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-   -   Saving fuel in a car by not using brakes (https://www.pprune.org/jet-blast/448931-saving-fuel-car-not-using-brakes.html)

 Loose rivets 16th Apr 2011 05:36

Saving fuel in a car by not using brakes

. . . well, not using them any more than necessary.

I have a mate, he's a mathematician among other things. He logs all his consumptions and is tickled pink with the savings. But I think he's thinking it through incorrectly.

I hasten to add, this is not a thing he does very often - being incorrect, that is.

He reasons the heat from the brakes is causing the losses to be increased. Fine, but his remedy is to slow down without much use of brakes. Fine again, but to think just saving heat from a specific source is doing the trick, is plain wrong. I think it's simply the average time over a go-stop sector. Nothing else.

 G-CPTN 16th Apr 2011 05:56

Using brakes almost by definition means destroying energy that has been expended (usually from application of the engine, though sometimes from potential energy from a gradient).
If you can avoid the excess energy (except from gradient) then you would save fuel.
If, even with input from gradients, you can sustain the speed rather than destroy it through braking then you will save having to replace energy using the engine (and thereby, save fuel).
Of course, there is another aspect - that of applying power using the engine. Excessive application will result in wastage of fuel.

My maxim, when I want to drive economically, is to think of the pedals as having a fragile object such as a hen's egg that would be fractured by heavy pressure on the pedal.
By 'stroking' the accelerator pedal instead of stabbing at it will save consuming excess fuel, and by gentle braking will avoid 'destroying' energy (which was generated from fuel, usually).

So, your friend is basically correct, though it is by avoiding applying too much power that can result in avoiding braking that destroys the energy that was (usually) created from fuel.

 wings folded 16th Apr 2011 06:09

If you never use the brakes, and wrap your car around a tree, there is an immediate and dramatic (and fairly permanent) impact on fuel consumption.
Car consumption tends to rise, however.

 mike-wsm 16th Apr 2011 06:25

Alas your friend is a mathematician, not an engineer. Simply "not using the brakes" and driving at the same speeds and accelerations will use the same amount of fuel. If he modifies his driving style to maintain lower levels of acceleration and deceleraton then he will save fuel, but he could equally do this without avoiding use of the brakes. The best way to waste fuel is to move the accelerator pedal suddenly, causing the ecu to inject a sudden increase in fuel (like the older style accelerator pump). Perhaps the best way to save fuel is to change ones footwear from outdoor clodhoppers to thin-soled deck shoes with more feel to them. Also drive in a slightly higher gear to avoid high rpm.

wf :D

 G-CPTN 16th Apr 2011 06:33

Yes, I upshift as soon as possible when in fuel-saving mode (though I do now drive a diesel which makes this more advantageous due to the torque).
The main thing is to avoid excessive engine revolutions whenever you can.

 Capetonian 16th Apr 2011 06:33

Nobody has mentioned slowing down by changing down and using a trailing throttle, thus reducing speed whilst consuming zero fuel. This involves predicting the need to slow down and is difficult particularly under heavy traffic conditions.

If, for example, when leaving a motorway, you approached your exit on a trailing throttle without braking, you would save fuel, but you would risk the wrath of drivers behind you in all probability.

The key is to minimise use of the accelerator, to avoid high revs and keep to the lowest possible throttle setting at all times. I am always amazed at how so many drivers will drive towards slower traffic at higher speed and then brake at the last minute, rather than gradually slowing down towards the obstruction.

By the way coasting in neutral, which some people do, is for several reasons, not a good idea. And turning off the engine when, for example, going down a long hill, is even stupider, specially in modern cars when you are likely to lose the power assisted steering and braking.

 rh200 16th Apr 2011 06:36

Basic conservation of energy, it doesn't work there's a plesebo effect here. The only way this could possibly work was if your mate had dynamic brakes putting that energy into a spinning fly wheel or some other potential energy device.

 spekesoftly 16th Apr 2011 06:37

He reasons the heat from the brakes is causing the losses to be increased.
I suspect your mathematician friend reasons that there is a direct correlation between the amount of heat generated by brake application and the increase in fuel used to regain speed.

Using brakes almost by definition means destroying energy ...
Energy cannot be destroyed, only converted to another form, in this case heat.

Good anticipation can avoid the unnecessary application of brakes, not only saving fuel, but also wear and tear on tyres, brakes and running gear.

 green granite 16th Apr 2011 07:19

Good anticipation will save fuel, for example coming up to a red traffic light lift your foot and slow gently from a 100yds away and the chances are it'll turn green before you get there, so you can accelerate away from perhaps 15mph instead of a standstill, same applies with approaching a parked car with another coming in the opposite direction. As for the theory that engine braking damages the car I'm afraid that's pure rubbish nowadays (providing your in the correct gear I'm not suggesting you put it in second at 40 and let the clutch in), same applies to tyre wear, much less wear than running hard and then slamming your brakes on.

 Checkboard 16th Apr 2011 07:46

Simply "not using the brakes" and driving at the same speeds and accelerations will use the same amount of fuel.
Not so - utilising engine braking saves energy over using friction braking. Any modern (within the last 15 years) electronic fuel injection system will reduce fuel flow to zero, if the engine is being driven by the drive train. Slowing down in gear uses zero fuel, slowing down in neutral on the brakes uses brake pad material AND fuel (to keep the engine idling).

 NZScion 16th Apr 2011 07:46

Yes the engine braking causing damage story is a load of rubbish. Particularly in (truck) diesel engines, many of which also have exhaust brakes to increase the braking achieved. I've also done it for years in petrol cars and it has caused no damage.

 ShyTorque 16th Apr 2011 08:03

Not so - utilising engine braking saves energy over using friction braking. Any modern (within the last 15 years) electronic fuel injection system will reduce fuel flow to zero, if the engine is being driven by the drive train. Slowing down in gear uses zero fuel, slowing down in neutral on the brakes uses brake pad material AND fuel (to keep the engine idling).
What really counts is when you close the throttle, closing it early so engine braking is sufficient reduces the fuel burn.

 Whirlygig 16th Apr 2011 08:15

He logs all his consumptions and is tickled pink with the savings.
Sounds like my kinda guy. :ok:

My new car display fuel consumption data and I've become fascinated with it over the last few months. I get excited when the av. consumption goes up another 0.1 mpg to 23.7. Watching this data has taught me to drive more sedately (almost, but not quite, like a native Norfolk driver) and better anticipate the road conditions and traffic. And cruise control helps.

Cheers

Whirls

 Parapunter 16th Apr 2011 08:17

There's some serious life getting requirements around here.:rolleyes:

 spekesoftly 16th Apr 2011 08:19

I get excited when the av. consumption goes up another 0.1 mpg to 23.7.
Only 23.7 mpg ! Obviously using the brakes far too often! ;)

 Whirlygig 16th Apr 2011 08:19

...and who are you to judge Para? :p

Spekes, doesn't that depend on the size of the engine, weight of vehicle and fuel used? Agreed, it's a tad high for 2.0 diesel but this is 3.2 petrol. :ok:

My driving instructor friend told me off for filling the tank up as it's less fuel efficient to be lugging all the extra fuel around whereas if I half-filled up more often, my average consumption would improve from not carrying the extra weight. I reasoned that the extra fuel used in stopping and starting more often at the petrol station would negate that.

Tried both - no discernible difference.

Cheers

Whirls

 BombayDuck 16th Apr 2011 08:28

I hope you're enjoying yours, then, Parapunter, and that we haven't caused you distress by forcing you to come in here and remind us how much we need one. :)

Back to the topic - one thing I've always marvelled is how people in Britain approach roundabouts, at least busy ones. Full speed to the edge and hard on the brakes. Friends, taxi drivers, even bus drivers. I've been raised in a country where fuel consumption matters above all, and if the roundabout is busy I always tend to coast the last few metres to a stop - why would I slam the brakes? If you're driving in Milton Keynes, this is especially good for fuel. Though I can understand the urgency to get out of Milton Keynes... :\

Whirls, I did the same with my rental last month - Tried three different driving styles on three different motorway stretch - was happy to see 40, 41+ and 36 mpg depending on how I drove, and now I know the best style (for that car). I do want to have a word with the Renault folks - what's the point of having a transmission (Megane) that needs 6th gear at 55 mph? Why not space them out a bit so that I can do that at 70 on the motorway and ave fuel?!

 BombayDuck 16th Apr 2011 08:32

Ah, I see you posted, while I was :)

My dad would say the same thing from a different angle - after I would fill up the tank he would say "be careful now, leave some extra distance to brake because you have a full tank".

It always troubled me but I couldn't place it why. Then one day..

"Uh, dad, what's the density of fuel? Less than one, right?"

"Yes"

"So assuming it is 0.9, and I know this car takes 40 litres, that would mean the weight of the fuel is at best 36 kilos"

(pause)

"That should mean it's not much weight added, and especially since the tank is low and in the centre of this car..."

"Point"

Probably the only time in my life I taught my dad something :)

 arcniz 16th Apr 2011 08:37

Particularly in (truck) diesel engines, many of which also have exhaust brakes to increase the braking achieved.

Usta stop often in a town that lay at the foot of a long, steep descent of straight, fast road traveled largely by very heavy trucks carrying process material from sources higher up the hill. The town actually had stop lights and speed limits across the road near where the descent flared, so the haulage drivers had to do whatever they could to arrive slow enough to maybe stop for the lights while also attempting to not set their brakes afire on the near-unstoppable descent.

One could hear the results from far away on a misty, cool morning: a sobering series of cannonades ricocheting off the many rock-faced hills surrounding. One truck would be a fusillade, two a barrage, and three or more running in formation fairly well resembled auditory apocalypse as their diesel engines chugged almost backwards against the pressing descent, doing the best they could do to hold gravity in check, while turning most of the surplus energy into sound, it seemed, full rich with the dissonant agony of wrenched and angry molecules, an aural spectacle that all in the miles surrounding involuntarily shared.

For people living near the road, the noise must have been unbearable at times. Somehow an informal curfew existed, known to the regular drivers. For the rest of the time, and especially for drivers who were unfamiliar, considerable art and effort had been spent to prepare an earnest but unofficial road sign planted near the top of the drop, its big letters proclaiming: "LAY OFF THE JAKE BRAKE!"

 Capot 16th Apr 2011 08:41

I can save fuel by driving more slowly, and not accelerating hard. My diesel 2L Mondeo gives 54mpg on the motorway at a constant 70mph, 47mpg at 80mph, and, when I have cut it too fine on refuelling and the next MSA is some way ahead, 65mpg at about 55 mph. (According to the computer, which is, I have found, fairly accurate over a longish period.)

Obviously I use the brakes less at lower speeds, but the saving comes from the lighter use of the right foot pedal, not the brake pedal.

These figures reduce by 10% if any tyre is more than 3 psi too soft, and the softer they are the worse the consumption.

They increase by 10% if I slipstream a white van.

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