More fuel efficient-idling vs switching off when stopped?

[TheOtherLeft]

I was wondering which save fuel most - idling or switching the engine off when stopped for more then a minute or so?

How much extra fuel is used when starting the engine compared to idling?

Does it make a difference if its petrol or diesel?

Cheers.

[MightyMouse]

Lots of OE work is going on now on engine shut down.... its a saver if the vehicles is set up to do it properly.

Unfortunately for us this involves fairly complex systems that use a special starter to move the crank to an optimum position for "instant restart", direct injection etc etc but is very effective especially in heavily built up areas. Expect to see much more of this in the near future.

However grinding away on a traditional starter etc isn't the way to go.

[KiwiBacon]

Diesels use maybe 1-2 litres per hour depending on your engine. Petrols 3-4 litre/hour again depending on your engine.

It's not a big difference in fuel, but idling isn't good for most engines and downright awful on others.

[Jacked]

It's not a big difference in fuel, but idling isn't good for most engines and downright awful on others.

In my experience it glazes the bores over time. But if its only for short while just leave it going. 10-15 minutes maybe not.
Unless its really cold and want to get the heater nice and toasty

[TheOtherLeft]

Diesels use maybe 1-2 litres per hour depending on your engine. Petrols 3-4 litre/hour again depending on your engine.

It's not a big difference in fuel, but idling isn't good for most engines and downright awful on others.

If a petrol uses 3-4l/hr when idling do you happen to know how much petrol would be used to start an engine, assuming we have yet to do the starter mod as described by MightyMouse?

[Gwagensteve]

MM is describing OEM technologies, not readily adapted to an existing car. VW and MINI have used these for a while. The fact they are doing it to cut emissions clearly indicates that an engine turned off will use less fuel than an engine running, even allowing the restart "puff" of extra fuel.

The OEM systems do stuff like monitor which cylinder is on it's compression stroke and turn the motor with the starter just enough to get that cylinder to pop. You can't do that with the key, and it takes starter control technology, and timing and fuel control technology that's just not present in an engine not designed to do it.

If you did it yourself, could you measure it? I'm sure if you religiously killed the motor at all reasonable idling opportunities, you would see a fuel consumption improvement. I don't think it would be worth the inconvenience- the gain would be too small - might get a bigger improvement with an energy polarizer or a magnet on the fuel line. ( I joke, but really, it's not worth the hassle

Steve.

[berad]

Off topic, but take taxi's/courier vans etc, those motors run on average more than most peoples everyday car, something has to be said for shut off and cooldown of motors.

Obviously idleing 24 hours a day is not optimum either for the motor.

[ISUZUROVER]

VW and MINI have used these for a while. The fact they are doing it to cut emissions clearly indicates that an engine turned off will use less fuel than an engine running, even allowing the restart "puff" of extra fuel.

It was a while ago, but I read a paper once which stated that if you were going to be idling for longer than 30 secs, it was better to switch off.

[RockyF75]

Off topic, but take taxi's/courier vans etc, those motors run on average more than most peoples everyday car, something has to be said for shut off and cooldown of motors.

Obviously idleing 24 hours a day is not optimum either for the motor.

I think its actually better for a motor not to cool down and warm up a lot (metal expands and contracts, and creates wear).
Hence why taxi's seem to sometimes go for half a million clicks without any trouble. The engines only get switched off a couple of times a month

[BundyRumandCoke]

Also consider starting current draw and the extra wear and tear on the starter motor, and the fact that repeated use of the starter, without sufficent time between starts for the alternator to recharge the battery, is going to eventually wind the battery flat.

[-Scott-]

If the engine is warm, and in good condition, the starter draw to crank & fire isn't particularly significant - I wouldn't worry about flattening the battery.

Now that I own a (petrol) car with a trip computer I'm driving more with economy in mind. It didn't take long to determine that a minute or two idling at lights would sent the consumption figure up a notch, but switching off and re-starting doesn't. If I know I'll be sitting there for a minute or two, and have a few seconds notice to re-start, I switch off.

But only in my new(er) car - the other two don't have trip computers, so WGAF?

[alien]

i cant imagine doing that in my zuk - i have one of the push-in-the-dash style immobilisers.... so in traffic i'd have to take the key out of the ignition, push the immob. insert key, turn and start... lol

[tweak'e]

Also consider starting current draw and the extra wear and tear on the starter motor, and the fact that repeated use of the starter, without sufficent time between starts for the alternator to recharge the battery, is going to eventually wind the battery flat.

+1

thats one problem i tend to get. stop at one place, then start up and drive up the paddock 100m and stop, repeat all day. battery gets drained fairly badly.
if only going to be stopped for a short time its better just to leave it running, just to get some charge back into the battery.
if doing a lot of short runs and a lot of starts i usually need to get the charger out and top the battery up.

edit: with turbo motor you don't really want to be turning motor on/off a lot either. not to good for the turbo.

[ISUZUROVER]

edit: with turbo motor you don't really want to be turning motor on/off a lot either. not to good for the turbo.

Only if you went straight from hard driving to stopped. In traffic your turbo wouldn't get hot enough to need to idle.

[tweak'e]

edit: with turbo motor you don't really want to be turning motor on/off a lot either. not to good for the turbo.

Only if you went straight from hard driving to stopped. In traffic your turbo wouldn't get hot enough to need to idle.

debatable. depends on of your just coasting along or having to go full noise between stops (hill, heavy load etc). its just something to keep in mind thats all.

[Chucky]

If you have to leave your diesel idleing for extended time, it is also a good idea to give it a bit afterwards. It just helps reduce glazing.
Also most diesel start with the fuel pump in full fuel position. So yu would most likely use more fuel constantly shutting down and restarting.

[MightyMouse]

Chucky - your obviously not talking about current technology common rail systems. The days of pure mechanical injection are gone.

I'd say your correct about old technology motors..... not engineered to even consided idle/stop, so gains / losses are very argueable.

However when done with the right technology, it can be a real winner in some driving cycles, obviously dense traffic in built up areas is most promising.

[ISUZUROVER]

Chucky - your obviously not talking about current technology common rail systems. The days of pure mechanical injection are gone.

I'd say your correct about old technology motors..... not engineered to even consided idle/stop, so gains / losses are very argueable.

However when done with the right technology, it can be a real winner in some driving cycles, obviously dense traffic in built up areas is most promising.

Exactly, and indirect mechanical systems at that. Most direct engines inject little/no extra fuel on startup, and you are instructed to start the engine with your foot OFF the accelerator.

Btw - the WA DEC (EPA) here has been running a campaign getting trucks (delivery trucks etc) to shut their engines down rather than idle them. Many companies have reported fuel economy improvements of 10-20% across their fleet.

[tweak'e]

mind you, ultimate way would be to use hybrid ie electric motor and batteries. thats how hybrids get their good fuel economy, the motor turns off instead of idling and electric is used in stop/start traffic.

tho i can't imagine the weight of the batteries and electric motor that would be required to push a 3 ton 4x4.

[tweak'e]

Also most diesel start with the fuel pump in full fuel position. So yu would most likely use more fuel constantly shutting down and restarting.

i don't know about that. no idea on inline pumps but most rotary ve style pumps i know there is an start setup which increase fuel a bit but its minor and certainly not full fuel.
only way to do full fuel is to start it with foot to the floor.

[KiwiBacon]

mind you, ultimate way would be to use hybrid ie electric motor and batteries. thats how hybrids get their good fuel economy, the motor turns off instead of idling and electric is used in stop/start traffic.

tho i can't imagine the weight of the batteries and electric motor that would be required to push a 3 ton 4x4.

Sniff Petrol was taking the piss from landrover about that. One of their publicists said they were investigating hybrid drivetrain options.

Sniff said a hybrid landrover would be soo heavy the earth will be pulled out of orbit and into the sun.
Genius that man.

[berad]

for a saving of 10% i wouldnt bother, and thats a courier thats stopping and starting more than the average commute, if the motor did it automaticaly it would be a good thing,

10% is a fair amount of extra cash over a year, but i dont think like that unfortunately haha.

[RockyF75]

for a saving of 10% i wouldnt bother, and thats a courier thats stopping and starting more than the average commute, if the motor did it automaticaly it would be a good thing,

10% is a fair amount of extra cash over a year, but i dont think like that unfortunately haha.

Probably not THAT much actually, I worked out once I was spending about 4k a year on diesel, that was driving in Sydney everyday, not sure on how many k's.

Even if it was double that, and I saved 10%... $800 a year is not enough IMO to risk possible engine damage over a long term, and almost certain loss off battery/starter life.

[berad]

Thats a rudd bonus every year though .

[Chucky]

Chucky - your obviously not talking about current technology common rail systems. The days of pure mechanical injection are gone.

I'd say your correct about old technology motors..... not engineered to even consided idle/stop, so gains / losses are very argueable.

However when done with the right technology, it can be a real winner in some driving cycles, obviously dense traffic in built up areas is most promising.

I should have been clearer and excluded common rail systems.
And from what I have seen every governed diesel engine when shut down (Fuel rack will go to the no fuel position) will reset to full fuel in rest position, so when it starts up it is at full fuel, even if for just a very short time.

[ISUZUROVER]

And from what I have seen every governed diesel engine when shut down (Fuel rack will go to the no fuel position) will reset to full fuel in rest position, so when it starts up it is at full fuel, even if for just a very short time.

Yes but this is probably like the Fluoro light bulb issue. When you run the calcs with them, it will be better to switch off, unless you are only leaving the room for 0.2 seconds.

[fester2au]

The miniscule amount of fuel you will save in everyday driving (assuming you are not a courier or taxi driver) will be outweighed by the medical bills if I get stuck behind you at the lights and don't make it through the first chance because you are too slow in getting going again.

I read a comment from a pensioner in the RACQ magazine suggesting we all do this and all I can see is the congestion. It's bad enough now that it takes so long to take off in a line of traffic let alone adding the restart as well.

I see it now ..sit at lights on netural with handbrake on...light goes green...put car in gear....start engine....take handbrake off....slowly release clutch and move away...2 people get through 1 change of lights.

Please I beg you save it till you can buy a car that does it for you.

[ISUZUROVER]

The miniscule amount of fuel you will save in everyday driving (assuming you are not a courier or taxi driver) will be outweighed by the medical bills if I get stuck behind you at the lights and don't make it through the first chance because you are too slow in getting going again.

Doing a good job to change the stereotype about people from Gympie...

[tweak'e]

.....
And from what I have seen every governed diesel engine when shut down (Fuel rack will go to the no fuel position) will reset to full fuel in rest position, so when it starts up it is at full fuel, even if for just a very short time.

its been a long time since i looked at it but not from what i can remember.
shut off at idle, control arms almost at minimum, governor will retract when its stopped so it will move the control arm a little bit toward more fuel.
but to go full fuel it would have to fight against the control spring. control spring tries to keep it at idle because thats where the throttle is.

[KiwiBacon]

.....
And from what I have seen every governed diesel engine when shut down (Fuel rack will go to the no fuel position) will reset to full fuel in rest position, so when it starts up it is at full fuel, even if for just a very short time.

its been a long time since i looked at it but not from what i can remember.
shut off at idle, control arms almost at minimum, governor will retract when its stopped so it will move the control arm a little bit toward more fuel.
but to go full fuel it would have to fight against the control spring. control spring tries to keep it at idle because thats where the throttle is.

My inline pump has a control cable which locks it into full fuel for startup. This remains until the governor senses enough speed to unlock it and return to governor control.