I Does Pulse and Glide only work with manual transmissions and a kill switch?

[russ_watters]

In the mountains, on the downhills it’s beneficial to be in neutral coasting as this drops the engine rpm from around 2500rpm while coasting to around 750rpm.

I also thought that back in the time, but it actually consumes more fuel being in neutral than being in gear on downhill (with no foot on the gas).
https://www.popularmechanics.com/cars/hybrid-electric/a5977/coasting-in-neutral-fuel-economy/

Again, this will depend on the slope and what you are trying to achieve speed-wise. If the slope is pretty mild and the speed high it probably makes sense to idle in neutral. But that's probably a pretty narrow range of situations. For my car anyway it doesn't take much slope before it will do a runaway acceleration from highway speed in idle. So maybe the situation can be summed-up: if you're in neutral and on the brakes you're wasting energy that could be used to spin your engine and run your accessories.

I did a quick calc earlier, but it may be possible to calculate exactly the range of speeds and slopes where it makes sense to idle (again, I expect it is very narrow)...

That link, btw, starts off talking about coasting in idle to a stop sign/light. Maybe that's some really lazy manual transmission stuff? At worst I'd just leave it in my top gear until I was going slow enough I had to get out or stall. In my newer automatic with paddle shifters I often downshift while coasting to a stop. Better for fuel economy and better for your brakes than being in neutral.

Carbureted cars can't achieve zero fuel flow when running.

That I wasn't aware of, but then I don't think I've ever driven one. Is that because there's no fuel flow control except via carburetor airflow throttling, and that never goes to zero (otherwise you pull a vacuum at the engine)?

 

[anorlunda]

That I wasn't aware of, but then I don't think I've ever driven one. Is that because there's no fuel flow control except via carburetor airflow throttling, and that never goes to zero (otherwise you pull a vacuum at the engine)?

There's an idle screw that keeps the throttle open enough to set idle speed.

You're no spring chicken Russ. I can't believe that you never drove a car prior to fuel injection. I never drove prior to the invention of the wheel, but it was close.

 

[russ_watters]

You're no spring chicken Russ. I can't believe that you never drove a car prior to fuel injection. I never drove prior to the invention of the wheel, but it was close.

Google tells me fuel injection went mainstream in the '70s. How old do you think I am?

I've had a few carbureted devices, but haven't dug into them enough for that. Thinking back to the RC plane I built as a kid, I remember it had a screw/pin for adjusting mixture, but no idle setting (probably because you want to be able to cut it off with the throttle). I'm flying real ones now, and I guess it probably has one, but it isn't for me to adjust.

 

[anorlunda]

Google tells me fuel injection went mainstream in the '70s. How old do you think I am?

LOL. I didn't mean to suggest ancient. But I did assume that most of us started with a 5-10 year old car.

Also the mainstream takeover of fuel injection was gradual. Wikipedia says:

Manifold injection was phased in through the latter 1970s and 80s at an accelerating rate, with the German, French, and U.S. markets leading and the UK and Commonwealth markets lagging somewhat. Since the early 1990s, almost all petrol passenger cars sold in first world markets are equipped with electronic manifold injection.

So up until 1995-2000 there were still numerous carbureted cars on the roads in the USA.

 

[Devin-M]

I also thought that back in the time, but it actually consumes more fuel being in neutral than being in gear on downhill (with no foot on the gas).
https://www.popularmechanics.com/cars/hybrid-electric/a5977/coasting-in-neutral-fuel-economy/

Again, this will depend on the slope and what you are trying to achieve speed-wise. If the slope is pretty mild and the speed high it probably makes sense to idle in neutral. But that's probably a pretty narrow range of situations. For my car anyway it doesn't take much slope before it will do a runaway acceleration from highway speed in idle. So maybe the situation can be summed-up: if you're in neutral and on the brakes you're wasting energy that could be used to spin your engine and run your accessories.

The article was an interesting read, and I didn't know the fuel injectors would bring the fuel flow to 0% while coasting in gear on a downhill.

That said I don't believe all factors have been considered in the article. When I was talking about coasting downhill in neutral in the mountains, for me, I meant going 60-70mph downhill for 5 miles or so at a time while in neutral on a hill like this:

From what I gathered in the article, even if fuel injectors aren't injecting any fuel at all while the car is in gear coasting downhill, surely keeping the engine turning at 2500rpm with no fuel being consumed is an additional source of drag above and beyond the air resistance and rolling resistance.

If my memory serves, and I do keep the car in gear while coasting downhill, it will lose speed faster than if it was in neutral, and so I might need to push the gas a little bit to maintain speed, whereas if I were to keep the car in neutral, it will maintain speed on its own from the release of gravitational potential energy.

 

[Motore]

Again, this will depend on the slope and what you are trying to achieve speed-wise. If the slope is pretty mild and the speed high it probably makes sense to idle in neutral.

When the speed is higher than the speed achieved purely with the torque of the engine, then leaving it in gear will consume no fuel. Leaving it in neutral will always consume fuel.

That said I don't believe all factors have been considered in the article. When I was talking about coasting downhill in neutral in the mountains, for me, I meant going 60-70mph downhill while in neutral on a hill like this:

Sure if you want to mantain constant speed, then it's another matter. It depends on the slope.

Every weekend I drive for 5 km down an average slope of 5% and I always leave it in the highest gear and it stays roughly in the 120 km/h (+-10km/h). It consumes no fuel at all, while in neutral it would consume about 0.6 l/h.

From what I gathered in the article, even if fuel injectors aren't injecting any fuel at all while the car is in gear coasting downhill, surely keeping the engine turning at 2500rpm is an additional source of drag above and beyond the air resistance and rolling resistance.

Sure, but from the safety concern it's actually a good thing. From my example above, if I would coast in neutral I would go around 150 km/h which on a twisty road is not really safe. And it would still consume fuel.

 

[russ_watters]

When the speed is higher than the speed achieved purely with the torque of the engine, then leaving it in gear will consume no fuel. Leaving it in neutral will always consume fuel.

For the scenario I'm describing, the slope is not enough to keep the car from slowing down. So fuel is needed either way (in gear or in neutral).

For the particular hill I was thinking of on my drive home, if I'm a gear or two below my top gear, the car slows down (not a highway).

 

[Motore]

For the scenario I'm describing, the slope is not enough to keep the car from slowing down. So fuel is needed either way (in gear or in neutral).

For the particular hill I was thinking of on my drive home, if I'm a gear or two below my top gear, the car slows down (not a highway).

Hmm it the slope is so big that the car wouldn't slow down, then leaving it in gear will consume no fuel and it will add some drag. If you have it in a lower gear the car will of course slow down faster and it still won't consume fuel until the speed because of the torque of the engine will surpass the speed of the car. This:

When the speed is higher than the speed achieved purely with the torque of the engine, then leaving it in gear will consume no fuel. Leaving it in neutral will always consume fuel.

is always true.

 

[anorlunda]

is always true.

Only for fuel injected engines.

 

[Motore]

Only for fuel injected engines.

Agree.

 

[Devin-M]

There's another stretch I often do with a 9.2 mile 1329ft descent which works out to 2.7% avg grade, and the speed limit is 55mph. I am thinking of doing a little experiment to see how well I can maintain 55mph in neutral vs in gear... & there's a gas station right at the bottom of the hill, so I should be able to fill up once, go up and down in gear, and then fill up again, go up, then down in neutral, then fill up and compare which used less gas while maintaining 55mph.

 

[Kenwstr]

I haven't read the article, not sure how to access it, so just some thoughts relating to comments:

If the car is an automatic, the transmission will generally be disengaged (apart from its minimal transmission drag) while coasting downhill, so the engine will be idling. You can of course put it in sports shift to lock the transmission & engine brake, but then your loading up and wearing the transmission rather than the brakes. I'd rather replace brake parts than trash a whole car due to worn transmission.

If it's a manual in gear, it's the same situation as sports shift above. If it's carburated, the engine will drag air/fuel past the throttle butterfly valve at a rate greater than idle, because the engine will be doing more revs than at idle.

I am uncertain about the situation with fuel injection, when the the transmission is driving the engine. It seems to me that the fuel control system could be made totally shut off the injectors, whether that is done, I don't know. I'm more familiar with maintaining carburated systems, none of which totally shut off fuel IME.

It should be noted that the most expensive component of a car is it's transmission. If you are engine braking through the transmission, this increases the load on the transmission, also increasing wear. When you wear out the transmission, the car is generally done as it's not worth the replacement cost. When you wear out brake parts, you just get them replaced, your choice. On very long steep decants, engine braking can save your brakes from overheating, which used to be more of a problem than it is now, especially when loaded up or towing unbraked trailers.

When I was younger (70s), on long straightish country descents, I used to just slip out of gear and coast without controlling speed (no braking), while illegal, this extended the coasting distance, sometimes considerably if it got me over another rise. Too much traffic, cops & radars these days. Probably not as beneficial as some may imagine because D = 1/2p x V^2 x S x Cd, that V^2 is the kicker.

 

[russ_watters]

If the car is an automatic, the transmission will generally be disengaged (apart from its minimal transmission drag) while coasting downhill, so the engine will be idling.

My last few cars haven't done that (though on my current car you can choose). If you have an rpm gauge though it will be easy to tell if it does or doesn't.
[Edit]
And by the way, this really annoyed me when I first saw it:

It's wrong and I have no doubt the engineers who designed the car know it is wrong. I'm sure it's there because marketteers think more features = better.

 

[anorlunda]

It should be noted that the most expensive component of a car is it's transmission. If you are engine braking through the transmission, this increases the load on the transmission, also increasing wear.

Are you sure about that? There's not much difference in the transmission providing braking force, as providing driving force to make the car go forward. Where is the evidence other than instinct that engine braking causes excess transmission wear?

My Toyota canceled cruise control when down shifting. Now I have a Mazda that leaves cruise control when I downshift. That's helpful holding a constant speed when going down steep hills where there are signs TRUCKS MUST USE LOWER GEAR. Sometimes, the transmission downshifts itself when going downhill in cruise control.

 

[Devin-M]

I just spent several hours testing this—

Let me recap-- I drive a 2008 Nissan Versa, 4 speed automatic, non-hybrid. It's advertised for 33mpg on the highway. When I drive it 61mph behind semi trucks for hundreds of miles I can usually average 40.5 miles per gallon (measured when I refill the tank - miles driven divided by gallons topped off - usually after ~350 miles). 40.5 mpg is about the highest MPG I've previously measured.

Now to today's experiment...

This is the test course:
-I began the experiment at the bottom of a mountain at a gas station in Lewiston, CA (at the bottom of the map), and used Gas Pump 1 to top off my tank (and by top off I mean the gas spigot would instantly turn off if I tried to pump more). All subsequent refills used this same Gas Pump 1, and were topped off in the same manner.

-Each "test" consisted of going up the mountain, back down to the gas station, then back up the mountain then back down-- 49.6 miles according to the Google map, 51.9 miles according to my odometer.

-55mph speed limit - though my speed varied at various points depending on terrain between about 38-60mph

-Neither test was I closely followed by another vehicle nor did I closely follow any other vehicles

-The first 51.9 mile test, I left the automatic transmission in "Drive" the entire way, both up and down, and I tried to get the highest possible MPG-- foot off the accelerator on the down hills, foot very lightly pressing the gas on the uphills. This test took 1hr:10min:54sec, and when I again completely topped off the tank on the same pump at the end, 1.12 US Gallons had been consumed, for a total of 46.3 MPG, the highest miles per gallon my car has ever achieved

-The second 51.9 mile test, I put the transmission in neutral as much as possible on the downhills in an attempt to save fuel, and I again tried to get the highest possible MPG-- foot off the accelerator on the down hills (in neutral), foot very lightly pressing the gas on the uphills. This test took 1hr:13min:38sec (2 min 44sec longer), and when I again completely topped off the tank on the same pump at the end, 1.91 US Gallons had been consumed, for a total of 27.1 MPG, considerably lower than the 40.5 MPG I get going 61mph at constant speed behind semis over a long distance

Conclusion: The results are consistent with considerably improved fuel economy when leaving the car in drive and foot off the throttle on long down hills (46.3 MPG), compared to significantly worse results when putting the car in "neutral" on long downhills (27.1 MPG), compared with medium results at constant 61 MPH behind a semi truck on flat ground (40.5 MPG)

 

[Devin-M]

 

[russ_watters]

Are you sure about that? There's not much difference in the transmission providing braking force, as providing driving force to make the car go forward. Where is the evidence other than instinct that engine braking causes excess transmission wear?

Where the rubber meets the road is actual lifespan and failure. I've never owned a car that didn't need several brake pad changes and most a rotor change, and I've also never owned a car that needed a transmission replacement.

A few years ago I was driving with a middle-aged client in the only car she'd ever owned, which she got in high school (I believe a Civic). It was most of the way back from the moon and really needed to be taken out behind the barn and shot, but the engine and drivetrain were fine. No major repairs ever (I asked).

Now, I've never gone above 180,000 miles myself but my sample suggests that transmission failure is fairly rare or by the time you get there it's time to take the car off life support anyway.

My Toyota canceled cruise control when down shifting. Now I have a Mazda that leaves cruise control when I downshift. That's helpful holding a constant speed when going down steep hills where there are signs TRUCKS MUST USE LOWER GEAR. Sometimes, the transmission downshifts itself when going downhill in cruise control.

My Kia takes that a step further and automatically downshifts (or declines to upshift) when going downhill to avoid accelerating. But the car doesn't know if that's what I actually want and I frequently override it. I don't think that feature can be turned off.

 

[russ_watters]

-The second 51.9 mile test, I put the transmission in neutral as much as possible on the downhills in an attempt to save fuel, and I again tried to get the highest possible MPG-- foot off the accelerator on the down hills (in neutral), foot very lightly pressing the gas on the uphills.

Good test. Not critical, but can you clarify if this required braking to avoid too much acceleration and/or if you allowed the car to accelerate as much as it wanted (and if so how much)?

 

[Devin-M]

Good test. Not critical, but can you clarify if this required braking to avoid too much acceleration and/or if you allowed the car to accelerate as much as it wanted (and if so how much)?

The road was curvy and had ups and downs during the descent so both runs required braking and acceleration. I treated the first run in “drive gear the whole time” like I wanted to get the highest possible efficiency so I wasn’t accelerating quickly, very light on the throttle (foot off the throttle as much as possible while descending). The second run I was also trying to get maximum efficiency, but for that run I assumed being in neutral as much as possible on the descents would achieve higher efficiency— but it clearly did not.

 

[Devin-M]

What surprised me greatly is I was able to achieve higher overall efficiency on the round trip up and down the mountain than I can at constant 61mph on flat ground behind a big rig… I ascribe that to the engine essentially being off half the time which is not the case on flat ground.

 

[Lnewqban]

What surprised me greatly is I was able to achieve higher overall efficiency on the round trip up and down the mountain than I can at constant 61mph on flat ground behind a big rig… I ascribe that to the engine essentially being off half the time which is not the case on flat ground.

It seems that car’s manufacturers have discovered that anything that keeps the engine rotating and legally connected to the wheels without burning fuel is one of the ways to best fuel efficiency.

Keeping the engine idling is expensier than using gravity to make it spin, overcoming useless compression and valve train drag, via automatic transmission and torque converter (which works both ways, once certain amount of rpm’s is achieved).

Thank you for sharing that good test, @Devin-M

 

[jrmichler]

I was able to achieve higher overall efficiency on the round trip up and down the mountain than I can at constant 61mph on flat ground behind a big rig… I ascribe that to the engine essentially being off half the time which is not the case on flat ground.

The engine efficiency has a lot to do with this. Most cars use less than 10% power at a steady 61 MPH on a level road, which puts the engine at a low efficiency point on the performance map. See the example performance map in Post #20. Most gasoline car engines have peak efficiency at about 80% of peak torque at 1/3 to 1/2 of peak RPM. That's several times the power needed to run at steady speed on a level road.

Extreme gas mileage driving, AKA hypermiling, uses that knowledge by accelerating at a power and RPM near peak efficiency, then shutting the engine off and coasting. It's called pulse and glide (P&G), and works best in gentle rolling hills.

 

[Devin-M]

While climbing I was maintaining the engine rpms below 2700rpm which meant the car could only go about 41mph max in 3rd gear on certain sections of the climb.

 

[Devin-M]

The engine efficiency has a lot to do with this. Most cars use less than 10% power at a steady 61 MPH on a level road, which puts the engine at a low efficiency point on the performance map. See the example performance map in Post #20. Most gasoline car engines have peak efficiency at about 80% of peak torque at 1/3 to 1/2 of peak RPM. That's several times the power needed to run at steady speed on a level road.

Extreme gas mileage driving, AKA hypermiling, uses that knowledge by accelerating at a power and RPM near peak efficiency, then shutting the engine off and coasting. It's called pulse and glide (P&G), and works best in gentle rolling hills.

I did a Pulse and Glide test today over 339.8 miles of basically flat terrain... the "pulses" were me accelerating from 55mph to 65mph (about 10 seconds), and the "glides" were me coasting from 65mph back down to 55mph (also about 10 seconds). Speed limit was 70mph. Supposedly during the glides my fuel injectors were off because my foot was off the throttle in Drive gear, so essentially my engine was off half the time. Engine rpms were maintained at all times between 2200rpm and 2700rpm -- 55mph was 2200rpm and 65mph was 2700rpm. This took 9.42 gallons so I averaged 36.07 MPG. While this did result in better fuel economy than my 2008 Nissan Versa 4 speed automatic non-hybrid's advertised highway miles per gallon (33 MPG), it was worse than when I simply follow a big rig at constant 61mph, which results in 40.5 MPG.

(round trip)

 

[jrmichler]

P&G requires a manual transmission and a kill switch. The driver shifts to neutral and kills the engine at the start of a glide, then bump starts the engine for the next pulse. Pulse and Glide does not work with automatic transmissions. The engine is still spinning, so the engine friction combined with driveline losses is slowing the car down.

P&G is most useful at lower speeds. My Canyon showed little benefit from it at 55 MPH on level roads. The primary benefit was at lower speeds and/or rolling hills. The glides need to be longer than the pulses to see a benefit.

And not all cars shut off the fuel when the throttle is closed. My old Canyon would not shut off the fuel at 2000 RPM, although I have a hazy recollection that it would shut off fuel above 4000 RPM.

 

[russ_watters]

P&G requires a manual transmission and a kill switch. The driver shifts to neutral and kills the engine at the start of a glide, then bump starts the engine for the next pulse. Pulse and Glide does not work with automatic transmissions. The engine is still spinning, so the engine friction combined with driveline losses is slowing the car down.

That's real hypermiler shstuff though. Most - excuse me - normal people will not want to turn off their air conditioning and power steering and go through the effort of the P&G itself. Point being: are you sure P&G does not work if the car stays in gear? I mean, certainly you won't get maximum benefit, but you should still get some, right?