Glide vs coast?

Here are some rough approximations for comparison.

A gallon of gasoline has about 44 horsepower-hours of thermal energy. While a Prius can extract about 17 hp-hrs of mechanical energy from that, a common non-hybrid probably gets closer to 13 hp-hrs.

My 1997 Subaru burned 0.25 gallons/hour at warmed-up idle. This is also what it burned in N-ICE-ON mode, gliding in Neutral.

When coasting in highest gear at highway speed, and computing the basic air pumping loss from the known engine displacement and engine monitor (ScanGauge for me, but lots of other choices available) readout of RPM and intake manifold vacuum, I'm remembering a loss of 6 horsepower. (As typical of North American manual transmissions of that era, it was geared short for performance, not tall for economy, so had a needlessly high RPM in top gear. Automatics allowed a bit lower RPM.) This doesn't even include the raw mechanical friction of engine spinning, which some others have suggested is at least 2 hp.

So figure 8 horsepower of total engine-spinning drag when coasting at 70 mph. That is equivalent to about 0.6 gallons/hour of fuel use. If that comes at a time that the car must stop with brakes or shed energy to limit speed, then at least it is not a true fuel burn. But if the driver is doing a pulse-and-glide pattern, or is going up and down shallow rolling hills where the journey continues at highway speed, then that in-gear coasting is costing real fuel, as it is shedding energy that must be replaced on the next pulse or uphill segment. Gliding in neutral with engine idling is then a true fuel saver.

My 2014 Subaru has the exact same engine size as that 1997. But the taller gearing (allowing lower highway RPM) and variable valves reduce the air pumping loss to an unknown amount. The idling fuel burn is also reduced, to 0.20 gal/hr.

Advanced exercise: compute the crossover speed, below which N-ICE-ON gliding no longer saves fuel. From the above estimated figures, that would be (very crudely) 28 mph for that specific car version. Other models and engine-transmission combinations would have different crossover speeds.

 

as much as i do agree with this and your conclusions, i don't think you factored in a few things:
1. you're using comparisons from your 20 year old subaru to argue for your 2014 car, which doesn't account for efficiency improvements in engine design (not to mention oil weight) that reduce overall frictional losses for the car (apparent in your 0.05 gal/hr improvement between the 2 cars), which would have a noticeable impact on the engine-spinning drag of 8 hp you had calculated again from your 20 yr old car. it's not just pumping losses that contribute to "engine spinning drag", so saying your new car has the same displacement as your old car is not apples to apples
2. revmatching out of neutral (as you should be doing whether manual or automatic) increases the fuel cost of neutral. however marginal it may be, but if its a rolling hills scenario as you mentioned, it'll add up
3. to roll downhill in gear at 70 mph means it's a really steep hill, steep enough that popping into neutral puts you well over the speed limit (dependent on state, but CA limits are ~55, tho we treat it more as a suggestion), and possibly the safe limit for any curves ahead
4. diminishing returns: the rate of air resistance increases exponentially the faster you're going, so popping into neutral at 70 mph saves proportionally less overall energy than popping into neutral at 40 mph--which you might have been accounting for with your 28 mph break even point

either way, its always gonna be a YMMV regardless of the method or scenario!

 

I'll reply to a few pieces at a time, as duties allow ...

You contradict yourself here, unless your claim is that every downhill must be either too shallow, or too steep, but never ever in between and approximately right.

I did acknowledge the need to sometimes leave it in gear for safety or speed limits. I can point you to several segments of I-90 between the Columbia River and Preston (outside Seattle) that are approximately right for my vehicles. And many many road segments elsewhere take require not merely being in gear, downshifted as well. Sometimes down several gears.

Are you claiming that the revmatching fuel will consume all the savings?

If not, then this becomes a 'So What?' Partial savings is still a savings.

Make that a low-order polynomial function, not an exponential function.

So what? A savings is still a savings. Most of my use of this is between 45 and 60 mph, where shallower slopes work.

 

I think you are picking at little nits, much too small to overturn the relative figures (already admitted as rough approximations) above.

To make N-ICE-ON gliding less efficient than DFCO (in-gear) coasting will require engine drag and pumping loss reductions of roughly 60% from that 1997 vehicle to the 2014 vehicle. But idling fuel use dropped only about 20%, and highway speed fuel consumption (derived from EPA rating) was reduced by only about 15%. Those are nowhere near enough to put the overall conclusion at risk.

 

It always amuses me to see the same car racing past me over and over again as I coast up to stop lights and generally go a nice even speed.

 

ah yes but hyper-miling is all about picking nits

i concede i concede! altho i still wont be popping my wife's subie into neutral


Posted via the PriusChat mobile app.

 

If she doesn't approve, then best not do it in her car. Keep it to your own, when she isn't along.

My spouse was doing extra long glides into stoplights, similar to the way hypermilers 'time' traffic lights, long before I met her decades ago. So expanding its use was no problem.

 

I don't consider myself as a hypermiler, but I've always paid attention to what's going on both immediately and well ahead of me and driven accordingly. That means changing lanes when someone is signalling a left turn and there's oncoming traffic that means he's going to end up blocking my progress, and it means watching the upcoming traffic lights so you get a sense of what portion of the cycle they're in to give you a heads up as to whether you need to hurry up a bit or ease off a bit. That's not hypermileing in my book, it's just common sense and situational awareness. I've never understood the people who race up to a red traffic light and then slam on the brakes at the last minute. I can't help but think that they're just not really paying attention to what's going on around them.

 

Probably busy playing Words with Friends.

 

Let me see if I can simplify....

Glide...was the nick-name of Clyde "The Glide" Drexler an All-Star NBA player...a nick-name given to him because of his seeming ability to "glide" through the air.
To my knowledge no NBA Player has had the nick-name "The Coast".
In fact to say an NBA player is "Coasting" would be considered an insult to their effort.

When I was a kid...occasionally we would go to "The Coast"...which meant we were going to the beach.
While you can take my PULSE at The Coast....while the tide comes Gliding in....
One should not confuse this activity as being related to any hyper miling technique.

Pulse and Glide, much like Abba...is a strange name for a musical group. But as a driving technique? I think I like it best ONLY second to the term "Feather The Brakes". Which has fallen out of favor since the advent of anti-lock brakes.

But whether you decide to Pulse and Glide or Pulse and Coast...or Coast to Coast with George Noory....remember that with any difficulty you encounter checking your 12 volt battery first is blood law.

I hope this clears any confusion up.

 

ROFLOL!!

 

Uncle Wayne [Mr. Gerdes, for newbies here] does include that on his hypermiling menu, because huge numbers of drivers don't do it and thus waste fuel.

While any of those drivers aren't paying attention, a certain portion are -- but are drag racing between lights, trying to beat the next red, or beat some other drivers to gain some positional improvement in the race to work or home, or just showing off. Or even trying to intimidate the slower drivers to speed up.