Effect of gradient on fuel consumption...

[maby]

... OK - clearly there will be one, but I was watching the numbers yesterday and I was surprised at how great it is - has implications for assessing performance...

I was driving back from the coast (Southampton) to our home (north of London) - the starting point was just a few metres above sea level. Following some of the discussions going on here, I was keeping an eye on fuel consumption. I knew that I had about a hundred miles in front of me - almost all motorway - so I was running in Save mode with about 75% battery charge. Ambient temperature was between 3C and 5C all the way and I had the heater on at 21C.

A few miles up the motorway (probably 10 miles from the start of the trip) I was surprised to see the fuel consumption showing 27mpg. I've done this trip many times and know that I generally get around 42mpg on it. I wondered if I had pulled the figure down very low in the first couple of miles, with a cold car in city traffic, so I reset the fuel consumption meter and it settled back on 27mpg within a couple of miles. I assumed that this was a symptom of the cold temperature and carried on.

About 30 miles into my trip, the fuel consumption suddenly dropped to 41mpg - and stayed at that figure more or less until I got home. I've just been checking on Google Earth and the break point in fuel consumption is just past the top of the South Downs. The altitude of the M3 motorway tops out at about 150m just past Micheldelver and the rest of my trip home is broadly flat - slightly down hill overall since my house is about 80m ASL.

So, a climb of 150m over a distance of 25 miles seems to be enough to increase fuel consumption by as much as 25% - that surprised me!

 

[Carnut]

Weird!
Mine does the same. I think all the readouts talk bollox! I don't trust any of them!
I am just starting a full to full (petrol) test to see what the REAL overall consumption is. Together with diarising each charge with cost.
My leccy is 13.1p per Kw X5 hours X2.7Kw =£1.70.. but screen shows last charge from flat cost £1.18. So it obviously took less than 5hours, or I got the 2.7Kws incorrect

 

[jaapv]

Charging is not linear, it decreases as the battery approaches its maximum charge, so it is not a simple multiplication.

 

[Neverfuel]

Weird!
Mine does the same. I think all the readouts talk bollox! I don't trust any of them!
I am just starting a full to full (petrol) test to see what the REAL overall consumption is. Together with diarising each charge with cost.
My leccy is 13.1p per Kw X5 hours X2.7Kw =£1.70.. but screen shows last charge from flat cost £1.18. So it obviously took less than 5hours, or I got the 2.7Kws incorrect

The battery will only take about 8.4kw from 30% full and will trickle charge towards the end so will not be drawing as much power as you think! even with conversion losses. If you assume 8.4 kW + about 9 - 10% for losses @ 13p per kW, the car is telling you fairly accurately what it has cost. Beware that any FREE rapid charging or charging at work will also be reflected in the overall monthly figure. I have found mine to be very accurate.

 

[anko]

WRT the gradient: Of course climbing comes at a cost. But do not underestimate the effect of a (really) cold engine. My previous car (Outlander Diesel) needed at least 40 km befor the fuel consumption would settle down. There is a good reason for me wanting to prevent cold starts as much as possible.

 

[Sunder]

... OK - clearly there will be one, but I was watching the numbers yesterday and I was surprised at how great it is - has implications for assessing performance...

You know, I was able to push my old 1.8 tonne car on flat ground. Surprisingly, I can't bench or dead lift more than 100kg. Gravity, hey?

You know that to overcome wind resistance and frictional resistance for most cars at 60km/h is in the single digit kilowatt range? But a decent hill climb on a 1.8 tonne car can push you over 100kw just to maintain speed.

Put into that perspective, maybe it should be surprising that hills only cost 25% more.

 

[GrahamRC]

Clearly gradient has a big effect on energy consumption...potential energy (m.g.h) is changing with height, then there is the energy expended in overcoming drag. I suspect that much of the variation in fuel consumption is due to differing wind strength, which can either assist or oppose the car motion depending on direction. I'm currently logging my phev's fuel consumption for my daily work commute, and I'm noting wind strength and direction (roughly) to assess the effect, as well as outside air temperature. Time will tell!

 

[anko]

... OK - clearly there will be one, but I was watching the numbers yesterday and I was surprised at how great it is - has implications for assessing performance...

You know, I was able to push my old 1.8 tonne car on flat ground. Surprisingly, I can't bench or dead lift more than 100kg. Gravity, hey?

You know that to overcome wind resistance and frictional resistance for most cars at 60km/h is in the single digit kilowatt range? But a decent hill climb on a 1.8 tonne car can push you over 100kw just to maintain speed.

Put into that perspective, maybe it should be surprising that hills only cost 25% more.

We are talking about 150 m over 25 miles. That is roughly a 1:250 ratio, which will add 7 kg gravitational force, or something like that? Sure you can push your car up a 1:250 ratio.

EDIT: To lift 1800 kg up 150 m you need 1800 kg * 9,81 m/s2 (at least where I live) * 150 m = 2648700 Joule. This is the equivalent of approx. 0.74 kWh. Lets say a liter of petrol contains about 9.7 kWh of energy and burns at an efficiency of 25% (which is a negative estimation), then you need about 300 cc of petrol to concur gravity. Normally, 25 miles will take about 3 liters or so? So, an additional 10% to 'get up there'?

 

[maby]

... OK - clearly there will be one, but I was watching the numbers yesterday and I was surprised at how great it is - has implications for assessing performance...

You know, I was able to push my old 1.8 tonne car on flat ground. Surprisingly, I can't bench or dead lift more than 100kg. Gravity, hey?

You know that to overcome wind resistance and frictional resistance for most cars at 60km/h is in the single digit kilowatt range? But a decent hill climb on a 1.8 tonne car can push you over 100kw just to maintain speed.

Put into that perspective, maybe it should be surprising that hills only cost 25% more.

We are talking about 150 m over 25 miles. That is roughly a 1:250 ratio, which will add 7 kg gravitational force, or something like that? Sure you can push your car up a 1:250 ratio.

EDIT: To lift 1800 kg up 150 m you need 1800 kg * 9,81 m/s2 (at least where I live) * 150 m = 2648700 Joule. This is the equivalent of approx. 0.74 kWh. Lets say a liter of petrol contains about 9.7 kWh of energy and burns at an efficiency of 25% (which is a negative estimation), then you need about 300 cc of petrol to concur gravity. Normally, 25 miles will take about 3 liters or so? So, an additional 10% to 'get up there'?

That is an interesting calculation which puts it in context - allowing for inaccuracies in my observations, in the car's instruments and other variables, my apparent 25% impact on fuel economy is not out of the question. Naively I would have thought that the effect of such a small gradient would be lost in the noise!

 

[Sunder]

We are talking about 150 m over 25 miles. That is roughly a 1:250 ratio, which will add 7 kg gravitational force, or something like that? Sure you can push your car up a 1:250 ratio.

EDIT: To lift 1800 kg up 150 m you need 1800 kg * 9,81 m/s2 (at least where I live) * 150 m = 2648700 Joule. This is the equivalent of approx. 0.74 kWh. Lets say a liter of petrol contains about 9.7 kWh of energy and burns at an efficiency of 25% (which is a negative estimation), then you need about 300 cc of petrol to concur gravity. Normally, 25 miles will take about 3 liters or so? So, an additional 10% to 'get up there'?

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 reminds me of a running joke in one of the world's toughest marathons. Winners tend to finish in the low 3h mark, rather than the typical low 2h mark. Yet it's marketed as:

The course climbs a total of 1,528m and drops a total 1,788m giving a net drop of 260m (it's a downhill course !!)

http://www.sixfoot.com/index.php/the-course/course-details

 

[greendwarf]

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.

 

[anko]

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.

 

[Sunder]

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.

 

[Neverfuel]

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.

 

[maby]

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.

 

[Sunder]

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.

 

[anko]

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.

 

[greendwarf]

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

 

[Sunder]

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.

 

[Sunder]

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.