Coasting vs engine braking

[bunglebus]

Thought I'd ask what everyone's opinions are on whether it's better to coast in neutral or stay in gear with your foot off the accelerator - for example when doing downhill on a motorway.

Not really after exact facts and figures, just opinions. Is coasting more fuel efficient? Is it better for car control to stay in gear instead?

The reason I ask is I like seeing an improvement in fuel economy especially on longer drives, and there are certain sections of motorway where I can knock my car into neutral at 60 and either maintain speed or even accelerate, depending how steep it is. However, as I understand it in pretty much anything with fuel injection/ECU, if you release the throttle in gear, the fuel supply is effectively cut off (but you slow down due to engine braking), whereas coasting the engine has to idle.

Just thought it would make an interesting discussion.

[Asimo]

I am a habitual coaster. It does significantly help economy. It is of course very dependant upon the road, seldom possible on motorways for example. But on the kind of roads I drive most, up and down, quiet B roads, it is easy to coast for 20+% of the distance, and fun to do so if I am in no hurry.

It is not dangerous.  (The so-called safety advice that it is dangerous comes from the age before synchromesh gearboxes and brakes that worked.)

[paulplom]

I'm not sure of the legalities of coasting. I don't think you're in full control of your car while you're doing it. 

[face]

My Golf has ‘Eco’ mode which amongst other things, knocks the DSG into neutral when you come of the throttle. It is constantly ensuring the right gear for the car’s speed is ready while coasting and engages it when you touch the throttle or brake. Doesn’t work on the adaptive cruise mind you. But it enables me to extract up to 45 mpg from a 300 bhp petrol car.

[St.Jude]

Engine braking, all the time. Makes a huge difference in maintaining the speed of the vehicle on a hill instead of burning the brakes up.

Also leaving the car in gear as you go down hill doesn't use petrol, well not in fuel injection cars anyway as there's no need for the ECU to supply fuel to the engine given that it's still moving and everything's tickety boo.

[wesacosa]

10 minutes ago, sgtberbatov said:

Engine braking, all the time. Makes a huge difference in maintaining the speed of the vehicle on a hill instead of burning the brakes up.

Also leaving the car in gear as you go down hill doesn't use petrol, well not in fuel injection cars anyway as there's no need for the ECU to supply fuel to the engine given that it's still moving and everything's tickety boo.

agree with all this, yet I still coast.  too many years driving with bad habits ?

[castros_bro]

Shirley you need a Saaab 96 and other numbers.

 

An unusual feature of the Saab drivetrain was a 'freewheel' (overrunning clutch). This allowed the transmission to run faster than the engine, such as when decelerating, or descending a long hill.

Freewheeling was retained in the four-stroke variant, until the end of production and in the Saab 99 with the 1709 cc Triumph engine. A minor drawback to the freewheel, particularly for drivers unfamiliar with the Saab, is that it makes engine braking unavailable although it could be manually engaged or disengaged by a control in the foot-well. Fixed wheel engagement, using the foot, could be difficult, as it involved pulling a 'T handle' intended for manual operation.^[10]

[lesapandre]

Some 50's cars had 'freewheel' on them that cut out engine braking when you took your foot off. You could switch it on or off. Rover's P4 had it and I think they dropped it once overdrive came in. Was an economy device. I had it on a 50's Austin Westminster - a heavy powerful car on marginal drum brakes - was terrifying.

[cobblers]

as sgtbebrvagatrottov says, Every single fuel injected or diesel car completely shuts off the fuel when you are coasting, but if you know it into neutral then it'll start using fuel again.

There's no advantage to coasting unless you're driving something with a carb, and even then the benefits are quite minimal.

[Stanky]

4 minutes ago, cobblers said:

as sgtbebrvagatrottov says, Every single fuel injected or diesel car completely shuts off the fuel when you are coasting, but if you know it into neutral then it'll start using fuel again.

There's no advantage to coasting unless you're driving something with a carb, and even then the benefits are quite minimal.

What these guys say. There is a section of the A32 that i used to drive daily by the Meon Hut services. coming home it was possible to coast for nearly 2 miles in 5th gear, keeping the speed at 50-60mph.

[bunglebus]

19 minutes ago, cobblers said:

as sgtbebrvagatrottov says, Every single fuel injected or diesel car completely shuts off the fuel when you are coasting, but if you know it into neutral then it'll start using fuel again.

There's no advantage to coasting unless you're driving something with a carb, and even then the benefits are quite minimal.

This was partly the reason for my initial question - there's one section where if I put my car into neutral at 60, I can coast for about two miles as my speed will increase to 70+, then I put it in 5th again at 60. If I was to stay in gear I'd need to use light throttle as the engine braking would slow me down. 

I only use this technique on motorways, everywhere else I use engine braking when I can see I'm going to need to slow down; limit changes, approaching lights etc.

[myglaren]

1 hour ago, paulplom said:

I'm not sure of the legalities of coasting. I don't think you're in full control of your car while you're doing it. 

Years since I read the Highway Code but do recall that it was said somewhere that coasting is illegal

Using engine braking is more economical with fuel injection as fuel is cut off completely.

I use engine braking at every opportunity and it was great with the C5 (diesel) but shite with the Civic (petrol)

Oddly enough my daughter, who is also a proponent of engine braking, was asking just this last week, she has a 2L petrol Megane and said it was disappointing compared with the diesel Rover 75 she had.

[bunglebus]

According to this https://www.drivingtesttips.biz/what-is-coasting.html coasting is not illegal. I agree that in general driving, engine braking is safer and more economic, but I still think there are situations where it can help economy.

Whether it's actually good for the car to be doing 60-70 MPh at idle (excessive cooling, having to bring the revs up quickly to put it back in gear etc) I don't know.

[sutty2006]

Plus 1 for coasting in gear as apposed to neutral. You will use more fuel on idle that in gear. As for legalities, well the way I see it is you would fail your driving test coasting in neutral. And it states somewhere in the Highway Code that you don’t have as much control over the vehicle. Merc Actros trucks have “eco roll” which (when accelerator isn’t pressed) disconnects the clutch but leaves it in gear to use the weight of the vehicle to save fuel. It’s all about keeping it legal. 

[danthecapriman]

Engine braking for me. It was how I was taught to drive and I find it a smoother more natural way to drive. 
As for fuel economy, I don’t honestly care! Given what I drive around in, it’s really not worth the worry! 

[St.Jude]

15 minutes ago, bunglebus said:

This was partly the reason for my initial question - there's one section where if I put my car into neutral at 60, I can coast for about two miles as my speed will increase to 70+, then I put it in 5th again at 60. If I was to stay in gear I'd need to use light throttle as the engine braking would slow me down. 

I only use this technique on motorways, everywhere else I use engine braking when I can see I'm going to need to slow down; limit changes, approaching lights etc.

Who knows. Obviously vehicle weight, tyres, drag, windows open etc all make a difference to the drag slowing the car down. But given (I think) 5th gear on most cars is near enough a 1:1 ratio to the engine the throttle input needed to maintain speed downhill would probably be either less or on a par with idle. 

[Jikovron]

I've noticed with the 1.0 suzuki Celerio that it has almost no engine braking , but the brakes are massively OTT for the size of the car really, so I think more modern cars must have the kind of egr system that flows enough to take away most of the vacuum pumping losses to preserve momentum without having to apply much throttle.

It works well as I've seen an average of 78mpg in that car where the daihatsu charade 1.0 would only deliver 65mpg in the same setting.

I tried the coasting trick in my skoda years back down a long descent on the m1, but it would quickly slow down to 50mph and drop further so I suspect wind resistance has a large part to play.

[Sir Snipes]

But if you're engine braking isn't that putting load on the engine? 

[sierraman]

Just sit on the brakes, that’s what they teach you these days. Doesn’t matter if it’s Porlock Hill, just sit in 4th with your foot on the brakes. ?

[320touring]

I tend to work it like this:

Engine braking (and associated Deceleration Fuel Cut Off(DCFO)) When I want to slow down or avoid acceleration - e.g. engine braking approaching junctions/hazards (with foot brake if required) or if going down steep inclines.

It's also good to make sure you're in the right gear to cane fuck out it as you exit the corner;)

 

Coasting can be particularly effective if used as part of a "pulse and glide" approach.

This is where you accelerate moderately heavily (about 60-75% engine load) to your upper speed (say 70mph) and then coast down to your lower speed (say 60mph).

It does reduce fuel consumption, but based on trying it a couple of times, it's a faff. So I don't. It's pretty popular in the States though..

[bunglebus]

Yeah CBA with coast and glide although it's been proven to be economic. Annoying for everyone around you and probably increases wear on the car. 

I do find it quite amusing overtaking people on the M25/M23 at idle though.

[Mally]

I'd forget to put it back in gear, and think the clutch was slipping.

[CreepingJesus]

I did a SAFED course donkey's years ago for work, out of curiosity. The whole 'defensive driving' thing was kinda how I drove anyway, cos my dad was keen on that so that's how I learned.

Long story short, a few years after, I got in a 'heated debate' with a TM at Asda after having the same route two days running, with similar loads and conditions, in nearly identical motors: I was over 1mpg - i.e., 10% - better with the manual box than the semi-auto. All due to time off the throttle, which I think is the more important part to focus on. 

Things like 'slow shifting' where I'd drop the throttle well before stepping on the clutch, giving the synchro time to do its' thing, and blending clutch and throttle back in nice'n'easy like. To do it properly, seriously, you have to get anal about it and look for those marginal gains! It matters more when your motor does 10mpg at best though.

I am nerdy enough to look up the gear set and SFC curve of anything I'm attempting that on. 

[CreepingJesus]

12 minutes ago, bunglebus said:

I do find it quite amusing overtaking people on the M25/M23 at idle though.

Engaging the 'Aberdeen Overdrive' at the top of Shap (in either direction, but southbound was better) and passing the express coaches that had breezed past me going up, was highly amusing. Focuses the mind somewhat, too.

[406V6]

I’ve been driving a brand new modern non-shite petrol-only vehicle (whose name I won’t mention here) that effectively has coasting built in. When driving along at higher speed (50 mph and higher) and taking the foot off the gas the revs drop to tickover and the autobox disconnects drive. There’s no engine braking when it does this.

When you get to electric cars there are all sorts of variants of engine braking, better known as regenerative braking where juice is put back into the battery. On some cars you can vary the strength of it, some not.

Regarding the original question I would just ease off and let the engine management do its thing rather than change gear into neutral.

[Spiny Norman]

I've always been taught that coasting was very bad practice and the only time I ever do it is maybe the last couple of hundred yards up to a red light or similar. In a modern fuel injected car I don't think it'll have much effect on mpg figures and I'm never that desperate that an extra 1mpg will make any difference to my financial stability. The idea of doing it at speed on a motorway horrifies me, there's a pronounced effect on the feeling of stability of  a car with the engine connected to the driven wheels even under no load compared to  when it's just freewheeling.

With the brand new stuff that does it automatically, any re-engagement of the gear will be handled so seamlessly and quickly that you'll not feel it happen, it won't be like the shunt you get at 60mph when you try and shove it into gear yourself with a bit of throttle because you suddenly need a bit of go.

[RayMK]

I remember my Dad coasting in his first car, a 1933 Austin 7.  He said it was partly to save fuel but it also enabled him to go faster at times.  With the engine bolted directly to the chassis (no rubber cushioning), anything above 30mph under power* sounded like mechanical suicide. Most journeys took forever, giving new validity to 'are we there yet?' which I constantly chanted from the back seat. He also coasted in his next car, a 1954 Hillman Minx.  This time, the reason was mainly economy.  Mum issued him with petrol money and Dad minimised its use for the intended purpose so that he could buy cigarettes.  Roads were generally much quieter back then (1950s/60s), making the speed variation caused by coasting less of a nuisance to others. 

My Mitsubishi Mirage has negligible engine braking from its CVT system. Although it may be possible to put the box in neutral on the move, I would half expect metal shards everywhere. I wonder if Vulg's LD10 can be coasted with the preselector gearbox? 

My opinion: Coast if you can do so safely. It must be more satisfying and far less irritating than stop/start economy systems.

[HMC]

I once used the silver box hatchback forum bike (xud7te) to coast my way to 67mpg on a trip along the a30 into Cornwall.

I understand that fuel cut off on the overrun is more economic Than coasting where the incline is sufficient - below a certain grade of of incline my impression is the cylinder pumping effect of the engine tends to slow the car- then needing then more throttle input rather than coasting along; if we are talking economy. 

[PhilA]

Engine braking. I was taught to read the road ahead and use the engine to scrub speed off the vehicle in preference to using the brakes, simply because you can use engine braking for a significantly longer time to gradually reduce speed rather than putting heat into the brakes, so as to save their braking efficiency for when you may need it most.

In terms of efficiency, foot off the throttle and pulling high vacuum the engine is drawing minimal fuel though with a carburetor and all my vehicles that have fuel injection cut the fuel on hard deceleration above about 1500rpm anyway.

 

But around here it's flat as hell and nobody knows how to drive without using their brakes...

[paulplom]

2 hours ago, PhilA said:

Engine braking. I was taught to read the road ahead and use the engine to scrub speed off the vehicle in preference to using the brakes, simply because you can use engine braking for a significantly longer time to gradually reduce speed

Very dangerous. I tried that at the track in the mx5 a couple of weeks back. I nearly ended up in the feckin tyres!