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Old 02-25-2013, 07:08 PM   #1 (permalink)
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Braking and city mpg

Scanning through many "MPG top ten tips" or "Getting best MPG" articles, I notice that the information regarding the impact of braking on city mpg is rarely emphasized and it is often just barely eluded to in most articles. Now in highway driving, the advice of avoiding high speeds, roof racks, low tire pressure, are usually presented correctly for the most part, the same can not be said with the advice concerning city driving strategies. You get the usual accelerate like you have an egg under the gas pedal, but very little specifics on optimizing your braking. The fact is that optimizing one's braking is in many cases, the number one driver controllable technique in positively impacting city mpg.

Judging from the way these articles are worded, most of these self-titled "auto experts" mistakenly think that a driver that accelerates quite quickly and minimizes the use of brakes would have worse mpg than the driver that accelerates very slowly and then brakes at the last second. Many writers and drivers cannot grasp that most braking is used to correct for the excessive motive power (or vehicle momentum) that was produced by the engine. Look at all the drivers that stay on the gas right up to a red light or stop sign and then brake quickly just before the stop. It drives me nuts watching drivers that accelerate really slowly but lay on the brakes almost as late as the aggressive drivers. Even in the new Prius hybrids only about 33% of the braking energy is recouped, so one is 3 times better off trying to not use any braking regen and glide as much as possible especially from the 50 mph down to 20 mph range. (40 mph has 4 times more kinetic energy as 20 mph) or in other words braking from 40 to 0 mph burns off 4 times the excess energy usage compared to braking from 20 to 0 mph (converting it from motion into heat). So laying off the gas early when your going at 40 mph such that you'll glide down to 20 mph near your stop has saved 75% of the motive power that was otherwise required to sustain the momentum (kinetic energy) at 40 mph. Gliding assumes no braking (brake pad or engine braking or regen), and engine not using fuel, which can be achieved in a Prius hybrid.

So my question is should optimizing braking be the number one technique presented for optimizing city mpg?

Also if done on a congested highway, optimized braking also minimize the traffic waves that reduces everyone's fuel consumption. I suspect if just 10% of drivers on a congested highway resorted to optimized braking, the throughput and capacity of said highway would increase significantly, not to mention the fuel $ saved by all. It's a win-win except it's hard to change human nature.


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Old 02-25-2013, 07:45 PM   #2 (permalink)
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Best rate of acceleration is fairly brisk, about the normal rate for traffic here which also has the benefit of not creating anger in those around you. Best mileage amounts to best conservation of momentum, which means avoiding braking at almost all costs, with the exception being, for me, light timing which when poorly executed will quickly destroy average mileage.
I don't average as good as many, but in my normal drive of 20 miles I have to contend with 23 traffic lights, most of which are bunched together in less than half of that drive. There is a very precise average speed (47-49MPH) that must be maintained to limit the number of lights caught from 0-7 in the 46 total in a round trip.
My blood pressure literally jumps when I get caught by one of the tripped lights that can never be timed, which is a shame when that could easily be corrected by placing a "no trip" time period exclusion in the light programming.

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Old 02-25-2013, 09:08 PM   #3 (permalink)
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Quote:
Originally Posted by briank View Post
So my question is should optimizing braking be the number one technique presented for optimizing city mpg?
Absolutely. It is whenever I talk to anyone about city driving techniques.

Ironically, the most frequent question is usually about acceleration technique, which I almost never talk about, because in the grand scheme of things it has a far smaller impact than the brake pedal.

The drivers I've coached have all also commented on how much more aware they need to be to "drive without brakes" (DWB) and avoid rushing into situations unnecessarily. It makes them safer drivers.

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Old 02-25-2013, 10:59 PM   #4 (permalink)
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I know the lights here from a lot of experience. Best possible scenario is to never slow down, while gliding down all of the very slight declines east of my home. Next best scenario is to be able to coast to the light and have it change before you are slowed down by all the rabbits who slammed on their brakes when they raced up to the red light.

After that you are wasting energy the instant you touch the brake pedal.

I used to worry about people running into my rear end when I applied the brakes. Now the only time I worry is when I get forced into a rapid stop by a tripped light that catches me just about perfectly wrong. I won't post my comments here.

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Old 02-26-2013, 09:45 PM   #5 (permalink)
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Old 02-26-2013, 10:23 PM   #6 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
Best rate of acceleration is fairly brisk, about the normal rate for traffic here which also has the benefit of not creating anger in those around you. Best mileage amounts to best conservation of momentum, which means avoiding braking at almost all costs, with the exception being, for me, light timing which when poorly executed will quickly destroy average mileage.
I don't average as good as many, but in my normal drive of 20 miles I have to contend with 23 traffic lights, most of which are bunched together in less than half of that drive. There is a very precise average speed (47-49MPH) that must be maintained to limit the number of lights caught from 0-7 in the 46 total in a round trip.
My blood pressure literally jumps when I get caught by one of the tripped lights that can never be timed, which is a shame when that could easily be corrected by placing a "no trip" time period exclusion in the light programming.

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Old 02-27-2013, 11:34 AM   #7 (permalink)
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As a 2010 Prius driver myself, I feel sorry when coasting to a red light, seeing all the rabbits change to the other lane beside me. I guess the rabbits are assuming that I will be accelerating as slow as a I decelerated. I can imagine their frustration when they see that they misjudged my acceleration intentions (I do like laying on a little bit on the acceleration - keeping it under or at 70 hp is still rather efficient in a Gen3 Prius in terms of BSFC, within 5% of max BSFC) Note I want to get from A to B almost as quickly as the next guy but with keeping my eye on efficiency. It can be amusing at times when I pass the typical gearhead driver that was trying with all his might, and the light turns green just as I'm passing him in the other lane. If he does make up a little time with his racing, those few seconds made up are all lost (and more) with the extra time at the pit stop (fueling).

Not just in driving but in any pursuit really, it's amazing how much more efficient people could become with a little more logic and a little less emotion. Now if Toyotal could get the next Gen Prius to recover a lot more regen and minimize the round-trip battery charge/discharge losses (maybe using supercaps), then even "not so smart drivers" could boost their city mpg.
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Old 02-27-2013, 12:08 PM   #8 (permalink)
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Quote:
Originally Posted by briank View Post
It can be amusing at times when I pass the typical gearhead driver that was trying with all his might, and the light turns green just as I'm passing him in the other lane. If he does make up a little time with his racing, those few seconds made up are all lost (and more) with the extra time at the pit stop (fueling).
The term "gear head" refers to an enthusiast of vehicles of one or more type that enjoys wrenching on their own vehicles. Gear head has nothing to do with driving style or techniques. Please don't insult me and millions more by stereotypically grouping us in with stupid drivers.

Time made up during a trip does not translate to a longer fuel stop. When a vehicle requires fuel typically it gives you notice and you refuel at your convenience. I have yet to see a driver peel off from a light then into a fuel station to top off the fuel he wasted.


If you ever have to come to Ohio just be aware that when coasting up to stop lights or stop signs don't be surprised the guy behind you guns it past you only to cut you off which causes you to brake.
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Old 02-28-2013, 09:39 AM   #9 (permalink)
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As a 2010 Prius driver myself, I feel sorry when coasting to a red light, seeing all the rabbits change to the other lane beside me.
I'm a 2005 Mustang driver and I always put in the clutch and coast to a stop, no matter how many people are behind me. A red light is a red light. If they don't like it, tough. I'm not going to waste gas to get to a full stop 5-10 seconds sooner.
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Old 02-28-2013, 01:35 PM   #10 (permalink)
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I'm a 2005 Mustang driver and I always put in the clutch and coast to a stop, no matter how many people are behind me. A red light is a red light. If they don't like it, tough. I'm not going to waste gas to get to a full stop 5-10 seconds sooner.
Yup. When the other drivers on the road are ready to buy gas for my car, I will drive faster to accommodate their need for my speed.

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