Environmental Engineering Reference
In-Depth Information
Chapter 4
Sizing the drive system
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 0.15-0.3
g
, which for a 1,500 kg vehicle requires
an accelerating force,
F
, of 2,205-4,410 N. Adding friction elements and aero-
dynamic loading boosts this to greater than 6,000 N. Aggressive acceleration levels
are ~0.6
g
, which amounts to a tractive wheel force of 8,820 N and 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. Typical rubber tyre to asphalt road surface coeffi-
cient of friction is
m
= 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
m
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. A growing trend is the use of
larger diameter tyres that can match or exceed the tyre patch area of wider tyres on
smaller rims.
The question arises frequently about why automobiles require a power plant
rated over ten times the power needed to sustain a constant speed cruise. On a level
highway and at constant speed the automobile power needed to sustain that speed is
simply the rolling resistance of the tyres combined with the aerodynamic drag of
the specific body style. These two contributors of propulsion power are described in
the following sections. The remainder of propulsion power needed at the engine
goes to overcome driveline losses such as gearbox clutch and gear mesh losses as
well as final drive and other viscous losses. We'll neglect these losses as they are
small contributors to the dominant aerodynamic and rolling losses.
The accelerating power for an automobile is a simple application of Newton's
third law; that is, the power needed to propel the vehicle is countered by the ability of
the road surface to react against the vehicle's tyre patches and the tractive
effort applied to these by the vehicle engine. This also means that if the tractive effort
(i.e. the torque) applied to the driven wheels exceeds the road surfaces' ability
to push back, then the wheels will slip. So, we consider only the case where
the propulsion power to accelerate is within the capacity of the driven wheels tractive
effort.
