Whatever the rights and wrongs of coasting, most cars these days will not lock the steering unless the key is removed from the lock. Ignition off is not normally a problem.
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"Whatever the rights and wrongs of coasting, most cars these days will not lock the steering unless the key is removed from the lock."
That's made me think. Like lots of other cars these days my car doesn't ever HAVE a key in the lock! So a stall might well lead to a steering wheel lock. Ooer.
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Bathtub tom,
If you didnt get my post or i have not explained myself clearly enough
I havent done this before or wont agin, like i said I was seeing what it did.
My steering lock only engages if i take key out, i dint take key out
Engine was only off for not even ten seconds
I doidnt bump start car, to start i dipped clutch to let engine stop and started as normal
My post wasnt concering driving with engine off, i was interested in the technical aspect of coasting in top gaer with engine on using no fuel, using drivetrain to turn engine round
Wheres screwloose when you need an good answer
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In my experience coasting works best with the engine running but out of gear. When the engine is in gear the additional braking greatly reduces the distance you will travel. The key to making it work is to read the road as far ahead as possible.
Dont forget that MPG = Distance travelled(miles)/fuel used (gallons)
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In my experience coasting works best with the engine running but out of gear. When the engine is in gear the additional braking greatly reduces the distance you will travel. The key to making it work is to read the road as far ahead as possible.
This is very true.
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I do hope I don't come across any of the members of the 'Coasting Club' during any tricky driving conditions out on the highway - not only might they be in neutral, but the 'ultras' are now debating the 'benefits' of turning the engine off altogether! Are people really willing to lose such a huge amount of immediate driving response & control (with the obvious possible consequences) for the sake of a few bob?
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Keep the engine idling at least to make sure you have plenty of vacuum on the servo,not just the residual.
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Can't see what the problem is, please explain why you think its dangerous. The obvious consequences are not obvious to me, please enlighten me!
What driving response and control is lost? Brakes and steering still work just fine. If you need to accelerate you just pop it back in gear and off you go.
If anything I think it makes for safer driving as you are more aware of the road ahead and tend to leave much bigger gaps from the car in front.
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I think woodbines is complaining about those who turn the engine off, not those who simply coast whilst the engine is idling.
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>>it must be purely compression and friction that makes a engine sound and not combustion
Yes, quite a bit of engine noise is just mechanical noise, and the noise caused by pumping air. On over-run, you probably wouldn't notice if your exhaust were suddenly removed.
However, when you are making a demand of the engine, the extra noise caused by the combustion and blowdown does make a difference - if youremoved the exhaust under these conditions, you would really know the difference.
When you are on over-run, and the fuel has been cut-off the engine provides a resistive torque that's made up of friction, and the difference between the work done on compression, and the work you get as the compressed gas expands (effectively, the compressed gas gets hot, and loses heat to the engine, and so, you don't get all the energy which is put into compression back again)
Number_Cruncher
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I think woodbines is complaining about those who turn the engine off not those who simply coast whilst the engine is idling.
Can we forget all talk of coasting with egnine off pplease, thats isnt what the thread is about. The thread is about coasting with engine on in gear and how an engine dient have to have fuel to keep running under these circs. The little test that i performed under the most safest circs which some of you are traumatised by was to prove my self wrong. I though by turning engine off in gerar while coasting would solw down more than with engine on. Like i said it was a one offI thought an engine needed at least tickover supply to turn over. I was wrong.
Thanks NC that was a good answer
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Can't see what the problem is, please explain why you think its dangerous. The obvious consequences are not obvious to me, please enlighten me!
I wish I could, really.
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@woodbines
No really, please explain why you think it's dangerous. Or was your post just a lame arguement without any reason to back it up? This is a discussion board, so lets discuss - I have no issue with being proved wrong.
Please describe just one situation where coasting could be considered dangerous and the reasons why.
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NC
I thought a large amount of retardation on the overrun was caused by moving the gasses past the valves (pumping losses?). I discovered this when I sold my first motorbike, a two-stroke, and bought a four-stroke.
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In terms of pumping losses, the big loss is dragging the air past the closed throttle, which, looked at another way was one of the advantage of diesel engines, having no restrictive throttle plate at all. But, yes, the work done in pumping the air contributes to the resistive torque provided by the engine on over-run.
Interestingly, if you have a turbo-charged engine, running under load, the effect of the inlet air being at high pressure is that you can actually get a **small** amount of positive, i.e., useful driving work out of the pumping, as the charge pressure pushes the pistion down during the intake stroke!
Number_Cruncher
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Please describe just one situation where coasting could be considered dangerous and the reasons why.
Ok, happy to Moonshine.
You've coasted (in neutral, engine running - I'm not quoting with the extreme case of engine off) down an incline & are just about to match your revs. to speed & engage gear again & re-engage the clutch, then an emerging car from a side street or the car in front suddenly applies brakes - you're engaged in engaging gear & lose vital metres stopping distance & hit the car - and/or car behind loses same - so hits you.
You've coasted down a dual carraigeway incline.. (well, I won't repeat the sequence) - this time, a vehicle is stopping suddenly in your left side lane (or an obstruction/piece of debris is spotted etc.) & it's difficult to stop in time, so you want move to the outer lane - but a vehicle is approaching quickly from behind, you'd have to accelerate quickly & pull out - guess what.
You're coasting.. (well, you know the drill..) & some fool tailgater is up your er, tailgate, you engage gear, your car slows a bit without brake light as the cluch bites - now he's much closer, you're distracted a bit.. , maybe even paying too much attention to the drama behind you..
You're coasting..etc.etc. - at the point you engage gear, there's some oil or maybe black ice or just a slippery manhole cover, just enough to make the clutch engagement compromise tyre grip a fraction, a slight skid - you might dab the brakes or the DSC may intervene - whatever, now there's more stuff going on to deal with.
You're coasting, a vehicle out of control a coming (from whatever cause) at you from an acute angle - from opposite direction or laterally - any braking will keep you in its path - you need to accelerate quickly..., let's just get in gear though.
You're..
Edited by woodbines on 07/02/2008 at 11:14
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Ok, some interesting scenarios, see my replies
"You've coasted (in neutral, engine running - I'm not quoting with the extreme case of
engine off) down an incline & are just about to match your revs. to speed & engage gear
again & re-engage the clutch, then an emerging car from a side street or the car in
front suddenly applies brakes - you're engaged in engaging gear & lose vital metres
stopping distance & hit the car - and/or car behind loses same - so hits you."
This situation will also occur when driving normally. Car pulls out/brakes just as you
are changing from 3rd to 4th gear etc. I don't believe being in nuetral would have any
effect on reaction times for stamping on the brake pedal. In fact I would even argue
that when coasting this will be less of a problem because a big part of it is reading
the road ahead.
"You've coasted down a dual carraigeway incline.. (well, I won't repeat the sequence) -
this time, a vehicle is stopping suddenly in your left side lane (or an
obstruction/piece of debris is spotted etc.) & it's difficult to stop in time, so you
want move to the outer lane - but a vehicle is approaching quickly from behind, you'd
have to accelerate quickly & pull out - guess what."
Ok, so lets say I'm cruising along in 5th gear and this happens. I need to pull out into
the outer lane but also need to accelerate quickly due to a fast approaching car.
Firstly, I would need to change down a gear to get the car moving, this is no different
(but maybe slower) than going from nuetral into 4th. Secondly, in 17years of driving
I've never been in this situation you describe. Thirdly, this type of manouvre is
extremely dangerous regardless of wether you are in gear or not.
"You're coasting.. (well, you know the drill..) & some fool tailgater is up your er,
tailgate, you engage gear, your car slows a bit without brake light as the cluch bites -
now he's much closer, you're distracted a bit.. , maybe even paying too much attention
to the drama behind you.."
The braking from engaging gear is small and only for a second, as you are putting it into gear to accelerate. Again, this is no different from any normall driving situation.
"You're coasting..etc.etc. - at the point you engage gear, there's some oil or maybe
black ice or just a slippery manhole cover, just enough to make the clutch engagement
compromise tyre grip a fraction, a slight skid - you might dab the brakes or the DSC may
intervene - whatever, now there's more stuff going on to deal with."
Out of all your responses so far this is the only one that may carry some wieght. lets start with black ice or oil, yes I agree that putting a car into gear form neutral could possibly cause the wheels to skid. However, it would only be for a moment. But lets play it safe and say no coasting when there's ice about.
"You're coasting, a vehicle out of control a coming (from whatever cause) at you from an
acute angle - from opposite direction or laterally - any braking will keep you in its
path - you need to accelerate quickly..., let's just get in gear though. "
This is the old 'accelerate out danger' arguement. But for the sake of the discussion lets assume that you can accelerate out danger. Again I don't see the problem with just putting the car into gear and accelerating...
Nice try, but you still have a long way to go before I'm convienced. I look forward to reading your counters (or anyones) to my responses :)
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I once owned a Saab 96. It had a lever under the dash, which disengaged gear but switched to a centrifugal clutch. There was no engine braking on over-run, but speeding up the engine re-engaged drive once it was running fast enough to "catch up" with the gearbox. Easing off again let the engine idle while one coasted downhill. It was presumably a great fuel saver, but it could feel quite scary at times. Apart from absense of engine braking, the lack of noise was uncanny - the car would race away downhill and would touch 100 mph before one realised it was going so fast.
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Cliff's post about going downhill and getting up to 100mph illustrates the point quite nicely. At 100mph you are going to travel for quite a distance when you get to the flat or next hill, this is where the savings really come in.
It's not just the fuel saved, it's the distance travelled that is equally important.
My experience is that my normal commute gets me around 33mpg at the moment. With coasting I can easily get over 40mpg. Under ideal conditions with coasting I can get 50mpg.
over the course of a year and many miles this can add up to a significant saving.
There is of course a downside - it makes driving 'hard work' you need to be very alert and concentrating all the time to be reading the road ahead and to get it right.
To be honest, some days (probably most days in fact) I'm too lazy or tired to put in the effort.
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>you need to be very alert and concentrating all the time to be reading the road ahead and to get it right. <
but aren't you supposed to do this all the time anyway?
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If only everyone did we would hardly ever have any accidents on our roads.
Sadly in life there is a huge difference between what 'should' be done and what 'is' done.
I wonder how many accidents are due to lack of concentration and awareness? I guess none as we are allways being told that they are all due to excessive speed...
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Not a centrifugal clutch-just a freewheel-fitted to number of other cars particularly pre WW II.Could change gear without the clutch.
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Not a centrifugal clutch-just a freewheel-fitted to number of other cars particularly pre WW II.Could change gear without the clutch.
I'm sure you are right technically. What I meant was it felt like some kind of speed-related ratchet. When engaged there was no engine braking whatsoever, in any gear. Just like a pushbike in fact. You had to rev the engine like mad to catch up the revs at say 65 if you wanted to get back up to 70.
Also I think from memory there were warnings about not re-engaging ordinary drive while on over-run. I don't think it liked suddenly being slammed into engagement at 5000 rpm.
It was presumably considered a perfectly safe arrangement in its day.
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Nice try, but you still have a long way to go before I'm convienced.
You carry on as you are then, that's your prerogative.
Edited by woodbines on 07/02/2008 at 23:13
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Of course coasting isn't dangerous unless the driver is an idiot.
It was a good idea with carburetted cars that used less petrol when they were idling.
But with modern cars it's more economical to leave them in a high gear on the overrun. Until the engine slows to idle speed, it uses no petrol at all. One of the great advantages of modern technology, and not the only one.
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Of course coasting isn't dangerous unless the driver is an idiot.
I agree completely!
But with modern cars it's more economical to leave them in a high gear on the overrun. Until the engine slows to idle speed it uses no petrol at all. One of the great advantages of modern technology and not the only one.
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I disagree!
I still maintain that it's better to coast in neutral because of the further distance travelled. The only time I think it's better to coast in gear is for example when approaching a red traffic light - i.e you will have to stop and in this case the engine braking is of benefit.
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Would disagree with coasting in neutral - I've tried both to maximise economy and find that coasting in gear - where as the previous poster said uses no fuel - to be far more efficient... In neutral you're still using fuel...
Edited by b308 on 08/02/2008 at 09:22
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A modern engine on over-run uses no fuel.
An engine idling obviously does use some fuel.
Therefore it is more economical to stay in gear.
Going into neutral and turning the engine off may theoretically reduce friction, but restarting and re-engaging the engine will involve wear or shock to the gearbox or clutch, and a burst of fuel as the engine fires.
Also I don't quite follow the procedure that would be involved. You have been whizzing down hill at 70-80 in neutral with the engine off, and then your speed starts to slow up. At 60 you decide you need to restart. Do you just slam it into gear and do a bump-start?
Or start the engine on the key and then increase revs appropriately until you can re-engage smoothly?
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Firstly, I never coast with the engine off, always keep it running. I wouldn't feel safe doing it with the engine for the reasons you state and becuase of the issues with steering locks, lack of power steering and servo assistance for the brakes.
popping the engine back into gear even at fairly high speeds isn't hard to do and I find it always re-engages just as smotth as any other gear change (in effect thats all you are doing - changing gear).
I'll write another post later to try and explain why it's better to leave in neutral.
Edited by moonshine {P} on 08/02/2008 at 10:14
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>>explain why it's better to leave in neutral.
Essentially, it's a trade off between the kinetic energy you put into friction with an over-running engine, and the fuel used at tickover during the the longer distance covered during in-neutral coasting.
Number_Cruncher
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Thanks to NC for helping to explain.
here are two key facts:
1 - A modern car will use less fuel on the over run if left in gear
2 - A car will coast further if left in neutral due to absence of engine braking.
here's some very crude maths to try and explain. Lots of assumptions in here, I'm hoping someone may be able to help with some more accurate figures, but regardless of the figures and accuracy it should illustrate the point.
'In gear'
Travel for 1 mile up hill, using 0.1 litres of fuel. Travel downhill for 0.5miles (due to engine braking) but use no fuel. Distance travelled is 1.5 miles using 0.1 litres of fuel.
Miles per litre = 15
'In Nuetral'
Travel for 1 mile up hill, using 0.1 litres of fuel. Travel downhill for 1 mile (due to no engine braking) but use 0.02 litres of fuel. Distance travelled is 2 miles using 0.12 litres of fuel.
Miles per litre = 16
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