Why braking affects fuel economy for beginners along with tips

[briank]

Most drivers don't get why good braking techniques can be the biggest contributor to improved fuel economy when driving in varying speed situations, so rather than explain the physics of it, I thought giving examples of how to implement the technique will work better for those who might happen to stumble on this page after a web search. These tips assume flat, open road conditions. Always prioritize safety first.

Tips

- Driving 45 mph and light turns yellow 500 yards ahead, immediately take you foot off the gas. Maintaining the same speed and then stopping in the last 100 yards (as most people do) doesn't get you there any faster, but burns extra gas (to maintain your speed) for no good reason. Light may more likely turn green when you get there as it took longer to reach the light. Same principle applies when you see stopped traffic a half mile ahead on a freeway.

- Driving 45 mph and you have to turn in 500 yards, immediately take you foot off the gas. When you get to the corner, and say your speed is still 20 mph, go around the corner at 20 mph even if that means enduring some more sideways g forces. Know your cars cornering ability particularly in slippery conditions. You will consume less gas to get up to speed once you are around the corner as you are already moving at nearly 20 mph.

- Accelerate Quicker: Don't accelerate slowly if the road is open ahead and you don't anticipate any reason to brake. Most modern cars are most efficient at producing acceleration in the 2000 to 3000 rpm range. It is a mistake to think accelerating slowly and then "slowing down" with the brakes is best for mpg. "Slowing down" in mpg speak, means driving at 55 mph instead of 75 mph, where air drag consumes 75%+ of your fuel, and doesn't mean using the brakes to slow down.

- In slowing moving traffic that is speeding up and slowing and stopping, keep a distance such that you will seldom have to use your brakes, in this case accelerate just to the amount of speed that will allow you to not have to slow using the brakes. The distance from the car ahead of you will likely "accordian" back and forth. But all the traffic following you on your lane (for a significant distance back) will be saving fuel as you will be smoothing the traffic flow. In fact many hybrids get their best mileage in such conditions.

- If you have a manual transmission car, lay into the gas pedal (~75% travel) after up-shifting at or below about 2000 rpm. When you have to slow down (unanticipated) use the gears to slow you down because this will cut off fuel to the engine while your slowing by engine compression. Neutral coasting can save you more fuel, but as these tips are for normal drivers, for safety's sake I will not go into this. Hybrid drivers can glide (engine off, no re-gen drag).

- At drive-thrus or extremely slow traffic, just let the engine idle speed power your movements, revving and then braking to move a few yards, is almost the most inefficient one can be to move a given distance. But almost 99% of people do it.

Remember Braking is to compensate for the excessive acceleration one did just prior to braking. What supplied energy for that excessive acceleration? That's right excessive gas. Be single-minded and don't follow the crowd, because the crowd are often wrong.
If one did an analysis of brake pad wear between 2 identical conventional cars that did the same city commute, but one driver had almost no wear and the other was half worn in the same number of miles, I would almost guarantee you the driver will the least worn brakes would have significantly better mpg than the other. If cars computers figured a displayed economy score based on the amount of cumulative deceleration, people might start to see the correlation to mpg.

[user removed]

I just walk in an order my food, eat it, and drive away. I also will slow down more rapidly when approaching a light to see if I can avoid stopping altogether. On a good day I will only have to stop for 4-6 of the 46 lights on my usual route. Try to time it so you go through the intersection at the highest speed possible, within legal limits. In some cases that means stopping, or slowing down fairly rapidly. Other times you can coast through the light just as it changes and pass all the dummies that blasted, by you, to the light when it was red.

regards
Mech

[PressEnter[]]

Good tips.

[mikeyjd]

braking=the devil

[briank]

Originally Posted by Old Mechanic

I just walk in an order my food, eat it, and drive away. I also will slow down more rapidly when approaching a light to see if I can avoid stopping altogether. On a good day I will only have to stop for 4-6 of the 46 lights on my usual route. Try to time it so you go through the intersection at the highest speed possible, within legal limits. In some cases that means stopping, or slowing down fairly rapidly. Other times you can coast through the light just as it changes and pass all the dummies that blasted, by you, to the light when it was red.

regards
Mech

I agree with you, but try teach that to most drivers. Better to give the 99.9% of drivers a whiff of the concept of efficient braking, than direct the effort to the 0.1% who have fairly good mpg already. Regarding the timing, I wish they had an impending light change warning flashing signal 200 yards or so ahead of every traffic light, one would then have a good chance of knowing whether to lay off the gas or go for it. The signal not flashing would mean one will make it if traveling at the speed limit.

[Diesel_Dave]

Pretty good summary. The only thing I tweak a little is the references to specific rpm ranges (i.e., 2000-3000 rpm). That range will depend on the vehicle & engine. For example, my engine almost never sees 2,000 rpm and 3,000 is nearly the red line.

[briank]

Originally Posted by Diesel_Dave

Pretty good summary. The only thing I tweak a little is the references to specific rpm ranges (i.e., 2000-3000 rpm). That range will depend on the vehicle & engine. For example, my engine almost never sees 2,000 rpm and 3,000 is nearly the red line.

I would have loved to say accelerate at max BSFC, but then that would stump the audience this is aimed at, who may happen on this article. I feel 2000-3000 rpm acceleration is a good average range for most engines which conveys to the masses that getting good (not optimum) mpg is not all about accelerating like "grandma". I personally found in my 2010 Prius that accelerating reasonably fast (in the HSI mid PWR region) has little impact on my overall mileage, but non-optimum braking has a big impact (the low hanging fruit if you will). It's nice that I can have the fun of quick acceleration and have a 60 mpg average (displayed) 58 mpg (calculated), which I know would be better if drove below 60 mph on the highway. I tend to cruise around 63 mph (displayed) on the highway plus or minus 5 mph to keep within 3% of the peak BSFC.

[eufo]

Originally Posted by briank

I would have loved to say accelerate at max BSFC, but then that would stump the audience this is aimed at, who may happen on this article. I feel 2000-3000 rpm acceleration is a good average range for most engines which conveys to the masses that getting good (not optimum) mpg is not all about accelerating like "grandma". I personally found in my 2010 Prius that accelerating reasonably fast (in the HSI mid PWR region) has little impact on my overall mileage, but non-optimum braking has a big impact (the low hanging fruit if you will). It's nice that I can have the fun of quick acceleration and have a 60 mpg average (displayed) 58 mpg (calculated), which I know would be better if drove below 60 mph on the highway. I tend to cruise around 63 mph (displayed) on the highway plus or minus 5 mph to keep within 3% of the peak BSFC.

I think I'm the audience because I don't know what BSFC is... LOL! Seriously, though, what is it? I have always felt like accelerating too slowly isn't beneficial to MPG, so it is nice to see this confirmed.

There are a few owners of CR-Zs like mine that claim better MPG while using SPORT mode (accelerating quickly). I have tried SPORT mode a few times and haven't noticed any particular detriment except for extra power (read drag) dedicated to the A/C and also it tends to idle at stops more often instead of killing the engine.

[Fat Charlie]

The brake pedal is a tool used to turn gasoline into heat and brake dust.

[Diesel_Dave]

Originally Posted by eufo

I think I'm the audience because I don't know what BSFC is... LOL! Seriously, though, what is it? I have always felt like accelerating too slowly isn't beneficial to MPG, so it is nice to see this confirmed.

BSFC stands for Brake Specific Fuel Consumption. It's a measure of engine efficiency. The "brake" doesn't refer to the brakes on a vehicle, but rather a dyno brake (a tool used to measure the power output of an engine).

The best way to think of BSFC is the ratio of fuel input to mechanical energy output (lower is better). The units are typicaly lbs/hp-hr in US units & g/kW-hr in metric units.

Note that BSFC doesn't capture how much energy is being produced, only how efficiently that power is being made--i.e., how much fuel does it take to produce a certain amount of work.

Vehicle parameters (vehicle weight, amount of braking, vehicle speed, etc, drag, rolling resistance, etc.) determine the amount of work required to travel a certain distance (hp-hr/mi), whereas engine parameters (speed, throttle, intake temp, etc) determine the BSFC (lb/hp-hr). Put those two things together: (hp-hr/mi)*(lb/hp-hr) and you get lb/mi. All you need then is fuel density (lb/gal) to get miles/gallon.