What is most likely to cause high fuel consumption?

Jez

Jez

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I appreciate that Jez, your auto box may make your car drive less efficiently at 50, but if you could force it to stay in top gear, it would be more fuel efficient at 50.

Not sure it would as the engine would labour surely? The engine would be at like 1000rpm which on an incline i doubt would fare too well pulling 2+ tons of car..
 
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Not sure it would as the engine would labour surely? The engine would be at like 1000rpm which on an incline i doubt would fare too well pulling 2+ tons of car..

I'm sure there are loads of variables which would change it, gradient and such, but I'm sure maintaining a constant 50 in top gear is well within the ability of your engine without it struggling.
 
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It seems to be quite a horrible question in general. Of course you'll get better economy (in a reasonably powered car) in 50 compared to 70 if you're using the same gear for both. I'm really confident that when i took my theory i was taught the answer was B in their official guide or something.
 
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It seems to be quite a horrible question in general. Of course you'll get better economy (in a reasonably powered car) in 50 compared to 70 if you're using the same gear for both. I'm really confident that when i took my theory i was taught the answer was B in their official guide or something.

They probably changed the question from 'aggressive driving' to 'aggressive braking' to throw people.

I don't remember a question about fuel economy when I did my theory.
 
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Standing still in traffic kills my fuel economy. I get 10mpg and that's only because the guage won't display any lower than that.

I had to explain that to myself like this:
When a car is stood still with the engine running it's getting 0 MPG.



Oh, and back to the question.... what about one of those new-fangled hybrid cars with regenerative braking, then, eh?
 
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Soldato
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50mph and 70mph may result in the car sitting at the same cruising revs.

Hence engine BSFC is the same. Now if 50mph is using more fuel that may be due to the viscous losses associated with the autobox not being in top where it can mechanically lock either side of the drivetrain and not waste energy heating the transmission fluid up that would account for the difference you are seeing.

My point on cruise control should have started with a question i guess. For your 50mph versus 70mph do you use cruise control? The point im making is the agressiveness of meeting that speed ends up some quite high engine loads/throttle opening that a human driver wouldnt do.

You may need to do the lesson some other time on cruise control Jez as I refered in no way as to how cruise control actually functions... I was making the assumption, as Ive stated, that you use cruise control.

You are missing the fact that the engine load will be different at those two speeds (more aero and tyre drag for a start) so the throttle opening will be different, hence there will be lower pumping loses running at the higher cruising speed for the same engine speed. I know this only applies to petrol but Jez's car isn't fuelled from Beelzebub's pump.
 
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Got a reply...

Hi Rob

The question was set by the DSA as part of the theory test syllabus,
however, it is true that cars burn more fuel when travelling faster,
especially over 50mph. This however is at a constant speed so a car that
drives a mile at 70mph will burn more fuel than a car that travels a mile at
50mph. So technically the question is right - remember the DSA and other
authorities don't like speed.

Best regards

Richard


Might be true for some cars but certainly doesn't seem to be the case for mine. There you go anyway, its a DSA answer. Which I still think is wrong :p

thanks for this Rob...
 
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in summary. Stopping and starting inertia is the biggest cuase of high fuel consumption.
No. Stopping inertia doesn't effect fuel consumption.

Only adjusting the accelerator effects fuel consumption. Anything that makes you push it harder, be that; increasing your speed due to previously having to slow down, increased wind resistance due to travelling at a higher speed, a hill, binding brakes, more weight, etc.
 
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No. Stopping inertia doesn't effect fuel consumption.

Only adjusting the accelerator effects fuel consumption. Anything that makes you push it harder, be that; increasing your speed due to previously having to slow down, increased wind resistance due to travelling at a higher speed, a hill, binding brakes, more weight, etc.

if you stop/slowdown, you have to start/speedup again, so yes it does effect fuel consumption.

smoothness is key.
 
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Does unburned fuel not get returned to the tank/fuel line anyway, so the efficiency with which fuel is burned is fairly pointless? Although I'm not sure how CO vs CO2 fits into this. :confused:

I think it's the extra fuel in the cylinders during combustion so will end up going out with the exhaust gasses.
 
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Surely you'll save a miniscule amount of fuel if, when you're coming to a stop, you brake gently earlier rather than slamming on at the end, cause over the distance you started braking, you're using less fuel, because you'll be going slower?

Sort of a graph of fuel consumption like
Code:
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versus
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I don't really know, just curious :)
 
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I just took my car out and tried some of this 'aggressive braking'.
I drove as normal except that when I was slowing down I pulled angry faces, shouted and swore, stuck my fingers up at other drivers etc.
It's too early to say if there's any effect on mpg.
 
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i would say A but it depends on the car.

if we are talking about an 800hp skyline with a big turbo and you drive around in positive boost all day, then your going to use way more fuel than normal.
 
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Surely you'll save a miniscule amount of fuel if, when you're coming to a stop, you brake gently earlier rather than slamming on at the end, cause over the distance you started braking, you're using less fuel, because you'll be going slower?

Sort of a graph of fuel consumption like
Code:
\
 \
  \
versus
Code:
--\
  |
  |

I don't really know, just curious :)

as soon as you take your foot off the accelerator fuel consumption goes down massively, as the kinetic energy of the car rolling keeps the engine turning over, not the fuel. so rolling upto lights off gas is better than going upto lights, then stopping and letting the car idle. idling uses more fuel than turning the engine by rolling.
 
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All modern cars are far more fuel efficient than they were a few years ago; a few things to remember, When you lift you foot off the accelerator providing that the engine is turning fast enough (above 1500rpm approx) still in gear you don't use any fuel at all (none) In the old days people used to knock there car out of gear going downhill to reduce mpg.
2nd, 56mph is the most efficient speed the faster you go after this mpg plummets with air resistance.
Reading the road ahead to avoid unecessary braking will mean that you also avoid unecessary acceleration too.
I have an Octavia 1.8 vRS turbo and I have got 47mpg! Tameside to Chester Zoo!
It all went out of the window driving back though!!
 
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