Overview

The vehicle power plant must be sized for the target vehicle mass, load requirements and performance goals. Vehicle propulsion system traction is set by the vehicle design mass and acceleration performance according to Newton's law, F = ma. Acceptable acceleration levels are 0.15 to 0.3 g, which for a 1500 kg vehicle requires an accelerating force F of 2205 to 4410 N. Aggressive acceleration levels are ~0.6 g, which amounts to a tractive wheel force of 8820 N or higher. The limit to tractive force is set by the vehicle mass in terms of normal force at the tyre patches in contact with the road surface. The typical rubber tyre to asphalt road surface coefficient of friction is ?=0.85; surface coefficient of friction is generally lower than these values due to air conditions, presence of dirt and oil films, etc. Tractive force limits at a tyre patch are given as ? F _Nqc, where the normal force is that due to quarter car mass. Tractive force at the tyre patch in excess of the traction limit results in wheel slip and a dramatic drop in tyre to road adhesion.

Passenger car propulsion power plants require peak power to vehicle mass ratios of 10 kW/125 kg for acceptable acceleration performance. Sports and luxury cars tend to raise this metric to over 13 kW/125 kg, whereas compact and sub-compact vehicles tend to ratios somewhat less than 10 kW/125 kg.

In this chapter essential guidelines...