Reading an article in a car magazine that gave tips on how to save fuel, you know the sort of thing - remove your roofrack, check tyre pressures etc!
One recommendation was to switch off your engine when stationary eg traffic lights.
Now I know VW and Citroen have previously sold stop/start cars that did this automatically but is there really a saving to be made? I understand it will put more pressure on a starter motor but I have never had a starter motor break down since my Talbot Samba so maybe they are up to it nowadays?
On what circumatances would it work - I would assume engine would need to be at full operational temperature for instance?
I tried it today coming in to work at some lights where I need to wait for 4 light changes before I got through and one of the hindrances was that people seem to edge forward. So lights change to red, everyone stops, and then everyone starts moving forward again closing up on the car in front meaning I had to to start the engine as there is now 2 car spaces in front of me!
{typo in subject line amended}
Edited by Dynamic Dave on 06/03/2008 at 12:30
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Some BMW 4 cylinder manual models automatically switch off when stationery.
When I was dithering at length about which one to order I asked the salesman why the feature wasn?t present on the sixes. His answer was that it cost more fuel to restart than was saved by stopping.
I am not qualified to judge on this but I was curious about the point at which became viable.
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ISTR that a figure of a minute was quoted, below that, not worth it - but bear in mind that diesels use a fraction of the fuel at idle compared to petrol engines.
Also consider the possible increase in wear/tear on the starter motor.
Edited by oldnotbold on 06/03/2008 at 10:52
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I do it if stuck in a traffic jam, partly for fuel economy and partly to stop the engine vibration! IMO its not worth doing it at traffic lights, it wastes time having to start the car again before setting off.
I read something about the Citroen Stop/Start system; they claim it improves fuel economy and reduces CO2 \"by around 10% over an urban cycle\", with 6% saving on combined cycle.
On Citroens they have a different type of starter motor specially designed for the purpose, which they say starts the engine quicker than a traditional one.
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If you do it when the engine is fully warmed through then there\'s very little wear on starter or major engine components, and modern injection systems will not overdose with fuel on the restart.
But as said above, the car will only use a sniff of fuel when idling. I seem to remember the figure of being stationary for more than 2 mins to have any noticeable benefit.
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Given the thread title, I imagine that should the engine fail to restart you\'ll be pushing the envelope.
{now amended}
Edited by Dynamic Dave on 06/03/2008 at 12:33
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I wouldn't do it, especially at lights. At lights you have just enough time to put the car into gear. Also, if you turn the engine off then there is a chance that it won't start the next time, especially if the battery is a bit dodgy.
I believe it saves fuel after something like 10s IIRC.
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I read somewhere the other day that BMW reckon the breakeven point for their latest stop/start system is 2 *seconds*.
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I think the doom mongers who are worried about their starter motors must still be running Morris Minors, with inertia engaged starter pinions clashing noisily into the flywheel ring gear every start.
As mentioned by Craig above, starting a warm modern engine doesn't cause much wear in the starter motor. Most cars go to the scrap yard with their original starter motor still in place - they aren't a fragile part nowadays.
I remember a colleague calculating a figure of about 10 seconds - this was over 10 years ago now, so I could imagine that the 2 seconds figure quoted by BP is feasible, and definitely impressive.
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I have never worn out a starter motor on any vehicle. The inertia engagement motors on my tractor (1949) and Triumph (1964) are both original, and the Volvo has done 390,000 miles.
The only difficulty I see is in correctly assessing the temperature of the engine and catalyst - the water temperature gauge will register long before the oil and cat are fully hot.
The other is in predicting how long you are going to be stationary. You know the situation - there is a hold up, but the traffic keeps creeping forwards. The stops get slowly longer, so at some point you decide to switch off. At that moment the queue suddenly moves forward again. Meanwhile someone ahead can't restart because his engine has overheated.
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The issue on some modern cars is that the cars likes to spend 5 seconds doing a system check every time it is restarted, if you pull away before it has done this it may not like it.
Ie Fuel gauge not reading, trip computer reset, idle revs not stable ?.
I guess on stop/start cars this type of thing is not done.
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If done in an indiscriminate way, you could see it causing more hold-ups (& therefore causing a nett increase in fuel usage) than it's worth. I guess most people already switch off at say, a level-crossing, but trying to mimic stop-start technology in a car not so equipped looks marginal, at best, to me.
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It is recommended in the manual for my VW Passat PD TDi sport that it is only worthwhile switching the engine off if the car is going to be stopped for 40 seconds or more. If it is any less there would be an increase in fuel consumption from re-starting it compared to just letting it idle.
My father-in-law-to-be's new mini cooper will stop of it's own accord at traffic lights etc even after a very short space of time. It is very disconcerting and takes some getting used to! It does seem to start pretty quickly either once your foot is placed on the clutch or when a gear is selected (not sure which).
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I presume it only actually stops the engine if the gear lever is in neutral, handbrake on, and feet off the pedals?
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I only do this at level crossings and the like - where there is little likelihood of the queue moving and some warning that the barriers will lift. Trouble with turning the engine off on the bike is that the immobilser will fire up and I then have to turn that off before starting the engine :-)
ISTR signs in germany along the lines of Bitte Motor Abstellen (that's from memory) near to lights etc, inviting drivers to switch off (I assume).
I wouldn't want a car that did it automatically.
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According to the fuel computer my Astra uses 0.2 gall/hr at idle when hot. That's 0.003333 gallons/min (0.015 litres/min). Doesn't sound much, but it equates to 1.6 pence per minute at current fuel prices.
JS
Edited by John S on 07/03/2008 at 17:25
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>>Most cars go to the scrap yard with their original starter motor still in place - they aren't a fragile part nowadays.
Really?
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Starter motors are not generally problematic these days - short haul vans don't seem to suffer unduly.
A diesel will idle at an air/fuel raio of 50:1 wheras thanks to our idiotic adoptation of catalytic convertors a petrol idles at about 15:1. There is therefore a much longer "payback" time with the diesel.
A diesel engine always requires excess fuel to start it and this is automaticlly selected by the fuel system whether it is a mechanical pump or is electronically controlled. This accounts for the tell tale puff of black smoke which occurs when a diesel is started, accompanied by a slight surge (with a mechanical pump) as excess fuel is automatically deselected and the engine then idles on the governor.
So, bearing in mind that excess fuel is a greater volume per shot than normal max fuel, I would recommend leaving a diesel running unless a stop of more than about 3 minutes is envisaged. Diesels are harder on starter motors too.
659.
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