Re: Manual trans driving, saving fuel - coast or downshift? - Saab General Bulletin Board - Saabnet.com
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Re: Manual trans driving, saving fuel - coast or downshift?
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Posted by Craig [Email] (more from Craig) on Wed, 15 Mar 2006 13:47:20 Share Post by Email
In Reply to: Manual trans driving, saving fuel - coast or downshift?, Caarma [Profile/Gallery] , Wed, 15 Mar 2006 13:06:19
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I think it’s really more a matter of energy conservation and how that pertains to fuel use.

It’s true that a modern engine management computer can shut the fuel off entirely when you are in gear but your foot is off the gas. However, the pumping losses (i.e. engine braking) will use far more energy (but not fuel) than the engine idling.

Thus you might think it’s always more economical to coast to a stop as it uses less vehicular energy. And in theory this is true if you allow unlimited stopping distances and are calculating fuel use in miles per gallon. However, in the real world you are usually confined to a certain stopping distance before you reach the stop light, etc. In this case you have no hope of recovering the energy that you loose through the pumping losses of the engine as you cannot use extra costing distance to recoup miles per gallon. Thus your goal because to use as little fuel as possible before you stop, in which case engine braking (and its consummate use of no fuel) is the most economical method.

In this very specific and theoretical example, I believe engine braking is more economical. Of course, this is just a microcosm of driving. To actually achieve maximum fuel economy you want to conserve energy, so in a general sense, I think it makes sense to coast as much as possible and use the brakes and engine braking as little as possible.

Wow, that was long and boring. Does it make sense?

Craig
’87 9kT 295k


posted by 65.114.26...

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