[Closed] So.... accelerating

Interesting question.

Relating it to bikes, I'd be more knackered overall if I pushed hard to get into a high gear quickly than if I pootled along, slowly increasing my speed and gear.

So I'm tempted to go with the former, but I'd be interested to hear a more scientific answer.

An article I read on fuel efficient driving suggested imagining there was an egg on the accelerator pedal - ie gentle acceleration.

IMO if you are driving in an urban environment with lots of starting and stopping it is more fuel efficient to keep speed down . If you accelerate quickly up to speed you inevitably end up having to break again, which is a waste of fuel

Modern cars are designed to be fuel efficient at highish revs and labouring them in high gears is counter productive. In 30 limits for example, I drive in 3rd gear, and unless I'm over 50mph, I'll generally be no higher than 4th, and my average fuel economy appears to improve when I do this.

A lot of people automatically associate high revs with a lot of fuel being used, but it's not the case. If you're going down hill, say, completely off the accelerator, you could be hitting 5k revs but there wouldn't be a drop of fuel being used in a modern car - gravity keeps the engine turning over...

I suspect it's six of one an half a dozen of another, for example taking a 30 second period:

I accelerate from 0 to 60 at a moderate rate, it takes me 20 seconds @25mpg to get to 60mph, then 10 seconds cruising @35mpg ... therefore avg mpg = 28-and-a-bit

If I accelerate from 0 to 60 at a brisk rate, it might take me 15 seconds @20mpg to get to 60, then 15 seconds crusiing @35mpg ... therefore avg mpg = 27.5

Have a graph, graphs are cool. It's from a [url= http://www.sciencedirect.com/science/article/pii/S1361920904000604 ]paywalled paper[/url]:

Comparative field evaluation of vehicle cruise speed and acceleration level impacts on hot stabilized emissions

The study demonstrates that as the level of aggressiveness for acceleration maneuvers increases, the fuel-consumption and emission rates per maneuver decrease because the vehicle spends less time accelerating. However, when emissions are gathered over a sufficiently long fixed distance, fuel-consumption and mobile-source emission rates per-unit distance increase as the level of acceleration increases because of the history effects that accompany rich-mode engine operations.

(meaning you spend more time in shorter, less fuel efficient gears)

I think this is a false premise. They're only 'less fuel efficient' relative to your speed. 200rpm in second is going to be as efficient as 2000rpm in fifth.

I think.

Modulate your speed with the throttle as much as possible, not the brakes.
-This requires forward observation, anticipation and avoiding unnecessary acceleration towards hazards/stops.

Accelerate 'positively', but don't take the engine near to the red line before changing up a gear (the optimum depends upon the car).

The emissions tests for which engines will be calibrated for feature brisk-ish accelerations. They certainly aren't fast, but you have to put a reasonable amount of pedal in to hit the target trace. If/when Europe goes to a more 'representative' test trace, then the accels will be even brisker as the aim is to mimic how the majority of people drive - the US FTP75 emissions test has some brutal acceleration parts in it!

The robustness of an emissions/FE calibration is pretty strong these days so the gains from accelerating one way or another will be minimal - it would be more efficient to concentrate on keeping momentum up so you do as little accel/deccel as possible rather than going for a perfect acceleration rate.

I think this is a false premise. They're only 'less fuel efficient' relative to your speed. 200rpm in second is going to be as efficient as 2000rpm in fifth.

Cougar - Member
I think this is a false premise. They're only 'less fuel efficient' relative to your speed. 200rpm in second is going to be as efficient as 2000rpm in fifth.

I think.

I don't think that's the case; at 2000rpm in first gear, I get 15-20 mpg, at 2000rpm in fifth I get approx 45. Assume it's drag on the drivetrain.

don't think that's the case; at 2000rpm in first gear, I get 15-20 mpg, at 2000rpm in fifth I get approx 45. Assume it's drag on the drivetrain.

I don't think that's the case; at 2000rpm in first gear, I get 15-20 mpg, at 2000rpm in fifth I get approx 45. Assume it's drag on the drivetrain.

Modern cars are designed to be fuel efficient at highish revs and labouring them in high gears is counter productive. In 30 limits for example, I drive in 3rd gear, and unless I'm over 50mph, I'll generally be no higher than 4th, and my average fuel economy appears to improve when I do this.

Don't think that's true - at least from the diesel perspective. BMW redesigned their transmissions to basically allow the car to idle in 4th gear around town, and look at taxi drivers, whose engine never exceeds 1200rpm for the life of the car. 🙂

I think some people worry too much.

If you accelerate quickly up to speed you inevitably end up having to [s]break [/s]brake again, which is a waste of fuel

Kinda. Fuel gives momentum, while braking takes momentum (converts it to heat), so braking takes fuel.

Ideally, don't brake at all. Practically you have to brake sometimes. To minimise braking, match your speed to the car in front with a large gap - larger than the [i]minimum[/i] braking distance in the Highway Code. This allows you to anticipate slowing gently by reducing your accelerator input alone, thus saving fuel. Tailgating is a very expensive way to drive because you have to brake often.

The downside of braking aversion is that you might go into a speed-restricted zone a bit hot. Which resulted in a speeding ticket for me last week. 😳

Don't buy an Impreza 🙂

Flaperon - Member
Don't think that's true - at least from the diesel perspective. BMW redesigned their transmissions to basically allow the car to idle in 4th gear around town, and look at taxi drivers, whose engine never exceeds 1200rpm for the life of the car.

Shouldn't think it'd be doing the engine mounts/DMF much good driving it at low RPM all the time?

ok, just to make it more difficult, what about in an automatic with a torque converter gear box?
from my days of paddling canoes, a firm pull of the blade gives less chance for the water to spill off it, hence more drive forward in your boat. As a torque converter is big impellers in fluid (like lots of paddles) at what point does the extra efficiency of rapid acceleration with said gearbox get out weighed by the extra fuel the engine burns?

match your speed to the car in front with a large gap - larger than the minimum braking distance in the Highway Code. This allows [s]you to anticipate slowing gently by reducing your accelerator input alone[/s] [i]short-sighted muppets to pointlessly overtake and slot into the gap[/i]

😀

Well there are many factors:

Turning the engine faster creates more drag.

At low revs, the combustion gas spends more time in the cylinder so ends up heating the walls more, and hence more heat is wasted

At high revs, the fuel might not have time to fully combust so will still be burning on the way out.

At low revs, in a diesel each bang needs to be bigger so there's more chance of some of it coming out the exhaust valve.

In a petrol, opening the throttle valve more reduces pumping losses, but this means either a higher reving small engine or higher speed, so that usually cancels out (but not always).

At low revs and higher load, particularly in a diesel, there are more stresses on the dmf since each bang contains more force, so it might fail earlier.

I've seen articles to support both but I drive to get to the desired speed as smoothly and efficiently as possible. It's the way I was taught for work and it's drilled into me, I find I get excellent MPG doing it this way. My wife drives the opposite way slowly working her way through the gears, she's a pretty good driver as her Dad was a driver by trade and taught her, she gets less MPG than me by about 5-10.

So the moral is drive how you want.

In a similar vein, on a motorway, I cruise at a more or less constant throttle opening. So I slow down a little up hills, but let the road speed and engine revs rise on downhills.

I reckon this is more fuel efficient than maintaining a constant speed (on cruise control, say), and the momentum gained on the downhills seems to last quite a way on empty-ish roads.

Easily get 40+ mpg out of a 2-litre turbo engine like this, and average speeds certainly not within the 'dawdling' range.