Why is 'hard' braking bad for fuel economy ?
 

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 ?

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.

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.

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 :)

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

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.

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).

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.

No GPH (MPGuino) ? :(;) It reads to 2 decimal places FYI and is on the front screen, or injectorHius is pretty much 1/2 of the duty cycle to like 6 digits.

:eek: That's gotta be a typo. Maybe 1 GPH.

P.S. Seriously, I love that Honda of yours :)

That was a typo. Change those GPM to GPH. It was still early in the morning.

My city MPG average is currently between 35 - 37 MPG. I only drive 7 miles one way to work, coast when I can, have memorized the timing of the lights, and hardly EVER use my brakes with more pressure than it takes to crack an egg ... err... an ostrich egg.

My mileage sucks so badly because I am driving an automatic.

On the highway, I have seen over 54 MPG at a steady 55-60 MPH

According to my SuperMID, I loose between 2.5 to as much as 4 MPG when stopping at a light. ( On the instant average readout )

I was just asking the question about braking out of couriousity. Thanks for the feedback.

From 100+ hypermiling / ecodriving tips to increase gas mileage - EcoModder.com

Hey - that's the exception! :)

On re-reading, I find myself stumbling at the words instant average. Seems like it would be one or the other. Does the manual for the SuperMID give a description of what instant average means?

I do use engine braking a lot, all you do is coast in your current gear or shift to a lower gear and the car slows quicker, you have to be sensible and not shift too low and rev the engine too high.
I asked my father about this as he's a mechanic and he said the engine doesn't use fuel while engine braking on a fuel injection car, since when you coast in gear it turns off the injectors, but the pistons and the other engine stuff is still moving so it still sounds like it's running although quieter.
Are we both wrong?

I'm not sure about the SuperMID, but on the cars I've driven that had an instant average, it calculated a rolling average over the last mile, so that the number is fairly accurate, but not jumping around like the instant display.

sometimes. In my metro if I coast in top gear above 40mph, it will kill the injectors till it gets down to 35mph, per the mpguino and the butt-meter.

Katana, you're describing DFCO; Deceleration Fuel Cut Off. (Let's not miss an opportunity to use an acronym!)
My Vibe does this down to about 1200 RPM. It's a more efficient option to using the brakes. :thumbup:

And yeah, I was trying not to muddy the waters with tip #49. :)

That is *NOT* an example of engine braking, it is an example of "how to break a transmission." Any time the car is shuddering, it indicates something is wrong. Some cars take to being shifted in and out of drive while moving, others, like yours, do not. You'll have problems down the line if you continue what you're doing, I guarantee it.

Engine braking involves letting off the throttle. Yes, it's that simple.

Mine only cuts the fuel every now and then, and only on a long steep downhill. My fuel cut is very noticeable and is a pretty powerful engine brake. Sometimes I can hold the throttle just right where itll fuel cut all the way down the hill and now slow down as bad.

Can someone clear this up for me? The OP asked why "hard braking" was bad; obviously, going from point A to B, you want to brake as little as possible because braking is pure wasted momentum.

But is hard braking bad in itself? That is; if you have to stop at a light, is slowly braking superior to slamming on your brakes (safety aside)? A strategy I like to use is slowing down to 15 or 20 well in advance of a light so it takes me longer to get there, the light turns, and I'm able to not be forced to brake to 0. Is there a reason not to brake hard when I do this?

Also, does braking itself decrease FE? I read on one of these websites that you could circle around a parking lot to bleed off extra momentum instead of just braking but that sounds ridiculous to me.

They do that to get a higher "score". Since it does not actually save fuel over a trip, I consider it cheating.

What you're describing is the same as the tip from the hypermiling/ecodriving list, posted above. Sound strategy.

Yes, that was this web site! Yes, it is a bit ridiculous, and it's splitting hairs. Orbiting a parking spot and coasting to a stop won't reduce your fuel consumption, but it'll increase your MPG. That specific technique is essentially a numbers game - something to do if you have come in with too much momentum.

Say you got a sudden red light a few hundred feet in front of you, and you're at speed. No matter what you will have to come to a stop because it's a hella long red. Also allow the condition you want your engine to stay on this whole time--you aren't into EOC and don't want to use the ignition when the light turns green. Finally say if you just coasted in neutral you would still be going very fast when you got to the light.

Under these circumstances the most fuel efficient option is to use DFCO for the longest time possible, using brakes minimally or not at all until you come to a complete stop.

In that kind of situation, some people will even downshift their transmission (as speed drops), to extend the DFCO opportunity.

The only advantage I can see to hard braking is as follows. Youre coasting up to a stop sign with the engine idling, downhill. Youve gotta stop anyway, and your wait time will probably be the same regardless of how long it takes to get there. So the faster you get there and make your stop, the less time your engine will be idling to get there and burning gas.

That makes sense, but alternatives of starting coasting earlier, or using engine breaking to slow down, would probably yield better results (if you stay in a moderate gear in relation to your speed and can DFCO most of the way to the stop you use LESS fuel on the journey from where you are to stopped-at-the-stop-sign).

Yes you would likely use more fuel if you gave it gas to get your speed up only to get to the stop sign faster.

I guess I just keep thinking of this one road on my way to work. Ive gotta crest a hill, then its downhill for like 1/2 mile to the stop sign and during that I can just idle in N. I can only crest the hill so slow. After that, the faster I let the car roll to the stop sign, the less fuel I use. If I gradually brake and take longer to get there, I burn more. If I EOCd, itd be a moot point, but I dont EOC for a few reasons.

I guess if you can't DFCO then that's a good option.

To us it does! I watch so many people, with automatics ride each others bumpers, and brakes like crazy. It just drives me nuts! When I first realized that also, I thought of it as conserving momentum...
:thumbup:

Dale Jarrett commentating in the ESPN booth at the Bristol NASCAR race in 2007; "The biggest fuel waste is that pedal right in the middle"

As a Hypermiler, you want to learn to "drive without brakes"...meaning use just enough acceleration and coasting to get you between stops on your destination. If you have to constantly brake hard between starting and stopping then you are over accelerating and under coasting.

However, "hard braking" has its place. The entire basis of hypermiling is to maintain as much momentum as possible. Let's say you are driving 45 MPH and the light ahead is still green...then all of a sudden, it changes to red. You quit accelerating and begin coasting but you realize that you're still going too fast and will have to stop ahead. INSTEAD, you can "brake hard" with a few hard brake pulses, reducing your speed to 25-30 MPH. You can extend your coasting time and hopefully make it through the light.

It takes much less energy to accelerate from a few MPH versus a dead stop. You also have to keep in mind your SAFETY and the wear and tear on your vehicle.

Braking hard and taking sharp turns at higher speeds increases wear on your suspension, tires and brakes. It also causes the gasoline to splash around inside your tank, which can offset your numbers.

All of this takes patience, timing and practice but once you master them then you will be that much better a driver.

Remember...NEVER ever risk your safety or your vehicle for a couple more MPGs. Your life is much more valuable than fuel economy.

Think about the following events that occur when BRAKE is pressed:

• DISTANCE coasting is reduced.
• TIME engine not loaded is reduced.
• MOMEMTUM is being converted into HEAT (by brakes).

I completely agree. Momentum should always be the first on a hypermiler's list. The less you have to stop, the more efficient your results. My only point was when faced in a particular certain situation, never compromise safety or the well being of your vehicle for fuel economy.

Keeping this in mind will make you a more conscious and efficient driver by ensuring that you take the additional care to give yourself plenty of space, time and patience.

A Q&A:

Why is hard braking bad?

Because it stresses and degrades your brake drums and shoes and/or disc pads and rotors faster than soft braking does. Your brakes will wear faster and that sucks $$$ out of your pocket faster. So don't make a habit of doing it.

Some of us here are only interested in saving fuel, at any cost. Some of us are primarily interested in saving $$$. Some are interested in both. It's your car, your money, and your choice. :)

Is all braking bad?

As others have said, all braking does is that it simply reduces momentum. A hypermiler's wet dream would be never to have to brake at all. But that's a fantasy - it's not real life in the real world. So let's be realistic and compare braking to '"when nature calls": when you gotta go, you gotta go. And when when you gotta stop (immediately, as in emergency situations) stopping is much better than NOT stopping. ;)

Metro mentioned the numbers game. When I drove Bacon, if I needed to stop while on a road trip, I shifted to neutral and waited it out, but when my Ultragauge showed GPH below my overall goal, I came to a stop and turned off the engine. It has an automatic transmission, so EOC was not an option. It was frustrating because I wanted to use the last of my momentum, but it just did not seem worth the fuel used while idling.