Kinetics, coasting, and you

[Loren]

Okay, I've been resisting, but you're overlooking something here.

You've examined how much kinetic energy you save or don't save on coasting down to 20 mph vs 10 mph or a full stop, and that's great. But, what about the other side of the coin? When you coast down to 20 mph and DON'T stop, you don't have to accelerate from a stop. So, a properly timed coast-down that allows you to MAINTAIN that 20 mph speed instead of slowing to 10 mph or stopping is more efficient.

What I often (very often) do is when I see something happen in front of me (say, a light just turned yellow 2 blocks ahead) is to hit the brakes immediately and scrub off some speed EARLY. This allows me to coast for a longer period of time. Coasting in DFCO, this costs me less than sitting at the light and idling would. (and I'm not one of those who wants to shut the ignition off at every light) My goal is to maintain as much momentum as I can rather than coming to a stop, thus avoiding the acceleration from zero mph.

I'll let you do the math, but accelerating from a full stop is about the least efficient thing you can do in a car, even with a light foot.

[sbergman27]

Originally Posted by Loren

Okay, I've been resisting, but you're overlooking something here. You've examined how much kinetic energy you save or don't save on coasting down to 20 mph vs 10 mph or a full stop, and that's great. But, what about the other side of the coin? When you coast down to 20 mph and DON'T stop, you don't have to accelerate from a stop. So, a properly timed coast-down that allows you to MAINTAIN that 20 mph speed instead of slowing to 10 mph or stopping is more efficient.

No. I'm not overlooking all the many possibilities. I'm just trying to start out simple. The intent is to present the table in an understandable way and then let people do creative things with it.

Of course my initial example with the profane and rude SUV driver (I always imagine him driving an SUV) was fanciful. But if people stop thinking linearly and start thinking in terms of the velocity squared relationship, it helps their intuition come to better seat of the pants FE decisions.

Avoiding braking from 40 to 30 might be significant. But 30 to 20? 20 to 10? Probably not. 10 to 0? No way. And the corresponding accelerations from 0 to 10, 10 to 20, and 20 to 30 are on the same, relatively minor scale.

-Steve