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Efficiency facts.
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Posted by Ravi [Email] (more from Ravi) on Thu, 31 Jul 2003 12:44:16 Share Post by Email
In Reply to: Re: disagree with one point..., TML [Profile/Gallery] , Thu, 31 Jul 2003 12:15:15
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The least amount of gas your engine can EVER use is at idle (yes it uses less when it is off, but that's another story).
The best mpg comes when running at idle but moving fast.
Hence pushing in the clutch and coasting downhill at 800 rpm idling will give you the absolute best mpg on a Saab.

If you stay in gear downhill, the drive wheels must turn the engine, which creates braking.
To overcome this braking effect, you must add fuel (if you want to keep going the same speed).
Hence going down hill in gear uses slightly more fuel than coasting with the engine at idle.

On the acceleration side, when you are running with a partially open throttle, you have an engine that is sucking through a straw, which takes work (energy). However WOT usually comes at the cost of fuel enrichment, so to first order, the most efficient acceleration will occur at the throttle opening that is just below fuel enrichment, typically 5000 rpm on Saabs.

Incidentally, the diesel's greatest efficiency compared to the gas engine is because it has lower pumping losses. There is no throttle and the intake pipes are always wide open. You simply squirt more fuel to go faster. The actually combustion efficiencies aren't that different.

-Ravi

posted by 198.20.40...

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