Progress at any Cost?

[jcochrane]

Having tried various things I've found I get marginally better mpg by taking a gear, immediately putting my foot to the floorboard then lifting back up to manage the acceleration. I will even use this technique commuting to London. Apart from mpg I'm getting from A to B in the shortest time without using higher speeds. I call that a win win situation.

[Gareth]

Let me put it another way.

The problem is you've picked an arbitrary constraint, a fixed cruising speed, and you're willing to adjust everything else to find the most economical way to use that fixed cruising speed. You're even willing to ignore variations in journey time.

The degree to which it is arbitrary is illustrated by your unwillingness to choose a lower cruising speed, which would be even more economical.

I can see that it makes a fun thought experiment but apart from that I see little value. Most people are either aiming to improve economy or to complete the journey within a fixed time.

[Kevin]

Let me put it another way.

The problem is you've picked an arbitrary constraint, a fixed cruising speed, and you're willing to adjust everything else to find the most economical way to use that fixed cruising speed. You're even willing to ignore variations in journey time.

The degree to which it is arbitrary is illustrated by your unwillingness to choose a lower cruising speed, which would be even more economical.

I can see that it makes a fun thought experiment but apart from that I see little value. Most people are either aiming to improve economy or to complete the journey within a fixed time.

I just wanted to focus on the acceleration bit, not on the whole drive. I agree that you can modify numerous things to improve economy, but I just want to analyse this particular aspect. My question is, what's the most economical way to accelerate? Quickly or slowly?

[GJD]

I would think the first example would be the most economic, but I'm not clever enough to do the calculations. Has anyone done the maths and come up with an answer? If they have, please show your workings.

I don't know the answer, but I'd note that for a given length of straight, the quicker you accelerate to your chosen cruising speed, the less distance you will cover in the acceleration phase, so the longer you will be maintaining that speed before it's time to slow down again for the next thing. The quicker you accelerate, the more fuel you will use in your cruise phase simply because it will be longer.

[YorkshireJumbo]

I would think the first example would be the most economic, but I'm not clever enough to do the calculations. Has anyone done the maths and come up with an answer? If they have, please show your workings.

I don't know the answer, but I'd note that for a given length of straight, the quicker you accelerate to your chosen cruising speed, the less distance you will cover in the acceleration phase, so the longer you will be maintaining that speed before it's time to slow down again for the next thing. The quicker you accelerate, the more fuel you will use in your cruise phase simply because it will be longer.

The problem is that maintaining a cruise uses much less fuel than accelerating - just watch an mpg display during acceleration. Over the years, I have seen stories that both ways of accelerating are more economical than the other, so I would guess that there are too many other variables to make it certain one way or the other.

My personal opinion is that accelerating gently in a higher gear is closer to cruising than hard acceleration, so would tend to be more economical, plus there's less stress put on the mechanical parts and the driver. The most economical drives I've had have been on busy motorways where the traffic is travelling at a fairly constant 50-60mph, whereas my least economical have been on busy A-roads where I've tried to maintain progress despite the Sunday drivers (and the protests from the kids in the back)

[Kevin]

The quicker you accelerate, the more fuel you will use in your cruise phase simply because it will be longer.

And would the converse apply? The slower you accelerate the more fuel you will use in your acceleration phase simply because it will be longer. There must be a trade off between the two, but where does that trade off lie?

There must be some evidence out there somewhere that has established the facts.

[GJD]

The quicker you accelerate, the more fuel you will use in your cruise phase simply because it will be longer.

And would the converse apply? The slower you accelerate the more fuel you will use in your acceleration phase simply because it will be longer.

No idea I'm afraid.

There must be a trade off between the two

*If* hard acceleration to a speed is more fuel efficient than gentle acceleration to that speed, then there will be a trade-off between the more efficient acceleration phase vs the longer cruise phase.

but where does that trade off lie?

Indeed....

There must be some evidence out there somewhere that has established the facts.

Must there be? Couldn't the answer depend on the type of engine (turbodiesel vs NA petrol for example), or be different for different engine models? I've no idea whether variables like that might have an impact, but I see no reason to assume they definitely wouldn't.

[YorkshireJumbo]

This sounds like the hypermiling technique sometimes known as "Pulse and Glide" or "Burn and Coast". Accelerate harder, but for a shorter period of time to reach the same cruise speed.

But they cut the engine totally once gliding, which is a very different technique to normal driving...

[fungus]

Lets not forget that those preparing for an Advanced Test need to be able to demonstrate safe, good progress. Whether you drive like this all the time is entirely up to you. That said, I don't think any examiner would worry about a candidate approaching a hazard too slowly as long as they get briskly up to the speed limit after the hazard (and if appropriate of course).

I can see repeated hesitation might be an issue but I don't believe that's the OP's main point.

Topper's book Very Advanced Driving has some interesting ideas about making progress, some of which I wouldn't think are really acceptable. One in particular has the 'very advanced driver' apparenly squeezing up the right-hand side of a car waiting at a roundabout and then slotting in behind a car on the roundabout ahead of the car waiting. A bit extreme and I'm not sure, even if I wanted to drive like that, such an opportunity would often present itself. I would say this goes beyond safe, good progress.

It is many many years since I have picked up my copy of Very Advanced Driving by A Tom Topper, but if IIRC, he recommended signaling right when waiting in busy traffic to turn left, then when some one invites you out, switch the indicator to left and emerge.

[michael769]

Sounds like Very Rude Driving to me!

[apple tango]

I think progress should be measured against objectives - if you need to get somewhere quick due to an emergency then I don't see any problem with progress at any cost, even if that rubs a few people up the wrong way, as long as it's done safely. However under most circumstances I would personally be looking to avoid that friction.

Regarding acceleration vs efficiency, I have tried gently accelerating through each gear (changing up as early as possible) and also a slightly more spirited approach of accelerating quite hard in each gear and only changing up as soon as the revs got high enough for the gear to become less effective. Personally I did not notice any significant difference in my mpg figures, so now I go with the 2nd approach. Perhaps I should add that I like to avoid using the brakes if I can, so when it's busy I may hold a lower gear and creep up to the back of a queue or traffic lights rather than accelerate hard then have to brake hard like most people seem to . I think it's the braking that "costs" you, so aiming to keep things moving (even slowly) is more pertinent than how you accelerate in my view.

[Kevin]

There must be some evidence out there somewhere that has established the facts.

Must there be? Couldn't the answer depend on the type of engine (turbodiesel vs NA petrol for example), or be different for different engine models? I've no idea whether variables like that might have an impact, but I see no reason to assume they definitely wouldn't.

Good point. The answer could well depend on these variables. I'd still have thought that someone would have performed the calculations, even if they've not done it for all engine types etc.

I see someone has suggested that brisk acceleration is part of a hypermiling technique, which makes me think that someone has worked out whether or not there is some benefit in it. I dunno, just putting it out there to those that might have a better knowledge or greater access to resources than I have.

[Kevin]

It is many many years since I have picked up my copy of Very Advanced Driving by A Tom Topper, but if IIRC, he recommended signaling right when waiting in busy traffic to turn left, then when some one invites you out, switch the indicator to left and emerge.

A bit cheeky! I've got my copy to hand so I'll have a browse later. Apart from a few potentially dubious driving techniques for making progress, I think there's some good driving advice in the book. I'd recommend anyone interested in advanced driving to have a read of it.

[YorkshireJumbo]

I see someone has suggested that brisk acceleration is part of a hypermiling technique, which makes me think that someone has worked out whether or not there is some benefit in it. I dunno, just putting it out there to those that might have a better knowledge or greater access to resources than I have.

But they still cut the engine once they've reached their set speed, so that the engine only runs for a small %age of the time. That technique IS proven to work better for mpg, but it only really works if you are not sharing the road with other users, as their speed varies widely. Unless you plan to drive like that, I'm not sure there are significant benefits.

We're talking about driving with the engine running to maintain a set speed.

[Kevin]

I see someone has suggested that brisk acceleration is part of a hypermiling technique, which makes me think that someone has worked out whether or not there is some benefit in it. I dunno, just putting it out there to those that might have a better knowledge or greater access to resources than I have.

But they still cut the engine once they've reached their set speed, so that the engine only runs for a small %age of the time. That technique IS proven to work better for mpg, but it only really works if you are not sharing the road with other users, as their speed varies widely. Unless you plan to drive like that, I'm not sure there are significant benefits.

We're talking about driving with the engine running to maintain a set speed.

Whatever speed you reach and whether or not the engine is cut once the desired speed is reached isn't the area of fuel saving that I'm exploring. The area of fuel saving I'm focusing on (and I accept that it is probably only a small area and that there are many other ways of saving fuel that probably have greater effect) is just the acceleration phase. My gut feeling, is that longer spent in lower gears means the engine uses more fuel for a given distance. So early changing up gears is probably beneficial.

Now, that could mean selecting higher gears very early, almost labouring the engine. But, lets say you want to make reasonable, every-day normal driving sort of progress.

You could gently accelerate, changing gear, lets just say purely for the sake of example, at about 2,500rpm, until you reach whatever speed you want to be travelling at. Or, you could accelerate briskly, still changing gear at about 2,500rpm, until you reach the speed you want to be travelling at. Before this is discected in minute detail (Gareth! luv 'im to bits, but he does like to over-analyse sometimes) I accept various engines, drivers etc may, do, or want to change gear at different engine rpms and that journey time may affect the way people may or may not want to drive at any given moment etc. Just concentrate on whether brisk acceleration or gentle acceleration is better for fuel economy if all other variables remain constant. Otherwise I'll get a headache. I can feel one coming on already.