Drlving Techniques Question

LEAFfan said:

Someone commented earlier that the shifter wasn't made for so many shifts

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I didn't see this comment, but FYI the Shifter mechanism is hall-effect, and has no contacts to wear out. It should outlast all other switches in the car easily.

-Phil

LEAFfan said:

If the hill isn't too steep, you will definitely do better in 'N'. Otherwise, on steep hills, you may build up too much speed.

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I think this is probably true in most cases, but it isn't "obviously" true due to the following reasoning:

• In the first place, let's exclude any hill not steep enough to cause a speed increase. I'm not interested at the moment in any possible advantages of pulse & glide, and leaving the car in gear on a slope exactly steep enough to maintain speed will not use any motor energy, so is equivalent to coasting.
• Speeding up will increase air resistance, and in fact the drag will go up as the square of the velocity. That's why you use more energy when you go faster.
• So it is a given that speeding up will use more energy than holding your speed. That loss of energy will show up as you try to continue coasting past the bottom of the hill.
• Conclusion: coasting which involves speeding up can never allow you to recover 100% of the energy.

Of course using regen can never recover 100% of the energy, either, so neither is "obviously" more efficient than the other.

• We don't have a very good handle on the efficiency of regen. Some people claim that it has to be far less than 50% due to all the energy conversions involved (mechanical, AC electrical, DC electrical, chemical, DC electrical, AC electrical, mechanical), while a few others claim they have measured as much as 80%. I think I remember reading 90% once on this board!
• But we should be able to calculate a theoretical efficiency for coasting.

Example: Driving at 60 mph, you allow yourself to coast up to 75 mph.
The relative drag forces between 60 and 75 are (75^2)/(60^2) = 5700/3600 = 1.58.
Since drag uses most of the energy at these speeds, we can say to a first approximation that you will be using 58% more energy at 75 than at 60.
Of course 75 was just your peak velocity, and your acceleration was probably fairly constant, so your average velocity was around 67.5. You would only be using 27% more energy at that speed.
But WARNING! That isn't the right number, because you are on an x-squared curve, so the top half will more than overbalance the bottom half. Since we are just approximating, let's say you lose 30% for this case.

So, 70% efficient for coasting, somewhere between 40% and 90% efficient for regen. Which is better? My personal guess is they might be about the same for this case. But note that the case I chose may be very close to LEAFfan's definition of "too much speed", if not already over that line. Coasting would have more of an advantage when the speed change was less.

As I said at the top, I think LEAFfan's statement is probably true in most cases.

Ray