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Old 11-18-2013, 02:26 PM   #1 (permalink)
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Why is 'driving with load' better than constant speed

Why is 'Driving With Load' (DWL) or 'constant throttle' (CT) better than constant speed? Given that we want to minimize fuel consumption but still arrive at our destination in a reasonable time, if we only consider the losses (aero and friction), the optimum solution is constant speed. There is a trade off between speed and economy. This is because the aero loss increases as the square of speed.

Is there some engine efficiency factor I'm missing? I see how pulse & glide works by putting the engine into an efficient regime on the pulse, and drastically reducing engine friction on the glide, but I don't see how that comes into play for DWL or CT.

Slowing down on the hills saves fuel on the hill, but also takes longer. Wouldn't it be better to just slow down the average constant speed?

Perhaps its just a conservation of energy thing? If you go up a hill and arrive at the top at 0 speed, then coast down the other side you've minimized the energy consumption for that event (up and down the hill). In practice you'd want to keep it in high gear and roll over the top at your minimum high gear speed. If you can't coast down the hill faster than your normal cruise speed and make up the time you've lost on the climb you're gaining economy but losing time. If you accelerate with throttle on the descent I don't think you'll gain much, if any, in fuel economy.

The articles I've found didn't (including the wiki here) did not explain the mechanics of DWL at all. If someone can explaing it or point me to something I'd appreciate it.

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