1634. Mass Properties and Automotive Longitudinal Acceleration

Paper

Abstract

Automotive longitudinal acceleration is dependent upon a large number of interconnected parameters, some of the most important of which are mass properties. The purpose of this paper is to explore the individual mass property effects.
The approach taken to achieve this purpose was to decouple the parameters by means of a computer simulation of an automotive acceleration 'run'. Each individual mass property parameter was then varied over a wide range while all other parameters were held constant. The acceleration results so obtained were plotted, and the conclusions were drawn from the behavior thus exhibited.
Several conclusions have been drawn from this effort. First, the effects of a mass property parameter variation are not necessarily constant over the entire speed range. For instance, increasing weight tends to cause an almost linear increase in the elapsed times for the lower speed ranges, but the higher speed ranges exhibit even greater time increases in an almost parabolic relationship. This is a matter of the increased rolling resistance associated with greater weight making itself felt at the higher speeds.
The longitudinal center of gravity (kg) and the vertical center of gravity (vcg) both affect acceleration through traction. If the situation is not traction critical then cg relocation can be of no help in obtaining better acceleration. When a situation is traction critical then acceleration is much more sensitive to change in kg then in vcg.
Increasing the vertical center of gravity tends to benefit the acceleration of rear wheel drive vehicles. For rear wheel drive vehicles the vcg generates increased traction through weight transfer. In the case of front wheel drive, the vcg can have no beneficial effect as the weight transfer is in the direction away from the drive axle; minimizing the vcg becomes the priority. Due to the effect of weight transfer, a front wheel drive vehicle will always be inferior in acceleration to a rear wheel drive vehicle if everything else is equal and the propulsive capability is great enough.
In general, a rotational mass is disproportionately detrimental to acceleration because it has to be accelerated both rotationally and translationally. The greatest return for the effort involved in mass reduction can be obtained from a reduction in rotational masses.
The engine rotational masses, other than the flywheel, represent a special case outside the scope of this paper. Vehicle characteristics and use demand a certain minimal rotational inertia for the flywheel to counteract engine stall-out tendencies at the onset of acceleration and to ensure smooth engine operation. In fact, a higher flywheel inertia can produce an initially quicker vehicle. This initial response has to be considered against the detrimental longer-term effects of accelerating a large flywheel inertia throughout the speed range. Flywheel design involves a high degree of compromise.
Rev A – 2023