What's the most fuel efficient engine speed?
Discussion
Hi
It is a pretty low speed really.
All you need to do is gear it so that it can maintain 56mph / 90kmh with the max amount of throttle (without triggering any enrichment for full throttle) with the least revs. You might well land up with a car that in real life needs knocking down 1 or 2 gears if you actually want to maintain 56mph as a road cruising speed (as otherwise any minor hill would result in losing speed) but it would have given a very good figure for the headline steady 56mph fuel consumption.
Drag does increase rapidly with speed (equivalent to the square of the speed) but at 56mph is is still fairly low. Rule of thumb the thrust to overcome rolling resistance is 1% of the weight in pounds, so with a (say) 3000lb car you need 30lbs of thrust to overcome rolling resistance, and that is pretty much constant with speed. Until about 40mph that constant rolling resistance is the largest factor (34lbs of thrust to overcome wind resistance at 40mph for a car with a cda of 8 - within reason). At 30mph the total thrust needed is about 49lbs (30 for rolling resistance, 19 to overcome drag), at 56mph the total thrust needed is about 97lbs (30 for rolling resistance, 67 to overcome drag) so 86% more speed for 98% more thrust, while at 70 it is 135lbs (30 for rolling resistance, 105 to overcome drag).
All the best
Keith
It is a pretty low speed really.
All you need to do is gear it so that it can maintain 56mph / 90kmh with the max amount of throttle (without triggering any enrichment for full throttle) with the least revs. You might well land up with a car that in real life needs knocking down 1 or 2 gears if you actually want to maintain 56mph as a road cruising speed (as otherwise any minor hill would result in losing speed) but it would have given a very good figure for the headline steady 56mph fuel consumption.
Drag does increase rapidly with speed (equivalent to the square of the speed) but at 56mph is is still fairly low. Rule of thumb the thrust to overcome rolling resistance is 1% of the weight in pounds, so with a (say) 3000lb car you need 30lbs of thrust to overcome rolling resistance, and that is pretty much constant with speed. Until about 40mph that constant rolling resistance is the largest factor (34lbs of thrust to overcome wind resistance at 40mph for a car with a cda of 8 - within reason). At 30mph the total thrust needed is about 49lbs (30 for rolling resistance, 19 to overcome drag), at 56mph the total thrust needed is about 97lbs (30 for rolling resistance, 67 to overcome drag) so 86% more speed for 98% more thrust, while at 70 it is 135lbs (30 for rolling resistance, 105 to overcome drag).
All the best
Keith
Dracoro said:
In general, as low a speed as possible but above the speed where the engine would labour.
As for the peak torque b
ks, that would mean me driving around at 7500rpm all the time 
There two things going on here though, are you asking which is the best for mpg or which is the most efficient (i.e. best performance per unit of fuel used etc.).
yours makes peak torque at 7500rpm? not peak BHP ?As for the peak torque b
ks, that would mean me driving around at 7500rpm all the time There two things going on here though, are you asking which is the best for mpg or which is the most efficient (i.e. best performance per unit of fuel used etc.).
why 56 miles an hour ? just cos trucks are limited to that speed does not make it the ideal speed for peak efficiency, it will vairy with final drive and gearbox ratios load on engine IE uphill or downhill i think it depends on VE of the engine + gearing + drag
Edited by NHK244V on Monday 7th November 14:01
NHK244V said:
Dracoro said:
In general, as low a speed as possible but above the speed where the engine would labour.
As for the peak torque bks, that would mean me driving around at 7500rpm all the time
There two things going on here though, are you asking which is the best for mpg or which is the most efficient (i.e. best performance per unit of fuel used etc.).
yours makes peak torque at 7500rpm? not peak BHP ?As for the peak torque b
ks, that would mean me driving around at 7500rpm all the time There two things going on here though, are you asking which is the best for mpg or which is the most efficient (i.e. best performance per unit of fuel used etc.).
Torque curve quite flat though, so much of that peak figure is available low down too.
kambites said:
And the answer is "it depend on the car but probably somewhere around 40mph". At least in my experience.
Yes, that's right. Fast enough that the engine/gearbox combination is working efficiently but you have to keep the speed well down to avoid the massive build-up of wind resistance as speed increases.The power needed to overcome wind resistance rises as the cube of any speed increase - which is mahoosive. Hence 80 bhp shopping cars can pull 100 mph but there's not a hope in hell of getting a sleek sports car to 200 mph with much less than 500 bhp.
NHK244V said:
why 56 miles an hour ? just cos trucks are limited to that speed does not make it the ideal speed for peak efficiency
Quite agree, but one of the headline figures that used to be used for fuel consumption was measured at a steady 56mph. Hence some stuff optimised for it. Then some people seem to think that as it is a fairly odd number it must be a specific speed where all cars are at their most efficient, rather than just being the mph equivalent of 90kmh (common continental A road speed limit).Ozzie Osmond said:
The power needed to overcome wind resistance rises as the cube of any speed increase - which is mahoosive.
Square of the speed rather than cube.All the best
Keith
Classic Grad 98 said:
allgonepetetong said:
...So I thought I would set the cruise at 65 to ensure i could get home without running out of fuel.
Using cruise control is the last thing you want to do- you will loose whole miles from your gallon, especially in hilly areas.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).
Classic Grad 98 said:
allgonepetetong said:
...So I thought I would set the cruise at 65 to ensure i could get home without running out of fuel.
Using cruise control is the last thing you want to do- you will loose whole miles from your gallon, especially in hilly areas.I would say under 60mph for an economy drive , or buy a diesel and don't worry about it ?
A labouring engine isn't efficient. You can bet your life that in top gear at 'economical' speeds, the engine will be below it's torque band, and therefore a wider throttle opening will be required to maintain a constant speed.
Anticipating the hill, allowing the car to build up some momentum on the downward slope, and then allowing the speed to gradually fall on the incline avoids the scenario where the dumb cruise control realises it's met with an incline, and opens the throttle much wider to maintain the speed.
Anticipating the hill, allowing the car to build up some momentum on the downward slope, and then allowing the speed to gradually fall on the incline avoids the scenario where the dumb cruise control realises it's met with an incline, and opens the throttle much wider to maintain the speed.
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.
Kickstart68 said:
Square of the speed rather than cube.
All the best
Keith
It seems to be a matter of "definition"!!All the best
Keith
- 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.
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
Surely it also depends on how you drive. All of the above is assuming a steady speed and a steady throttle, which doesn't often occur in the real world. If you need more or less throttle (to cope with hills for instance), the whole ballgame changes. For example, I think my 320d is efficient in 5th gear in a 30 limit at 1100rpm, but as soon as I need any throttle I'd be far better off in 4th or even 3rd.
A friend of mine calibrates engines for a leading global car manufacturer, he told me that you need to know the 3D fuel map for your particular car.
The engines I use at work are at their most efficient at peak torque, but you need to have them flat out 2300rpm and load the engines so they get pulled back to about 1700 rpm. They are using a huge amount of fuel, but doing a huge amount of work.
The engines I use at work are at their most efficient at peak torque, but you need to have them flat out 2300rpm and load the engines so they get pulled back to about 1700 rpm. They are using a huge amount of fuel, but doing a huge amount of work.
Willy Nilly said:
A friend of mine calibrates engines for a leading global car manufacturer, he told me that you need to know the 3D fuel map for your particular car.
The engines I use at work are at their most efficient at peak torque, but you need to have them flat out 2300rpm and load the engines so they get pulled back to about 1700 rpm. They are using a huge amount of fuel, but doing a huge amount of work.
The engines I use at work are at their most efficient at peak torque, but you need to have them flat out 2300rpm and load the engines so they get pulled back to about 1700 rpm. They are using a huge amount of fuel, but doing a huge amount of work.
It's all about the BSFC plot.
Even if you had your engine 'at peak torque', it's only peak torque at wide open throttle, so then you'd accelerate anyway (unless you're going up a hill that puts the perfect opposite load on the engine).
In reality, what someone said earlier of "about 40 mph in top gear" won't be far wrong.
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