Environmental Engineering Reference
In-Depth Information
1
2
3
4
90 %
80 %
+
60 %
Cruise speed
Vehicle speed
Figure 8.6
A diagram of the maximum engine power provided by a four-speed transmission as a function of
vehicle speed (solid lines). The dash-dot line is the steady-speed vehicle power requirement. Contours of
relative engine thermal efficiency for lesser power operation in fourth gear are shown as dashed lines, with
the 100 % peak being marked by
+
.
automatic transmission is less efficient than the manual one by about 10 percentage points. Most
passenger vehicles employ four or five forward speeds and one reverse. The speed ratio between
adjacent shifts is about 1.5.
The drive shaft connecting the engine/transmission to the wheels, either front or back (or both
in the case of four-wheel drive), utilizes a differential gear to apply equal torque to both drive
wheels while allowing different wheel speeds during maneuvering. The drive shaft provides for
the vertical motion of wheels with respect to the chassis and for the steering motion when using a
front-wheel drive.
The maximum engine power available to a vehicle, as a function of vehicle speed and trans-
mission gear ratio, is shown in Figure 8.6 (solid lines) for a four-speed manual transmission. Each
of these four curves is identical to the brake power curve of Figure 8.4, but is plotted here against
vehicle speed rather than engine speed. At any vehicle speed, the maximum engine power (which
occurs at wide open throttle) is the ordinate of the corresponding gear shift level (1, 2, 3, or 4).
At lower vehicle speeds, three or four levels are available, with the lowest yielding the greatest
possible power. At the highest speed, only the highest gear can be used to deliver engine power.
For constant-speed travel, the engine power demand, as determined from equation (8.15), is
shown as a dash-dot curve in Figure 8.6. It rises rapidly with vehicle speed, as the aerodynamic
drag outpaces rolling resistance. At any vehicle speed, the vertical distance between the “demand”
(dash-dot) curve and the “maximum supply” (solid) curve is the maximum power available for
vehicle acceleration and hill climbing. At low vehicular speeds, this difference is greatest for the
lowest shift level, while at the highest speed this difference is least, shrinking to zero at the vehicle's
top speed. The lower gear levels are needed to provide high acceleration at low vehicular speed.
It is clear from Figure 8.6 that only a small fraction of the maximum engine power is used
at steady vehicle speeds, reaching about 40% at cruising speed. The remainder is available for
acceleration and hill climbing. For average vehicle duty cycles, only a fraction of the time is used
in acceleration, and the portion of that at which the engine power is a maximum is small. The
vehicle capacity factor, the time-averaged fraction of installed power that is utilized by a moving
