Petrol consumption.... keep it running or switch it on & off?

[Fais]

Hey all...

My driving is mainly around west end of London; so i'm stuck in traffic, at level crossings, pulling up here and there...

Just wondering... is it more efficient to switch off the car for a few minutes and then restart it, or keep it running. I've noticed my SL320 takes a mouthful of petrol whenever i start... but is that better than leaving it running for about 3-4 minutes?

Thanks

 

[DITTRICH]

It is sort of possible to guestimate the fuel used in standing around in traffic jams. I did this by calculating miles per litre for a sustained motorway run starting with a full tank and then topping it up at the end. Kepp fairly constant speed and note the rpm.
Its possible to calculate how much fuel is used per revolution (very little). Given that a car sitting still idles at 700rpm, you can calculate how much it is costing in fuel to sit around. From memory, my W202 c230K drank fuel at the rate of 3 litres per hour standing still. However, I caution that this is VERY VERY rough and others will no doubt say what about this and that and so on. In my defence, its just an estimate and I was VERY VERY bored at the time!

Petrol cars use alot of fuel when starting from cold to quickly warm up the catalytic converter, but warm starting wouldn't need alot of fuel because everything's still at operating temperature.

It did strike me that the ASSYST system might take notice of the number of starts and reduce the interval to the next service, but since I don't know if it ignores warm starts or how it EXACTLY calculates when the next service is due, this is just supposition on my part. Other may know better here.

Regards

Les

 

[mark.t]

let it run

when the engines warm and running it should be doing it as efficiently and cheaply as possible...if its fuelling correctly ...I would assume if you where to keep turning off and on the engine this would possibly cause more wear on the parts that are shocked into life every time we start up .....just put traffic stand still down to life...and leave it running ...and leave the aircon on as well it will not make that much difference in the grand scheme of things ..lets get back to enjoying our cars and not fretting over spilt petrol ...oh and while your sat in traffic give the throttle a proud thump and let the 6 cylinders do the talking

 

[bangerman]

if we need to talk about fuel compustion & price etc shouldn't we be talking on a Vauxhall or Ford forum....


come on, we all love MB for their quality and MB motoring can be done on a budget, but not at petrol stations.

Big engines hate restarts, no oil pressure for a few seconds etc ( loads of wear ).


I used to live in West London ( long live QPR ), found it cheaper to move to a lovely cottage in Wales, Insurance cheaper, no traffic, but loads of rain !!
Lovely country roads to test on though....

 

[GrahamC230K]

if we need to talk about fuel compustion & price etc shouldn't we be talking on a Vauxhall or Ford forum....


come on, we all love MB for their quality and MB motoring can be done on a budget, but not at petrol stations.

Big engines hate restarts, no oil pressure for a few seconds etc ( loads of wear ).


I used to live in West London ( long live QPR ), found it cheaper to move to a lovely cottage in Wales, Insurance cheaper, no traffic, but loads of rain !!
Lovely country roads to test on though....

Are Fords and Vauxhalls more efficient? I doubt it.

You make the rather silly assumption that those with larger sums of spare cash than others wish to waste it.

I know some very thrifty poor people, I know some very careful poor prople and some very careful filthy rich people. I have never met a thrifty rich person!

This was not shall I buy the SL500 or the SL55 and can I afford the petrol question!!

Personally, I leave it running for less than a few minutes, more than two or unattended, switch it off.

 

[glojo]

It is interesting that some of the bigger top of the range Mercedes-Benz have cylinder management which runs the engine at its most fuel efficient manner. Only using the 12 cylinders when that type of power is required.

Mercedes-Benz diesel Sprinters offer an option which cuts the engine out after ticking over for so many seconds and starts up when you depress the throttle. I would appreciate 'Dieselmans' input on this regarding the wear and tear issues, but clearly the manufacturer is happy to offer it. Maybe it might be fitted on other manufacturers petrol powered vehicles.

Regards,
John

 

[altreed]

Fais,

Could I ask what MPG you get from your car? Have you done a tank to tank MPG check. That will give us a better idea of your fueling issue.

I enjoy a healthy 25mpg in town from mine and over 30mpg on a long run. I go through a small town with traffic design flaws to and from work. Rarely get over 40 and spend a few minutes at a standstill. So I consider my cycle to be mostly urban.

 

[Jukie]

I know this is about fuel consumption but should we also consider the battery when considering whether to leave the engine running or go for the "off-on" option?

 

[tim.100]

I think the injectors on my old BMW each put in 2.7lbs/hour when in idling neutral, and 2.9lbs/hours in drive. There were 6 of them.

so - assuming 3lbs/hour = 1.62kgs/hour per injector, times six = 8.172kgs/hour.

I'll use JetA1 fuel density - don't know value for petrol: 1244 litres per tonne.

8.172kgs/hour = 10.166 litres/hour

90p/litre = £9.15 per hour waiting for Ken's re-phased anti-car London traffic lights. And nearly £8 per hour to HM Government! That's outside the London congestion tax zone!

These calcs are for a 1988 BMW 730i. Any ideas for Diesels?

 

[Ser Demec]

on or off

Get a diesel running on SVO

 

[se97mlm]

Well now that i calibrate Diesel engines for my job, i can tell you!

Take a 4 cylinder 220 cdi with a manual gearbox so we don't have to worry about the power dissipated in an autobox held stationary on the brakes

6mg main fuel per injection + 1.5mg pilot at 2 per rev at idle. =15mg/rev
800 rpm = 13 1/3 rps.
fuel per second = 200 mg
fuel per hour= 200 x 60 x 60 = 720g = 0.720 kg

density diesel @ 40°C approx 860 kg/m3

Therefore 1/0.86 * 0.720 = 0.84 litres per hour.

Not much really is it! There must be a mistake with the BMW calculation above, because a 2.8L 24v Golf V6 4 motion only uses 1.5 ltr per hour at idle

It may be useful to remember the engine only needs to put out enough power to overcome it's internal friction to idle.

 

[bangerman]

@Grahamc230k, I have never made a silly assumption in my life.
The point i was trying to make was that
When u chose to purchase a car you have to take into account,
Fuel consumption
Running costs
Insurance etc
The important point was that big engines do not like short sharp starts etc

An engine that spends it live up & down the motorway will in general always outlast one that does the school run & short journery's ( mile 4 mile etc ).

Good point about the money side though,

credit where credit is due

 

[GrahamC230K]

There are some amazing calculations in this thread.

@Grahamc230k, I have never made a silly assumption in my life.

I thought that might have sounded a bit harsh. I bet you have though!

 

[bangerman]

ok, hands up!

Maybe?

 

[GrahamC230K]

ok, hands up!

Maybe?

Deal.

I still can't believe the science in this thread - I was thinking there would be a bit of speculation etc, but no, someone has the equation to work out the fule consumption at idle!!

What are the theories on wear and tear on stopping and restarting a warm engine?

 

[del320]

And what about the wear and tear on the ignition lock and key, rattling it back and forth to ensure the immobilisor doesn't kick in?
And the embarrasment when it fails to start 'cos you turned it off in D

 

[Fais]

Cheers guys.... looks like the engine is staying on....

I purchase the car using my student loan which i have saved over the years.... while i am affording the maintence, insurance etc... i may as well treat the SL as a 'privilege'... whatever i can save financially, great, whatever i can save on wear and tear even better.

But the SL is proving costly. Unfortunatley i haven't worked out the exact MPG... not long ago i did find that a £60 full tank gave me just over 200 city miles .

My shorter journeys seem to be the culprit. My office is close to home, so the car doesn't get a chance to warm up (raised this thread which confirmed high revs on cold start was normal: http://mbclub.co.uk/forums/showthread.php?t=9919) ... when i go out for appointments soon after i'm again on a cold start... since the journeys are relatively short the car doesn't warm up, if does, it's time to switch it off. Add traffic lights (plus traffic!) and useless train level crossings and i'm sure you'll start to see my problem (*please no comments on the fact that the car isn't suitable for my needs*).

I love the car to bits, those journeys with the top down can make a boring routine journey far more pleasant...

... whatever i can do to keep the SL as long as possible i'm happy to try...

Fais,

Could I ask what MPG you get from your car? Have you done a tank to tank MPG check. That will give us a better idea of your fueling issue.

 

[altreed]

Ooo, full tank and 200 miles

Just to make you feel a bit better, I get about 250 from a full tank, so your mpg isn't that bad!! It will be around 20mpg.

 

[bangerman]

I hate to mention a french company( Citr***) , but what about the new C2/3/4 that stops the engine at the lights when foot is on brake and then restarts in milli seconds...

i don't know the Techno behind this, but same question about wear and tear comes to mind.

All the big engined cars i have owned whilst in regulary town traffic, have always suffered from premature wear!!
Either bottom end problems or cam wear.. surely down to retarts & operating temp etc

Comments welcomed.