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Graph of car fuel consumption versus speed



 
 
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  #51  
Old October 8th 08, 08:35 PM posted to uk.d-i-y
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Posts: 677
Default Graph of car fuel consumption versus speed

In message , The Natural
Philosopher writes
I m not convinced that slow acceleration is as effective as its made
out to be.

Under heavy acceleration the engine management system will go open loop
and injects more fuel than is strictly necessary for very good reasons,
the excess fuel is burned in the cat and wasted so slow acceleration
will use less fuel even if the eventual speed is the same.


the energy needed to get a car up to speed is the same.

Of course, but unfortunately real life internal combustion engines don't
run best with theoretical energy inputs so even though the amount of
energy required is the same you'll find that a heavy foot will still use
more fuel.


--
Clint Sharp
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  #52  
Old October 8th 08, 09:09 PM posted to uk.d-i-y
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Posts: 477
Default Graph of car fuel consumption versus speed

Roger wrote:

It is more than 40 years since I did any thermodynamics and we didn't do
much on IC engines anyway but through the mists of time ISTR that an IC
engine is most efficient at max bmep (brake mean effective pressure
IIRC) and max bmep equates very closely to max torque.


Me too, that's why it should be better to get to your target speed by
keeping the revs in the max torque band as you change gears, which implies
full throttle but I suspect the fuel map is such that its best to back off
from this a bit.

AJH
  #53  
Old October 8th 08, 09:22 PM posted to uk.d-i-y
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Posts: 132
Default Graph of car fuel consumption versus speed

On 8 Oct, 13:26, Martin Bonner wrote:

I *think* that the force exerted by drag goes as the /square/ of the
speed. *


Only for a constant drag coefficient. Cd itself, however, depends on
the flow pattern around the car and that in turn depends on the speed
- it's often expressed in term of the Reynolds number. As speed
increases the flow becomes more turbulent and drag increases
enormously. Conversely, when the speed is low the flow is (largely)
laminar and drag is low.

Ian
  #54  
Old October 8th 08, 09:25 PM posted to uk.d-i-y
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Posts: 132
Default Graph of car fuel consumption versus speed

On 8 Oct, 11:20, The Natural Philosopher wrote:
Andy Burns wrote:


yes they are, I've seen several artics with stickers on the back saying
"this vehicle is restricted to X mph" where X is lower than the 56 mph
limit (and I don't mean the 40 mph limit on single carriageways).


50mph on single carriageways....


40mph. Highway Code, section 123.

http://www.direct.gov.uk/en/TravelAn...code/DG_070304

Ian
  #55  
Old October 8th 08, 09:33 PM posted to uk.d-i-y
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Posts: 9,056
Default Graph of car fuel consumption versus speed

Clint Sharp wrote:
In message , The Natural
Philosopher writes
I m not convinced that slow acceleration is as effective as its made
out to be.

Under heavy acceleration the engine management system will go open loop
and injects more fuel than is strictly necessary for very good reasons,
the excess fuel is burned in the cat and wasted so slow acceleration
will use less fuel even if the eventual speed is the same.


Yes, I know that, but the real point is where is the optimal throttle
setting for efficient acceleration. It ain't idle, and as you have
pointed out, it ain't full welly either.



the energy needed to get a car up to speed is the same.

Of course, but unfortunately real life internal combustion engines don't
run best with theoretical energy inputs so even though the amount of
energy required is the same you'll find that a heavy foot will still use
more fuel.


But a mouse foot may be just as bad. That's the point.

If what the engine is optimised for is top gear cruise at 56mph, thats
medium revs and maybe 1/3rd throttle.

Possibly that's where we should be accelerating?

I've been dong research. Its fascinating.
some facts seem to be well agreed.

1/. Full throttle is not efficient.It tends to cause over rich mixture.

2/. Max RPM is inefficient. It wastes power in engine friction.

3/. On other than racing engines, peak torque is arrived at at lower
than max revs. This is generally true even with a turbo, as the sort of
breathing or boosting that will make an engine develop peak torque at
peak RPM is totally intractable in a road car.

So that definitely means we want to not use full throttle or max RPM
when accelerating..

Down at the bottom end, it seems to be the case that wide throttle on a
very low RPM is not efficient either.

Surprisingly, this area of car engine performance seems ill understood
by almost all the sources I can find. Or its simply ignored.

Interestingly, one poster on a US group said that sub 50mh, he needed to
drop a gear to get his mpg back. Thats just cruising along.

I can't be sure, but I get the distinct impression that medium light
throttle and medium revs works best on my petrol machine, and in fact
the turbodiesel as well.







  #56  
Old October 8th 08, 09:39 PM posted to uk.d-i-y
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Posts: 9,056
Default Graph of car fuel consumption versus speed

andrew wrote:
Roger wrote:

It is more than 40 years since I did any thermodynamics and we didn't do
much on IC engines anyway but through the mists of time ISTR that an IC
engine is most efficient at max bmep (brake mean effective pressure
IIRC) and max bmep equates very closely to max torque.


Me too, that's why it should be better to get to your target speed by
keeping the revs in the max torque band as you change gears, which implies
full throttle but I suspect the fuel map is such that its best to back off
from this a bit.


I think so too.

Certainly maximum welly at ultra low RPM is not ideal.



AJH

  #57  
Old October 8th 08, 09:43 PM posted to uk.d-i-y
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Posts: 9,056
Default Graph of car fuel consumption versus speed

The Real Doctor wrote:
On 8 Oct, 11:20, The Natural Philosopher wrote:
Andy Burns wrote:


yes they are, I've seen several artics with stickers on the back saying
"this vehicle is restricted to X mph" where X is lower than the 56 mph
limit (and I don't mean the 40 mph limit on single carriageways).

50mph on single carriageways....


40mph. Highway Code, section 123.

http://www.direct.gov.uk/en/TravelAn...code/DG_070304

Ian

Ah, Thats for over 7.5 tonnes tho, and Im not licensed for them, so I
never learnt that ;-)


Mind you, none of them keep to it. I drive the A11 frequently in a 3 ton
camper, and I keep pace with the artics.;-)

If they slow to 40mph, it creates a tailback from Thetford to the start
of the dualled section..all ten miles of it :-)
  #58  
Old October 8th 08, 09:45 PM posted to uk.d-i-y
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Posts: 2,231
Default Graph of car fuel consumption versus speed

On Wed, 08 Oct 2008 13:22:05 -0700, The Real Doctor wrote:

On 8 Oct, 13:26, Martin Bonner wrote:

I *think* that the force exerted by drag goes as the /square/ of the
speed.


Only for a constant drag coefficient. Cd itself, however, depends on the
flow pattern around the car and that in turn depends on the speed - it's
often expressed in term of the Reynolds number. As speed increases the
flow becomes more turbulent and drag increases enormously. Conversely,
when the speed is low the flow is (largely) laminar and drag is low.

Have to diasagree the Reynolds numbers for object of size car, in air,
mean that even a few mph and you are into unambigously turbulent flow,
where the drag is proportional to speed squared.

The Cd can be affected by things like a half closed door/hatch. But is
largely unaffected by speed for Reynolds numbers of 10K+.




--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

  #59  
Old October 8th 08, 09:51 PM posted to uk.d-i-y
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Posts: 2,231
Default Graph of car fuel consumption versus speed

On Wed, 08 Oct 2008 03:42:58 -0700, Nige Danton wrote:

On Oct 8, 3:35*pm, Adrian wrote:
Nige Danton gurgled happily, sounding much like
they were saying:

within any given class are likely to be fairly similar. As a rough
rule of thumb, increasing drag starts to come seriously into play
from about 60mph upwards.
Drag cubes with velocity and so it may become important at speeds
lower than 60 mph.


Trust me on this... I've got plenty of experience with low-powered,
unaerodynamic vehicles. It starts to come into play at about 60.


I'm a cyclist and aerodynamic friction plays a huge role in determining
speed and above ~30kph the benefits of drafting behind another cyclist
are considerable. There's an energy saving of ~20% for the first cyclist
in a pace line and that rises to maximum of ~30% for the fourth cyclist.

I really would be surprised in cars are so slippery that aerodynamic
friction does not play a significant role at speeds slower than 60mph.

What sort of vehicles are you referring to?


IIRC about 20 years ago, 30 mph, was about evens between the air drag and
other resistances (mainly tyres). These days the shapes are more slippery
so 40mph might be the balance point.

It all depends on what you mean by 'significant' and 'dominant'
If 40 mph was the balance between aero drag and other friction then
At 60mph aero drag is about 70% of total which I call significant.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

  #60  
Old October 8th 08, 10:12 PM posted to uk.d-i-y
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Posts: 2,749
Default Graph of car fuel consumption versus speed

It happens that Ed Sirett formulated :
IIRC about 20 years ago, 30 mph, was about evens between the air drag and
other resistances (mainly tyres). These days the shapes are more slippery
so 40mph might be the balance point.

It all depends on what you mean by 'significant' and 'dominant'
If 40 mph was the balance between aero drag and other friction then
At 60mph aero drag is about 70% of total which I call significant.


I've done a bit of caravan towing rough research using the fuelling
computer and given still conditions and fast roads, my most economical
speed is around the 50 to 55mph mark - surprisingly, because you would
think the poor aerodynamics would make it very uneconomical. When in no
particular hurry, I can manage economy figures only slightly worse than
when not towing.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk


 




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