Vehicle performance

The performance of a vehicle is determined by the following factors:

• maximum cruising speed

• gradeability

• acceleration

Maximum Cruising Speed

The maximum speed of a vehicle is defined as the constant cruising speed that the vehicle can achieve with full power plant load on a flat road. The maximum speed of a vehicle is determined by the equilibrium between the tractive effort of the vehicle and the resistance and maximum speed of the power plant and gear ratios of the transmission. This equilibrium is:

(30)

equation 30 shows that the vehicle reaches its maximum speed when the tractive effort, represented by the left hand side term, equals the resistance, represented by the right hand side. The intersection of the tractive effort curve and the resistance curve is the maximum speed of the vehicle, Figure 9.

 

Figure 9: Tractive effort of an electric motor powered vehicle with a single speed transmission and its resistance

For some vehicles, no intersection exists between the tractive effort curve and the resistance curve, because of a large power plant. In such a case the maximum speed of the vehicle is determined by the maximum speed of the power plant. This maximum speed is given by

(31)

Gradeability

Gradeability is defined as the grade angle that the vehicle can negotiate at a certain constant speed. For heavy commercial vehicles the gradeability is usually defined as the maximum grade angle that the vehicle can overcome in the whole speed range.

When the vehicle is driving on a road with relatively small grade and constant speed, the tractive effort and resistance equilibrium can be expressed as

                                                                      (32)

Hence,

                                                       (33)

where

                                                                                (34)

The factor d is called the performance factor. When the vehicle drives on a road with a large grade, the gradeability of the vehicle can be calculated as

                                                                                     (35)