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Old 07-22-2009, 09:42 AM   #32 (permalink)
Deezler
Master EcoModder
 
Join Date: Apr 2008
Location: Michigan
Posts: 261

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

The Beast. - '03 GMC Sierra 2500HD SLT
90 day: 12.86 mpg (US)
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Quote:
Originally Posted by bwilson4web View Post
The average speed of the EPA City cycle is 28 mph so 30 mph happens to be very close.
Exactly, I said steady state, and you are talking about average speed. Even the city drive cycle consists of multiple periods of acceleration followed by cruising at speeds a good bit higher than 30 mph. Furthermore I still think that driving a car in an ultra urban environment is a pretty stupid way to get around.

Quote:
Originally Posted by bwilson4web View Post
Because of the low vehicle power demands, ~5 hp, it can be treated as a constant for engines of similar displacement. So we don't really have to bother with "a function fo speed (rpm) primarily and load second." A steady speed eliminates another red herring.
Playing with the aerodynamic calculator on this site just now one can find required road load power levels at 30 mph of anywhere from ~2.5 HP for a small car to over 10 HP for a truck or SUV. But I was referring to engine speed (rpm) here, not vehicle speed.

Quote:
Originally Posted by bwilson4web View Post
Actually it is fairly accurate in the case of vehicles that must run their engine to maintain a constant speed. As pointed out, the 5 hp starting motor power gives a good approximation of the engine, internal power loss at low power settings. To provide an equal amount of motive power, 30 mph requires pretty close to the extra 5 hp needed.
Again you are lumping all IC engines into one number. I measure internal engine friction at work (auto industry supplier dyno lab). A large V8 engine we just ran consumes 17 HP just to spin at 2000 rpm. This engine of course has the low end torque capability to drive most cars at less than 2000 rpm, so its FE performance is going to depend entirely upon vehicle gearing. A small 4 cyl engine will often show less than half that much power loss.
So your average engine / vehicle approximations are crude at best. The balance point of vehicle power requirements to engine friction power consumption will vary.

Quote:
Originally Posted by bwilson4web View Post
Seriously, do diesel advocates really think they'll get in excess of 100-115 MPG as the Prius team did in August 2005?
Probably not (although 80+ mpg wouldn't surprise me). I hadn't seen your data, and it is impressive. How long a period of time do you obtain your data for? I would imagine once the battery is drained your mileage plummets. Hence the advantage gained in city driving with regen braking. But long stretches of road where one can drive steady state at 30 mph don't really exist, so I still find the whole conversation kind of moot. For the same reason I'm not going to offer to test my own vehicle. I bought it for highway commuting.
Oh and please refer to AMCI obtaining 38 mpg city for the new VW diesel. EPA is a pile of Sh*t.
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