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Old 06-17-2008, 04:36 AM   #1 (permalink)
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100+ hypermilling / ecodriving tips article

There's a point on it, if anyone could clarify for me, I'd like to take up.

43) The most efficient way to slow down

When you *have* to slow down, here's an approximate heirarchy of methods, from best to worst.

1) coasting in neutral, engine off (ie. roll to a stop);
2) coasting in neutral, engine idling;
3) regenerative coasting (hybrid vehicles)
4) regenerative braking (hybrid vehicles)
5) coasting in "deceleration fuel cut-off" mode (in gear, above a certain engine RPM)
6) conventional friction braking (non-hybrid or hybrid)

Choosing the right method depends on traffic conditions (following vehicles) and how quickly you need to stop.
#5, I would think should be placed at #2, and safer/better than #1. It's my understanding that whilst coasting in gear in a modern injection engine (although this could perhaps just be a modern diesel?), the ICE stops consuming fuel, and runs off the kinetic energy you have consumed. Whereas, if you go into neutral, your engine will still be ticking over, at say 800rpm, and consuming fuel. The only reason I can see that #2 would be higher is because you can coast for longer like that, although that means you don't stop as quickly, which of course equates to energy not lost, but when you're idling, you're still burning fuel. Have I overlooked something major? Or perhaps American petrol engines are different?


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Old 06-17-2008, 07:24 AM   #2 (permalink)
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You are correct in assuming that #5 slows you down much faster. Since we are talking about distance per amount of fuel used, #2 is better overall since you do not loose speed very fast. Therefore, your fuel use increases, but the distance you travel greatly increases.
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Old 06-17-2008, 10:28 AM   #3 (permalink)
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The difference in coasting distance is huge. You're absolutely correct about it still consuming fuel. I'd say it depends on your speed. If you're going 55mph or more, coasting at idle isn't much different than with the engine off. If you're going 5mph, you're almost idling at a stop, which gives you zero mpg. As always, "it depends".


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