# Vehicle-specific power

The concept of vehicle-specific power (VSP) is a formalism used in the evaluation of vehicle emissions. The idea was first developed by J. L. Jiménez (Jiménez 1998) at the Massachusetts Institute of Technology.[1][2] Informally, it is the sum of the loads resulting from aerodynamic drag, acceleration, rolling resistance, and hill climbing, all divided by the mass of the vehicle.[1] Conventionally, it is reported in kilowatts per tonne,[1] the instantaneous power demand of the vehicle divided by its mass.[2] VSP, combined with dynamometer and remote-sensing measurements, can be used to determine vehicle emissions.[2]

The United States Environmental Protection Agency held a "modelling shootout" in 2001, to help with the development of its (then) new MOVES (motor vehicle emissions simulator) vehicle emissions model. Two of the four modelling metholodogies in the shootout, one from North Carolina State University (Frey 2002) and one internal to the EPA, used vehicle-specific power metrics.[3][4] MOVES was eventually implemented using vehicle-specific power as its primary metric.[4] (See Koupal et al. 2002, § 7.1.3.1 for the EPA MOVES draft VSP specification.)

## Formulae

VSP can be represented by several mathematical formulae. Haibo Zhai of North Carolina State University provides the following formula:

${\displaystyle VSP=v\times (a+g\times \sin {\phi }+\psi )+\zeta \times v^{3}}$[5]

where:

v
vehicle speed (in metres per second)
a
vehicle acceleration (in metres per second per second)
g
acceleration due to gravity (gee, in metres per second per second)
ϕ
ψ
rolling resistance coefficient (metres per second per second)
ζ
drag coeficient (reciprocal metres)

Jiménez' own formula is:

${\displaystyle VSP={\frac {power}{mass}}={\frac {{\operatorname {d} \over \operatorname {d} t}(E_{kinetic}+E_{potential})+F_{rolling}\cdot v+F_{aerodynamic}\cdot v+F_{internal}\cdot v}{m}}}$ (Jiménez et al. 1999)

## References

### Cross-reference

1. ^ a b c NAP2001, p. 108.
2. ^ a b c NAP2000, p. 184.
3. ^ Barth, Younglove & Scora 2005, p. 10.
4. ^ a b Huai et al. 2005, p. 9595.
5. ^ Zhai 2007, p. 56–57.