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Old 08-28-2008, 05:56 PM   #1
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coming to a complete stop?

One thing I try to do when I am driving is leave a lot of space in front of me so I can coast up to a stop light. I try to time them so I don't have to stop. It really bothers me when I have to come to a complete stop and the light changes almost immediately.

So what I am wondering is how big of a penalty in MPG does it cost to come to a complete stop? Would it take the same amount of fuel to accelerate a car from 0 to 10 Kph as it takes to accelerate from 10 to 20 Kph? It seems to me that it would take a lot of energy to just get the car moving from a stop, as opposed to going from 10 to 20 Kph, even though it is the same amount of acceleration.

I am sure this is easily solved mathamatically, but that is a bit beyond me.

Thanks

Peter
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Old 08-28-2008, 06:39 PM   #2
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The biggest expendature is just to get the vehicle to move once it has come to a complete stop. Remember Physics class? A body in motion will want to stay in motion, a body at rest will want to stay at rest... Once you get it moving it will want to keep moving so not as much energy is required . By not coming to a complete stop you are saving the energy required to get the car moving again.

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Old 08-29-2008, 04:36 AM   #3
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another factor is the kinetic(probably misspelled) energy you gain from it too. from the first example 0-10 vs 10-20 lets say it takes the same amount of fuel. if you were to let off the gas afterwards, you would roll much farther at the higher speeds even if it took you more gas to get to the 20 thus paying for the gas you used.

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Old 08-29-2008, 04:53 AM   #4
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I've been trying to figure out the best way to word this, so it can be added to the list of gas savings tips.

The basic idea is this: If you see a red light down the road, hit the brakes hard and fast. If you are doing 40 MPH, you want to slow down to 20 MPH as quickly as possible. Then, keep going at the reduced speed toward the stoplight, with the hopes that you'll be able to keep your 20 MPH.

What most people do is keep going 40 until the last possible second, then slow down to a dead stop. Obviously it takes more fuel to accelerate from 0MPH than 20 MPH to resume your speed.

It can be tricky, especially if it is a traffic light that you're unfamiliar with. But if you have a good idea of the lightcycle timing, you'll be able to take a decent guess at the speed you'll need to slow down to, based on your distance from the light.

I'm sorry I can't quantify it, but it is definitely a way to save some gas (and wear on your brakes)!

-Bob C.
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Old 08-29-2008, 06:21 AM   #5
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Hitting the brakes is never good. Popping it into neutral to coast or changing down to attempt DFCO might be okay. I think you'll lose more maintaining 20mph plus what you burned off in the brakes, than DFCO coasting even if you need to stop, at least that momentum went into stopping the motor sucking fuel for 30 secs or so.
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Old 08-29-2008, 07:04 AM   #6
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Quote:
Originally Posted by RoadWarrior View Post
Hitting the brakes is never good.
In the case I tried (and apparently failed) to describe, you have to hit the brakes sometime. Therefore it's better to slow to 20 MPH early, then 0 MPH at the last possible second. It's all about timing the stoplight.

And you don't have to maintain the 20 MPH- perhaps you use the brakes to slow to 25, then coast to 20 MPH by the time you reach the redlight (just as its turning green).


-BC
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Old 08-29-2008, 07:22 AM   #7
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What I do is if I see the light red far up ahead I immediately take my foot off the accelerator and coast. If I've seen it far enough ahead I will just idle @ 3 MPH until the light changes. If the light changes when I'm a "medium" distance I will downshift & DFCO to the light. If I'm close to the light when it changes and I know its going to be a long light I will shift into Neutral, and cut off the engine, then restart when I see the cross street's light turn yellow. Of course for these strategies to work you really have to know how long they are going to stay red, so you can select the best one for that time.

-Jay
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Old 08-29-2008, 08:07 AM   #8
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The more speed you can maintain, the less fuel you will use getting back up to full speed. Stopping is the worst, 5mph is better than 3mph etc. It takes more fuel to accelerate from 10-20 mph than from 0-10 mph, but only because in addition to changing the speed 10mph, you also have to overcome the increased air resistance. There is 4 times as much air resistance at 20mph as at 10mph. One thing you're forgetting Jay is that not only does a body at rest tend to remain at rest, but a body at 10mph tends to remain at 10mph, so anytime you want to make a change in velocity, the amount of energy required depends solely on how much of a change you want to make, not the starting velocity (we're talking theoretical here, not including friction or air or anything else)
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Old 08-29-2008, 08:19 AM   #9
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Quote:
Originally Posted by bobc455 View Post
The basic idea is this: If you see a red light down the road, hit the brakes hard and fast. If you are doing 40 MPH, you want to slow down to 20 MPH as quickly as possible. Then, keep going at the reduced speed toward the stoplight, with the hopes that you'll be able to keep your 20 MPH.
Quote:
Originally Posted by RoadWarrior View Post
Hitting the brakes is never good. Popping it into neutral to coast or changing down to attempt DFCO might be okay. I think you'll lose more maintaining 20mph plus what you burned off in the brakes, than DFCO coasting even if you need to stop, at least that momentum went into stopping the motor sucking fuel for 30 secs or so.
A combination of those two ideas is what I do.

The most important part is to carry as much kinetic energy as possible through the light, so you want to get there when you can remain going as fast as possible. If you can get there doing 20mph and the vehicles in front of you are doing 20mph by the time you catch up to them, that's the best you can do.

Now, the question becomes how and when to bleed off the extra speed. I do BC's quick brake stab first, then DFCO the rest of the way in my car; and in my truck, which doesn't DFCO worth a damn, I just do the brake stab and coast in neutral.

Discarding energy early with the brakes sucks, but having to come to a complete stop is far, far worse -- then you have to discard the rest of the energy anyway. DFCO coasting and then having to stop saves a little bit of energy idling, but burns a LOT of energy accelerating.

Also, it doesn't always work so well with traffic behind you...
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Old 08-29-2008, 08:33 AM   #10
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Well, it has been 15 years since I've been in a physics classroom.

-jay
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