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Radio 2's homwork query today at 6.45pm was 'at what speed would a car cover a set distance using the least fuel?'
The answer was rather flimsy, but apparently 52-56 mph [ignoring all the car variables etc]. Most time was spent waffling on about making sure the boot was empty and so forth.
But if I travel at 62 mph I arrive quicker so the engine would be burning fuel for less time. I don't think the expert had thought it through fully.
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I think you haven't thought it through properly.
Your argument basically states if you drive faster, you use less fuel.
Try driving a 100 mile trip at 50mph and then at 100mph (Law permitting) and share the results.
Edited by a900ss on 13/11/2012 at 22:18
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Radio 2's homwork query today at 6.45pm was 'at what speed would a car cover a set distance using the least fuel?'
For most cars, the answer is the slowest speed you can comfortably maintain in its highest gear. Modern cars with relatively high gearing means about 40 - 45 mph in top gear.
Not really practical if you are going on a 100 mile journey, and you would probably be pulled up on a motorway for going so slow.
The other question they should ask is "how valuable is your time?"
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the example does not factor in any hills.
if your driving at 56mph in top gear. there is hardly any torque to climb hills hence the economy drops.
my drive to and from work has some steep gradiants to contend with and i can prove that driving slower used more fuel to climb a hill.
if i was at the bottom of the hill at 70mph it was easier on the engine and used less fuel.
if i drove at 60mph the engine struggled to maintain speed and used more fuel
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If you keep it theoretical then the slower you go the less total fuel you'll use.
Main problem with theoretical exercises like that is that engine efficiency varies across the rev range so the torque curve of the specific engine can't be ignored.
I tow a caravan with a turbo-diesel automatic, limited to 60mph - I tried cruising at 56/58/60 mph using GPS and the cruise control - the most economical for my outfit is 58mph, better marginally than 56 (because it's nearer the BMEP Maimum point at 2,000 rpm) - but that can't be carried forward to any other combination.
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Anyone who has tried to gain fuel economy knows that "pulse and glide" is the way to go.. As diddy says, hills are the issue...
Hybrids don't do well in hilly country.
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ISTR that it is thought that the most efficient way to drive is with the engine at the RPM that gives maximum torque, broadly speaking.
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Hybrids don't do well in hilly country.
Rather the reverse actually, scavenging energy downhill on the overrun to play back later on an uphill drag is their meat 'n potatoes.
It's long, flat bits that they're rubbish at. There's some jiggery-pokery involving fiddling with revs ("energy recycling") which attempts to alleviate this, but it's not what they're cut out for.
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The point that's being missed is that aerodynamic drag scales with the CUBE of speed. Thus, once aerodynamic drag overtakes tyre rolling resistence and engine friction as the dominant force, it is game over.
For most cars this is depressingly low. Around 40mph.
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Nope, it's speed SQUARED.
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Yes, but Power=Fdrag*V
So power required to overcome aerodynamic drag scales to the cube of speed.
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Yes, but Power=Fdrag*V
So power required to overcome aerodynamic drag scales to the cube of speed.
Although my schoolboy physics is well rusty, that was always my understanding, and at that once 60mph (ish) is reached it's an almost exponential increase.
But as I say, it's well rusty....
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That's roughly the observed result.
But at low speeds, aerodymanic drag is trivial-although the power required to overcome aerodynamic drag still follows the cube function.
Just don't tell the mimsers that the most economical speed is ~40mph (or lower) in some cars!
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I guess every vehicle is different due to gearing and engine torque characteristics.
I have a Seat 1.9 diesel and 2000rpm in top gear equates to an indicated 70mph. The car does not feel happy pulling such a tall gear from lower revs, with peak torque at around 1900rpm, but presumably the aerodynamic drag at that speed would negate keeping the revs at peak torque or peak BMEP in top gear, so the thing to do would be to run at lower speed in a lower gear but still at peak torque, e.g: 60mph in 5th or 50mph on 4th, even though this seems slightly counter-intuitive?
Maintaining a constant speed for any distance would I guess be more of an issue on congested UK roads, especially motorways when running at less than 75-80mph means getting baulked by trucks and needing to accelerate to pass them...
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Squared works for purely aerodynamic "things" like aircraft.
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The equation for aerodynamic drive is:
Force=Coefficient of drag*effective frontal area of object*density of air*Velocity of object relative to air^2
Straight away, we can one of the reasons why economy is worse in winter-the air is denser.
But all these terms have a linear effect-with the exception of velocity.
So if you change the coefficient of drag from say 0.35 to 0.3 ie average to very good, it only reduces drag by about 14%. But if you increase your speed from 70mph by 14% to 80mph, your drag term increases by 30%.
So tweaking the shape of the car, or the gearing has a smaller effect than changing speed. This is why the maximum economy for all cars occurs over a fairly narrow range of speeds.
Alas.
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The equation for aerodynamic drive is:
Force=Coefficient of drag*effective frontal area of object*density of air*Velocity of object relative to air^2
That is the correct equation. If I may weigh in.....I did this topic for my PhD.
The car can be approximated to a half-ball shape; drag coeff for a hemisphere is about 0.4.
We're very limited with the range of drag coefficients based on shape, because the car must fulfil functions such as safety, having passengers etc. So speed is the key. It must, as all real-world systems, be a combination of all factors, and it would be impossible to calculate how they all fit together exactly. You've also got to factor in the Navier-Stokes equation......
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Hybrids don't do well in hilly country.
Rather the reverse actually, scavenging energy downhill on the overrun to play back later on an uphill drag is their meat 'n potatoes.
It's long, flat bits that they're rubbish at. There's some jiggery-pokery involving fiddling with revs ("energy recycling") which attempts to alleviate this, but it's not what they're cut out for.
Err..
Hybrids will do well in hilly country iif the hills are short as the downhill section recharges the battery. But as battery capacity is finite and as the battery can only - in theory- be charged to 100% - long downhills means the battery charges to its maximum capcity and then all the following charge is incapable oof beinmg stored.
So in the kind of hills we have - often miiles long and 50 to 150metres + high.. the hybrid cannot work as effectively as on slight gradients..
..
Edited by madf on 14/11/2012 at 14:00
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Only a900ss has picked up the point I was getting at, which is that m.p.g. is not the only factor determining fuel use (as the proponents of stop/start have shown).
However, although a 100 mph journey over 100 miles would take 1 hour, and not the 2 hours it would take at 50mph, even I can see that spending an extra hour in the car would be worth it - fuel-wise at least.
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Oh indeed. These sums are all being carried out under ideal conditions, with everything else held constant.
Driving at 40 probably won't get your oil or tyres up to operating temperature which means more friction and more frequent oil changes. Diesels will have to waste more fuel on DPF regens.
The fuel savings of driving at 50 vs 100 are worthwhile. But if you compare 50 to 70, the economic case is less clear. How much do you value your time?!
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Only a900ss has picked up the point I was getting at, which is that m.p.g. is not the only factor determining fuel use (as the proponents of stop/start have shown).
However, although a 100 mph journey over 100 miles would take 1 hour, and not the 2 hours it would take at 50mph, even I can see that spending an extra hour in the car would be worth it - fuel-wise at least.
??
MPG is everything on a fixed journey as far as fuel consumed goes - distance divided by MPG = fuel used.
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Only a900ss has picked up the point I was getting at, which is that m.p.g. is not the only factor determining fuel use (as the proponents of stop/start have shown).
However, although a 100 mph journey over 100 miles would take 1 hour, and not the 2 hours it would take at 50mph, even I can see that spending an extra hour in the car would be worth it - fuel-wise at least.
??
MPG is everything on a fixed journey as far as fuel consumed goes - distance divided by MPG = fuel used.
The OP is trying to make the point that total fuel used could be lower when driving faster as the total time the engine is running is shorter.
I disagree hence my post earlier (the immediate post after the OP's first post).
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If Bobbin and Unthrottled agree, they are almolst certainly right: Bobbin is a physicist and Unthrottled either a physicist or an engineer, possibly both.
Forgive me for a typical arts graduate's viewpoint, but I suspect that as so often there isn't an absolutely right answer. Different types of engine may vary in their optimum speed / economy formula, and even within the same type a torquey petrol engine like mine (VAG 2.0 TSI) may differ from, say, Honda or Alfa engines which thrive on high revs.
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Well, as you're an accountant, think of air resistance as inflation. Inflation is one variable of many in an economic system, but once inflation starts to run away, it becomes the dominant force in an economy and changing other variables has little effect.
You're absolutely right in that there is no magic single number. But all the types of car on the road have a maximum economy within a fairly small range. Given the massive variation in engine characteristics, body styles etc, that is quite an incredible result.
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Quote:..... (re hybrids) ""It's long, flat bits that they're rubbish at. There's some jiggery-pokery involving fiddling with revs ("energy recycling") which attempts to alleviate this, but it's not what they're cut out for.""
Depends on the type of hybrid. For the type where electric and mechanical drive is combined, surely a direct mechanical drive for cruising on motorways would be a good idea - either selectable or engaged automatically. Perhaps some hybrids have this, I don't know.
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