[bimmer456]

So when running in Eco-Pro mode on the highway, the car will enter a coast mode where it goes into Neutral when you're not pressing the gas. I read somewhere that all modern cars turn off the fuel injectors when in gear and you're not pressing the gas and the engine is above say 2000 rpm. If the engine drops below maybe 1500 the fuel injectors come back online. So in this condition you're not using any fuel, vs. in coast mode the transmission is in Neutral and the engine is idling, so it's using fuel otherwise it would stall. Wouln't it make more sense to leave it in a higher gear to prevent engine braking while coasting and turn off the fuel injectors? Or maybe turn off some of the fuel injectors like cylinder deactivation on some cars.

[Billfitz]

If you could turn off the fuel injectors the engine would stop.

[bimmer456]

Originally Posted by Billfitz

If you could turn off the fuel injectors the engine would stop.

Actually the engine would keep spinning if it's in gear, though there would be engine braking to some degree due to the compression in the cylinders. It would stall if you came to a complete stop.

[Billfitz]

Originally Posted by bimmer456

Actually the engine would keep spinning if it's in gear, though there would be engine braking to some degree due to the compression in the cylinders.

There's a lot of engine braking. If you had a car with a standard and no power anything, especially steering, you could test it by turning the ignition off, though I wouldn't recommend it. Wherever you saw the idea that injectors are turned off when coasting is wrong. The injectors on unused cylinders are turned off with a variable cylinder engine, but only on the unused cylinders, and in order to work the valves on those cylinders are also kept closed.

[bimmer456]

Originally Posted by Billfitz

There's a lot of engine braking. If you had a car with a standard and no power anything, especially steering, you could test it by turning the ignition off, though I wouldn't recommend it. Wherever you saw the idea that injectors are turned off when coasting is wrong. The injectors on unused cylinders are turned off with a variable cylinder engine, but only on the unused cylinders, and in order to work the valves on those cylinders are also kept closed.

I don't know if it's on a BMW, I saw it on documentation on a honda that the injectors are shut off when the engine is above a certain rpm even in neutral, but can bring the injectors online before the engine stalls and sustain an idle.

[HighlandPete]

Originally Posted by bimmer456

I don't know if it's on a BMW, I saw it on documentation on a honda that the injectors are shut off when the engine is above a certain rpm even in neutral, but can bring the injectors online before the engine stalls and sustain an idle.

Yes, in normal driving mode fuel is off, on the over-run, above specific revs, (some engines as low as 1,500rpm). Been like this for years with fuel injection. The idea being "foot off the gas" to ensure fuel cut off and fuel saving.

I also question how coasting saves more fuel, only possible if drive train losses exceed the fuel used to idle the engine.

[-435UK-]

I would say coasting does save fuel on my 435. Especially on a long downhill stretch. Without it the engine slow the car so much that you need to depress the accelerator repeatedly to maintain speed. I've noticed mine doesn't always coast though, it seems to be linked to the satnav. When the reduce speed image is present on dash it automatically engaged gear, wish I could change that.

[Billfitz]

Originally Posted by bimmer456

I don't know if it's on a BMW, I saw it on documentation on a honda that the injectors are shut off when the engine is above a certain rpm even in neutral, but can bring the injectors online before the engine stalls and sustain an idle.

I doubt they're all shut off. Honda is one brand that uses variable cylinder management on sixes.

[bimmer456]

Originally Posted by Billfitz

I doubt they're all shut off. Honda is one brand that uses variable cylinder management on sixes.

That's different from the fuel cut off. That's only on some sixes vs all cars have the fuel cut off. I think the manual sixes don't have VCM as this impacts responsiveness when you get on the throttle.

[bimmer456]

Originally Posted by -435UK-

I would say coasting does save fuel on my 435. Especially on a long downhill stretch. Without it the engine slow the car so much that you need to depress the accelerator repeatedly to maintain speed. I've noticed mine doesn't always coast though, it seems to be linked to the satnav. When the reduce speed image is present on dash it automatically engaged gear, wish I could change that.

I would think it would be based on speed and try to use 8th to minimize engine braking, once you are going slower than it allowed in 7th or 8th maybe engage coasting as that would save more fuel. I haven't used eco pro much but when I did, it seem to enter coast mode immediately when I came off the gas, basically shifting into neutral and then back into drive when I pressed the gas.

[otay]

Related story...

We have a 2011 Honda Odyssey with ~80k miles that had an issue last year. Engine light turned on; I thought it was just a misfire or something else minor. But it turned out that there was a TSB on VCM. Apparently the original VCM programming wasn't quite right, and excess fuel was left in the 3 cylinders that get shut off (when coasting and/or other conditions make it OK). The fix was to replace the piston rings in the 3 cylinders (!) and reprogram the ECU. Funny that the 3 cyls were at the back of the engine and they some how did this repair with the engine still in the car. Even though we were way beyond warranty, the work was done at no cost, plus had a loaner for the 3 days it was in the shop.

About 12k miles later, it hasn't blown up yet.

Modern cars are incredibly complex.

[bimmer456]

Originally Posted by otay

Related story...

We have a 2011 Honda Odyssey with ~80k miles that had an issue last year. Engine light turned on; I thought it was just a misfire or something else minor. But it turned out that there was a TSB on VCM. Apparently the original VCM programming wasn't quite right, and excess fuel was left in the 3 cylinders that get shut off (when coasting and/or other conditions make it OK). The fix was to replace the piston rings in the 3 cylinders (!) and reprogram the ECU. Funny that the 3 cyls were at the back of the engine and they some how did this repair with the engine still in the car. Even though we were way beyond warranty, the work was done at no cost, plus had a loaner for the 3 days it was in the shop.

About 12k miles later, it hasn't blown up yet.

Modern cars are incredibly complex.

Even Hondas aren't immune to recalls and engine problems. Someone I spoke with in person commented that the 3 cylinders that are deactivated would receive more wear than if they alternated banks of cylinders, but that would just make it more complex with all the plumbing and electronics now that I think of it and in this case all 6 cylinders would need the piston rings replaced. 80K miles is at least a decent amount vs. Bimmers needing whole new engines with less than 20k miles on the clock in many cases. Good thing they took care of it out of warranty. I had a 2011 Toyota transmission go bad after 115k miles, cost $$ to fix. Another thing I noticed is that BMW will typically replace rather than repair or refurbish which I think is better than Honda and Toyota which typically try to repair an engine instead of replace, though in your case it's out of warranty and something that could be repaired. I saw a story about a recall on the Lexus IS where they basically rebuilt the entire engine for a production run of cars form the mid 2000's. That must've been a headache for the dealers, BMW would have just replaced the entire engine with a new one from Germany. My friend's brother also had a brand new 2016 civic that had the engine blow up and they refurbished/rebuilt rather than replace. She said it was pretty lame to get a refurbished engine in a brand new car, I said maybe they should have just given him a new car.

[chiefneil]

I knew I'd heard somewhere that while coasting (foot off the gas while car is still moving in Drive, not the BMW eco-pro definition of coasting whatever that is) fuel is shut off.

I had to google and voila, Car and Driver confirms it:

"We went in search of an expert opinion on this one. According to Paul Williamsen, the product education manager at Toyota, “All contemporary Toyota and Lexus vehicles (and every other car built since the 1990s that I’ve looked at) can detect the condition when engine revs are higher than idle with a fully closed throttle: Under these conditions, all current to the fuel injectors is stopped, and no fuel is injected.” That means if your foot is off the gas while the car’s in gear, you’re not using any fuel.

Tom Read, GM’s powertrain spokesman, agrees: “Shifting into neutral in an automatic will cancel fuel cutoff. Thus, it is better to remain in gear and let the drive wheels pull the engine airflow down to where fuel cutoff can be enabled or where fuel flow is minimized.”"

http://www.caranddriver.com/features...neutral-page-3

[pz619]

I've wondered the same. My guess is that in coast mode it uses so little fuel that it outweighs the engine breaking effect with the injectors off in regards to distance.

[Billfitz]

Originally Posted by chiefneil

“All contemporary Toyota and Lexus vehicles (and every other car built since the 1990s that I’ve looked at) can detect the condition when engine revs are higher than idle with a fully closed throttle: Under these conditions, all current to the fuel injectors is stopped, and no fuel is injected.” That means if your foot is off the gas while the car’s in gear, you’re not using any fuel.

Tom Read, GM’s powertrain spokesman, agrees: “Shifting into neutral in an automatic will cancel fuel cutoff. Thus, it is better to remain in gear and let the drive wheels pull the engine airflow down to where fuel cutoff can be enabled or where fuel flow is minimized.”"

This is where it gets complicated. If you're in drive and take your foot off the gas the injectors can stop putting fuel into the cylinders. The transmission drag will keep the engine turning, so there's no need for fuel. With a system like Eco-pro that puts the car into free-wheeling there's no transmission drag to keep the engine turning over, so if the fuel flow was stopped entirely the engine revs would go to zero, instead of idle speed. That's the situation described where it says “Shifting into neutral in an automatic will cancel fuel cutoff.
This makes the Eco-pro system counter-intuitive, since in theory complete fuel shut off with the transmission active would use less fuel than with the car in freewheeling mode. A BMW engineer would have to explain why that's not the case.

[HighlandPete]

Originally Posted by Billfitz

This makes the Eco-pro system counter-intuitive, since in theory complete fuel shut off with the transmission active would use less fuel than with the car in freewheeling mode. A BMW engineer would have to explain why that's not the case.

Has to be a balance of where the parasitic losses add up to greater losses, (total road load) than simply coasting on idle.

As I see it, if you are driving at a steady speed and lift off the gas, will the conditions cause the car to slow down too quickly, or will the speed hold quite steady, like down a slight hill?

There are conditions where you don't need to add more gas, to keep up the pace, like the steady downhill scenario. You don't want or require engine braking, (not steep enough), so coasting must be more efficient in that condition. If you are driving in gear, (adding more engine losses) you will need to add (use) more gas to keep the speed, compared to the idle consumption.

I note this in a VW the family own, with the coasting function. Surprising how often coasting cuts in even on country roads, in what appears to be steady state driving.

[bimmer456]

So driving in eco pro just now I noticed it does kick in back in gear when slowing down to a slower speed to allow engine braking but when coasting at a cruising speed it idled. Presumably during this slowing down portion at lower speed the injectors are off. The parasitic losses and engine braking would require presses of the gas to maintain speed which uses more gas while coasting maintains more momentum if that's what you want using only idling gas. Overall it may save more fuel using that momentum. It doesn't appear to use that idle time to charge the battery however which maybe uses more idle gas unless you let it idle slower than normal. The battery charging only happens when applying the brake, maybe using resistance from the alternator to help slow the car while charging the battery

[kern417]

just remember it's not a linear relationship between rpm/throttle and fuel consumption. That's why 6th gear at 60mph uses less gas than 5th gear at 60mph. yes you may have to input less throttle to maintain speed since more torque is going to the wheels in 5th gear, but the higher rpm negates the benefit of less throttle.

cruise control at say 3000rpm may be low fuel consumption by just compensating for drivetrain friction, but it'll be more than the consumption at 800rpm no matter what. the only time i've heard it's bad to go into neutral is when you're going downhill and the momentum/weight of the car is enough to overcome the drivetrain losses in itself. in that case, it makes sense to stay in gear. but on flat ground that's almost never the case.

[metallicpea]

I love this feature. I will take the minimal fuel it takes to idle the engine without any load over the losses to have the car drivetrain breaking when off the throttle during my usual hwy driving any day of the week. This is the only reason I use the eco pro mode during mundane hwy trips.

I would imagine the calculation of fuel savings would be pretty well evaluated before BMW would implement such a feature. Fuel savings is such an area of focus nowadays.

[bimmer456]

Originally Posted by metallicpea

I love this feature. I will take the minimal fuel it takes to idle the engine without any load over the losses to have the car drivetrain breaking when off the throttle during my usual hwy driving any day of the week. This is the only reason I use the eco pro mode during mundane hwy trips.

I would imagine the calculation of fuel savings would be pretty well evaluated before BMW would implement such a feature. Fuel savings is such an area of focus nowadays.

I like the jekyl and hyde change of the car's attitude when jumping from comfort to sport + with two quick presses of the button. There's too many presses required to jump from eco-pro to sport +, would be nice if there's a way to code a quicker jump between the two modes. Wonder what the difference is between cylinder deactivation vs. coasting at idle, as you could maintain speed with cylinder deactivation, basically cruising vs. coasting. With coasting you eventually have to get back on the gas to maintain speed on a long road trip so it may be worse depending on how you drive vs. setting cruise control and utilizing cylinder deactivation.

[BAA435I]

If the instantaneous fuel consumption display is accurate, Eco mode uses 1.2-1.5 l/100km of fuel while coasting on a flat surface. When coasting in gear, the same display will show 0.00 l/100km.

[kern417]

coasting in gear is like dragging the brakes. you can't use the instantaneous consumption display for cruising speeds because it will go up and down as the throttle tries to maintain speed. one second it will be 0, the next it'll be 2+.

There is only rolling resistance when coasting in neutral, so the 1.2-1.5 is constant.