Fuel price discussion thread (was ‘chaos’)
- Thread starter watercooledman
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Completely made up, why would there be more friction when the engine is going the same speed and the wheels are going slower?? and why are you generating more torque when you are traveling slower?
Im seriously amazed that you guys have been driving for how long? and haven't noticed that driving slower on the motorway uses less fuel. Instead we are making up formulas to try and prove physics wrong. Lower engine speed less friction, lower vehicle speed less drag. Engine speed needs to be around optimum BSFC for really optimal but this concept is why idling at 50mph wouldnt work either.
The point made about accelerating slowly is a different discussion and based on how ICE work rather than speed vs drag.
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we are talking about the frictional energy(engine/transmission) losses during the time is takes to drive a mile (mpg efficiency) not the frictional energy expended over time, which would, indeed, be greater for the 6th gear/1500rpm/faster situation. - #notdissingnewton.
Who is 'we'?
It was you who said more friction in 1st gear to cover a mile! (not time) I quote your post below which i assume you made rather than copy and pasted from somewhere else? unless you are going to argue 50J is more that 2000J?
"so to travel a mile in 1st @1500 say 2000J friction + 50J aero (driving that mile might take 10minutes?)
6th @1500 50J friction + 1000J aero"
Its literally painful how much you dive in to details to try and find holes in posts, in this case the subject is about how to save fuel (eg drive slower!) and maybe help some people, eg try 65mph rather than 75mph on the motorway, or stop ragging away from lights to save fuel. Yet you turn up with some Joules conversation which will be lost on most people.
it was budforces question mpg at 1500rpm 1st vs 1500rpm 6th so, related to fuel saving
you can't escape from talking about energy used to cover the distance , which relates to mpg, on ev topic people are familiar with talking about Kwhrs of energy, but using those might have conflated topics, so I went with joules, you could use CC's of petrol
frictional energy, to cover the mile in 1st is more than 6th 2000>50 ? I don't see the issue.
so, to cover the mile in 1st1500rpm you use 2000+50=2050J , and in 6th1500rpm 50+1000=1050J, so your mpg is theoretically nearly doubled at 60mph (e: the 6th gear case)
(e2: or if you did the mile in 5th@1500 say 40mph , perhaps friction is 100, aero 800, so total is 900J - the winner)
If I had found somewhere to paste from I would have done so/linked it; I'd like to see real car data.
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You have made up frictional Energy numbers. How can it be less at higher speed ? I see you have now changed your tune to talk about friction and aero drag…. Why is friction 2000 then 50 at higher speeds. Stop making things up
I have real car data. It’s called driving your car at different speeds and see the difference. Complicated I know.
I have real car data. It’s called driving your car at different speeds and see the difference. Complicated I know.
Ok humour me again then.
If you are making a journey that is 60 miles.
If you drive at 60mph the entire way, it'll take you 1 hour.
If you drive at 30mph, it'll take you 2 hours.
Hopefully I am right so far
But your cars engine uses fuel just by running, on the second trip it's running for twice as long, right so far?
I'm not saying you will use twice as much fuel doing the trip at 30mph, but you still need to factor that in? No?
If you are making a journey that is 60 miles.
If you drive at 60mph the entire way, it'll take you 1 hour.
If you drive at 30mph, it'll take you 2 hours.
Hopefully I am right so far
But your cars engine uses fuel just by running, on the second trip it's running for twice as long, right so far?
I'm not saying you will use twice as much fuel doing the trip at 30mph, but you still need to factor that in? No?
On a completely flat 60 mile road no interruptions you would likely use less fuel in 2 hours
If left idling for 2 hours vs 1 it would have course use more, as mentioned above it is the miles you travel for the gallon that give you the indicator for efficiency.
You can see that when you look at an instantaneous fuel gauge, when I am pootling along on a 30mph road maintaining momentum my car is saying ~70+mpg, when I am doing the same at 60 it drops to ~50odd, when I get on the turbo and go full knob mode, well lets not talk about that
If left idling for 2 hours vs 1 it would have course use more, as mentioned above it is the miles you travel for the gallon that give you the indicator for efficiency.
You can see that when you look at an instantaneous fuel gauge, when I am pootling along on a 30mph road maintaining momentum my car is saying ~70+mpg, when I am doing the same at 60 it drops to ~50odd, when I get on the turbo and go full knob mode, well lets not talk about that
Another way to look at it would be. If me and you have identical cars each filled with exactly 1 gallon of fuel, if I drive at 50 mph which gets me 50 mpg and you travel at 70 which gets you 40mpg what will happen?
You will take off and leave me behind but eventually you will run out of fuel, I will pass you a few minutes later doing the clarkson smug face and will carry on for another 10 miles.
You will take off and leave me behind but eventually you will run out of fuel, I will pass you a few minutes later doing the clarkson smug face and will carry on for another 10 miles.
NoOk humour me again then.
If you are making a journey that is 60 miles.
If you drive at 60mph the entire way, it'll take you 1 hour.
If you drive at 30mph, it'll take you 2 hours.
Hopefully I am right so far
But your cars engine uses fuel just by running, on the second trip it's running for twice as long, right so far?
I'm not saying you will use twice as much fuel doing the trip at 30mph, but you still need to factor that in? No?
Caporegime
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But have you taken into account that he will free-wheel far further when he runs out?
Yea sorry, I understand the mpg calculation you see has already factored that in, it is what it is.
I understand if it's showing 70mpg at 30 mph and 35mpg at 60mph, your twice as efficient doing 30, and it's not the same because doing it at 60 took you half the time, I 100% get that.
I think where I was going, is behind the mpg calculation, part of that calculation will include just the running of your engine regardless of speed, so at some point, if you go slowly enough there will be a point where your efficiency drops?
I understand if it's showing 70mpg at 30 mph and 35mpg at 60mph, your twice as efficient doing 30, and it's not the same because doing it at 60 took you half the time, I 100% get that.
I think where I was going, is behind the mpg calculation, part of that calculation will include just the running of your engine regardless of speed, so at some point, if you go slowly enough there will be a point where your efficiency drops?
The problem is turning it into actual real world meaningful gains on a larger scale and without other potential unintended side effects.
Dropping all the speed limits indiscriminately by 10 MPH for instance probably won't have the effect it might look like on paper.
Predominantly because people ignore speed limits now never mind ones that have been reduced with no real substance behind it, I mean if people want to save money/use less fuel, it is obvious to drive slower, well to many, perhaps some in this thread need a limit or some Eco driving tips though
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Might have a stronger effect instead
More cars fit on the road at lower speed.
And there will be less spread between low speed and high speed lanes.
So lowering speed limits will indirectly reduce congestion, which also improves average fuel consumption
unless everyone overthinks the problem
Problem is it is something you do need to "overthink" to actually turn the science and ideals into real world gains.
For instance many of the articles on it use smoothed graphs and scales which suit whichever side of the argument they are presenting - if you look at the raw data most car's fuel economy is actually quite notchy within the "green zone" and there can be dramatic changes just from small MPH differences in some parts of a car's fuel economy vs speed graph (often around 40MPH for instance many cars can be worse than at ~30 and ~50-55 in some cases quite a bit worse but it is often smoothed out when trying to present the lower speed = better fuel economy argument) - coming back to the point sandys makes above - a good percentage of drivers on the road ignore speed limits either driving a bit above them or a bit below them - so if we dropped all 30 MPH limits down to 20 for instance then those who are exceeding the speed limit and pushing on towards 30 the difference in fuel economy vs them at 30 or even just above 30 as many do is actually very low, most cars you don't have to drop much below 20 before their fuel economy sinks massively - so those going a bit below 20 will end up causing significantly worse use of fuel than if the limit was 30. Despite the fact that if everyone was doing a smooth 20 it would save on fuel and then you have the reality that often in 30 zones it is stop-start traffic any way.
Keeping traffic moving does have by far the biggest impact on fuel economy - by far the biggest meaningful difference comes from people not constantly slowing and speeding up again - but it isn't as simple as just lowering speed limits - that can work in some environments and layouts better than others. Generally it seems to work quite well on the M25 for instance but in other places where you can't take advantage of shorter safety distances, etc. it might actually go the other way and reduce how many vehicles can get through in a given time or result in vehicles actually driving at speeds which either make no material difference or is worse.
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