Why is 'hard' braking bad for fuel economy ? - Fuel Economy, Hypermiling, EcoModding News and Forum

Cd · 11-08-2008, 11:17 PM

I have read several times that 'hard' braking and sharp corners are bad for your gas mileage.
Why is this ?

Is it just the fact that you have to accelerate once you have lost your momentum, or is there also some other factor.

Also, can you explain engine braking ?

I notice when coasting to a stop I will shift my automatic from neutral back to drive, and the car will shudder and suddenly slow as the engine 'catches ' and revs up.

Is this an example of engine braking ?

Tango Charlie · 11-08-2008, 11:47 PM

When you brake, you are turning gas into heat and brake dust. Since burning the least amount of gas is our game, the brake pedal is our enemy. Braking indicates that you have accelerated more than you really needed to.

Bicycle Bob · 11-08-2008, 11:53 PM

Any braking wastes momentum; typically, people coast a bit to use the momentum and make the braking "easy."
Hard cornering also wastes energy, but not as much as braking to a lower cornering speed. Tires with a side slip angle are like tires running up a slope of that angle, bearing the side force as weight. This action will do the work of brakes to use up any surplus momentum, but trying to maintain speed or accelerating through the corner defeats that automatic fine-tuning.
If you don't have time or inclination to coast down to a moderate cornering speed, the most economical option is to scrub off some speed in the corner and then accelerate on the straight. Unfortunately, this is not as safe a way to get sideways as going in slow and accelerating in the corner.

Engine braking is usually used on manual transmissions, but yes, you are finding it. On diesel trucks, the engine braking is sometimes enhanced by a noisy, spring-loaded exhaust restriction, or Jake Brake, which is what those signs are about.

trebuchet03 · 11-08-2008, 11:54 PM

Engine Braking - keeping it in gear without throttle. You're dissipating energy by compressing air.

For hard braking, that's probably the case... I'm not familiar with it being excluded to hard braking only....

Sharp corners.... Simply explained (and ignoring any other loss), the power necessary to turn to is inversely proportional to the radius of the turn. Bigger turn = less power. If you want me to get more into the physics, just let me know

jamesqf · 11-09-2008, 01:13 AM

Though hard cornering might actually save energy, if you refrain from braking going in, and accelerating out.

basjoos · 11-09-2008, 07:35 AM

One way to avoid inadvertant engine braking is to note what GPM (Scangauge) or injector pulsewidth (SuperMID) your warm engine requires to maintain your typical cruising rpm (1 GPM at 2000rpm for my F150) and then to avoid dropping into that GPM/pulsewidth range when driving, because whenever you do, your engine has switched over from propelling your vehicle to braking your vehicle. If you find yourself dropping into that range a lot while driving on a particular stretch of road, then its time for a coast or FAS.

Cd · 11-09-2008, 08:08 AM

So if I understand correctly, braking has no effect on FE. It is just the fact that you are wasting your momentum and having to rebuild it up again once you apply the brakes.

Just as I thought. ( Seems like common sense really )

brucepick · 11-09-2008, 08:47 AM

Quote:

Originally Posted by Cd

So if I understand correctly, braking has no effect on FE. It is just the fact that you are wasting your momentum and having to rebuild it up again once you apply the brakes.

Just as I thought. ( Seems like common sense really )

Exactly. Except of course, wasting your momentum and building it up again has a huge negative effect on your FE.

You could make a pretty good case that braking and the acceleration that come after are what make EPA City ratings typically lower than the EPA Highway ratings for any given vehicle (except some hybrids, but that's another story).

Tango Charlie · 11-09-2008, 09:23 AM

You're on the right track, Cd. But it may help you hone your driving technique to realize that the hit on your mileage has already been taken before you hit the brake pedal. Think of it as an indicator of your skill in judging your needed acceleration, or your ability to judge conditions ahead of you. I am constantly griping at myself for coming up on stop signs above 20 mph. To me, it indicates that I left my foot in the accelerator too long to get there.
If you need to come to a stop, the acceleration back up to speed is required, anyway. Where you have the opportunity to increase your efficiency is how you came up to the stop.
Remember the first law of thermodynamics. Turn that energy in the gas tank into forward momentum ONLY. Don't let any go to waste being converted into heat through the brakes.