Fuel economy and acceleration

[Kimosabe]

It is said that flooring (in each gear if you have a car with a manual gearbox designed by victorians) it upto the speed limit and then maintaining that speed for as long as possible is a more fuel efficient method than slowly reaching the desired speed and therefore holding it for less time over the same route or distance.

I would like to see the evidence compared for each model of hare/tortoise driving and also to consider the wear on the engine, gearbox, tyres etc.

http://www.efficient-mileage.com/acceleration-test.html


Tesla cars and free solar charging: https://medium.com/editors-picks/32eeaacc207a

[Gareth]

It is said that flooring (in each gear if you have a car with a manual gearbox designed by victorians) it upto the speed limit and then maintaining that speed for as long as possible is a more fuel efficient method than slowly reaching the desired speed and therefore holding it for less time over the same route or distance.

It's pretty academic but, anyway, the two cases aren't the same because the slow acceleration method means the journey takes longer.

If you want to compare like for like, you'd need to make the journey time the same. All the things being equal, and they're almost certainly not, using the slow acceleration option you'd need to use a higher top speed. The higher top speed may be less efficient because efficiency losses due to wind resistance would probably out-weigh the gains from less acceleration over a longer period.

[Kevin]

If you want to compare like for like, you'd need to make the journey time the same

Fuel efficiency is usually measured in miles per gallon, not minutes per gallon. Is not Kimosabe asking about fuel consumption over a given route or distance?

over the same route or distance.

[Gareth]

Is not Kimosabe asking about fuel consumption over a given route or distance?

He could be but where do you draw the line? With slow enough acceleration a journey could take much much longer - would there be any point to the comparison?

Kimosabe added an unnecessary constraint when he said that the brisk acceleration option used the speed limit as the peak cruising speed, which necessarily means a slower acceleration option will result in a longer journey time if also constrained by the speed limit. I wasn't sure whether he'd considered that aspect.

Anyway, to be useful in terms of practical application, constant journey time makes much more sense and helps to clarify some of the issues.

[jont]

Is not Kimosabe asking about fuel consumption over a given route or distance?

He could be but where do you draw the line? With slow enough acceleration a journey could take much much longer - would there be any point to the comparison?

Or you could walk/cycle - or if time is no object at all even take public transport

[TR4ffic]

As with most things, isn't the answer found in the compromise..?

Brisk but steady acceleration as opposed to ‘slow’ or ‘flooring it’.

Planning your progress so that, if you can see you need to slow down ahead, don’t accelerate.

Picking an appropriate speed for the conditions; not necessarily the speed limit.

Less braking means you’re not scrubbing off speed you’ve just paid (in fuel used) to gain*

Appropriate use of engine braking where modern engine management cuts off fuel supply completely*

* ...and I'm not advocating practically no braking and endless downshifts thro the 'box

You could go on forever with the ‘journey completed over the same timeframe’ or ‘not exceeding the same top speed/speed limit’ discussion…

[jcochrane]

It is said that flooring (in each gear if you have a car with a manual gearbox designed by victorians) it upto the speed limit and then maintaining that speed for as long as possible is a more fuel efficient method than slowly reaching the desired speed and therefore holding it for less time over the same route or distance.

I would like to see the evidence compared for each model of hare/tortoise driving and also to consider the wear on the engine, gearbox, tyres etc.

http://www.efficient-mileage.com/acceleration-test.html


Tesla cars and free solar charging: https://medium.com/editors-picks/32eeaacc207a

Just to complicate things there is another way. Short maximum acceleration (foot on the floor) right at the start of the acceleration phase followed by an early and long complete lift off. If completely off in many newer cars I'm told it will actually cut off the fuel supply. So maybe this could be quite a good technique for these cars.

In my own car if I want the best mpg then cruising at 50mph in 6th gear on motorways probably gives the best compromise of fuel economy and journey time. However as I don't do massively high mileage on motorways each year driving this way, when on a motorway, would have little impact on my yearly annual fuel costs.

I'm not entirely sure that for, many non very high mileage drivers, how the throttle is used makes a significant difference to the overall cost of car ownership.

The link you provide compares throttle openings/fuel used for 0-30. A maximum throttle opening from a standstill (drag racing style) I would hope that for road driving this style of "getting away from the traffic lights" is not applicable. I would imagine that constantly accelerating from a standstill like this could well have adverse effects on wear and tear on tyres and car.

[trashbat]

Wide open throttle is the most efficient way to produce power.

However, this is not the same thing as the most efficient way to use fuel.

Brake specific fuel consumption is part of the answer: http://www.autospeed.com/cms/article.html?&A=112611

[TR4ffic]

Brake specific fuel consumption is part of the answer: http://www.autospeed.com/cms/article.html?&A=112611

Thanks for that... A nice conversation piece for the pub tonight...

An interesting point in the article re the Prius (which I drive - although it’s a CVT transmission which wasn't part of the OP); I find best results are achieved by accelerating fairly firmly up to your desired speed - two reasons for this, firstly, the CVT (what I describe as) 'digs in' to improve acceleration (under light acceleration, engine revs can be quite high for little improvement in speed) and, secondly, once at your desired speed, if you lift off the gas a fraction the hybrid drive (including electric motor) will maintain the speed.

Perhaps I’ve found this BSFC ‘sweet spot’ without realising it. I have recently achieved 70.1 mpg over nearly 400 miles (ave speed 38 mph) on my daily commute. Sorry – Didn’t want to be an mpg bore

[TripleS]

I have recently achieved 70.1 mpg over nearly 400 miles (ave speed 38 mph) on my daily commute. Sorry – Didn’t want to be an mpg bore

Quite right; leave the boring stuff to me. In fact I can be exceedingly boring on a variety of topics, not just my eco-driving.

70.1 mpg though. Hmm, yes, I like that. Much more relevant than all this 'making progress' business.

Best wishes all,
Dave.

[Kimosabe]

So is it better in terms of fuel economy to :

a) spend longer at the desired target speed ?
or
b) spend less time at the desired target speed ?

I was told answer 'a' is correct (and that sounds about right to me) but want to know what lies behind it before quoting it to others. The fuel cut off answer seems about right.

ps: BSFC charts = good!

[Astraist]

The article supplied adresses the subject of acceleration rate through a simple mechanical point of view. However, when it is put into context during road driving, things become a bit more complex. Sudden acceleration tends to be less accurate, especially since it correlates with 'edgy' or 'rushed' driving.

Therefore, it is feasible to assume that sudden acceleration will push the car to a speed well over the desired speed or the speed in which the traffic flows. This will be followed by deceleration and than be re-acceleration. This pattern of accelrating and decelerating ad infinitum to maintain a certain progress instead of coasting through a constant, linear, speed - would produce very low fuel to mileage efficiency.

Also, sudden acceleration leaves little room for abortion before the 'desired' speed is reached. If the conditions around the driver change in a way that makes the desireable speed too fast, than the driver would find himself already too fast, where in a more brisk acceleration he might be able to abort the acceleration in just the right timing to be right on the required speed.

Also, I don't believe that wear and tear can really be distinguished from fuel efficiency. Sudden acceleration, as a habit, and particularly if opening the throttle is unsmooth, will cause much more wear and tear to the engine, especially if this habit is done when the motor is cold.

We also failed to consider that acceleration to a higher speed would mean more fuel loss through speed itself, higher revs in automatic or automated gear boxes or simply through the lack of accuracy caused by rapid acceleration and a quick rise in the RPM, the effects of acceleration to a higher speed, the dependency on road conditions (wind, high friction surfaces), the car (lorries, different tyres) and other factors.

Overall, rationale and empirical experience as well as personal observations show that a smoother, more linear and yes - a more gentle approach, leads to higher fuel-milleage efficiency.

[Kimosabe]

I'm so glad everyone missed my reference to manual gearboxes being made by 'The Victorians'.

[Mr Cholmondeley-Warner]

Not missed, just ignored

When you get more drawn in by this AD lark, you'll start wanting a car with a Victorian gearbox again

[Standard Dave]

I'm so glad everyone missed my reference to manual gearboxes being made by 'The Victorians'.

As opposed to automatics gear boxes driven by people born in the Victorian age.