Projeto eletrico

Mechanical Engineering ME 481 Vehicle Design Fall 2000

Lecture Notes

By Richard B. Hathaway, Ph.D., PE Professor Mechanical and Aeronautical Engineering

Section 1 Energy Consumption and Power Requirements in Design

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Aerodynamics and Rolling Resistance
GENERAL FORMULAS - AERODYNAMIC Dynamic Pressure:

Pd
Drag Force:



1  v2 2
1  v 2 A f ( RE ) 2

Fd



Fd
Aero Power



1  v 2 Cd A 2

Fd



1 (1.2) Cd A ( v  v0 ) 2 2

P=
Cd = A= v= coefficient of drag projected frontal area (m2) vehicle velocity (m/sec)

 3 V A f(RE) (2) f(RE) V0 =  = air density  1.2 kg/m3 = Reynolds number headwind velocity

ENGLISH UNITS
-6 3 HP aero = (6.93 X 10 ) C d A V

where: A = area (ft2)

V = velocity (MPH)

Cd = drag coefficient

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SI UNITS

     1 kW  1 .2  V 3  PKW = Cd A  3  1000 W   (2)   3600          1000  
-6 P aero = (12.86 x 10 ) x C d A V (V  V0 ) 2

P V Cd

= power (kw) = velocity (KpH) = drag coefficient

A V0 

= area (m2) = headwind velocity = 1.2 kg/m3

GENERAL FORMULAS – ROLLING RESISTANCE
ENGLISH UNITS

HPrr = Crr W x

V 375
W = weight (lbs) V = velocity (MPH)

where: Crr = coefficient of rolling resistance
SI UNITS

 9.81   x C rr x M x V Prr =   3600  Prr = (2.72 x 10- 3 ) x C rr x M x V where: P
M = power (kw) = mass (kg) Crr V = coefficient of rolling resistance = velocity (KpH)

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TRACTIVE FORCE REQUIREMENTS.
Vehicles require thrust forces, generated at the tires, to initiate and maintain motion. These forces are usually referred to as tractive forces or the tractive force requirement. If the required tractive force (F) is broken into components the major components of the resisting forces to motion are comprised of acceleration forces (Faccel = ma & I forces), Gradeability requirements (Fgrade), Aerodynamic loads (Faero) and chassis losses (Froll resist ). F = Faero + Froll resist + Fgrade + Faccel = (/2) Cd A v2 + Crr m g