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Old 07-20-2009, 12:40 PM   #30 (permalink)
Master EcoModder
Join Date: Apr 2008
Location: Michigan
Posts: 259

Bio Deezler (sold) - '03 Volkswagen Jetta GLS TDI
90 day: 50.78 mpg (US)

The Beast. - '03 GMC Sierra 2500HD SLT
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Who the heck drives steady state 30mph anyway? Speeds this low are reserved for neighborhoods and ultra urban areas where you probably ought to just ride a bike or walk anyway. Even a 35mph limit street is usually able to handle 40mph where this almost becomes a non issue.
I really don't understand how engine friction at 30 mph is a "problem". Internal engine friction is a function of speed (rpm) primarily and load second. Reductions in pumping losses and inherent efficiency increase in combustion under higher loading more than make up for the latter, too. Engine speed to maintain constant vehicle speed is dictated by gearing alone, so why would 30 mph be the "problem" spot? Whether you are traveling 30 mph or 60 mph at 2000rpm, you nearly have the same internal frictional losses (and hence fuel used to overcome those losses). Its really more a problem of poor efficiency at low speed and load requirements, and even then its a fully sliding scale.

And the comparison between engines is not moot. A small 4cyl engine will consume far less fuel due to internal friction compared to a large v8 dropped into the same vehicle. Saying 50% at 30mph is a very poor blanket statement. Downsizing the engine and focusing on internal friction reduction still offer great promise for fuel consumption improvement and the continual evolution of the ICE.

I would be interested to compare the FE of a prius steady at 30mph vs. a diesel VW allowed to run P & G in the 25-35 range as well.....
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