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Old 02-13-2012, 12:54 AM   #21 (permalink)
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Quote:
Originally Posted by bandit86 View Post
240V and 90A is 21450 Watts. at 746W per hp it's taking 28.8 hp to push that car along. how can 28.8 hp equal 200 pounds of force required to push it along? something not right

is there a page for how much hp is required to push a car along at highway speeds? would like to see some figures
Hello bandit86,
To answer your second question, if you mean a list of actual cars, I dunno. But if you look at the top of this page, see Garage & Tools? Click Tools and then click "Aerodynamic & rolling resistance, power & fuel consumption calculator" you get here.
And you can see the HP for any car if you can think of the weight and wind drag and stuff like that.
Now about your first comment. the power required is the force times the speed. So 200 lbs X 60 mph. Of course we don't think of power in terms of lb mi/hr, we like ft lb/ sec. And 550 ft lb/sec is one hp. So 200 X 60 = 12000
now divide by 3600 seconds in an hour = 3.3 multiply that by feet in a mile 5280 X 3.3 = 17600 ft lb/sec. Divide by 550 and get 32 hp. So 200 lb of force at 60 mph is 32 hp.
There is a little less than 50 hp-hr available in a gallon of gas. Engine efficiency at cruise might be between 10% and 20% say 15% So you'd get about 7.5 hp-hr per gallon. Using 30 hp for an hour takes 4 gallons, and you went 60 miles, so 15 mpg. And that's just cruising. If you need to get to that speed from a stop, and then throw away your momentum braking again, well 10 mpg is reasonable for a big heavy old brick.

-mort

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Old 02-13-2012, 01:17 AM   #22 (permalink)
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Originally Posted by bandit86 View Post
Holly crap it's close. What about diesel?
Hi bandit86,
If you mean replacing a gasoline engine with a diesel, what should you expect fuel consumption wise? Diesel has about 10% more energy per gallon than gasoline. But the diesel cycle is way more efficient at cruise conditions. Maybe better than 30% efficiency at the 30 hp or so needed. So about 50 hp-hr per gallon at 30% efficiency is 15 hp-hr available. So to get 30 hp you need 2 gallons per hour, and at 60mph that's 30 mpg. Now this assumes that the diesel engine is plenty big enough to produce cruise power at high efficiency, and yet not so heavy that more force is needed to push the truck.

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Old 02-13-2012, 05:29 AM   #23 (permalink)
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I meant the diesel version, 1/3 smaller. It turbocharged gets an easy 30 mpg
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Old 02-13-2012, 09:29 AM   #24 (permalink)
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Quote:
Originally Posted by bandit86 View Post
I meant the diesel version, 1/3 smaller. It turbocharged gets an easy 30 mpg
I don't understand your question.
The diesel version gets 30mpg because just like mort said, diesel fuel has more energy in it per gallon and diesel engines are more efficient so they pull more of that energy out of the fuel, wasting less of it, because a diesel engine is always running wide open throttle and only limiting the fuel, then a throttled turbo (dump gate) can help boost power output when needed without hurting fuel mileage, allowing a smaller, more efficient, lighter weight engine to be used.

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