RE: What is a 'coasting' function? PH Explains
Discussion
Essel said:
My Dad bought a Saab 96 in 1972 ish, It was a manual, and also had a coasting function. There was a push/pull lever in the footwell. Push it in and when you lifted off the accelerator it freewheeled. Bit of a novelty, but not used very often. Our Tiguan auto has rthis function, but apart from trying it out, we dont use it. (The stop/start is usually turned off as well, as I find it annoying!)
I was gonna say, this is just another example of old technology coming round again in a modern form - like how Mazda's i-Stop stop/start doesn't use the starter motor but instead primes whichever cylinder has stopped on compression with a squirt of fuel and fires the plug. Which is a modern automatic version of a starting method that used to be taught to chauffeurs using the priming cups, trembler coils and manual ignition timing controls of their Rolls-Royces and Hispano-Suizas. In this case Rovers, Cords, Packards and other cars with a luxury bent had 'coasting functions' via a freewheel in the gearbox input shaft so the engine would disengage and return to idle on the overrun. It made things quieter, reduced oil consumption (old engines used to pump a lot of oil past the rings when at high rpms and low cylinder pressures), stopped the characteristic 'bandsaw' noise of the straight-cut gears working on the 'back' of the teeth and in the days before baulk-ring synchromesh made it easier to change gear.
Saab had its freewheels to overcome the lubrication problem of two-stroke engines on the overrun and then continued it due to the economy/noise benefits. Citroen fitted the centrifugal 'trafficlutch' to the 2CV which effectively turned it into a 'twist-and-go' transmission in first and second gear which was great for zipping around central Paris.
I do a lot of motorway driving in my M140i (70 mile each way commute) so started experimenting a lot with Eco Pro mode, since whatever I fuel I save on the boring motorway run means I've got more to burn on the B roads 
Anyway, I'd say on a typical 70 mile M6/M60/M62 run I gain at least 5mpg in Eco using coast compared with comfort mode. In fact, I'd go so far as to say that in flowing mway traffic the coast function improves economy far more than the softening of throttle response and all the other stuff the eco mode does. In my previous M135i which didn't have coast function it was just pointless using Eco, made no difference.
With coasting you need to leave a much greater gap to the traffic in front than you'd ever normally consider and anticipate more what the the cars in front are doing. Everyone else accelerates, lifts off, accelerates, lifts off... all of a sudden when you lift off as the cars in front do, you feel like you're accelerating towards them! It therefore needs a change of driving driving style to make the most of it. Some may scoff at having a performance car and using this kind of function to save fuel but for me, with the mileage I do it means my fuel bills are still (relatively) sensible and a 45+mpg commute is easily achievable without going any slower than the rest of the traffic.
Anyway, I'd say on a typical 70 mile M6/M60/M62 run I gain at least 5mpg in Eco using coast compared with comfort mode. In fact, I'd go so far as to say that in flowing mway traffic the coast function improves economy far more than the softening of throttle response and all the other stuff the eco mode does. In my previous M135i which didn't have coast function it was just pointless using Eco, made no difference.
With coasting you need to leave a much greater gap to the traffic in front than you'd ever normally consider and anticipate more what the the cars in front are doing. Everyone else accelerates, lifts off, accelerates, lifts off... all of a sudden when you lift off as the cars in front do, you feel like you're accelerating towards them! It therefore needs a change of driving driving style to make the most of it. Some may scoff at having a performance car and using this kind of function to save fuel but for me, with the mileage I do it means my fuel bills are still (relatively) sensible and a 45+mpg commute is easily achievable without going any slower than the rest of the traffic.
Max_Torque said:
What should be made clear is that "coasting" is not really anything to do with "engine braking" just that this term is used because it is familiar to drivers, ie they lift the throttle, the car slows down. In reality, what coasting actually does is to provide the vehicle with a massively tall (infinitely so in the case of engine off coasting!) top gear! As such, coasting is really an extension of "downspeeding" which is the reduction, where ever possible of engine parasitic load from the vehicles roadload (roadload = total drag of the car, ie aero + rolling(tyres, transmission etc) friction). You might ask er, what?" and that's ok, because most people dont' actually understand how and engine works and how it drives the wheels of their car!
Consider the torque produced at the flywheel of the engine:
At wide open throttle, that torque is at the maximum positive value the engine can make (max air/fuel being burn't)and the vehicle accelerates positively at it's maximum rate (for the gear it is in)
At zero throttle, that torque is at the maximum negative value the engine can make (engine friction and parastic (ie water/oilpumps etc) loads combined, no combustion is occurring thanks to DFSO (Deccel Fuel Shut Off) and the vehicle is deccelerating at it's maximum rate (without additional retardation from the wheel friction brakes)
Now, what happens between these two points is the interesting bit. As the driver opens the throttle, the engine is allowed to start burning fuel (DFSO is exited) but the net flywheel torque is still negative (vehicle still deccelerating). This is because up until a certainly fuel burn rate, the engine is not producing enough torque to overcome it's own friction. Here, fuel is being burnt just to keep the engine going round, and is not being used to move the car (ie is not contributing to the vehicles road load). These conditions exist any time the driver is not fully off the throttle, and yet not applying enough throttle to get to zero net flywheel torque.
this can be summarised as: It's important to realise the BMEP = IMEP - FMEP or BRAKE mean effective pressure = INDICATED mep minus Frictional mep.
For a typical engine, FMEP can be around 2bar, and BMEP typically for a modern turbo engine say 20bar. This in effect is the same as saying your car deccelerates on a closed throttle at 10% of the rate at which it accelerates at WOT, which most drivers would have felt themselves. The issue, is that when driven gently (ie not at WOT) that 2 bar is a considerable portion of the actual road load, which might be just 4 bar BMEP. Drive at that load (about 40mph in top gear) and the engine itself is taking 1/3 of the energy to just turn it round!
So what a "Coasting" system does is to try to remove (partially for engine idle, and completely for engine off) the frictional losses (FMEP) of the engine from the vehicles roadload during operation whenever possible. It can do this at all times when the driver demand is bellow the zero net torque line. ie it operates before DSFO.
I came here wondering about just the DFSO, and you've already explained it & much more. Thank you!Consider the torque produced at the flywheel of the engine:
At wide open throttle, that torque is at the maximum positive value the engine can make (max air/fuel being burn't)and the vehicle accelerates positively at it's maximum rate (for the gear it is in)
At zero throttle, that torque is at the maximum negative value the engine can make (engine friction and parastic (ie water/oilpumps etc) loads combined, no combustion is occurring thanks to DFSO (Deccel Fuel Shut Off) and the vehicle is deccelerating at it's maximum rate (without additional retardation from the wheel friction brakes)
Now, what happens between these two points is the interesting bit. As the driver opens the throttle, the engine is allowed to start burning fuel (DFSO is exited) but the net flywheel torque is still negative (vehicle still deccelerating). This is because up until a certainly fuel burn rate, the engine is not producing enough torque to overcome it's own friction. Here, fuel is being burnt just to keep the engine going round, and is not being used to move the car (ie is not contributing to the vehicles road load). These conditions exist any time the driver is not fully off the throttle, and yet not applying enough throttle to get to zero net flywheel torque.
this can be summarised as: It's important to realise the BMEP = IMEP - FMEP or BRAKE mean effective pressure = INDICATED mep minus Frictional mep.
For a typical engine, FMEP can be around 2bar, and BMEP typically for a modern turbo engine say 20bar. This in effect is the same as saying your car deccelerates on a closed throttle at 10% of the rate at which it accelerates at WOT, which most drivers would have felt themselves. The issue, is that when driven gently (ie not at WOT) that 2 bar is a considerable portion of the actual road load, which might be just 4 bar BMEP. Drive at that load (about 40mph in top gear) and the engine itself is taking 1/3 of the energy to just turn it round!
So what a "Coasting" system does is to try to remove (partially for engine idle, and completely for engine off) the frictional losses (FMEP) of the engine from the vehicles roadload during operation whenever possible. It can do this at all times when the driver demand is bellow the zero net torque line. ie it operates before DSFO.
It seems a clever idea to me. Got to be honest, and I'll get my badge ready to hand in as I leave, for most commuters and drivers in traffic, this sort of thing does seem a good idea. Of course, total net cost is something we'll struggle to work out - were the cost savings or pollution savings worth the extra load on any clutches/starter motors etc. But on the face of it, seems a relatively sensible idea.
Max_Torque said:
All completely irrelevant in 2018.
1) No modern car will "loose control" if you lift off the accelerator (under conditions where it wouldn't also have loose control if you braked, and as most people dont' left foot brake, you always have to lift off first.......)
2) modern engines make 20 to 30 bar BMEP, the transmission is (necessarily) sized to deal with that. The 2 bar of FMEP is caused by engine braking is
therefore effectively completely irrelevant and will never ever wear the transmission out
The reason driving "systems" are useful is entirely a Human one, ie they force the driver into driving systematically,which is proven to develop positive habit forming actions, and hence they reduce the occurrence of human error!
The car DGAF how you drive these days (which is lucky, because most drivers i see couldn't drive a greasy stick up a pigs a*se...... ;-)
Was I the only one looking through the previous post to work out what the DGAF function was?! 1) No modern car will "loose control" if you lift off the accelerator (under conditions where it wouldn't also have loose control if you braked, and as most people dont' left foot brake, you always have to lift off first.......)
2) modern engines make 20 to 30 bar BMEP, the transmission is (necessarily) sized to deal with that. The 2 bar of FMEP is caused by engine braking is
therefore effectively completely irrelevant and will never ever wear the transmission out
The reason driving "systems" are useful is entirely a Human one, ie they force the driver into driving systematically,which is proven to develop positive habit forming actions, and hence they reduce the occurrence of human error!
The car DGAF how you drive these days (which is lucky, because most drivers i see couldn't drive a greasy stick up a pigs a*se...... ;-)
Flat6 said:
With coasting you need to leave a much greater gap to the traffic in front than you'd ever normally consider and anticipate more what the the cars in front are doing.
If you do this then you'll get considerably better fuel economy even in a car without a coast function though, so are you sure that the 5mpg improvement is really down to the tech or are you just driving more with economy in mind? Either way 45mpg from an M140i is amazing - hats off to you!I have a Skoda Octavia on lease, its a 2L diesel with DSG gear box, its my second one, the 1st had the eco-mode that disconnected the gear box all the time and with the stop and start doing 15.000 to 20.000 miles a year I was doing about 60 to 70 mph. Now I have a new one you have to manually switch it on every time. I do miss it when its not engaged and the consumption has dropped down to 50 to 55 mph 
I'm just about to return a Mercedes C200 with both stop start and "gliding mode". It probably does improve MPG to a degree, however it's bloody infurating to have the engine shut off as soon as the wheels stop, it would even do it between reverse and drive and vice versa.
Driving home from work last night in my 9 year old, diesel, conventional auto barge was far more pleasant than listening to the sound of the engine cutting out and restarting and the car happily telling me I was in gliding mode.
I presume this type of gubbins makes it acceptable to fit a 41L fuel tank to such a large car. It really doesn't save that much fuel.
Driving home from work last night in my 9 year old, diesel, conventional auto barge was far more pleasant than listening to the sound of the engine cutting out and restarting and the car happily telling me I was in gliding mode.
I presume this type of gubbins makes it acceptable to fit a 41L fuel tank to such a large car. It really doesn't save that much fuel.
Roger Irrelevant said:
Flat6 said:
With coasting you need to leave a much greater gap to the traffic in front than you'd ever normally consider and anticipate more what the the cars in front are doing.
If you do this then you'll get considerably better fuel economy even in a car without a coast function though, so are you sure that the 5mpg improvement is really down to the tech or are you just driving more with economy in mind? Either way 45mpg from an M140i is amazing - hats off to you!I genuinely think though that the coasting accounts for the saving due to the extra ground you can cover. This is highlighted on the frequent descents travelling over the M62 on the Pennines where there are probably several sections where I can travel a significant distance (>1 mile?) with no throttle whereas in comfort mode (no coasting) you need to keep on the gas in order to maintain speed.
cmvtec said:
I presume this type of gubbins makes it acceptable to fit a 41L fuel tank to such a large car. It really doesn't save that much fuel.
Unfortunately it's something else much more annoying that causes a 41L fuel tank to be fitted to a large car. I'll try to explain but it has been a few years since I was directly involved in this sort of thing!When a vehicle is tested for fuel consumption, it is done so at a mass which has specific constraints around it. One of those constraints is that you have to a fuel tank filled to 90% of the maximum capacity.
On the old NEDC test procedure, the car was not tested at it's actual weight but was tested at an "inertia" mass. So if you could take a few kgs out of a car and drop down an inertia class then you could test a car that could be over 75kg lighter than in real life - hence improving the cycle specific fuel consumption.
Add both of those things together and you get manufacturers trying to save a few kgs in several places to drop an inertia class and suddenly you have tiny fuel tanks.
Now some manufacturers would have range targets for their cars so you would think that reducing the fuel tank size would shrink the range. Unfortunately, a lot of those range targets would be based upon that massively unrealistic cycle fuel consumption so, because your car is now magically more efficient, the fuel tank could be smaller....
t you don't need.Max_Torque said:
So what a "Coasting" system does is to try to remove (partially for engine idle, and completely for engine off) the frictional losses (FMEP) of the engine from the vehicles roadload during operation whenever possible.
Sorry to be a bit picky but that's not entirely true (and given how good the rest of your explanation is I feel a bit rude to pick up on this!). What coasting is doing is to always remove the frictional losses of the engine from the roadload regardless of whether it's engine on or off coasting.What engine off coasting does for you is to remove that fuel required to keep the engine turning at idle - which as you mentioned can be considerable due to the parasitic losses.
Idle is especially inefficient as the spark timing is never (well, hardly ever) at an optimal timing as the calibration will retard the spark to keep torque available in reserve in case of a sudden additional parasitic load (such as A/C) so that you don't get engine stumble when that load comes in.
Nanook said:
Essel said:
My Dad bought a Saab 96 in 1972 ish, It was a manual, and also had a coasting function. There was a push/pull lever in the footwell. Push it in and when you lifted off the accelerator it freewheeled. Bit of a novelty, but not used very often. Our Tiguan auto has rthis function, but apart from trying it out, we dont use it. (The stop/start is usually turned off as well, as I find it annoying!)
In the Saab, it was a necessity for the 2-stroke, to maintain adequate lubrication.They left it in when they fitted the 4-stroke Ford V4.
Ah, I see someone mentioned this a couple of posts after that one.
The extremely advanced (for a 60s design) aerodynamics helped as well.
stavers said:
cmvtec said:
I presume this type of gubbins makes it acceptable to fit a 41L fuel tank to such a large car. It really doesn't save that much fuel.
Unfortunately it's something else much more annoying that causes a 41L fuel tank to be fitted to a large car. I'll try to explain but it has been a few years since I was directly involved in this sort of thing!When a vehicle is tested for fuel consumption, it is done so at a mass which has specific constraints around it. One of those constraints is that you have to a fuel tank filled to 90% of the maximum capacity.
On the old NEDC test procedure, the car was not tested at it's actual weight but was tested at an "inertia" mass. So if you could take a few kgs out of a car and drop down an inertia class then you could test a car that could be over 75kg lighter than in real life - hence improving the cycle specific fuel consumption.
Add both of those things together and you get manufacturers trying to save a few kgs in several places to drop an inertia class and suddenly you have tiny fuel tanks.
Now some manufacturers would have range targets for their cars so you would think that reducing the fuel tank size would shrink the range. Unfortunately, a lot of those range targets would be based upon that massively unrealistic cycle fuel consumption so, because your car is now magically more efficient, the fuel tank could be smaller....
No less infuriating, but all the same useful.
It came as a genuine surprise the first time I had to fill it that it would only take £45. It became beyond tiresome when I had to do it every few days. This, twinned with some technology that I really don't want or need, sullied the driving experience of a new Mercedes.
stavers said:
Max_Torque said:
So what a "Coasting" system does is to try to remove (partially for engine idle, and completely for engine off) the frictional losses (FMEP) of the engine from the vehicles roadload during operation whenever possible.
Sorry to be a bit picky but that's not entirely true (and given how good the rest of your explanation is I feel a bit rude to pick up on this!). What coasting is doing is to always remove the frictional losses of the engine from the roadload regardless of whether it's engine on or off coasting.What engine off coasting does for you is to remove that fuel required to keep the engine turning at idle - which as you mentioned can be considerable due to the parasitic losses.
Idle is especially inefficient as the spark timing is never (well, hardly ever) at an optimal timing as the calibration will retard the spark to keep torque available in reserve in case of a sudden additional parasitic load (such as A/C) so that you don't get engine stumble when that load comes in.
The engine itself is completely physically decoupled (declutched) from the wheels in either case (at idle or engine off) but it is not practically decoupled from the vehicles total energy consumption profile when it is at idle, because fuel is still being burnt to keep it turning, and as all the energy that moves your car around must come solely from it's fuel, then even physically decoupled, the engine friction is still, in effect, included in the roadload, all-be-it indirectly! And what the car owner cares about is the total amount of fuel used for any given journey. They don't care where it was "used" ie in engine friction or in aero drag or whatever, but they do care that it was used!
Roger Irrelevant said:
The savings made when the car is coasting for an extended period, however, could deliver a claimed fuel saving of around 10 per cent
Hmmmmmmmmmmmmmmmmmm....the word 'could' should be in bold italics and double underlined here. It would doubtless take a very specific set of circumstances to achieve anything close to a 10% fuel saving, and it would make virtually no difference in the real world as most people don't seem to realise that if you take your foot off the throttle your car will still move forwards - stay on the throttle until the last minute then brake seems to be the modus for a lot of people (including my wife, drives me mad!).
I've found that cars don't slow down that quickly once you're off the throttle anyway, especially if they've got a long top gear, and it only takes a slight downhill grade to maintain quite high speeds. I'd file this firmly under 'unnecessary complication for no real benefit'.
I believe that VAG ACT 1.4 TSI engines improve fuel consumption by 7% by shutting down 2 cylinders whilst on light load & I’d assume the new 1.5 EVO which supersedes it would do a little better. Is the last 3% really worth it?Hmmmmmmmmmmmmmmmmmm....the word 'could' should be in bold italics and double underlined here. It would doubtless take a very specific set of circumstances to achieve anything close to a 10% fuel saving, and it would make virtually no difference in the real world as most people don't seem to realise that if you take your foot off the throttle your car will still move forwards - stay on the throttle until the last minute then brake seems to be the modus for a lot of people (including my wife, drives me mad!).
I've found that cars don't slow down that quickly once you're off the throttle anyway, especially if they've got a long top gear, and it only takes a slight downhill grade to maintain quite high speeds. I'd file this firmly under 'unnecessary complication for no real benefit'.
mac96 said:
Nanook said:
Essel said:
My Dad bought a Saab 96 in 1972 ish, It was a manual, and also had a coasting function. There was a push/pull lever in the footwell. Push it in and when you lifted off the accelerator it freewheeled. Bit of a novelty, but not used very often. Our Tiguan auto has rthis function, but apart from trying it out, we dont use it. (The stop/start is usually turned off as well, as I find it annoying!)
In the Saab, it was a necessity for the 2-stroke, to maintain adequate lubrication.They left it in when they fitted the 4-stroke Ford V4.
Ah, I see someone mentioned this a couple of posts after that one.
The extremely advanced (for a 60s design) aerodynamics helped as well.
I’ve got this feature on my 7 Speed DSG 2.0 tdi Passat and it does save fuel. Noticeably.
However I intend to keep this car for circa 200k miles and for that reason I don’t use it. I just can’t help but think that the extra wear (and hence potential replacement costs) on the DSG clutches over a prolonged period will outweigh any MPG savings.
I might be wrong but I’ll drive around in ignorant bliss.
However I intend to keep this car for circa 200k miles and for that reason I don’t use it. I just can’t help but think that the extra wear (and hence potential replacement costs) on the DSG clutches over a prolonged period will outweigh any MPG savings.
I might be wrong but I’ll drive around in ignorant bliss.
Max_Torque said:
stavers said:
Max_Torque said:
So what a "Coasting" system does is to try to remove (partially for engine idle, and completely for engine off) the frictional losses (FMEP) of the engine from the vehicles roadload during operation whenever possible.
Sorry to be a bit picky but that's not entirely true (and given how good the rest of your explanation is I feel a bit rude to pick up on this!). What coasting is doing is to always remove the frictional losses of the engine from the roadload regardless of whether it's engine on or off coasting.What engine off coasting does for you is to remove that fuel required to keep the engine turning at idle - which as you mentioned can be considerable due to the parasitic losses.
Idle is especially inefficient as the spark timing is never (well, hardly ever) at an optimal timing as the calibration will retard the spark to keep torque available in reserve in case of a sudden additional parasitic load (such as A/C) so that you don't get engine stumble when that load comes in.
The engine itself is completely physically decoupled (declutched) from the wheels in either case (at idle or engine off) but it is not practically decoupled from the vehicles total energy consumption profile when it is at idle, because fuel is still being burnt to keep it turning, and as all the energy that moves your car around must come solely from it's fuel, then even physically decoupled, the engine friction is still, in effect, included in the roadload, all-be-it indirectly! And what the car owner cares about is the total amount of fuel used for any given journey. They don't care where it was "used" ie in engine friction or in aero drag or whatever, but they do care that it was used!
What can be interesting, in terms of energy consumption with a decoupled powertrain, is what can happen on HEVs & BEVs. I know that some manufacturers, to "help" with coastdown numbers for fuel consumption, allow the electric machines to "push" a little when the car is in neutral and has a high state of charge as the motor is never fully decoupled from the wheels!!
cmvtec said:
stavers said:
cmvtec said:
I presume this type of gubbins makes it acceptable to fit a 41L fuel tank to such a large car. It really doesn't save that much fuel.
Unfortunately it's something else much more annoying that causes a 41L fuel tank to be fitted to a large car. I'll try to explain but it has been a few years since I was directly involved in this sort of thing!When a vehicle is tested for fuel consumption, it is done so at a mass which has specific constraints around it. One of those constraints is that you have to a fuel tank filled to 90% of the maximum capacity.
On the old NEDC test procedure, the car was not tested at it's actual weight but was tested at an "inertia" mass. So if you could take a few kgs out of a car and drop down an inertia class then you could test a car that could be over 75kg lighter than in real life - hence improving the cycle specific fuel consumption.
Add both of those things together and you get manufacturers trying to save a few kgs in several places to drop an inertia class and suddenly you have tiny fuel tanks.
Now some manufacturers would have range targets for their cars so you would think that reducing the fuel tank size would shrink the range. Unfortunately, a lot of those range targets would be based upon that massively unrealistic cycle fuel consumption so, because your car is now magically more efficient, the fuel tank could be smaller....
No less infuriating, but all the same useful.
It came as a genuine surprise the first time I had to fill it that it would only take £45. It became beyond tiresome when I had to do it every few days. This, twinned with some technology that I really don't want or need, sullied the driving experience of a new Mercedes.
It is very infuriating! That's the problem with regulations - there's always someone trying to be clever with them!
Gassing Station | General Gassing | Top of Page | What's New | My Stuff