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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 1:53 am 
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There is also the effect of momentum. On the flat there is unlikely to be a continuous steady fuel consumption rather a series of small acceleration/coast phases (whether on manual or cruise control) but when climbing it is more likely to be a steady burn to maintain speed when using the ICE.


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 2:11 am 
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Location: Netherlands, Utrecht area
Sunder wrote:
That assumes a straight lift. I doubt you'd get 25 miles of road that is perfectly and uniformly 1:250. Most likely it would rise and fall many times over that 25 miles.
It doesn't make such assumption, as it doesn't really matter: use the formula I gave to calculate the sum of all the little bits of extra energy needed for all the little climbs and then the sum of all the little bits of energy 'saved' because of all the little drops of altitude. Then subtract these numbers and you will get to the same value as I did by just looking at the overall difference in altitude. What happens in between doesn't matter that much.

All that can be said is than engine load is not X all the time, but a little bit more than X at some moments and a little bit less than X at others. So, the overall efficiency of the engine could be impacted. But I think this will be minimal.


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 4:32 am 
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anko wrote:
Sunder wrote:
It doesn't make such assumption, as it doesn't really matter: use the formula I gave to calculate the sum of all the little bits of extra energy needed for all the little climbs and then the sum of all the little bits of energy 'saved' because of all the little drops of altitude. Then subtract these numbers and you will get to the same value as I did by just looking at the overall difference in altitude. What happens in between doesn't matter that much.


Hmm? Surely you don't think driving up 100m then driving down 100m would be the same as cruising on a flat do you? Well, I guess it wouldn't matter if we lived in a perfect frictionless, lossless world, but we don't.

Just like the six foot track isn't really a downhill run, it does matter how much elevation is gained and lost for that net 150m gain.


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 4:46 am 
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Location: Doncaster, South Yorks
Sunder wrote:
anko wrote:
Sunder wrote:
It doesn't make such assumption, as it doesn't really matter: use the formula I gave to calculate the sum of all the little bits of extra energy needed for all the little climbs and then the sum of all the little bits of energy 'saved' because of all the little drops of altitude. Then subtract these numbers and you will get to the same value as I did by just looking at the overall difference in altitude. What happens in between doesn't matter that much.


Hmm? Surely you don't think driving up 100m then driving down 100m would be the same as cruising on a flat do you? Well, I guess it wouldn't matter if we lived in a perfect frictionless, lossless world, but we don't.

Just like the six foot track isn't really a downhill run, it does matter how much elevation is gained and lost for that net 150m gain.


It would be another difficult area to "prove" as on such a long run, ambient temperatures and barometric pressures will change during the journey and the engine will adjust the mixture of the fuel accordingly through the Linear Air-Fuel Sensor and / or the Throttle Valve Control Sensor. There will be some difference explained by gaining height, but a figure couldn't be relied on unless the test was done in a perfect environment with static temperatures, pressure and relative humidity.

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Last edited by Neverfuel on Tue Jan 19, 2016 5:14 am, edited 2 times in total.

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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 4:49 am 
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Sunder wrote:
anko wrote:
Sunder wrote:
It doesn't make such assumption, as it doesn't really matter: use the formula I gave to calculate the sum of all the little bits of extra energy needed for all the little climbs and then the sum of all the little bits of energy 'saved' because of all the little drops of altitude. Then subtract these numbers and you will get to the same value as I did by just looking at the overall difference in altitude. What happens in between doesn't matter that much.


Hmm? Surely you don't think driving up 100m then driving down 100m would be the same as cruising on a flat do you? Well, I guess it wouldn't matter if we lived in a perfect frictionless, lossless world, but we don't.

Just like the six foot track isn't really a downhill run, it does matter how much elevation is gained and lost for that net 150m gain.


Well, in the case I described, it is a relatively steady climb according to Google Earth. That is not to say that there are downhill drops, but they are few and not significant - it's a 150m climb over a distance of almost 30 miles.

Other driving conditions were pretty constant - temperature between 3 and 5 degrees, running on cruise control at just over 60mph, road surface reasonably dry and all decent quality, not a lot of wind and the entire route was more or less in the same direction.


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 5:04 am 
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Fair enough.

I don't really want to get bogged down in the details. My point only was that lifting an object up is surprisingly energy intensive, especially compared to just maintaining cruising speed.

Think of it this way: if you were to apply 9.8ms^2 laterally instead of vertically, you would hit 100kmh in a hair under 3 seconds. Thats a fast car and a truck load of power in any's parlance.


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 5:37 am 
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Joined: Mon Dec 01, 2014 11:30 am
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Location: Netherlands, Utrecht area
Sunder wrote:
anko wrote:
Sunder wrote:
It doesn't make such assumption, as it doesn't really matter: use the formula I gave to calculate the sum of all the little bits of extra energy needed for all the little climbs and then the sum of all the little bits of energy 'saved' because of all the little drops of altitude. Then subtract these numbers and you will get to the same value as I did by just looking at the overall difference in altitude. What happens in between doesn't matter that much.


Hmm? Surely you don't think driving up 100m then driving down 100m would be the same as cruising on a flat do you? Well, I guess it wouldn't matter if we lived in a perfect frictionless, lossless world, but we don't.

Just like the six foot track isn't really a downhill run, it does matter how much elevation is gained and lost for that net 150m gain.

Yes, I do. Taking into account what I said before:

An engine running for 10 minutes at 80% load and then for 10 minutes at 60% load may operator a little bit more or less efficient than an engine running all 20 minutes at 70% load.

To be honest, I don't see what friction and / or losses have to do with it, until the gradient becomes so steep that you could actually cost or even recover energy during the descents.


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 1:40 pm 
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Sunder wrote:
Fair enough.

I don't really want to get bogged down in the details. My point only was that lifting an object up is surprisingly energy intensive, especially compared to just maintaining cruising speed.

Think of it this way: if you were to apply 9.8ms^2 laterally instead of vertically, you would hit 100kmh in a hair under 3 seconds. Thats a fast car and a truck load of power in any's parlance.


Which is why climbing stairs is such good exercise (or knackering :lol: ) and why cyclists standing on the pedals are idiots ;)


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 2:03 pm 
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Quote:
An engine running for 10 minutes at 80% load and then for 10 minutes at 60% load may operator a little bit more or less efficient than an engine running all 20 minutes at 70% load.

To be honest, I don't see what friction and / or losses have to do with it, until the gradient becomes so steep that you could actually cost or even recover energy during the descents.


Okay. There is definitely a misunderstanding here. I'm not sure why you think a car's engine would be running at 60% while going down hill?

I was thinking of a situation of 10 minutes at 40% load and 10 minutes of regen, vs 20 minutes at 5% load. Friction and losses would come into play because we can't regen all the potential kinetic energy we stored going up the hill.


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 Post subject: Re: Effect of gradient on fuel consumption...
PostPosted: Tue Jan 19, 2016 2:05 pm 
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greendwarf wrote:

Which is why climbing stairs is such good exercise (or knackering :lol: ) and why cyclists standing on the pedals are idiots ;)


I agree 90% :) The 10% is that humans are not engines. We have fast twitch and slow twitch muscles and they tire at different rates.

You might want to stand on the pedals for strategic positioning (effectively blow your energy in fast twitch muscles just to pass on that hill). Otherwise you're 100% right.


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