Which uses less fuel?

Submitted: Friday, Mar 03, 2006 at 11:01
ThreadID: 31366 Views:2064 Replies:8 FollowUps:8
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Hi all,
I had a thought (as I often do) and the thought led to a ponder. The suituation was this: I was heading down to the fish'n'chip shop last night for dinner and at a tad under 60km/h the 80 (petrol/auto if it matters) was sitting at about 1200rpm. Which made me think:

Does a car running at say 1200rpm in neutral use the same amount of fuel as it does driving (ie under at least some load) at 1200rpm?

Now my first reaction was, of course it does, but I can't back that up with any specific knowledge or whatever, it's just my common sense sayin so. But THEN I thought if the engine is doing exactly the same thing in both scenario's then all things are equal and the same amount of fuel is used. Now I don't know what to think - but it's interesting to think about!

Whaddya reckon??? Exercise the brain on a Friday! :-P
Cheers
Scoey.
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Reply By: mick - Friday, Mar 03, 2006 at 11:20

Friday, Mar 03, 2006 at 11:20
Scoey,

I imagine it would use more. The load would be different, no load as compared to some load. I suppose it would be like riding a pushy, 100 revs per min with no load (exercise bike on min setting) to 100 revs per min up a steep incline.
Same revs different workload, the hill will stuff ya before the excercise bike would.

A bit like changing into top gear to early, low revs but the ole motor is working far harder,

thats my 2 bobs worth!

Mick
AnswerID: 158318

Follow Up By: Scoey - Friday, Mar 03, 2006 at 11:40

Friday, Mar 03, 2006 at 11:40
Yeah interesting. That's my basic theory too I guess. I reckon someone on the forum will be able to give us a scientific reason for it tho! could be just that simple too! I just thought the arguement against had some merit too. ;-)

Cheers, Mick
Scoey
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Reply By: Member - Doug T (QLD) - Friday, Mar 03, 2006 at 11:51

Friday, Mar 03, 2006 at 11:51
No wind resistance would meen less fuel but....ummm ....why is it running at 1200 rpm in nuetral i thought idle speed was about 700-800 rpm.
Ive seen these old guys [ i'm gettin close too ]about 80yo 3/4 deaf with clutch in and engine screaming at about 3000 revs "ridin the clutch" , your NOT like that are you Scoey.....yet
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Follow Up By: Scoey - Friday, Mar 03, 2006 at 12:11

Friday, Mar 03, 2006 at 12:11
Haha! No, I'm a spritely 27 Doug although most weekends after a game of Rubgy I do feel about 80yo! About the question. you're quite right, the truck idles at about 800rpm but I was being purely hypothetical - although the 80's are renown for an idle speed of anywhere up to 1400rpm when cold so I guess it's not out of the question! Just a thought and nothing more - it would probably be easier if I could switch the ol' noggin off but alas I can't! ;-)
Cheers
Scoey
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Follow Up By: Member - Doug T (QLD) - Friday, Mar 03, 2006 at 14:25

Friday, Mar 03, 2006 at 14:25
hi Scoey
just trying an experiment with url s and how they work, i notice that when i just copy and paste a url it is not active so ppl have to copy and paste it back to the address bar so if i don't put http// and instead put www. it might become active so here goes

www.members.westnet.com.au/dtilley/icons.htm
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Reply By: Michael ( Moss Vale NSW) - Friday, Mar 03, 2006 at 12:12

Friday, Mar 03, 2006 at 12:12
1200 rpm under load will have a different throttle position than idle, of course open more, so more fuel will be used. As for the 1200 RPM at speed would only be 50kph I guess so the wind drag on the car would be minimal...Well thats my theory.. Best regards, Al Einstien
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Retired 2016 and now Out and About!

There's time to rest when you're dead,
Get out and do something instead!

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AnswerID: 158325

Follow Up By: Scoey - Friday, Mar 03, 2006 at 12:30

Friday, Mar 03, 2006 at 12:30
Yeah good theory Albert! I guess the thoery holds more water if you used a different scenario. Say truck doing 3000rpm heading up a snotty big real rough mongrel of a hill, or 3000rpm - hyopthetically ;-) - in Neutral. She'd be wide open heading up the hill but probably not so much in neutral?
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Reply By: 3.0turbob - Friday, Mar 03, 2006 at 12:27

Friday, Mar 03, 2006 at 12:27
If you apply the factor of the co-efficient of drag of the vehicle at a given tyre pressure (remaining constant), opposed to the friction induced by the surface of the road at it's relative temperature and humidity, then include the barometric pressure of that time of the day, factor in the fuel flow rate, .... then the answer would be ..yes?

..................................................
sorry, should be in the friday funny section
Rob
AnswerID: 158331

Follow Up By: Scoey - Friday, Mar 03, 2006 at 12:39

Friday, Mar 03, 2006 at 12:39
Well exactly! All that goes without saying, plus it would depend greatly whether or not I was running a Fitch too! Great! This thread just lost all credibility! haha!
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Reply By: Dave Thomson - Friday, Mar 03, 2006 at 15:40

Friday, Mar 03, 2006 at 15:40
Scoey, the earth spins at 1000 miles an hour {24 hrs in a day 24000 miles an hour}
if you could travel at 25000 miles an hour, would you meet youself coming back ??? We both need to get out more I think,
regards,
Dave
By the way , why do they call flats apartments when they're all together ???
AnswerID: 158365

Follow Up By: Scoey - Friday, Mar 03, 2006 at 16:26

Friday, Mar 03, 2006 at 16:26
Easy Dave, of course you would meet yourself coming back! ;-) Nice one but! ;-) Hey, you might know.... Where do you suppose ants have room for their insides?? Oh and the one that people seem to have heaps of trouble with about how long thier piece of string is? It's twice, half it's length! ;-)
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Follow Up By: D-Jack - Friday, Mar 03, 2006 at 18:04

Friday, Mar 03, 2006 at 18:04
I'm currently working on a philosophy that we are all just brains sitting in jars on a shelf thinking that all this is happening, sure we feel the body hurting, but could it be some kind of trick the brain is playing to make us believe we have bodies???????????????
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Reply By: F4Phantom - Friday, Mar 03, 2006 at 21:59

Friday, Mar 03, 2006 at 21:59
In a petrol engine if you use engine braking down a hill no matter what the RPM the engine uses ZERO fuel. The butterfly valves are shut and so do the spark plugs, I am actually not sure if the fuel is cut off but it must although I cant see how it would be. Anyway, this is a situation where the engine is under no load, the rpm is high and it uses no fuel. If you reverse this, it's simple, engine under load uses more fuel, more load more fuel. And as already stated about throttle positions add to that computer intervention, my car is sluggish in economy and way faster in sport mode.
AnswerID: 158410

Follow Up By: Mike DiD - Saturday, Mar 04, 2006 at 11:35

Saturday, Mar 04, 2006 at 11:35
In a fuel-injected vehicle, the Injectors remain shutoff if coasting and the RPM is well above idle. i.e. you only use fuel to provide power or to keep the engine idling.

Mike
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Reply By: Gerhardp1 - Saturday, Mar 04, 2006 at 10:45

Saturday, Mar 04, 2006 at 10:45
I have recently got one of these and it answers all these questions.

www.scanguage.com
AnswerID: 158504

Reply By: Member - Collyn R (WA) - Saturday, Mar 04, 2006 at 15:25

Saturday, Mar 04, 2006 at 15:25
Quick response.

For both petrol and diesel, consumption will be less whilst idling because consumption is related to work being performed. And whilst idling the only load is internal friction, drive belt and fan losses etc. Once the vehicle is moving friction, rolling and wind loads add to the load on the engine - and more fuel in burned.

Whilst idling however a petrol engine is 'throttled down'. That its cylinders are only partially filled with air - and thus its 'effective' compression ratio is reduced. This reduces efficiency and thus increases consumption.

A diesel engine however full cylinder-loads of air almost regardless of speed an/or load. Its effective compression is thus much the same as its nominal compression (of 18:1 - 22:1) throughout most its rev range. This enables its to to run closer to optimum efficiency.

This is part of the reason why diesels are so much more efficient in heavy traffic and whilst slowing traversing dirt tracks etc.
Collyn Rivers

AnswerID: 158559

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