What's the most fuel efficient engine speed?
Discussion
Ozzie Osmond said:
It seems to be a matter of "definition"!!
Mmm, definition wise I think it is because power is a rate of doing work, so factors in time already. Hence if you double the speed you need 4 times the force to overcome the drag (ie, the square) but you are also doing it more rapidly (making it the cube), but as you won't be doing it for as long to cover the same distance that last part disappears.- Power to overcome air resistance increases roughly with the cube of the speed, and thus
- Energy required per unit distance is roughly proportional to the square of speed.
All the best
Keith
5lab said:
BarnatosGhost said:
5lab said:
disputable. that claim relies on the 'truth' that slowing down up hills, and speeding up down them, is a good thing to do.
Certainly, not using the brakes at all, and maintaining the highest possible gear are good things to do for economy runs, but slowing down up hills is something I can't understand the logic behind (as long as you're not dropping a gear to do so).
It's because adding, say, 10mph to your speed needs just a little squirt on the throttle going downhill, but a prolonged, flat-out drag when going uphill. That same principle applies to maintaining a steady speed.Certainly, not using the brakes at all, and maintaining the highest possible gear are good things to do for economy runs, but slowing down up hills is something I can't understand the logic behind (as long as you're not dropping a gear to do so).
If you can ease off the throttle before the top of the hill and coast to the top then you get the additional altitude (potential energy) at the cost of your momentum, but not at the cost of your fuel.
Once on the way back down, you can replace your lost momentum by burning fuel - but to do so you use a lot less fuel as you're also expending some of your potential energy.
Slowing-down going uphill and speeding-up going downhill creates the net effect in fuel-consumption terms of 'flattening out' the countryside.
Smaller uphills, and smaller downhills = less overall work.
It'll always take xx kw of power to lift the car 1000ft (someone good at maths can work out how much). I can't see how doing that at 60mph uses more fuel than at 50mph, other than the additional fuel used by driving on the flat at 60mph vs 50mph
The rationale being that in the first instance you've gained the potential energy without losing the momentum - in order to do that energy has to have been expended from somewhere - the fuel.
In the second case the additional potential energy has partially been 'earned' from the reduced momentum, so that same value hasn't been spent in lost fuel.
If we can also agree that speed is 'cheaper' or 'easier' to gain when going downhill than uphill (short squirt on the throttle vs long press on throttle) then I think the case is proven.
Momentum lost going uphill is 'cheap' to replace when going downhill, compared to the longer periods on-throttle spent trying to keep the momentum equal all times.
Practical experiment (if you have a instant consumption display) - drive a steady 2 mile hill at a steady 60mph - you might be down to 20mpg for a mile and then up to 60mpg on the way down for a mile.
Now drive that same hill by starting at 70mph dropping steadily to 50mph at the top. You'll average maybe 30mpg on the way up. At the top of the hill a squirt of throttle (maybe 15mpg for 0.05 of a mile) brings you quickly back to 60mph and 60mpg for the downward mile. Average speed is the same, but less fuel is used.
That's why cruise control harms fuel economy.
BarnatosGhost said:
That's why cruise control harms fuel economy.
...but does make the situation safer overall. It is better to achieve and maintain a steady speed (assuming road traffic allows) than to vary the speed due to inclines etc. in the same way you will be able to get a tow from heavy if you get close enough but the savings are insignificant compared to the inevitable repair costs. blank said:
It's all about the BSFC plot.
For real world, the point at which rolling resistance from drivetrain/tyres = aerodynamic drag normally is the most efficient speed to go at, which is typically around 40mph. On a motorway, 56mph a safe distance behind a lorry will be the most efficient, safe thing to do IMO.
Starfighter said:
BarnatosGhost said:
That's why cruise control harms fuel economy.
...but does make the situation safer overall. It is better to achieve and maintain a steady speed (assuming road traffic allows) than to vary the speed due to inclines etc. in the same way you will be able to get a tow from heavy if you get close enough but the savings are insignificant compared to the inevitable repair costs. (love cruise, me)
Ozzie Osmond said:
BarnatosGhost said:
love cruise, me
Did you know there are some muppets who want to ban Cruise in UK? Apparently it's being linked to some accidents and the arguments are that a) our roads are too crowded, and b) drivers lose concentration.... BarnatosGhost said:
Ozzie Osmond said:
BarnatosGhost said:
love cruise, me
Did you know there are some muppets who want to ban Cruise in UK? Apparently it's being linked to some accidents and the arguments are that a) our roads are too crowded, and b) drivers lose concentration.... IIRC, on average 1600rpm is the most efficient operating speed on average for a diesel engine, and just a little more, 1800rpm for a petrol engine.
So probably around 40mph for a small petrol hatch, 75-80mph for an Audi A8 4.2TDI (55mph/1000rpm in top)
Obviously drag is an issue, and there comes a point where the loss in fuel efficiency created by drag overcompensates for the gain created by tall gearing.
So probably around 40mph for a small petrol hatch, 75-80mph for an Audi A8 4.2TDI (55mph/1000rpm in top)
Obviously drag is an issue, and there comes a point where the loss in fuel efficiency created by drag overcompensates for the gain created by tall gearing.
If I was out in a desolate area and I suddenly noticed I was about to run out of fuel (and it was imperative that I got as far as I could before this happened) I would go as slow as it was possible to go in first gear (effectively a little above the engine idle speed). This would reduce the amount of energy lost to rolling resistance and air resistance and so reduce the amount energy being used to overcome these forces - leaving more energy available to move the car. This is not the engine's peak torque or peak HP.
It would be sensible to change up if I encountered a slope in my favour (that was sufficient to allow me to do this) but it would be even more sensible to allow the car to coast in neutral so long as your speed did not fall below the one you were travelling at on the flat.
This is the most efficient speed for the vehicle and it depends primarily on the idle speed of the engine and the ratio of first gear.
I think it is easy to get confused about efficiency as it somehow seems to involve the element of time and we assume that there is some trade-off between speed, time and efficiency - i.e. the less time you spend travelling the less time you have to use fuel and that there is an optimum point at which this trade-off ceases to work in your favour.
However, the equation for fuel efficiency is E = distance travelled / fuel used - time is absent.
Competitions for fuel efficiency usually stipulate minimum speeds to stop entrants going round at walking pace (or below) which was how some of the earliest records were set.
It would be sensible to change up if I encountered a slope in my favour (that was sufficient to allow me to do this) but it would be even more sensible to allow the car to coast in neutral so long as your speed did not fall below the one you were travelling at on the flat.
This is the most efficient speed for the vehicle and it depends primarily on the idle speed of the engine and the ratio of first gear.
I think it is easy to get confused about efficiency as it somehow seems to involve the element of time and we assume that there is some trade-off between speed, time and efficiency - i.e. the less time you spend travelling the less time you have to use fuel and that there is an optimum point at which this trade-off ceases to work in your favour.
However, the equation for fuel efficiency is E = distance travelled / fuel used - time is absent.
Competitions for fuel efficiency usually stipulate minimum speeds to stop entrants going round at walking pace (or below) which was how some of the earliest records were set.
Edited by jim23mac on Tuesday 23 September 17:01
jim23mac said:
If I was out in a desolate area and I suddenly noticed I was about to run out of fuel (and it was imperative that I got as far as I could before this happened) I would go as slow as it was possible to go in first gear (effectively a little above the engine idle speed). This would reduce the amount of energy lost to rolling resistance and air resistance and so reduce the amount energy being used to overcome these forces - leaving more energy available to move the car. This is not the engines peak torque or peak HP.
It would be sensible to change up if I encountered a slope in my favour (that was sufficient to allow me to do this) but it would be even more sensible to allow the car to coast in neutral so long as your speed did not fall below the one you were travelling at on the flat.
This is the most efficient speed for the vehicle and it depends primarily on the idle speed of the engine and the ratio of first gear.
I think it is easy to get confused about efficiency as it somehow seems to involve the element of time and we assume that there is some trade-off between speed, time and efficiency - i.e. the less time you spend travelling the less time you have to use fuel and that there is an optimum point at which this trade-off ceases to work in your favour.
The equation for fuel efficiency is E = distance travelled / fuel used - time is absent.
Competitions for fuel efficiency usually stipulate minimum speeds to stop entrants going round at walking pace (or below) which was how some of the earliest records were set.
Idling in 1st wouldn't be as good as idling in 2nd. In both cases air resistance is negligible, but in 2nd, each rotation of the engine takes you further up the road.It would be sensible to change up if I encountered a slope in my favour (that was sufficient to allow me to do this) but it would be even more sensible to allow the car to coast in neutral so long as your speed did not fall below the one you were travelling at on the flat.
This is the most efficient speed for the vehicle and it depends primarily on the idle speed of the engine and the ratio of first gear.
I think it is easy to get confused about efficiency as it somehow seems to involve the element of time and we assume that there is some trade-off between speed, time and efficiency - i.e. the less time you spend travelling the less time you have to use fuel and that there is an optimum point at which this trade-off ceases to work in your favour.
The equation for fuel efficiency is E = distance travelled / fuel used - time is absent.
Competitions for fuel efficiency usually stipulate minimum speeds to stop entrants going round at walking pace (or below) which was how some of the earliest records were set.
Dracoro said:
SpeckledJim said:
Idling in 1st wouldn't be as good as idling in 2nd. In both cases air resistance is negligible, but in 2nd, each rotation of the engine takes you further up the road.
But would be under more load so uses more fuel....allgonepetetong said:
My question is, what is the most efficient speed at which to cruise? (speed at which greatest distance travelled per unit of fuel)
Not quite as simple as that. Yes in general you need a low engine speed which doesn't labour, as stated above by another poster. But if the road goes downhill then let it speed up! Read the road ahead and slow down up a hill if you can carefully get to the top of it by doing that in the same gear .. etc .. etc.Even at constant speed, for any given speed the gear doesn't make as much difference as you might think - because you are moving the same car weight (mass) at the same speed with the same air resistance and the same road resistance - all you will change by altering the gear is the piston speed and internal engine losses - which does make a difference, but you'd lose more by speeding up.
There really isn't a simple answer!
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