2nd Gen Non-Engine/Transmission Ideal RPM's.....Is it true?

[camnicolson]

I always thought engines were at there most efficient at or close to their peak torque rpm, which would put it right around 1,800-2,000 rpm on the Cummins.



Cam

 

[truckstom]

Originally posted by jponder

I'll bet almost any truck will see a gain of 5MPG just by slowing from 70 to 50. I bet that truck that was running 80 would gain 7-8MPG by slowing to 50. [/B]

But who want's to drive 50 mph..... you'd get run plum over down here in Texas... ...

 

[nwilliams98]

tbrudder,

In case you hadn't considered this before:



Fd=Cd*Af*V^2



Fd=drag force

Cd=coefficient of drag

Af=frontal area

V=velocity



Power=Fd*V



Therefore the amount of horsepower required to go a certain speed increases with the cube of velocity. You need 8 times as much horsepower to go twice as fast. This of course neglects rolling friction but applies quite well to your planes.



BTW. I'm doing my Mech E thesis on light truck drag for the Army's national automotive center. It basically consists of a CFD optimization. So far I've reduced the drag on a Ram by 33%. I'll post the results, and methodology to the board when I finish. (If the Army does not mind).

 

[Steve Campbell]

Nwilliams 98....

I, for one, would be very interested in that information.

 

[tbrudder]

Originally posted by nwilliams98

Fd=Cd*Af*V^2

Yeah, aeronautical and automotive aerodynamics are basically the same, here, though we usually use wing planform area instead of frontal area, and the Cl term factors in a lot more at higher speeds. If I were in a nitpicky mood, I'd point out there's a missing 0. 5*rho term in there, too

Power=Fd*V



Therefore the amount of horsepower required to go a certain speed increases with the cube of velocity. You need 8 times as much horsepower to go twice as fast. This of course neglects rolling friction but applies quite well to your planes.

Forgot to mention that part. We're basically on the same page here. Airplanes are a bit simpler for drag--don't have to figure the rolling resistance and driveline losses are a lot simpler.

BTW. I'm doing my Mech E thesis on light truck drag for the Army's national automotive center. It basically consists of a CFD optimization. So far I've reduced the drag on a Ram by 33%. I'll post the results, and methodology to the board when I finish. (If the Army does not mind).

I, too, would love to see this when you're done. I wish I'd done more stuff with the SAE back in college. I don't really have any CFD capabilities anymore, either, though I did try a simple freestream drag calculation using values for Cd and Af I found on the internet, though 0. 500 and 32. 9 ft^2 don't sound quite right to me. You have any initial Cd values handy?



Good luck with you thesis; keep us posted.



--Ty

 

[nwilliams98]

I'm not sure of the exact Cd but think it is around . 55.







I'm running simulations to calculate the viscous and pressure forces directly so the Cd will be calulated later once I settle on a shape.



At 70mph you are looking at about 1000 N of drag for a stock 4x4.



I've never thought the Fd equation (mentioned before) was a perfect fit using Cd as basic shape fudge factor anyway. There is probably dependence between Cd and V.

 

[tbrudder]

Originally posted by nwilliams98

I'm not sure of the exact Cd but think it is around . 55.







I'm running simulations to calculate the viscous and pressure forces directly so the Cd will be calulated later once I settle on a shape.



At 70mph you are looking at about 1000 N of drag for a stock 4x4.



I've never thought the Fd equation (mentioned before) was a perfect fit using Cd as basic shape fudge factor anyway. There is probably dependence between Cd and V.

Somewhere else, I found a Cd value of 0. 500, though I'm not sure of the accuracy. Too bad Dodge doesn't publish this stuff--I know BMW and Mercedes put the Cd in their brochures.



Cd is pretty accurate when you've got fairly simple shapes in a uniform flow. Works quite well on airplanes, though it gets stickier the faster you go. Cd gets replaced with a half-dozen coefficients (parasite drag, form drag, wave drag, etc). Cd itself shouldn't change much (that is, the individual Cd coefficients shouldn't), but as velocity increases, some become dominant over the others.



Also, the underside of an auto is not at all simple or streamlined, and the disturbance of the road surface comes into play.



If you've got the equipment, Navier-Stokes and CFD are the way to go.



--Ty