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In the middle of December i was forced into finding a replacement for my trusty Ford Transit Connect 1.8TDCI (90PS). This turned out to be a VW Caddy 2.0 SDI and it has resulted in my changing driving style to suit the less powerful n/a engine. With the Connect, i would change up earlier and use the turbo's torque, whereas with the Caddy i need to hold on to the gears longer, especially uphill. Conversly, the Caddy feels much smarter off the line, the Connect feeling more flat until the turbo woke up. Also, getting the Caddy coincided with finding definitive information regarding van speed limits on the single carriageway roads i spend most of my time on (for the record, all vans or 'LCV's are legally allowed to do 50mph, regardless of weight). Before this, i had mistakenly thought that it had to do with whether or not the GVW was up to 2 tons (60mph) or more than (50mph). Even though i was certain i should have been doing 50mph with the Connect (being the 2.3t lwb version) i still generally drove around 60mph, which, along with the short stop start running once i got to wherever i was working, gave me an average of 40mpg. The Caddy, i expected to be a fair chunk more efficient than the Connect (known as being amongst the thirstier of it's ilk), so was dissapointed when the first time i worked out the economy at 42.8mpg. It did gradually improve once i started with the 50mph regime, but only to around 44-45mpg. However since i also removed the roof rack, it is now as near as dammit 50mpg. So i am quite pleased at the savings, and am also finding the lower speed more relaxing!. Driving at 60mph and coming upon slower drivers, you kinda feel obliged to overtake, so you have to look for gaps, judge whether or not they are big enough, positioning yourself to get the best view, etc. But now, i rarely need to overtake as i'm going slower than most cars, and even if i do find myself behind a truck doing less than 50mph, it usually is not by a big enough margin for me to be bothered by it or unduly held up.
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Fuel consumption is mainly about driving styles and conditions. The VED on my car, Renaultsport Clio, was higher than on my father’s, Modeo 2.0 auto, despite it being more economical.
But mine varied immensely on conditions.
Around town, urban not inner city, 27mpg.
Longer journey, using dual carriageways, 32mpg.
Motorway 38 mpg.
Blizzard coming back to UK and from Dover to London I got 48.5 mpg. Not just driving carefully because of conditions but trying to see best consumption.
Driving down to South of France at 110 mph all the way, 28mpg. Cheaper than overnight stop.
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I think it comes down to having 'mechanical sympathy', which I'm sure most of us here have. For economy it is usually about trying to keep the engine somewhere near the peak of the torque curve, which for a diesel is around 2000rpm. For petrols more like 3000+ ? Basically listening to the engine and learning what the sound means - especially the unexpected wrong ones, of course.
Agree....though sadly, that "sympathy" seems to be getting rarer these days.
The basic "interest" in cars and how they work has diminished IMO...it seems now to be simply a metal box sensory deprivation device that gets you from A to B...engine sounds are ignored in favour of peak volume ICE/bluetooth gizmos' etc.
The "old skool" interest has largely gone..,,but I guess that's a generational thing...and the basic reliability and complexity of modern vehicles.
I remember when we HAD to know stuff...or find it out... and then often fix it ourselves to keep our cars on the road.
Times is changed methinks.!
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Times indeed changed. I'm no mechanic but my old Viva was easy for changing plugs, points, timing, etc. loads of stuff you could do yourself. There were lads in the band who could do more advanced stuff and we helped each other out. Now anything wrong and I haven't a clue, 'can't fix it sir, needs the part replacing' followed by 'how much?!' Multi function indicator switch last month or so, connection faulty...can't be fixed...£500 for new one. As an auto driver I just let the car do it's thing keeping to about 2000 rpm but occasionally for fun put the 'S' mode on which livens things up.
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As an auto driver I just let the car do it's thing keeping to about 2000 rpm but occasionally for fun put the 'S' mode on which livens things up.
I'm "guilty" of that too.!...I let mine do its thing generally...it is largely a 3 litre cruise machine after all...but I confess I have a "hooligan" moment every now then.
I love to hear my V6 on full song as it were, so it gets an "Italian tune" sometimes,.
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I also practise economical driving (most of the time!), but people pay too much attention to fuel consumption. A few mpg here or there matters little to the average motorist averaging 8000m per annum. Last year our three old cars (TR7,Focus 1.6 Zetec, Audi W12) did 10,700m, our lowest ever total as now fully retired pensioners. Petrol cost £1915. That's 17.9 pence per mile. At circa £5.80 per gallon (sorry, still in old units) that’s an average of 32.4mpg. If they were tiny boring cars with an average of a stunning 50mpg, petrol would have cost £1241, a saving of £225 per car. Wow. Compared to what most suffer in depreciation costs – plus possibly contractual unnecessary ‘servicing’ and unforeseen repairs to complex ‘economical’ engines - this is peanuts.
More important than driving technique for getting the best from your vehicle is looking after it wisely and carefully to preserve its longevity - and choosing a well built well engineered model that is likely to endure.
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At circa £5.80 per gallon (sorry, still in old units) that’s an average of 32.4mpg. If they were tiny boring cars with an average of a stunning 50mpg, petrol would have cost £1241, a saving of £225 per car. Wow. Compared to what most suffer in depreciation costs – plus possibly contractual unnecessary ‘servicing’ and unforeseen repairs to complex ‘economical’ engines - this is peanuts.
For me, the question of 'economy' is nearly all about using less fuel, not less money. Money recirculates and can be reprinted if necessary. We burn fuel once only, and what we burn cannot be used by anyone else - it's turned into water and CO2 and all that business. So I have always looked for economical cars and have tried to drive economically (tho not to the exclusion of everything else).
The first car which I monitored was a Morris 1100 (1962) which I bought in Canada and drove over much of the US. On one 3500-mile trip it averaged 44mpg (British) which at the then exchange rate cost £19. Today's vehicles average >60mpg (207 diesel) and 47mpg (205 petrol). 32.4 I would feel disappointed with.
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Last year our three old cars (TR7,Focus 1.6 Zetec, Audi W12)
I remember you mentioning that previously. Possibly only the V10 petrol is rarer in the A8! Were you specifically looking for one, or did it cross your path and it was too good to refuse?
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Last year our three old cars (TR7,Focus 1.6 Zetec, Audi W12)
I remember you mentioning that previously. Possibly only the V10 petrol is rarer in the A8! Were you specifically looking for one........
Yes. There were indeed only about 50 RHD 'sport quattros' (shorter,firmer and lower geared than the more common W12 LWB limos) imported to the UK in 2005 - I searched for 6/12 before one I liked appeared for sale. Despite being streets ahead of the competition (the similar powered but more primitive engined absurdly expensive 2WD Aston Martin DB9 appeared at the same time) I think no UK car mag ever tested them. If you are interested, here is a link to an old I think Polish mag review. - even this is mis-titled as 'L' WB.....
www.avtomobilizem.com/arhiv-2002-2006/audi-a8-w12-.../
The Lambo V10 is much more common, superceding it and badged as a reborn S8; same set-up and power but more raucous and less torque. The unstressed high quality NA engine needing minimal maintenance should last me forever with luck!
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The Lambo V10 is much more common, superceding it and badged as a reborn S8; same set-up and power but more raucous and less torque. The unstressed high quality NA engine needing minimal maintenance should last me forever with luck!
Thanks for that - from what info I could find, the V10 petrol seemed to be relatively short lived. For some reason I thought the V10 diesel was available in the A8 too, but apparently not.
I do have an affection for the niche variants of mainstream models that VW, Audi and MB seem to specialise in .... like the Passat W8!
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"For economy it is usually about trying to keep the engine somewhere near the peak of the torque curve, which for a diesel is around 2000rpm. For petrols more like 3000+ ?"
That's not going to work for at least some petrols. The Mazda 2 litre engine in the CX-5 produces maximum torque at 4,000 rpm. Top gear (6th) gives about 30 mph per 1,000 rpm. So at maximum torque the car would be exceeding 100 mph, at which speed wind resistance would produce a lot of drag.
I find driving at around 60 mph in top on a long journey produces around 45 mpg; in road works with a speed limit of say 50 mph consumption improves further, which suggests to me that the advantages of keeping the engine "somewhere near the peak of the torque curve" are cancelled out by other factors.
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Due to where I live outside London it`s a waste of time trying to get good economy unless I go for a drive to the coast (almost a straight run of 52 miles each way)
most of the time except school holidays your lucky to get to 30mph, last week there was roadworks on every A road to where I travel to, so a usual 12-15 minute drive was over an hour
I decided for the first time ever to try BP Ultimate diesel, but to do more than fill up once or twice and filled up on it for 8 weeks to make sure the tesco fuel I usualy use did not affect the results
I found the BP was terrible consumption wise my usual mpg dropped roughly 50-100m tank and more depending on traffic, before anyone mentions stop/start that only works for a short time in severe traffic jams but swiched engine off instead if it was that bad traffic
when I first got the car it was doing same mpg as the old 2.2 civic 45mpg but acceleration was better on that car, after first service the 1.6 new civic started improving to 60+, on a run had 80mpg but with BP drops really heavily, imo the only thing I can say positive about BP is it improved acceleration by a fair bit though its not needed where I live, I would have prefered better fuel consumption
But I know now, where I wondered if other drivers reports of BP Ultimate diesel were justified, but I don`t think so, I will stick to Tesco diesel in future its better fuel economy and runs as well......
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Craig PD, sound like you make the most out of your vehicle's strengths, do you stll miss that PD engined Passat, one of the best cars of all time?
Leif, yes gears on hills is one of my pet hates, on my commutes there are two small valleys on the road, the number of people who brake and go slowly down only to hit the throttle when climbing the other side for a 10/20 mph faster speed is amazing, instead of allowing weight and gravity to do its thing meaning momentum takes you most of the way up the other side, wasteful.
Andrew T, yes mechanical sympathy goes hand in glove with maintained economical progress, and a longer vehicle life to boot.
Badbusdriver, isn't it odd how that slower speed doesn't lead to noticeably slower journey times, and your figgures prove it (interesting how much difference the roof rack made) i'll take one example from the fuel readings last week of the difference in progressive driving, a colleague who did the same journey as me (its hard on the vehicle loaded outbound) came flying past me just before a roundabout where he was on the throttle till the last second then braked hard...he stayed in sight through the peak period journey, on the fuel reading he got 5.9, i got 8 mpg for the same run, not cos i'm any better but make use of terrain and overrun, and always remember that when your foot is off the throttle your journey is free, i also seldom use the service brakes, having the instant fuel useage display up (can program the display from the menu) makes a world of difference here...by the way peak torque is about 1200 rpm and i run at 53mph mostly which is just under the 1200.
i do the same in my car, lifting off a long way from junctions and driving up on a trailing throttle making use of terrain where possible for extended free running, timing lights and junctions for no stopping makes a huge difference in any vehicle, possibly bar a good hybrid which recovers the lost energy.
KJB 123, these differences in MPG are amazing from just one car, the high speed run shows just how much air and road friction takes its toll on fuel, just out of interest your urban and sensibly driven figures are not far different from my Landcruiser, though i suspect the performance is not in any way comparable :-)
Nellyjack, yes times have changed, i'm just surprised how little interest so many people have in whats going on under the bonnet, maybe more people have such a high disposable income these days they don't need to fix things, or is it access to easy credit or the myriad of alternative car use/finance methods i wonder...of an V engines make a wondrous sound if an expensive soundtrack.
Barney, the multi function indicator stalk failed on the W124 we still have, so less than 10 years ago, £60 new! Yes the VIva and most traditionally driven cars of that era were lovely to work on (FWD spoiled that), engines out in less than an hour and two strapping lads could stand on the inner wings and lift it out with a rope, i still find Landcruisers reasonably easy to work on though i won't be heaving the engine out with a rope nor dropping the gearbox on me ample belly and wiggling it out, and ye i like proper autos though i know they arn't the last word in fuel economy but for smooth progress they're wonderful.
JohnF, though we might disagree on the method and time scale of servicing, i think we read off the same hymnsheet generally, buying the right car in the first place is paramount, however i get a perverse pleasure at work from beating the vehicle's computers over fuel economy and progress, i believe some vehicles are now just a bit too clever by half and likely to have quite short economic lives because of that.
FP, yes i agree, the higher the speed generally the lower the economy, 60 mph seems to be the sweet spot for most normal vehicles in the progres versus economy battle, once you get above 70 unless the vehicle is very streamlined and narrow tyred, the two seldom bedfellows, then fuel takes a nosedive, though it's not nice playing with the lorries at 50 odd mph and i would rather travel that bit faster to avoid doing so.
Bolt, thats really odd how the BP Gucci fuel has made your consumption worse, i too have found Tesco Diesel to be good for running in my Diesel, though confess i seldom buy from there because i disliked their business methods for varios reasons and have avoided them for years i buy from them if i need fuel early morning because of their 24 hour machines, i have tried Shell's posh stuff too but noticed no difference other than the cost, and i'm back on Morrisons/Sainsbugs with a genrous helping of Millers in each tankful.
again in the traffic and cut and thrust of traffic you describe economy driving can go out of the window, i'd have thought a hybrid would be the ideal drive train for that type of going, though whether cost effective overall is another question.
All, many thanks for the responses.
Edited by gordonbennet on 29/07/2018 at 12:57
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Bolt, thats really odd how the BP Gucci fuel has made your consumption worse, i too have found Tesco Diesel to be good for running in my Diesel, though confess i seldom buy from there because i disliked their business methods for varios reasons and have avoided them for years i buy from them if i need fuel early morning because of their 24 hour machines, i have tried Shell's posh stuff too but noticed no difference other than the cost, and i'm back on Morrisons/Sainsbugs with a genrous helping of Millers in each tankful.
TBH, Tesco during the time I tried BP only had 4 diesel pumps working out of 12 so decided untill they fixed these pumps would try BP.
I may give millers a try, though have never been one to use any additives, I was taught during my apprenticeship they were not advisable and seen damage to engines caused by oil additives, which put me off using them!
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Nellyjack, yes times have changed, i'm just surprised how little interest so many people have in whats going on under the bonnet, maybe more people have such a high disposable income these days they don't need to fix things, or is it access to easy credit or the myriad of alternative car use/finance methods i wonder...of an V engines make a wondrous sound if an expensive soundtrack.
True...the soundtrack is lovely when you give it some boot particularly..but thankfully I don't have to overly concern myself with fuel costs.I only do about 5k miles p.a. so I'm prepared to pay the price at the pumps to have the power and the luxury when I DO use it.
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<< isn't it odd how that slower speed doesn't lead to noticeably slower journey times >>
Every couple of weeks I take a 40-mile trip into Welsh Wales. The first 3 miles are urban, followed by 20 on motorway or dual, then all but the last two on single-lane usually quiet A roads. The last stretch is narrow B road ending in a village.
The strange thing is that unless I meet a serious hold-up somewhere, the journey time is almost always 62 minutes plus-or-minus 1. Bimblers in cars or convoys of cyclists don't make that much difference. However the trip time seems likely to lengthen because of spreading Welsh 50mph limits with average-speed cameras - the latest is to extend the 50 zone on Ewloe Hill (been there for many years) right through the newish 3-lane stretch to the border where it becomes M56. Daft. No doubt cameras will appear soon.
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It took about three months to find what I call the sweet spot. The speed on the A12 that gives highest speed with maximum fuel efficiency quietest engine etc. For the Vitara S 1.4 Auto I've found it's 67mph.
Coincidentally once it hit over 5k miles from new it seems to be giving up to 10pct better mpg.
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JohnF, though we might disagree on the method and time scale of servicing, i think we read off the same hymnsheet generally, buying the right car in the first place is paramount.......
I'm sure we do, gb - looking ahead to avoid braking as much as possible, snicking the auto in and out of 'N' to coast down hills and up to junctions, lights and slip roads (I don't suppose that's feasible for HGVs!), gentle acceleration out of roundabouts and corners, no full throttle or high revs until engine fully warm (that's long after the water temperature is up to normal - shame most cars no longer have oil temp gauge...) - and regular preventive corrosion maintenance on brake pipes, fuel line, subframes.....
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snicking the auto in and out of 'N' to coast down hills and up to junctions, lights and slip roads
I seem to remember reading somewhere that coasting a torque convertor auto in neutral is bad for the transmission and that they should be towed with the drive wheels off the ground. No doubt some one better informed than I am will be along soon to confirm or deny this.
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snicking the auto in and out of 'N' to coast down hills and up to junctions, lights and slip roads
I seem to remember reading somewhere that coasting a torque convertor auto in neutral is bad for the transmission and that they should be towed with the drive wheels off the ground.
Not bad as long as the engine is running - the transmission oil pump will still be working. Towing is another matter - see the handbook. Mine says no faster than 50km/hr and no further than 50km.
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I'm sure we do, gb - looking ahead to avoid braking as much as possible, snicking the auto in and out of 'N' to coast down hills and up to junctions, lights and slip roads (I don't suppose that's feasible for HGVs!), gentle acceleration out of roundabouts and corners, no full throttle or high revs until engine fully warm (that's long after the water temperature is up to normal - shame most cars no longer have oil temp gauge...) - and regular preventive corrosion maintenance on brake pipes, fuel line, subframes.....
The IAM and others specifically advise against coasting as it is dangerous. (I'm not an auto driver but I assume N is the equivalent of having the clutch down in a manual car.) Basically you are not in control of the car when coasting. In any case, for many cars you will use MORE petrol doing as you suggest, not less. If you do down hill without pressing the accelerator, and in gear, the engine will turn over of its own accord and the engine management unit will cut petrol to the engine. If however you coast out of gear, the ECU will need to keep the engine fueled so as to keep it idling.
Your other advice, gentle acceleration etc is good.
My experience is that forward observation counts above all, as that allows you to get the correct speed without undue acceleration and/or braking, and you can often merge onto roundabouts seemlessly.
Edited by Leif on 30/07/2018 at 12:14
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<< If you go downhill without pressing the accelerator, and in gear, the engine will turn over of its own accord and the engine management unit will cut petrol to the engine. If however you coast out of gear, the ECU will need to keep the engine fueled so as to keep it idling. >>
When driving an old-style petrol car (manual choke, no ECU) I sometimes free-wheel down long gentle inclines - in fact I recall doing that for over 10 miles somewhere in Utah back in the 60s. I have no evidence that it saves much fuel, only the intuitive logic that the engine is idling more slowly.
However when I try it with my current diesel car the instantaneous MPG figure shows 999 (i.e. no fuel) if in-gear, while in neutral the value varies between 415, 564 or some other high figure - confirming Leif's point above.
I don't feel that I lose much control of the vehicle by coasting in this manner, as it takes only a moment to re-engage gear - brake assistance and all other functions still operate. Switching off ignition however ...... :-(
Edited by Andrew-T on 30/07/2018 at 12:58
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<< If you go downhill without pressing the accelerator, and in gear, the engine will turn over of its own accord and the engine management unit will cut petrol to the engine. If however you coast out of gear, the ECU will need to keep the engine fueled so as to keep it idling. >>
When driving an old-style petrol car (manual choke, no ECU) I sometimes free-wheel down long gentle inclines - in fact I recall doing that for over 10 miles somewhere in Utah back in the 60s. I have no evidence that it saves much fuel, only the intuitive logic that the engine is idling more slowly.
However when I try it with my current diesel car the instantaneous MPG figure shows 999 (i.e. no fuel) if in-gear, while in neutral the value varies between 415, 564 or some other high figure - confirming Leif's point above.
I don't feel that I lose much control of the vehicle by coasting in this manner, as it takes only a moment to re-engage gear - brake assistance and all other functions still operate. Switching off ignition however ...... :-(
Yes, I think cutting off fuel to the engine when it does not need it is a 'recent' innovation, so not the case for classic cars for example.
I think the argument is twofold, firstly the engine can provide some braking which downhill can be a good idea, and secondly although it only takes a 'moment' to engage gear, you still have to switch your attention to the clutch and gear lever for a second or two, which takes your concentration off the road ahead, and during that time you can't use your engine for speed control. The latter probably doesn't really matter if going down a long hill with good visibility, but coasting to a roundabout for example is a big no no in my book.
I know you didn't suggest switching off the ignition, and would not suggest it of course, but certain systems such as the brakes lose or have limited power when the ignition is off. I wonder if anyone out there actually does turn the engine off when moving? Hard to believe, but ...
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The IAM and others specifically advise against coasting as it is dangerous. (I'm not an auto driver but I assume N is the equivalent of having the clutch down in a manual car.) Basically you are not in control of the car when coasting. In any case, for many cars you will use MORE petrol doing as you suggest, not less. If you do down hill without pressing the accelerator, and in gear, the engine will turn over of its own accord and the engine management unit will cut petrol to the engine. If however you coast out of gear, the ECU will need to keep the engine fueled so as to keep it idling.
Oh dear, Leif. This was argued out 8yrs ago - and you still don't understand it which, if you never passed any physics exams, is entirely understandable and forgivable.
www.honestjohn.co.uk/forum/post/index.htm?t=86509
Eventually, 'AndyTheGreat' and 'Cliff pope' seemed to be the only ones that understood basic physics and agreed that I was right. No engine can 'turn over of its own accord', as you put it! It requires energy from somewhere and if getting no fuel, it uses the momentum (mass x velocity) of the car.
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<< Oh dear, Leif. This was argued out 8yrs ago - and you still don't understand it which, if you never passed any physics exams, is entirely understandable and forgivable.
www.honestjohn.co.uk/forum/post/index.htm?t=86509 >>
That's an amusing thread which I must have missed the first time round. But on the question of coasting, I am surprised at the small amount of engine braking provided by a diesel engine, which given no fuel - just air - and a compression ratio of about 20, I would have thought would be more of a damper?
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I am surprised at the small amount of engine braking provided by a diesel engine, which given no fuel - just air - and a compression ratio of about 20, I would have thought would be more of a damper?
Possibly one or both of the following reasons?
1. Diesel engines are usually mated to 'tall' gearboxes and don't spin as fast as petrol engines.
2. Your deceleration fuel cut-off system (assuming it has one) might have its fuel resumption rpm set highish.
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I am surprised at the small amount of engine braking provided by a diesel engine, which given no fuel - just air - and a compression ratio of about 20, I would have thought would be more of a damper?
Possibly one or both of the following reasons?
1. Diesel engines are usually mated to 'tall' gearboxes and don't spin as fast as petrol engines.
2. Your deceleration fuel cut-off system (assuming it has one) might have its fuel resumption rpm set highish.
Diesels tend not to have a throttle plate, hence no manifold vacuum and reduced retardation, is my assumption.
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The IAM and others specifically advise against coasting as it is dangerous. (I'm not an auto driver but I assume N is the equivalent of having the clutch down in a manual car.) Basically you are not in control of the car when coasting. In any case, for many cars you will use MORE petrol doing as you suggest, not less. If you do down hill without pressing the accelerator, and in gear, the engine will turn over of its own accord and the engine management unit will cut petrol to the engine. If however you coast out of gear, the ECU will need to keep the engine fueled so as to keep it idling.
Oh dear, Leif. This was argued out 8yrs ago - and you still don't understand it which, if you never passed any physics exams, is entirely understandable and forgivable.
www.honestjohn.co.uk/forum/post/index.htm?t=86509
Eventually, 'AndyTheGreat' and 'Cliff pope' seemed to be the only ones that understood basic physics and agreed that I was right. No engine can 'turn over of its own accord', as you put it! It requires energy from somewhere and if getting no fuel, it uses the momentum (mass x velocity) of the car.
Are you trying to be condescending and arrogant? If so, well done, you have succeeded.
With the somewhat elementary credentials of a 37 year old grade A in physics A level, a 34 year old First in physics from Imperial College and a 31 year old PhD in Physics from the University of Cambridge, I beg to disagree with your rather simplistic argument.
Highway Code rule 122 explains why coasting may reduce car control. The IAM teach that it is dangerous and to be avoided. If you coast during a driving test, you will fail. Obviously on a long hill with no hazards, it'll be safe, but that's not a typical driving scenario, and I bet that's not typical of your coasting.
As to fuel efficiency, a modern engine such as the 3 pot turbo in my VW Polo does not lose much energy from friction. On my drive to work I have many hills, and I can cruise down each hill whilst in gear without accelerating, thereby using little or no fuel, and in practice I often brake to avoid going too fast.
There is a reason for modern stop start technology, namely that an engine does use significant amounts of fuel while idling.
This is a nice clear explanation:
www.lightfoot.co.uk/news/2018/02/02/bad-technique-.../
And here is another from a school of motoring:
www.chrischambers-som.com/pdf/whatiscoasting.pdf
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Are you trying to be condescending and arrogant? If so, well done, you have succeeded.
No, just knowledgeable and authoritative. Trying to correct false assertions and to criticise unhelpful anachronistic dogma.
With the somewhat elementary credentials of a 37 year old grade A in physics A level, a 34 year old First in physics from Imperial College and a 31 year old PhD in Physics from the University of Cambridge, I beg to disagree with your rather simplistic argument.
I find it difficult to believe that someone with such academic qualifications could write 'energy....is supplied by gravity' (2008. Gravity is a force, not energy) - and - 'the engine will turn over of its own accord' - and - ' 99% of the time coasting downhill does not save fuel'.
It is undeniable that turning over an engine plus its ancillaries at 600rpm will require more energy than turning it over at 1800rpm. Whether that energy comes from injected fuel or the momentum of the vehicle is immaterial, because both sources of power come from burning fuel. Gravity is a red herring, providing no energy, and doesn't come into the argument.
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Are you trying to be condescending and arrogant? If so, well done, you have succeeded.
No, just knowledgeable and authoritative. Trying to correct false assertions and to criticise unhelpful anachronistic dogma.
With the somewhat elementary credentials of a 37 year old grade A in physics A level, a 34 year old First in physics from Imperial College and a 31 year old PhD in Physics from the University of Cambridge, I beg to disagree with your rather simplistic argument.
I find it difficult to believe that someone with such academic qualifications could write 'energy....is supplied by gravity' (2008. Gravity is a force, not energy) - and - 'the engine will turn over of its own accord' - and - ' 99% of the time coasting downhill does not save fuel'.
It is undeniable that turning over an engine plus its ancillaries at 600rpm will require more energy than turning it over at 1800rpm. Whether that energy comes from injected fuel or the momentum of the vehicle is immaterial, because both sources of power come from burning fuel. Gravity is a red herring, providing no energy, and doesn't come into the argument.
You are remarkably pompous. I have no idea what the ‘energy ... is supplied by gravity’ nonsense is all about. Coasting down a hill uses more energy because the engine must burn fuel to keep it ticking over, whereas if you keep a modern engine in gear while going downhill, it will turn off the fuel supply. Simples. I do this all the while. I’m currently getting about 65 mpg from my petrol VW Polo, calculated using fuel in and miles covered, for a commute with plenty of hills.
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It is undeniable that turning over an engine plus its ancillaries at 600rpm will require more energy than turning it over at 1800rpm. Whether that energy comes from injected fuel or the momentum of the vehicle is immaterial, because both sources of power come from burning fuel. Gravity is a red herring, providing no energy, and doesn't come into the argument.
You are remarkably pompous. I have no idea what the ‘energy ... is supplied by gravity’ nonsense is all about.
Pompous or not, here is what you said 8yrs ago....complete with erroneous final sentence. Gravity is a force, not free energy to be tapped, like the futile inventors of perpetual motion tried to do. Did you really get a physics degree?
Wed 1 Sep 2010 13:27
Any - Coasting - HJ Daily Telegraph today - Leif
I refer you to your original post. When I go down a shallow hill I do not need to accelerate. So the fuel used is zero. If I coast, fuel is used. That is the key. Your original post is confused because you say "It must surely need less energy to keep it rotating at 700rpm than at twice that speed - or more." Yes, that is true. But most if not all the energy required to keep it rotating at twice that speed or more is supplied by gravity.
What you fail to appreciate is that the momentum (a force, like gravity) of the car at the bottom of the hill is far greater after coasting using fuel for idling than if it was in gear using momentum (plus possibly some fuel if the deceleration fuel cut-off system resumption rpm is set high) to keep the revs at 1800 or more. The effect of gravity on the car is exactly the same in both scenarios - and will have no effect at all if on the level coasting or decelerating to a roundabout or slip road exit.
Edited by John F on 01/08/2018 at 10:07
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Given that they say not to argue with a fool, I'll call it a day.
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Leif and John-F - it doesn't look if you will be easily reconciled. To my simple mind (I only have a chemistry doctorate) you are comparing two similar scenarios.
If your car coasts (A, box in neutral) it will arrive at the foot of a hill under gravity only, but having used a little fuel to keep the engine idling. If it runs down in gear (B), the fuel is cut off, engine braking takes effect and it arrives at the foot more slowly (and thus a bit later). Fuel consumption is less, but travel time is more. Just like anywhere else? You may have wasted some fuel by having to brake in either scenario, more in A than in B.
Don't know what all the fuss is about ..... :-)
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Leif and John-F - it doesn't look if you will be easily reconciled. To my simple mind (I only have a chemistry doctorate) you are comparing two similar scenarios.
If your car coasts (A, box in neutral) it will arrive at the foot of a hill under gravity only, but having used a little fuel to keep the engine idling. If it runs down in gear (B), the fuel is cut off, engine braking takes effect and it arrives at the foot more slowly (and thus a bit later). Fuel consumption is less, but travel time is more. Just like anywhere else? You may have wasted some fuel by having to brake in either scenario, more in A than in B.
Don't know what all the fuss is about ..... :-)
Andrew: You might want to read my posts. I keep the engine in gear, and still have to use the brakes downhill. Engine braking is not as per keeping your foot on the brakes, it's actually a small effect, that's why they say when going down a steep hill, use a low gear.
A mere chemistry doctorate, you should be ashamed of yourself. ;) (Just kidding. What an educated forum this is ... )
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<< Andrew: You might want to read my posts. I keep the engine in gear, and still have to use the brakes downhill. Engine braking is not as per keeping your foot on the brakes, it's actually a small effect, that's why they say when going down a steep hill, use a low gear. >>
Leif, I can assure you I have read all these posts. I have no knowledge of the particular hills you describe - I was imagining a long and fairly gentle hill where keeping the engine in gear can limit speed. Steeper places need different treatment, clearly. But my simple argument still applies despite the use of brakes.
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<< Andrew: You might want to read my posts. I keep the engine in gear, and still have to use the brakes downhill. Engine braking is not as per keeping your foot on the brakes, it's actually a small effect, that's why they say when going down a steep hill, use a low gear. >>
Leif, I can assure you I have read all these posts. I have no knowledge of the particular hills you describe - I was imagining a long and fairly gentle hill where keeping the engine in gear can limit speed. Steeper places need different treatment, clearly. But my simple argument still applies despite the use of brakes.
I am sure you can imagine an artificial scenario with a long road with no corners, no junctions, no hazards, you coast downhill, reach a dangerous speed, and then coast uphill. But we're discussing driving on real roads.My commute involves many hills, and even in gear I need to occasionally dab the brakes to keep to a safe/legal speed.
By the way, when you are coasting downhill out of gear, foot on the brake, have a think about the cost of replacing worn brake pads/disks. And that ignores the increased.
Do you really coast out of gear? I hope not. This fool JohnF does. And from the sounds of it he hurtles through villages and round bends out of gear, at excess speed.
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Don't know what all the fuss is about ..... :-)
No fuss, just an attempt to portray the truth. I am beginning to feel what Copernicus must have felt like when intelligent folk told him how foolish he was to think the sun doesn't go round the earth like the moon does. Anyway, those who resort to insults have usually lost the argument.
Andrew-T, gravity is irrelevant, as I have pointed out. Fuel consumption in (B) might well be less, but the saving will be more than negated by having to burn fuel to restore the momentum of the car to that which it would have had if coasted. You are nearly there in appreciating that 'engine braking' occurs when in gear. All braking, whether by turning an engine, squeezing a disc or driving a generator, consumes the car's kinetic energy which has been produced either by burning fuel or draining a battery.
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Andrew-T, gravity is irrelevant, as I have pointed out. ... You are nearly there in appreciating that 'engine braking' occurs when in gear.
John, I can't make out why you seem to think that my view differs seriously from yours. Nothing you have just said contradicts what I said earlier, but for some reason you want to look for more discussion points. I don't think I suggested that the fuel used during engine braking would balance that needed to return to the 'A' speed.
Gravity is not irrelevant as it is contributing force in all the scenarios, but as that force is universal it can be left out of the discussion. Do I feel more like Galileo or the papal inquisition?
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John, I can't make out why you seem to think that my view differs seriously from yours.
I don't. It doesn't. Your posts always make good sense.
Gravity is not irrelevant as it is contributing force in all the scenarios, but as that force is universal it can be left out of the discussion.
Quite so. Leif seemed not to understand this.
Do I feel more like Galileo or the papal inquisition?
Probably Galileo - he took the flak!
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Don't know what all the fuss is about ..... :-)
No fuss, just an attempt to portray the truth. I am beginning to feel what Copernicus must have felt like when intelligent folk told him how foolish he was to think the sun doesn't go round the earth like the moon does.
Anyway, those who resort to insults have usually lost the argument.
You opened with several insults ...
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Anyway, those who resort to insults have usually lost the argument.
You opened with several insults ...
Where? Please provide quotes. You have called me a fool in several posts. I merely questioned your credentials and said you didn't understand my arguments. And yes, I do coast at high speed sometimes - e.g. A21 Sevenoaks bypass over the Downs - lovely stretches of circa 70mph coasting. This clearly excercises you greatly! But not often through villages;-)
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Anyway, those who resort to insults have usually lost the argument.
You opened with several insults ...
Where? Please provide quotes. You have called me a fool in several posts. I merely questioned your credentials and said you didn't understand my arguments. And yes, I do coast at high speed sometimes - e.g. A21 Sevenoaks bypass over the Downs - lovely stretches of circa 70mph coasting. This clearly excercises you greatly! But not often through villages;-)
Frankly I’ve long since tired of your pompous, abrasive and condescending manner. You started out with snide put downs. If you don’t want stones throwing at you, don’t throw stones at others. You are indeed a fool. You should take some driving lessons, improving your forward observation and planning will gain you far more mpg than imagined gains from unsafe driving habits. As an aside, I drove from Basingstoke to Fareham, 41 miles, in a manual VW Polo petrol engined car, the computer recorded 81 mpg for the journey. Curious that you compare yourself to Galileo, and you object to me calling you pompous. Oh for goodness sake. There is a slight difference between Galileo and his argument with the church, and a minor little online argument, or can’t you see that? Pompous oaf.
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<< Frankly I’ve long since tired of your pompous, abrasive and condescending manner. You started out with snide put downs. >>
I was going to weigh in here, but on brief reflection, there is probably no point ....
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Well said, Andrew - there isn't.
Thread closed, as it has descended into insults.
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In any case, for many cars you will use MORE petrol doing as you suggest, not less. If you do down hill without pressing the accelerator, and in gear, the engine will turn over of its own accord and the engine management unit will cut petrol to the engine. If however you coast out of gear, the ECU will need to keep the engine fueled so as to keep it idling.
Absolutely true - but - by shutting the throttle in gear downhill you may scrub an extra 10-20% off your speed through engine braking, speed you may have to make up again to get up the other side - the net fuel usage will be worse than if you coasted maintaining or even increasing road speed on a sip of fuel. Obviously if you're coming to a halt then staying in gear to take advantage of the fuel cut-out is king.
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<< Obviously if you're coming to a halt then staying in gear to take advantage of the fuel cut-out is king. >>
Don't we all remember listening to wagons changing down through the gears before coming to a stop? I still do that on some stretches of road, for example a half-mile of NSL which becomes a quarter-mile at 40 finishing at a roundabout, which I reach in third gear, ready to continue if the road is clear. Very little fuel used during the final stretch .....
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In any case, for many cars you will use MORE petrol doing as you suggest, not less. If you do down hill without pressing the accelerator, and in gear, the engine will turn over of its own accord and the engine management unit will cut petrol to the engine. If however you coast out of gear, the ECU will need to keep the engine fueled so as to keep it idling.
Absolutely true - but - by shutting the throttle in gear downhill you may scrub an extra 10-20% off your speed through engine braking, speed you may have to make up again to get up the other side - the net fuel usage will be worse than if you coasted maintaining or even increasing road speed on a sip of fuel. Obviously if you're coming to a halt then staying in gear to take advantage of the fuel cut-out is king.
That is one of those theoretical points that sounds good but in practice it ignores the real world. Firstly if you do coast out of gear down a hill, you will soon reach a dangerously high speed. Don’t come crying to me if you kill yourself, or someone else. I do a commute with many hills, and do an adequate speed downhill by staying in gear and not accelerating. And I’m no mimser. Secondly, such hills as you describe are pretty rare, most have side roads and other hazards, as well as reduced speed limits in parts, and the chances are you will have to brake anyway, either to avoid crashing into the back of another car, or killing a cyclist, given traffic levels, or because you don’t want to go through a village with sharp bends at 80 mph. So in reality, the safest and most fuel efficient way to drive is to stay in gear downhill.
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- but - by shutting the throttle in gear downhill you may scrub an extra 10-20% off your speed through engine braking, speed you may have to make up again to get up the other side - the net fuel usage will be worse than if you coasted maintaining or even increasing road speed on a sip of fuel. Obviously if you're coming to a halt then staying in gear to take advantage of the fuel cut-out is king.
By George - he's got it!
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I find driving at around 60 mph in top on a long journey produces around 45 mpg; in road works with a speed limit of say 50 mph consumption improves further, which suggests to me that the advantages of keeping the engine "somewhere near the peak of the torque curve" are cancelled out by other factors.
Yes, clearly if your engine's peak torque lies well above normal cruising speed you will see the benefits of reduced air and rolling resistance. That probably won't be the case with a small diesel engine.
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I have done a couple of experiments; one forced and my own choice. They prove that air resistance is the main cause of increased fuel economy at higher speeds.
Experiment 1 (forced) - having to drive through a long stretch of 50mph average spped camera monitored road works. The MPG for that 20 mile route (M6 in Cheshire) in my car can be 50+mpg.
Experiment 2 - same car, same road, but keep the car in fifth gear (not in D in a seven speed gearbox). Barely any change in economy despite higher revs.
Do the same revs in D; so a much higher speed and the economy drops, so the mechanical and friction losses in the drivetrain are minimal compared to air resistance. The solution is to drive slower and more smoothly to improve economy and the biggest user of fuel is acceleration.
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Well this coasting lark is a funny old thing, us old in the tooth lorryists always used Irish/Aberdeen overdirve (out of cog) in the odl rattlers for miles of free wheeling, but it was always regarded as illegal or at least unapproved.
Fast forward to today and when the all seeing systems on my automatic lorry decide the speed and terrain mean above a certain distance can be travelled in neutral it does indeed automatically coast, the display showing N and the engine on tickover, does this mean coasting is no longer illegal or what?
As Leif mentions the instant display will show fuel usage for keeping the engine ticking over, but if it's on overrun at the same speed still in drive then the readout is either far higher or shows nothing.
I suspect the system has worked out that the limited engine braking if in gear will see cruise speed and therefore fuel usage resumed faster, so i assume the machine has decided keeping the engine ticking over will use less fuel than the earlier resume due ro engien braking, i don't know of course the amounts are too small for someone like me to work out...the vehicle is very clever in that it reads the topography ahead (GPS) and say its set at 53mph, my normal cruising speed, it will automatically increase to 55mph max just before a long climb (but only if loaded, empty it knows the vehicle has enough power to maintain cruise, and will cut the power at any weight just before the peak is reached allowing speed to drop by up to 7kph before gravity takes over and pulls the lorry down the next hill.
Obviously with our roads as busy as they are, this variation of speeds due to this system can cause traffic problems, so i often override the slowing up at the peak to prevent unecessary elephant races, but i can't actually switch the system off as such unless i cancel cruise control and use foot throttle, on cruise the GPS is permanently linked unless i set the gearbox to power range (it has three settings, eco normal and power), a setting i don't use because the revs are held pointlessly high for no benefit.
I don't actually chuck my own auto cars in N and coast, there is so little engine braking if you leave the things in D that it would make little difference anway.
I too would like some gauges back again in cars, oil pressure and temp would be a good start, i really would not like a car with a warning light only for water temperature, oh and for modern Diesels a display for the the condition and state of play for DPF's is desperately needed as well as an override trigger so owners embarking on the right journey can instigate sensible regens.
PS, my lorry has now done 80k kms and is approaching its first oil change, apparently due at 90k! shudder :-)
Edited by gordonbennet on 30/07/2018 at 16:10
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The 130 PD is such a hard act to follow, Craig.
That's why I turned back to one when I found how gutless the modern diesels are.
Haven't come across any engines that have the low end torque of the PD and this is quantified after a day trying out the latest cars from most of the manufacturers .
So what is the replacement going to be once all the PDs are gone ?
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The 130 PD is such a hard act to follow, Craig. That's why I turned back to one when I found how gutless the modern diesels are. Haven't come across any engines that have the low end torque of the PD and this is quantified after a day trying out the latest cars from most of the manufacturers . So what is the replacement going to be once all the PDs are gone ?
They were (and still are) a very good engine. The throttle response was/is particularly satisfying, which I believe is related to the PD injectors' high pressure, and the fact that the motors are not encumbered & stifled by as many emissions controls as modern engines
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