Coasting vs engine braking

[OliverDavis]

Thing is, I've got the hang of it. I'm a realist. Whilst I fully understand your post, you, and most others seem to think that that downhill will last for 113 miles and therefore the car should tell me that I'm "achieving" 113mpg. The hill won't, obviously last for 113 miles and would need to be at a considerable incline to even secure a prolonged 30mph. Yes, I appreciate that the car would have to know what was coming up in terms of flat surface or up hill, but I've never looked at my "instantaneous" mpg, coasting in neutral, and marvelled at my 113mpg.

Interestingly, the 5 series F10 hyrbid does read the road ahead and uses the engine / electric motors accordingly

So why are you showing your point? You are saying that the instantaneous MPG read out is pretty much pointless. What we are getting at is what is using more fuel, coasting or engine braking. Using the instantaneous readout will give us the answer, the correct answer. No one would coast/engine brake up hill to save fuel obviously, so we are basing it all on just downhill driving. Once the downhill driving stops, we are not bothered about the MPG, as that is not what OP is asking, because you cant coast then.

As for the 5 series Hybrid, it would need to be able to predict lights and other traffic ahead of it to be able to figure out an MPG value for your ahead journey. It cannot do this, it can only increase economy by giving you a bit of an extra push/changing gear ahead of time. It cannot see the whole journey ahead.

 

[acemastr]

To throw a spanner in the works:

You will roll further while coasting, if the gradient is such that you don't gain masses of speed and have to use the brakes, is the minimal amount of fuel used to idle the engine countered by the fact you can roll twice as far as engine braking?

 

[Django x2]

well, it's always been a trade off between no fuel engine braking vs further distance travelled in neutral. engine braking correctly, or coasting in gear, it's possible to travel some distance, but not as much as in neutral obviously.

F10 hybrid uses sat nav to see traffic, elevation, etc, and gives the option to re-route to save fuel, which is pretty epic. They must be rubbish though as there are loads for sale at the mo

 

[Simon]

A car reading 0mpg would have to be able to read the road ahead and see what you are going to do. If it saw a flat/uphill and knew you will stop and not lightly accelerate then yes, it would read 0mpg, but this would still only happen once you come to a stop. As currently you are getting 0 miles out of a gallon.

Or just sit at idle not moving

 

[Jonnycoupe]

To throw a spanner in the works:

You will roll further while coasting, if the gradient is such that you don't gain masses of speed and have to use the brakes, is the minimal amount of fuel used to idle the engine countered by the fact you can roll twice as far as engine braking?

Yes exactly. Depends if you're about to stop or not though as if you maintain loads of speed you will need to brake hard later for a junction. This is where driving a hybrid is different and Im on the brakes lightly stuffing all the excess into the battery for later and it does it at a rate that means I dont have to come off the throttle 3 miles before I get to that stop point. Regardless what you do in a petrol car its a bit of a waste ultimately having to stop.

 

[mattyfez]

There's a lot of down hill travel on my commute home,

Through experimentation I've discovered I'm better off using engine braking as it cuts the fuel down to about 1200 rpm, but below that I think it idles and uses fuel, so sometimes I need to down shift to bring the revs up an fool it. It's a bit of a balancing act but i think I've got used to it.
There's also a long very slightly downhill straight, with a 30mph limit so i pop it in 5th and put the cruise control to 30 and that save quite a few mpg.

 

[Devrij]

No, you will tend to stop quicker with less pad wear with the car in gear (for most of the rev range) due to engine braking. As above, if you're at low revs e.g. <1500rpm only then will the engine be fed a bit of fuel which would counteract braking. Besides, you are less likely to lock your wheels under hard braking.

It depends what rpm the engine is at I guess, and in this situation where you'd be at low rpm anyway you may well be right. I was trying to point out that since your brakes can slow you down faster than your engine rpm would drop, the momentum of the flywheel will counteract your braking slightly. Marginal, I know, and only a concern at higher speeds and emergency stops. I would always clutch in when performing an emergency stop for that reason.

There's a lot of down hill travel on my commute home,

Through experimentation I've discovered I'm better off using engine braking as it cuts the fuel down to about 1200 rpm, but below that I think it idles and uses fuel, so sometimes I need to down shift to bring the revs up an fool it. It's a bit of a balancing act but i think I've got used to it.
There's also a long very slightly downhill straight, with a 30mph limit so i pop it in 5th and put the cruise control to 30 and that save quite a few mpg.

This is what I'm now doing. I can't really tell when the cutoff is, so I'm adjusting my method to go 4th down the 30 zone rather than 5th, although this involves blipping the throttle every now and then so may be counter-productive

 

[acemastr]

Anyone know how it works with automatics? obviously I can roll for miles down even the slightest incline and keep the speed I'm doing (do it on the way home in a 30, sits nicely) and the mpg meter does go to the left(but that's based on throttle position, i'm sure of it)

 

[Clarkey]

Believe it or not, the mpg gauge is based on fuel consumption If it goes all the way to the left then you're in fuel cut, simples.

 

[Freefaller]

Irrespective of fuel consumption, it's also better to keep the gears engaged as you then have control over the engine and your car.

 

[miniyazz]

It depends what rpm the engine is at I guess, and in this situation where you'd be at low rpm anyway you may well be right. I was trying to point out that since your brakes can slow you down faster than your engine rpm would drop, the momentum of the flywheel will counteract your braking slightly. Marginal, I know, and only a concern at higher speeds and emergency stops. I would always clutch in when performing an emergency stop for that reason.

Interesting point, I guess that would vary depending on the engine/flywheel weight then. Most cars I suspect would drop from e.g. 4000rpm to 1000rpm (if you clutched) within a few seconds - whether you would brake quicker than that would depend on your brakes and your speed (4000 to 1000rpm in 6th might equal slowing from 100 to 25, i.e. probably not slowing quicker w/ braking, vs 4000-1000rpm in 2nd which might be 40mph to 10mph).

 

[Devrij]

Interesting point, I guess that would vary depending on the engine/flywheel weight then. Most cars I suspect would drop from e.g. 4000rpm to 1000rpm (if you clutched) within a few seconds - whether you would brake quicker than that would depend on your brakes and your speed (4000 to 1000rpm in 6th might equal slowing from 100 to 25, i.e. probably not slowing quicker w/ braking, vs 4000-1000rpm in 2nd which might be 40mph to 10mph).

Exactly. It was more relevant when I rode a bike because you have more stopping power/weight, and my big slow V twin didn't drop revs very quickly, but I think it still applies in some (but not all of course) situations in a car. I think a lot of my car habits are carried over from my biking days (learnt to ride before I learnt to drive), so perhaps some of it doesn't take the clever engines in cars into account.

 

[miniyazz]

Exactly. It was more relevant when I rode a bike because you have more stopping power/weight, and my big slow V twin didn't drop revs very quickly, but I think it still applies in some (but not all of course) situations in a car. I think a lot of my car habits are carried over from my biking days (learnt to ride before I learnt to drive), so perhaps some of it doesn't take the clever engines in cars into account.

I did a couple of tests this afternoon. Firstly, my Golf (V6) dropped from 4krpm to 1krpm in 2-2.25s. Some back of the envelope calculations with the official government stopping distances, assuming linear deceleration, suggests that it takes 4.8s to stop from 70mph, 2.7s from 40mph and 2.1s from 30mph once you hit the brake (so excluding thinking distance, which is irrelevant here).
Essentially, this broadly means (for my car) that at motorway speeds I should be emergency braking in gear no matter what rpm I'm doing, until I've nearly stopped of course. While at 30mph I should be in gear if I'm doing below around 3.5k rpm; higher than this and I should disengage the clutch - and at higher speeds correspondingly higher rpms are allowed before it's best to disengage the clutch!

Practically speaking, it means for almost all situations, it's better to leave in gear until close to stopping.

This is what I'm now doing. I can't really tell when the cutoff is, so I'm adjusting my method to go 4th down the 30 zone rather than 5th, although this involves blipping the throttle every now and then so may be counter-productive

Blipping the throttle uses a fair bit of fuel relative to idling, but it's only for a short space of time. Idle fuel supply in my car is around 20-25ml/minute, or 0.35-0.4ml/second. Maintaining revs at 1000rpm = 0.6ml/s, 2000rpm = 1ml/s, 3000rpm = 1.6ml/s, 4000rpm = 2.2ml/s, 5000rpm = 2.9ml/s, 6000rpm = 4ml/s

So assuming it takes a second to blip the throttle/engage gear while maintaining those revs, for it to be a worthwhile manoeuvre it depends how much longer you'll be able to 'coast in gear' by changing gear. If you'll be in that gear for a further couple of seconds it's worth blipping up to around 2000rpm; for around 5 seconds up to around 4000rpm, and for around 9 seconds up to around 6000rpm.
However there's also reduced wear on the pads which you could try and work into the calculation. I think I'll continue to throttle blip in most situations..

 

[Tom_ed1987]

I did a couple of tests this afternoon. Firstly, my Golf (V6) dropped from 4krpm to 1krpm in 2-2.25s. Some back of the envelope calculations with the official government stopping distances, assuming linear deceleration, suggests that it takes 4.8s to stop from 70mph, 2.7s from 40mph and 2.1s from 30mph once you hit the brake (so excluding thinking distance, which is irrelevant here).
Essentially, this broadly means (for my car) that at motorway speeds I should be emergency braking in gear no matter what rpm I'm doing, until I've nearly stopped of course.

I wouldn't worry about that. I'm fairly sure during an emergency stop the limiting factor to your deceleration will be tyre grip/ABS action

 

[mattyfez]

It depends what rpm the engine is at I guess, and in this situation where you'd be at low rpm anyway you may well be right. I was trying to point out that since your brakes can slow you down faster than your engine rpm would drop, the momentum of the flywheel will counteract your braking slightly. Marginal, I know, and only a concern at higher speeds and emergency stops. I would always clutch in when performing an emergency stop for that reason.



This is what I'm now doing. I can't really tell when the cutoff is, so I'm adjusting my method to go 4th down the 30 zone rather than 5th, although this involves blipping the throttle every now and then so may be counter-productive

Yeh the only giveaway on mine is the mpg goes to 99.9mpg if I'm coasting in gear downhill, which is as high as it goes, but if the revs drop below 1200 it changes to about 30mpg.

As for cruising at 30mph in 5th I think it really depends on your gearbox ratio, if I drop to 28ish mph you can hear the engine labour if you then accelerate, but it seems happy enough at 30mph. Different cars will be different though.