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Old 02-25-2013, 07:08 PM   #1 (permalink)
briank
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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.


Last edited by briank; 02-25-2013 at 07:13 PM..
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