet at 70 mph and virtually nothing at 100 feet.  I don't recall any gain drafting a car at about 25 feet. Also,  I am sure it annoys others drafting that close especially the truckers as it does me.  So, I don't do it.

 

Here's the link to the Mythbusters test. 

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If you're drafting a tractor trailer, you're playing a deadly game. If you can't see their mirrors, they can't see you.

ICC bumper standards still don't require the updated reinforcement to keep your vehicle from going under the trailer,

if you do rearend one. If you're lucky, the trailer might be fitted with the voluntary updated version and it could save you.

Edited by drdiesel1
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Yes, Paul, that's coasting.

 

Scott, I'm going to disagree with fotomot... glide is coasting by definition.

 

Keep in mind that hypermiling greatly predates when hybrids hit the market, but some techniques are so much easier with a hybrid drivetrain they're become associated with hybrids. Pulse and glide is one of them.

 

 

From the link:  Pulse and glide works like this: let's say you're on a road where you want to go 60 km/h. Instead of driving along at a steady 60, you instead accelerate to 70 (that's the pulse), and then coast in neutral with the engine off down to 50 (that's the glide).

 

That link is 8 years old for traditional ICE vehicles.  The technique and definition (for hybrids) have evolved.  The 3rd gen Prius came with a power meter and an ECON button to allow the driver to easily P&G without illegally or unsafely operate in a neutral gear. Just apply enough accelerator pedal to cancel out regen and get the power flow (in and out) to zero.  It's harder to manipulate in the CMax so I prefer to err slightly on the positive side and get slightest hint of a blue bar showing on the meter(s).

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From the link:  Pulse and glide works like this: let's say you're on a road where you want to go 60 km/h. Instead of driving along at a steady 60, you instead accelerate to 70 (that's the pulse), and then coast in neutral with the engine off down to 50 (that's the glide).

 

That link is 8 years old for traditional ICE vehicles.  The technique and definition (for hybrids) have evolved.  The 3rd gen Prius came with a power meter and an ECON button to allow the driver to easily P&G without illegally or unsafely operate in a neutral gear. Just apply enough accelerator pedal to cancel out regen and get the power flow (in and out) to zero.  It's harder to manipulate in the CMax so I prefer to err slightly on the positive side and get slightest hint of a blue bar showing on the meter(s).

My technique is about the same as ScubaDadMiami stated above.   I understand this P & G for a traditional ICE vehicle but I don't understand why you would want to do this on a Hybrid?  Would you not want to use the battery power to drive the car?  Would you drain off the battery and then Glide?  For me by the time I drain the battery I am low on speed anyway so I don't know how I would do that?

 
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 why you would want to do this on a Hybrid?  

 

You need to consider that in a hybrid, ALL energy comes from gasoline; even the electricity in the battery.  Even if it comes from regeneration while coasting down a hill, you had to burn gas to get to the top of it.  No free lunch, etc.

 

Gliding is basically reducing the most amount of drag that you can.  Coasting involves drivetrain drag.  In a conventional car, that comes from the engine/transmission still being driven by the wheels while in a hybrid it comes from regeneration.  Either quickly slows the vehicle down relatively speaking.  Gliding removes those losses and you will travel much further from a Pulse than simply coasting.  This is not a method to maintain a constant speed.  You pulse up and glide slowly back down; rinse, lather, repeat.  Do it enough and you can actually develope muscle memory and do it subconsciously.  :headspin:

 

To be in a CMax glide, both the up and down chevrons that are around the battery indicator need to be off.  It's not easy to modulate and, like I said, I err slightly on the EV side just as ScubaDad described.  50's and 60's are easily obtained but here is one of my best over a decent distance (warm weather, warm ICE):

 

null_zpsb40f84d1.jpg

 

Jus-a-CMax has even reported trips in the 80's.  :drop:

Edited by fotomoto
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You need to consider that in a hybrid, ALL energy comes from gasoline; even the electricity in the battery.  Even if it comes from regeneration while coasting down a hill, you had to burn gas to get to the top of it.  No free lunch, etc.

 

Gliding is basically reducing the most amount of drag that you can.  Coasting involves drivetrain drag.  In a conventional car, that comes from the engine/transmission still being driven by the wheels while in a hybrid it comes from regeneration.  Either quickly slows the vehicle down relatively speaking.  Gliding removes those losses and you will travel much further from a Pulse than simply coasting.  This is not a method to maintain a constant speed.  You pulse up and glide slowly back down; rinse, lather, repeat.  Do it enough and you can actually develope muscle memory and do it subconsciously.  :headspin:

 

To be in a CMax glide, both the up and down chevrons that are around the battery indicator need to be off.  It's not easy to modulate and, like I said, I err slightly on the EV side just as ScubaDad described.  50's and 60's are easily obtained but here is one of my best over a decent distance (warm weather, warm ICE):

 

null_zpsb40f84d1.jpg

 

Jus-a-CMax has even reported trips in the 80's.  :drop:

I think the issue might be that some may not appreciate the magnitude of the electrical losses associated with regeneration and use of the stored regenerated electrical power from the HVB in EV mode.

 

I think one needs to define /look at the losses that are eliminated with "gliding" in the C-MAX by feathering the throttle such that there is no energy flow into the HVB vs foot off the throttle "gliding" with regeneration.  MG1 and MG2 spin in either case in proportion to vehicle speed (since ICE is off and not spinning).  So, the frictional losses of the drive train should be about the same in both.  But, the losses that are eliminated by feathering vs foot off the throttle "gliding" are virtually all electrical losses associated with reducing / eliminating regeneration and the reuse of such in propelling the C-Max in EV mode.  

 

This should save fuel since by reducing / eliminating these losses, the C-Max will glide further.  IMO, gliding losses with foot off the throttle associated with regeneration are at least 25% of the KE at the wheels.  So, "gliding" with foot off the throttle is returning only 75% of the KE back to the wheels.  By feathering the throttle the C-Max can use nearly 100 % of the KE at the wheels (but for some energy to maintain a zero HVB power flow) and glide further until minimum speed is reached.

Edited by Plus 3 Golfer
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  • 7 months later...

I agree with most of what Plus 2 had to say - especially:

 

Since there are electrical related losses associated with regeneration, IMO it's more efficient to coast in N.

 

I have been coasting down long, moderately steep hills in Neutral (for example, Davelo, back in the day when I lived in the Boston area, down Rte 2 Belmont Hill toward Fresh Pond) since the 70's, and I still do it with the C-Max and my Buick Lacrosse (mild hybrid).  If I can coast, instead of feathering the throttle to maintain speed. that has to be a gas saver.  If the hill is too steep and I will pick up too much speed, then of course I use D or even hill descent mode   I do not coast in Neutral up to stop signs or red lights.

A regular fuel injected car (not one with start-stop) turns off the injectors when coasting in drive. By putting your car in neutral, gas is being used to keep the engine spinning instead of using the inertia built up but traveling forward so by shifting into neutral you are using MORE gas, not less. Plus it's a dangerous thing to do. Ever need to accelerate in a hurry to avoid an accident? Hard to do when you're in neutral. So there is actually no reason to do this. it wastes fuel and is dangerous. 

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A regular fuel injected car (not one with start-stop) turns off the injectors when coasting in drive. By putting your car in neutral, gas is being used to keep the engine spinning instead of using the inertia built up but traveling forward so by shifting into neutral you are using MORE gas, not less. Plus it's a dangerous thing to do. Ever need to accelerate in a hurry to avoid an accident? Hard to do when you're in neutral. So there is actually no reason to do this. it wastes fuel and is dangerous. 

 

The engine doesn't necessarily keep spinning in a hybrid when placed in neutral, it depends on how the car is programmed. 

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  • 1 month later...

I agree with most of what Plus 2 had to say - especially:

 

Since there are electrical related losses associated with regeneration, IMO it's more efficient to coast in N.

 

I have been coasting down long, moderately steep hills in Neutral (for example, Davelo, back in the day when I lived in the Boston area, down Rte 2 Belmont Hill toward Fresh Pond) since the 70's, and I still do it with the C-Max and my Buick Lacrosse (mild hybrid).  If I can coast, instead of feathering the throttle to maintain speed. that has to be a gas saver.  If the hill is too steep and I will pick up too much speed, then of course I use D or even hill descent mode   I do not coast in Neutral up to stop signs or red lights.

Edited by Redshift
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