Tuesday 6th November 2018

What is a 'coasting' function? PH Explains

The concept of coasting to save fuel isn't a new one but automated systems take things one step further

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A coasting mode is a powertrain technology designed to improve the claimed efficiency of a car. These systems achieve improvements in fuel consumption and emissions by effectively uncoupling the engine from the transmission in appropriation conditions. When the driver lifts off the accelerator in a conventional fuel-injected car, the fuel supply to the engine is typically shut off - or greatly reduced - and engine braking will begin to slow the car. No fuel is used but the car's deceleration will, in many cases, require that the driver soon begin accelerating and burning fuel again.

The use of a dedicated coasting mode can instead prove more efficient; because the engine is disconnected from the transmission, no engine braking takes place and the car decelerates far more slowly. It consequently travels a longer distance before the engine needs to be called upon, consuming less fuel overall. It is also safer than coasting manually (either using the clutch or by engaging neutral) as drive can automatically and immediately be restored should it be required.

The simplest set-ups just disconnect the engine from the transmission, and drop the engine speed to idle, allowing the car to sail along unimpeded. More advanced systems shut the engine off when coasting, which permits further benefits such as a greater reduction in fuel consumption. There are many examples of these systems but manufacturers do brand them differently. Volkswagen, for examples, calls its engine-off coast mode the 'eco-coasting' feature.

How do coasting systems work?
A coasting system is effectively an extension of the stop-start set-up already found in many a car with an automatic transmission. When the vehicle is in motion, the system monitors numerous parameters - including accelerator pedal position, brake pedal actuation and speed. If the driver takes their foot off the accelerator and the vehicle is still rolling, and then doesn't touch the brake, the coasting system uncouples the engine from the transmission - typically by disengaging a clutch within the gearbox - and brings the engine back to idle.

This allows the car to roll along for an extended period, as it is not slowed by engine braking, and reduces its overall fuel consumption and emissions. An idling engine still burns fuel, though, which is why more advanced set-ups will shut the engine off when it is uncoupled from the transmission; this permits greater efficiency and emissions improvements. As soon as the driver touches either the accelerator or the brake, the engine is restarted - if necessary - and the transmission re-engaged.

Coasting systems can be found in cars with dual-clutch transmissions and those featuring torque converter-based automatics, too; the ZF 8HP transmission, for example, permits both engine-on and engine-off neutral coasting at speeds of up to 99mph if the manufacturer requires it. Manufacturers could also employ coasting systems in manual transmission-equipped cars, if so inclined, by employing a clutch-by-wire system. This would grant the automatic operation of the clutch, allowing the engine to be disconnected from the transmission and shut down to permit engine braking-free coasting.

In certain situations, that said, the engagement of a coasting mode may prove a disadvantage. Consequently, some systems - including the one used in seven-speed DCT Minis - make use of both the navigation or forward-looking safety systems to assess whether the driver is likely to require engine braking to ensure better control and safety. If it's deemed that the car is approaching a junction or similar, then the coasting mode may be automatically locked out.

Hybrid vehicles can also benefit from coasting set-ups; Porsche, for one, has a special 'sailing' mode for its hybrids that features a clutch that disconnects the engine from the transmission but leaves the electric motor engaged. The motor can then be used to gently nudge the hybrid along, helping extend its engine-off range. Porsche also makes use of engine-on coasting in several of its other models, including the 911 and Macan.

Are engine-off coasting systems really that beneficial?
During the average trip, according to Bosch, around 30 per cent of the time is spent coasting - but the engine couldn't be shut off for the entirety of that time, as many coasting moments would be very brief. The savings made when the car is coasting for an extended period, however, could deliver a claimed fuel saving of around 10 per cent - and notable reductions in CO2 emissions. Furthermore, the larger the engine then the greater the potential saving made through coasting.

Bosch isn't the only company to claim such improvements in fuel consumption. Transmission manufacturer ZF, for example, states that its coasting function can similarly deliver a 10 per cent improvement in fuel consumption and a 10 per cent reduction in CO2 emissions. The use of a hybridised coasting set-up is claimed to have more significant benefits - with some manufacturers stating an overall fuel saving of up to 25 per cent in real-world conditions.

One further minor benefit to the use of coasting systems is a reduction in both vibration and noise, particularly in engine-off systems. In any case, as emissions and economy targets become increasingly tough to meet, expect to see coasting systems on a more regular basis.

PH Explains homepage

[leakymanifold]

Colour me a skeptic but Bosch and others also promoted similar amounts of fuel savings from stop start systems which in the real world do very little.

[Cardinal Hips]

"Yes sir, the technicians have found the problem with your car and the random disengaging and engaging of power in low speed traffic, it's the coasting solenoid in the gearbox. To replace it we will have to drop the entire sub-frame to remove the gearbox and then strip it down to replace the part. Estimate is £5632.78, £479.32 is parts, we will put together another estimate to replace the front bumper and headlights from where the car kept leaping in to the vehicle in front, would you like us to proceed"

[Oz83]

What are the long term effects of starting and stopping an engine tens of thousands of times?

I'm sure they have done their homework, but I noticed in my friends BMW that when it was brand new, the start-stop feature was smooth and quiet. 3 years on, it's noticeably louder and shakes the car.

[Essel]

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!)

[hondansx]

Had it on a 981 Boxster S, and to be fair it was imperceptible and - I assume - helped it get to 40mpg on a run.

[MX6]

This seems like common sence really, it's just the automated means of dipping the clutch when your steadily slowing up or going down hill.

It was always said that this is an incorrect way to drive as one would supposedly be out of control of the vehicle (a driving test fail). You can save a noticeable amount of fuel driving that way though.

Presumably if this side of driving to left to the ECU to decide rather than a human then everything is okay...

[clarkey]

In a hybrid, are you better off using the down hill bits to recharge the battery ('comfort' mode in a 530e) or coasting (eco-pro mode). I'm not sure.

[jet_noise]

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!)

Left over from their 2-stroke days where coasting on a closed throttle with no oily petrol being delivered accelerated engine wear (bores/rings?).

[mrbarnett]

I like to use the gears in place of brakes wherever possible to lose speed; this would prevent that, and would consequently annoy me greatly.

[NotBenny]

Sounds excellent, but I can't help but feel like this is a system to improve drive cycle fuel consumption, and not provide real world, long term improvements in costs and emissions. Bigger batteries, bigger starter motors, additional control valves making already over-weight cars even heavier still, increasing the cost and environmental impact to built and maintain the car for what may be a negligible real world improvement is a real shame in my opinion, I really hope it does work though as it sounds like a great idea in principle.

[SonicShadow]

article said:

It is also safer than coasting manually (either using the clutch or by engaging neutral) as drive can automatically and immediately be restored should it be required.

The simplest set-ups just disconnect the engine from the transmission, and drop the engine speed to idle, allowing the car to sail along unimpeded.

I thought it was generally accepted that this was generally *less* efficient due to the previously mentioned fact that pretty much every EFI system from the past 30 years sets injectors to 0% when throttle input is 0%?

[Salamura]

I'm not sure why I need this on a car. If I want to coast with the engine at idle I can press a pedal and pull a lever and put the car into neutral. If I want engine braking I can lift off and stay in gear. I don't need a computer to do that for me, and potentially do the opposite of what I want if its programming is not perfect. It's just one more thing to go wrong and break, and one more useless automation of a simple manual process...

[MB140]

I have an m135i that has stop start and coasting in eco-pro mode.

The stop start is annoying as fk. It turns the engine of too soon. Literally the second the wheels stop rotating. Which is alright until your in traffic and are just constantly inching forward. Stop, start, stop, start. It’s just too fking on and off too early. If it waited 5 seconds to see if I was actually waiting rather than inching in traffic then it wouldn’t be so bad.

Worse still it defaults to on every time I start the car. Why can’t it remember my last setting. (I know it can be coded but BMW won’t do it so it would mean me paying someone to code it or buy an app and dongle to do it myself which is a pain.

As for coasting and eco pro mode. The only time I use that is in town to dull the throttle response. Other than that pointless.

[Mammasaid]

mrbarnett said:

I like to use the gears in place of brakes wherever possible to lose speed; this would prevent that, and would consequently annoy me greatly.

And hasn't been best practice for more than 40 years....

Brakes to slow, gears to go.

You do know it's cheaper to replace brake pads than gearboxes??

[CharlesA]

How does the engine off coasting function work with power steering and power assisted brakes? I guess electrical power steering would be fine, but not hydraulic; for the brakes, is it only brake-by-wire (but even brake-by-wire has a hydraulic system?) or is there a hydraulic accumulator?

[anonymous-user]

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.

[anonymous-user]

Mammasaid said:

Brakes to slow, gears to go.

You do know it's cheaper to replace brake pads than gearboxes??

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...... ;-)

[Mammasaid]

Max_Torque said:

Mammasaid said:

Brakes to slow, gears to go.

You do know it's cheaper to replace brake pads than gearboxes??

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.......

But still can cause driven wheels to lose traction in wet/greasy conditions in a manual gearbox which is what I was driving at. It's what ABS was invented for, stamp on the brakes and the driver can steer out of trouble (that's the idea anyway). However changing down and releasing the clutch rapidly can cause the loss of traction and sometimes no amount of electronic intervention can overcome this.

[Roger Irrelevant]

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'.

[anonymous-user]

Mammasaid said:

Max_Torque said:

Mammasaid said:

Brakes to slow, gears to go.

You do know it's cheaper to replace brake pads than gearboxes??

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.......

But still can cause driven wheels to lose traction in wet/greasy conditions in a manual gearbox which is what I was driving at. It's what ABS was invented for, stamp on the brakes and the driver can steer out of trouble (that's the idea anyway). However changing down and releasing the clutch rapidly can cause the loss of traction and sometimes no amount of electronic intervention can overcome this.

Modern cars include dynamic stability control that includes "Torque up", ie the engine can apply positive torque to prevent the drivewheels underspeeding. ie, if you clumsily downshift on a large trailing throttle and stability is going to be compromised (ie measured vehicle yaw / trajectory != driver demand), the system, the DSC will apply the throttle for you to regain stability. Generally this system only operates on actual loss of stability and not just an underspeed of the driven wheels that doesn't lead to a loss of stability because to drivers, the re-application of torque feels like "speeding up" and hence is to be avoided where-ever possible, even if the reason it had to act was the fact the driver is awful (see a theme appearing here?? lol!)