Truth or Myth – What's the science?

I drive a 2000 VW T4 Caravelle with a 2.5 TDI engine.

Most often (whenever I can), I turn the motor off at red lights and train crossings etc., but have been told by some that the amount of fuel I burn re-starting wastes more than I would have burnt had I just left it running for the 2 minutes (or whatever).

Can anyone tell me what the facts are?

Myth, that’s why we have stop start engines now.

Myth – but wait until the engine is warm before you start turning it off (as stop-start engines do).

Perhaps in ye olden days, but not anymore. I always turn my engine off if I get to a level crossing that has just gone down…

Not sure I’d bother at every red light – your battery might not like it.
Stop-start systems are designed with this in mind & won’t stop the engine if the battery voltage drops.

Would that not drain the battery if it’s not been specifically designed as a stop start engine, or wear the starter motor out more quickly?
Just asking, not an informed opinion at all.

@tommyhine

I’ve never thought about that, myself, but nor have I experienced any bad effects. I was once told that the Scandinavians have always done the on/off thing, and that was long before stop/start engines.

I have a Volvo V50 with stop / start and it says in the handbook that a certain type of battery is required.

I think the stop/start things have a faster more powerful starter motor too?

Most stop start engines have two batteries – the main is used for restarting and the support battery for everything else. They also usually use an Integrated Starter Generator (ISG) which actually uses the alternator to start the engine. These both make auto stop starts smoother, but doing it ‘manually’ shouldn’t put too much more wear on the starter or battery as long as you aren’t doing it every minute or so fr a long time! (I.e. at crossings etc it would be ok, but not if used like a real stop start at every idle!)

My thoughts, based on absolutely no facts or science whatsoever, is that whilst stop-start systems save fuel, engine/starter/battery damage caused by repeated cycling may become significant?

Just out of interest with stop-start engines what happens on cold mornings when you pull up at the lights with the heater/fan/radio/lights on?

I’ve looked into this a while ago, and from various different sources worked out that for my vehicle at the time (2001 VW T4 2.5 TDi 150) I would save roughly 75p for each hour of avoided idling (@ £1.35/Litre)

That’s taking into account the additional wear and tear on starting components (as estimated by various different studies I found, all which said that was around £10/year)

I never turn off my engine at traffic lights, but at level crossings etc, and in traffic jams that are not moving I do.

There is a load of logic as to when stop start turns on. Depends on the car – some don’t stop start with A/C on, some do, some need seatbelt on etc.

Just out of interest with stop-start engines what happens on cold mornings when you pull up at the lights with the heater/fan/radio/lights on?

Mine won’t turn the engine off if the temp is below 4c. The rest of the time it will monitor the battery and turn off if there is enough battery available regardless of how much a/c and other power drains I’m using. The radio uses tiny amounts of power so I doubt that would ever be an issue.

You don’t get out much do you neal?

You don’t get out much do you neal?

ahahahaha! 🙂 I was just about to post the exact same thing!

the amount of fuel I burn re-starting wastes more than I would have burnt had I just left it running for the 2 minutes

Not true.

It only has to be off for a few seconds to make it worth stopping, according to a VW bloke on the telly I heard talking about this once.

However as above you don’t want to be stopping every single time you come to a set of lights – that would really wear the starter I’d imagine. Level crossings though – good idea.

You don’t get out much do you neal?

🙂 it was more down to the fact I was buying a new van, and the option of a Stop/Start engine was available (for £1750) and I was working out if I was going to pay for it or not.

(I didn’t !)

My Freelander 2’s got the ECO start-stop on it – I asked the Salesman when it would save fuel and he said that studies had been done to show that if you were stationary for longer than 30 seconds then the fuel saved by it switching off would be greater than the fuel used to start it up again.

So now I play the guessing game when I’m at traffic lights or in queues, do I take my foot off the clutch (that’s the trigger for the start-stop) or keep it in???

It is thought that one of the reasons taxis get so many miles out of their engines is because they leave them running.

So this thread has totally cleared up the facts! good old STW 🙂

It is thought that one of the reasons taxis get so many miles out of their engines is because they leave them running.

LTI Fairways (there’s one on autotrader with 390k miles) have 2.7 litre 4-cylinder diesel engines (IIRC, someone may correct me) but in their normal urban environment, this would mean they rarely go above what most of us would consider a fast’ish idle so little wear and tear.
that and taxis are usually properly maintained.

I think the reason that taxis do so many miles is because age is more significant than miles when it comes to wear and tear.

Rocketman:

Lots of calcs done by the engine management system to determine when to enable/disable stop start. Lots of factors like temps, length of time running, ambient conditions, engine load (AC etc.) taken into account.

They have a large starter-generator that acts as an alternator an a quick starter. Technically they’re classed as micro-hybrids. Just let it do it’s thing and it will generally save you a bit.

However, they’re designed to give you a better economy figure on the Euro (or US) drive cycle. The Euro one has several longish stops in it, so stop start systems give around 4-5% better economy figures on the official test. In reality, with shorter stops, it’s probably a bit less than that.

EDIT: And you don’t really use/waste a significant amount of fuel starting a warm engine.

it was more down to the fact I was buying a new van, and the option of a Stop/Start engine was available (for £1750) and I was working out if I was going to pay for it or not.

At that price I reckon less than one second I could work out if I was going to get that as an option.

http://www.scotland.gov.uk/Publications/2003/04/16936/21249

I think the reason that taxis do so many miles is because age is more significant than miles when it comes to wear and tear.

A comparison of my old car which was 13.5 years old with 158k miles and my “new” one which is 4.5 years old with 121k miles would tend to back that up.

At that price I reckon less than one second I could work out if I was going to get that as an option.

True enough, although I do like a bit of “recreational maths” to be honest 😉

Economics too – if you drive 150k miles in a tax in what, 3 years, and the turbo goes, it’s easily worth £750 to fix the turbo on a 3 year old car that’s in good nick otherwise.

It’s not as attractive to have to shell out £750 to fix the turbo on a 15 year old car that only cost you £500, for instance.

I turn mine off for ‘long’ waits, level crossings for example but not necessarily traffic lights. Amazed how many people I see queueing 5-10minutes at the petrol station (we have one in town which is much cheaper than the others and always has queues) with the engine running the whole time.
.
Also, I tend to buy old bangers, for environmental reasons but also financial. The age of my cars means that there is usually something wrong. When modern stop/start cars get old enough for me to have one how is this going to affect me? If I have a problem getting it started in the first place am I going to be left stranded every time I come to a red light? If I have an issue now I just have to worry about not stalling it, I know not to turn it off.

http://www.warwickshire.gov.uk/Web/corporate/pages.nsf/Links/B73E7DCC51542EE0802572FD0073F398/$file/Final+report+anti-idling+march08.pdf