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DoT advice is designed to be simple to understand rather than accurate-as such it is simplified to the point of absurdity.
Coasting saves fuel if you want to preserve your momentum. If you have to come to a halt, obviously leave it gear.
But by far the biggest howler is the advice to mimse the accelerator pedal and shift gear at a fixed RPM, say 2500 RPM. This is both obnoxious to the drivers behind you and inefficient.
The best way is to accelerate sharply in a lowish gear until about you hit your target speed. Then change up two ratios and leave it in that gear until you need to accelerate again. Very simple, quick and fuel efficient. Yet, virtually no one uses the technique. So from a standing start, my shift pattern is often 1,2 accelerate to 30 or 40, then straight into 5th. if I need to get to 50-60, I use 1,2,3, 5.
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Coasting, ie transmission in neutral engine at idle does use more fuel than simply taking foot off accelerator but leaving transmission in gear - as so many modern cars have fuel cut-off systems which operate above the idle speed.
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So what keeps the engine spinning? Your momentum, that's what. It's not a free lunch unless you have to slow down anyway.
I've tried every fuel saving trick in the book. Coasting does work, but the gains aren't huge.
The best 'free' ones are: preserving momentum-usually speeds up journey time too.
Sharp acceleration followed by cruising in high gear.
Blocking off the about 1/2 radiator grille yields good gains too, but you'll want an accurate temperature gauge!
I've never noticed any gains/losses from different tyre pressures.
I've used cardboard side skirts on the rear wheels and folded in wing mirrors in the past. The former looks ridiculous and the latter is downright dangerous.
I also gave up slipstreaming lorries because it is stressful to both the tailgater and the tailgated.
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Having to sit at a steady 55mph (1,700rpm in 5th) on the 20-odd mile stretch of the M62 monitored by average speed cameras did wonders for my fuel economy this morning, if nothing for the journey time :-/
Edited by craig-pd130 on 29/03/2012 at 18:11
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It's agony though, isn't it?! I've done that run a few times and have to treat myself to a blast at 85ish afterwards. There are some great inclines on the '62 for coasting though!
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In SWMBO's car, which has an updated-every-minute (or so) average fuel consumption readout (miles-covered/fuel-used since last reset), leaving the car in (a high) gear but taking one's foot completely off the accelerator at every possible opportunity is definitely the technique which bumps up the mpg.
On one occasion I managed to get 83mpg over a 50+ mile trip along A and B roads doing 45-50mph (which was the speed of the rest of the traffic in any case).
I've heard previously that a short time accelerating quite briskly up to a given speed is more economical than spending a longer time accelerating at a lower rate only tickling the throttle, but is there objective test data proving this? and why should it be so?
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I've heard previously that a short time accelerating quite briskly up to a given speed is more economical than spending a longer time accelerating at a lower rate only tickling the throttle, but is there objective test data proving this? and why should it be so?
Look at an efficiency map for a typical diesel or petrol engine and the answer is obvious.
What most people don't realise is that it takes a fixed amount of work to accelerate a mass from a given starting speed to a given final speed, independant of the rate of acceleration.
The variable is the efficiency of the engine. Engines are always more efficient when loaded. When you tickle the accelerator most of the fuel is simply used to keep the engine rotating and only a little useful work.
Edited by unthrottled on 29/03/2012 at 18:34
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What about overfuelling, if you're thrashing it?
If F=ma, then is the most efficient acceleration produced in a reasonably narrow band of revs around, or just above, the torque peak?
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My 08 Berlingo doesn't have the auto fuel shut-off thingjmejig, So I'll still action the "pop it in neutral" wotsit, as I am sure the savings, whilst miniscule, are there.
The last time I travelled the M62 (last week) the 50mph roadworks lasted for 17mls.
& the number of foreign truckers going past at easily doing well over the restiction, is mind boggling.
Also my 4th gear must still have the burrs on from production, as I too usually go from 3rd to 5th.
("There are some great inclines on the '62 for coasting though!")
I'll second that.
Edited by Kanberlingoo on 29/03/2012 at 18:44
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Yeah - that giant sweep around Saddleworth on the way back toward Manc. Foot off!!! Wind assist!
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Yeah - that giant sweep around Saddleworth on the way back toward Manc. Foot off!!! Wind assist!
That's the one :) If you're doing 80 when you pass the Lancs / Yorks border marker, it's possible to to still be doing 80 at the bottom of the hill with scarcely any throttle input
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Yeah - that giant sweep around Saddleworth on the way back toward Manc. Foot off!!! Wind assist!
That's the one :) If you're doing 80 when you pass the Lancs / Yorks border marker, it's possible to to still be doing 80 at the bottom of the hill with scarcely any throttle input
I will therefore assume that rule holds for faster than that :-))
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"There are some great inclines on the '62 for coasting though!"
DPF diesels could probably get a free passive regen going up the hills too. But not many seem to avail themselves of the opportunity. (the amount of work required to climb a hill is also independant of speed). This is the trouble with fuel confusers. Your instantaneous fuel consumption is irrelevant. It is the total amount of fuel consumed over the journey that matters. It's maddening to watch people slowing down as they go up hills because it is pointless!
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My best coast was along the A537 from Buxton to Macclesfield. As soon as I passed the Cat and fiddle pub, roll into neutral, engine off. Stayed off until I hit Macc 6 miles later.
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Engine off = no servo brakes.
I would crash.
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Engine off = no servo brakes.
Only once there's no more vacuum. I think you should have enough for one or two applications, even with the engine off. Not sure if the vacuum leaks away with time though.
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I get about half a dozen applications before the servo is exhausted. There's always my faithful handbrake/drums which don't need vacuum assistance. Blimmin grabby though. When a brake shoe engages, it engages hard.
My old man used to test drive trucks with drums all round. With a bobtailing tractor unit, an emergency stop would send the back axle up in the air.
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Engine off = No brakes,
Engine off = No steering??? Try it!
Power steering = best thing since the proverbial sliced bread. I thought a poster would have mentioned the safety factor regarding coasting, even though I suggested in my OP to ignore it for the sake of the discussion, but engine off, no way!!! You'd look pretty stupid if the engine failed to fire up at the precise moment you try to regain control of your journey to the grave.
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Engine off = No steering??? Try it!
I think that's only a problem at low speeds, like for parking. I've not done this for ages, but my recollection was that at normal road speeds I didn't really need the power steering.
But at low speeds, my current car is a bear to steer with the engine off. I do it all the time if I'm just pulling forward a couple of feet, but I need to use both hands on the wheel.
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What happens with electric power steering? Can the battery supply the required power with the engine off?
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But don't some people claim they get slightly better economy when NOT using cruise control because rather than maintaining the same speed when going up a hill, they instead maintain the same throttle opening, and allow the car to slow down a bit?
Provided there's enough torque to do it, without the engine going into terminal revs reduction, aren'yt you then increasing the load on the engine and therefore making it run more efficiently?
(Presumably provided you avoid excessive EGT)
;-)
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Ah, The classic constant throttle vs constant speed argument!
The constant throttle argument might give better economy because your overall speed is lower -you go up the hill at 40, but come down at 55-average speed=47.5mph.
The constant speed argument means you go up the hill at 50, and come down the hill at 50. Average speed=50mph
Aerodynamic drag is greater at 50 than 47.5. That is the variable, the hill is irrelevant.
But going up a hill at 40, and coming down at 60 will use more fuel than maintaining a constant 50 even though the average speed is the same.
Think about it what you do on a bike. You pedal hard up the hill because you have to do the climb. But you freewheel down the hill, because pedalling against air resistance is largely futile. Why change behaviour in a car.
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Also very much depends on the car.
Toyota's hybrid system returns far better economy at speed using cruise control than it does with the throttle operated manually.
At a guess, I'd say that activation of cruise causes the computer to recognise that it's likely to be doing the same speed for a while and act accordingly. Certainly it seems to spend far more time in its juice-sipping "energy recycling" mode with the cruise engaged than it does without.
I hadn't considered whether it might be possible to wring a bit more out of it by disengaging cruise before a climb and allowing the speed to drop. Not sure I can be bothered to find out (company fuel card).
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Sorry, but I placed this post in the wrong order.
Anyway. Engine off = No brakes,
Engine off = No steering!!! Try it.
Power steering = best thing since the proverbial sliced bread. I thought a poster would have mentioned the safety factor regarding coasting, even though I suggested in my OP to ignore it for the sake of the discussion, but engine off, no way!!! You'd look pretty stupid if the engine failed to fire up at the precise moment you try to regain control of your journey which could end up being to the grave.
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Engine off = No steering!!! Try it.
No power steering. I suppose the physically frail might not be able to turn the wheel themselves, in which case this trick is not for them. Above about 5mph any able bodied person should not require power steering.
You'd look pretty stupid if the engine failed to fire up at the precise moment you try to regain control of your journey which could end up being to the grave.
Over-egging the pudding makes one look pretty silly too. Why would the engine fail to fire? Why would I end up in my grave if it didn't?
Now let's talk safety. If you had a mechanical failure occur with your vehicle, would you be able to calmly retain control of the vehicle? Probably not.
Whereas I who have practiced with reduced car function under controlled conditions would be much more likely to avoid panic.
Edited by unthrottled on 30/03/2012 at 10:39
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You then, are obviously the exception to the rule. Anyone who switches off their engine whilst in motion, is foolhardy. And "Why would the engine failed to fire"
You come across as someone who has been engaged in the motor industry & possibly in R&D. So if that is correct, you of all people should know that anything, mechanical & or electrical, can fail at any time.
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You of all people should know that anything, mechanical & or electrical, can fail at any time.
So it is helpful to know what it feels like if you lose power assistance, no?
In an emergancy the engine could be started very quickly by dropping into gear and dumping the clutch. I wouldn't recommend it though. Don't panic Jones! I've only do it on roads where you can see a long way ahead.
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The only car that I ever had an issue with there was a Talbot Alpine. Perennial issues with blocked idle jets in the Weber DCNVH carb meant it was prone to cutting out when off throttle (e.g. during gear changes) and a ludicrously low number of turns lock-to-lock on the steering made the transition from having power assistance to not having it distinctly unpleasant.
Even that could be steered when it failed though.
As for the engine failing to fire afterwards, it doesn't matter. If it's back in gear and connected to the wheels it'll be going round, whether or not it fires. Thus the power steering pump will be running and vacuum for the brake servo will be being generated (either by pump of manifold depression - makes no odds). My only comment would be that "pretty silly" is a fairly kind description of this particular piece of idiocy.
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My only comment would be that "pretty silly" is a fairly kind description of this particular piece of idiocy.
What-running without the engine off on a quiet road with good visibility? Why?
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>>"going up a hill at 40, and coming down at 60 will use more fuel than maintaining a constant 50 even though the average speed is the same."
No, what I meant was, and the technique I use on SWMBO's car is:
Keep the accelerator in the same position when going up the hill as it was when going along the flat preceding the hill.
As the car crests the hill, still keep the accelerator in the same position, the car now going downhill, until the car regains speed.
When the car has reached a speed just above the flat road speed, take foot completely off accelerator pedal for the rest of the downhill.
Apply accelerator back to original position just before the hill runs out, to allow gravity to assist in accelerating back to the flat-road speed.
i.e. I am not deliberately trying to go faster downhill than on the flat. That may happen, but only if the hill is steep enough to cause it.
So yes, it is similar to riding a pushbike.
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Power ateering is nice, but as said, above 5mph, not essential.
I've driven a Discovery with broken power steering, ie none, after having made the checks I was given by the dealer. Bit of an effort turning out of my drive, but after that, the 15 miles to dealer to replace the broken pump was surprisingly no major effort. Indeed, I'd argue the car was better on the motorway with no assisance.
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F1 drivers would disagree.
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Going slightly off-topic, but a close acquaintance chose to be a good soul and NOT panic-buy. However, with still a day or two's essential journeys to make, they now find themselves getting worryingly low in the tank and none of the stations in the vicinity have ANY diesel left...
...if it really came down to it, how risky would it be to put engine oil / gear oil / fresh veg oil in the fuel tank of a Euro4 turbodiesel?
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Engine oils are designed to be oxidation resistant ie not burn, so they wouldn't work very well.
straight veg oil? If the ambient temperatures aren't too low (which they aren't) it should start and run ok. But it will form a laquers inside the fuel system which can be difficult to clear up afterwards. I doubt that it would kill the fuel pump or anything catastrophic if used in the short term. Fuel filters can be quite prone to blocking too.
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I had to run from the west country to eastbourne to visit a poorly FIL in a care home so journey was neccesary ! Train was £136.00 and 4 changes and about 7 hrs so took ma chance in the car.
Our 1.6 TDCi Fiesta has va small tank so range can be limited. We filled up and lept speed to 60 mph , I also added 200 ml of two stroke oil. Ran a drean and returned 69.7mpg over 474 miles and managed one tank !
Petrol stations in Sussex were dry and those that had fuel had queues a mile long and those that were quiet had stuck the fuel up to £1.52 ltr !!!!!!!!
so keeping speed and rpm down got the best mpg ever
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Keeping the cruising speed down to 50 or 60 really does help, but it is so boring over a long distance.
Not wanting to enter a game of one petty upmanship but I did manage computer read outs in the 80s with a fabia VRS kept at 50mph over a 10 mile stretch of M6.
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I find it helps to relieve the boredom travelling the M1 J10 - 13 through the 50MPH limited roadworks to play with the fuel consumption readout just to see what silly numbers come up.
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Sadly, I haven't got one. So I have to try to work it out from first principles. The annoying thing is that any fuel injected car is capable of estimating fuel consumption since it knows both the speed and the flow rate of the injectors. Definitely a feature that is purposely deleted on cheaper models.
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What model car unthottled? If its modern CANBUS type electrics, you might be able to just turn it on in the ECU with the correct diagnostic equipment. Have enabled the Trip computer on my Astra "Life" poverty spec, all I had to do was buy a matching screen and head unit. Same goes for all the rest of the kit, steering wheel controls, cruise control etc. Most the kit is still there ready to go.
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It's not OBD2 alas. Otherwise I'd run a scan gauge. All that data. In real time. *wistful sigh*
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So Chaps & Chapesses, after 40 replies/postings, is it worth the fuel saving to "coast" at every opportunity? Or do we say more fuel is saved on the "over-run" I run the 2008 1.6HDI Berlingo Multispace Desire, which doesn't have the engine shut off on idle. To refresh your memories, my query is the OP on this thread.
BeeJay
Edited by Kanberlingoo on 03/04/2012 at 15:43
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Yes, unless you have to slow down anyway. In which case leave it in gear!
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I'd suggest that if you can't work out which is the best then leave it in gear.
I can't believe that an '08 vehicle hasn't got a fuel cut-off on overrun. What's your reasoning?
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I'd suggest that if you can't work out which is the best then leave it in gear.
Agreed. You need to be on the ball with this trick. You need to remember that you're out of gear and be adroit at getting it back in again. That means memorising roughly what RPM corrosponds to each speed for your gears so you can rev match before you let the clutch out. Otherwise your car will nose dive. For instance I know that 50mph corrosponds to ~2400 RPM in 5th, 3000 RPM in 4th, and 3700 RPM in 3rd. So getting back into gear is as quick as changing gear-and quite a bit smoother.
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Okay, after talking to Citroné technical, they say that, the "fuel cut-off on over-run" is only applicable to some petrol driven vehicles. My 2008 HDI has the EGR "Exhaust Gas Recirculation" device fitted, but that has no bearing on my query, but do agree that selecting neutral, technically, does save fuel, but not recommended.
With regards "getting it back in gear" thereby avoiding an emergency stop if selecting too low a gear, scaring the hell out of the poor sod behind you, is really a none starter regarding this debate, although point taken. Having had this motor from new, I'd be a pretty poor driver if, by now, I couldn't associate speed with gear selection when re-engaging.
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I coast whenever possible, but, do not turn off your engine , doing so will no doubt activate your steering column lock.
I'm sure everyone on here realises this, but thought it best to make the point.
Currently I get 60mpg from my Citroen c3 Picasso 1.6hdi, pretty much happy with that.
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Coasting with the engine switched off? I just never get that sort of chance, nor much chance to coast any other way.
The only chance for "coasting" I get is good observation and anticipation of speed limit reductions, traffic lights and junctions - the best I can do is use the fuel cut-off on zero throttle while the revs are above 1200rpm.
Having lived with 20-something fuel consumption for years, I'm quite happy with 33mpg - after all it's a 2 tonne, automatic SUV driven the same way I used to drive the Subaru !!!
Never take life too seriously - no-one ever gets out alive anyway !!
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Never take life too seriously - no-one ever gets out alive anyway !!
Ha ha! Very true. Its a just a game RT. If you can't go fast (which these days youseldom can) you need something to amuse yourself.
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doing so will no doubt activate your steering column lock
Well there is doubt, because it doesn't. With the ignition in position 1 (accessories on) the steering lock is not applied. I'm not suggesting you should do it, but fabricating straw men arguments is silly.
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you have to turn the ignition off before going to the accessories position, steering locks, just tried it. No straw involved either...
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Well, it doesn't on mine! While the key is in the ignition barrel, the steering lock cannot be engaged. If this was a problem I think I would have discontinued the engine off coasting since I don't live in South Dakota!
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No chance of steering locking, on traditional a Saab. ;) Hence much safer!!!!
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