Turn engine off?

Mick L · 13-09-2005 11:50am #1

At what point is it more economical to turn your engine off than leave it running if you're stopped for a period of time? Does anyone have any idea what kind of formula you could use to calculate this? I assume it would depend on the type of engine, size, idle rpms. Anything else?

lazygit · 13-09-2005 11:58am

hehe .. i have no idea where about this.. i was alsways told that its less economical to turn it off.. but..

while i was doing me driving lessons the instructor made me shut the engine off when stopped at level crossings to reduce pollution and noise..

Mick L · 13-09-2005 12:03pm

lazygit wrote:

hehe .. i have no idea where about this.. i was alsways told that its less economical to turn it off.. but..

while i was doing me driving lessons the instructor made me shut the engine off when stopped at level crossings to reduce pollution and noise..

reduce noise at a level crossing??

surely the noise of a train passing will make that a bit irrelevant. Unless of course you're turning the engine off to try and hear the train coming.

I've heard its more economical to keep the engine on also but at some point it must change.

fletch · 13-09-2005 12:08pm

Yeh I turn my engine off at level crossings and at some traffic lights that I know take ages to change....not sure at what point it is better to leave it on tho

Bluetonic · 13-09-2005 12:15pm

fletch wrote:

Yeh I turn my engine off at level crossings and at some traffic lights that I know take ages to change....not sure at what point it is better to leave it on tho

pauln · 13-09-2005 12:22pm

There was a guy on the radio about this the other day from the SEI crowd I think, I was only half listening to it but he said if you were expecting to be in the same spot for 30sec or more then it was more efficient to turn off your engine.

As for calculating this I suppose you'd get the power usage of your starter motor, time how long it takes to start the car, then there's all the auxiliary systems ignition fuel pump etc to take into account as well. That would give you a figure for your power usage to start the car.
You could do all this by putting a ammeter in series with your battery and taking measurements that way I suppose but wouldn't recommend it unless you knew what your at and the alternator would probably have to be disconnected to get more accurate readings because as soon as you start to crank it over it would start charging the battery and make it harder to get a reading.

Then you could calculate how long it would take your alternator to return this energy to the battery at idle and that would be roughly the minimum amount of time that you would have to exceed in having your engine switched off to save power.

lazygit · 13-09-2005 12:28pm

pauln wrote:

Then you could calculate how long it would take your alternator to return this energy to the battery at idle and that would be roughly the minimum amount of time that you would have to exceed in having your engine switched off to save power.

or you could just leave your engine running ;p

fletch · 13-09-2005 12:32pm

Bluetonic wrote:

<checks calander to make sure it's not April 1st>

Some traffic lights I use can take upwards of 5mins....& particularly in the morning when the choke is on, i def save fuel by turning the car off

pauln · 13-09-2005 12:36pm

Here's a link to some tips from the SEI about how to save energy when driving http://www.sei.ie/content/content.asp?section_id=516

BrianD3 · 13-09-2005 12:38pm

It would also depend on temperature and how long the car has been running for. A cold start followed immediately by a stop and then another cold(ish) start with a rich mixture would probably use more fuel and cause more wear than leaving it running. Also, AFAIK it takes 5-10 miles driving to fully recharge a battery after a cold start. If the weather is cold and/or the battery is not in great nick, trying to start the car again within a short time might not work.

testicle · 13-09-2005 12:38pm

Newland Cross crossing from Belgard Road to Fonthill Road, and vice versa is about 5 mins...

opto · 13-09-2005 12:51pm

And don't forget most of your engine wear occurs during starting, and what about pollution when the cat is warming up again?

MrPudding · 13-09-2005 1:08pm

The Prius stop the petrol engine whenever it can. I know it also has an electric engine but I am sure they would not engineer it to stop the engine if there were not a benefit to doing so.

I think that modern elecronically managed & properly serviced engines start more ecconomically than older engines.

MrP

fletch · 13-09-2005 1:51pm

pauln wrote:

Here's a link to some tips from the SEI about how to save energy when driving http://www.sei.ie/content/content.asp?section_id=516

Thanks for the link.....love this quote "use public transport, cycle or walk instead. This will also reduce unnecessary wear and tear on your engine"....well DA!!

fletch · 13-09-2005 1:53pm

pauln wrote:

Here's a link to some tips from the SEI about how to save energy when driving http://www.sei.ie/content/content.asp?section_id=516

"Even if you only have to wait for 30 seconds, it is more economical to switch the engine off and start it again when necessary." - Answers the thread I think...

pauln · 13-09-2005 2:03pm

fletch wrote:

"Even if you only have to wait for 30 seconds, it is more economical to switch the engine off and start it again when necessary." - Answers the thread I think...

It does but I'd say much of what is said can be taken with a pinch of salt, that figure will vary and some of the recommendations are just plain unrealistic. Then again it's their job to take things to there extreme so they're doing a good job in that regard.

naughtyadam · 13-09-2005 2:17pm

I wonder if the 30 second rule applies to this motor when it's cold. (7MB WMV)

adam

HelterSkelter · 13-09-2005 2:33pm

pauln wrote:

Here's a link to some tips from the SEI about how to save energy when driving http://www.sei.ie/content/content.asp?section_id=516

"Do not approach traffic lights at speed. If it is clear that you will have to stop, begin slowing down well in advance."

Don't you HATE these people who slow down well before the lights and crawl up to them at 10kph. I know I was told this is what you should do when I attended the ignition course but I don't agree with it. There is more chance of an accident because you are moving when you could be stopped at the lights/junction. Also it is inconsiderate to other drivers as you are taking up more road space on our already very congested roads.

fletch · 13-09-2005 3:15pm

HelterSkelter wrote:

"Do not approach traffic lights at speed. If it is clear that you will have to stop, begin slowing down well in advance."

Don't you HATE these people who slow down well before the lights and crawl up to them at 10kph. I know I was told this is what you should do when I attended the ignition course but I don't agree with it. There is more chance of an accident because you are moving when you could be stopped at the lights/junction. Also it is inconsiderate to other drivers as you are taking up more road space on our already very congested roads.

I have to admit to doin this all the time to try save petrol....I vaguely remember something in my physics class that the biggest force needed to move an object is the initial force to move it from it's stationary position so if i apply this to my drivin, I'm sure I'll save a few cent every year

fjon · 13-09-2005 3:55pm

In my car manual it states I should turn the engine off if not moving for 30 seconds or more.
I used to do this all the time, although I don't any more. I was on the Naas road, waiting to turn right into Clondalkin. Anyone who knows the stretch knows it takes ages, so I turned my engine off. Once the lights went green, I turned the ignition, and nothing. I had to sit on the road with my hazards on for nearly an hour. Luckily I got someone to give me a hand after a few minutes to push the car onto the central reservation.
Turns out the electric cable for my starter got disconnected, and it was a tiny little job to fix it. Still, I don't want to be in that situation again!

naughtyadam · 13-09-2005 4:13pm

I don't think I've ever tried to save petrol in my life.

adam

mike65 · 13-09-2005 4:16pm

I always take my time reach red lights or traffic which is backed up and clearly not going to get through. Its not hogging road space unless you (behind me) are trying to rush to every traffic light junction. There's no point, if everyone drove in a smooth, progresive, thoughtful fashion we'd all save money on fuel, wear and tear and finally - hair!

I vaguely remember something in my physics class that the biggest force needed to move an object is the initial force to move it from it's stationary position

Exactly, its like switching on the immersion heater from cold or tepid the latter takes no time to heat the former takes forever on a cold morning.

Mike.

pauln · 13-09-2005 5:17pm

On a slightly related topic there was a guy in the states who carried out a few "Experiments" on traffic jams and driving speeds. He found that if you can find the average speed at which the traffic is moving and drive at that speed then you'll find that after a while all the traffic behind you will start moving smoothly along at that speed, no unnecessary stops and starts.
Anyone fancy trying it on the M50 :rolleyes:

Link: http://www.smartmotorist.com/wav/wav.htm

mike65 · 13-09-2005 5:51pm

Londons M25 they have tried something similair with variable limits, in heavy traffic they'd actually drop the max speed limit and this moved traffic faster as the mean speed increased due to less "bunching".

Mike.

unkel · 13-09-2005 6:11pm

BrianD3 wrote:

It would also depend on temperature and how long the car has been running for. A cold start followed immediately by a stop and then another cold(ish) start with a rich mixture would probably use more fuel and cause more wear than leaving it running. Also, AFAIK it takes 5-10 miles driving to fully recharge a battery after a cold start. If the weather is cold and/or the battery is not in great nick, trying to start the car again within a short time might not work.

Indeed the outside temperature and engine temperature are important. The engine might be flooded if you switch off on a cold day only a very short time after starting up

I switch the engine off typically if it is going to be at least a minute or two

Turn engine off?

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