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.