Page 188 - Mechatronics with Experiments

174 MECHATRONICS
300
Gear 1 C B
Torque (% of max engine torque) 150 Gear 2 Y X Gear 3 Gear 4 Engine lug
250
200
A

Z
100
curve
Gear 5
50

0 500 1000 1500 2000 2500 3000
Engine speed (rpm)
FIGURE 3.22: Engine lug curve between torque–speed, and scaled versions of it at different
gear ratios of the transmission (assuming 100% transmission efficiency), which shows the
speed and torque profile that is delivered at the transmission output shaft, over laid with
resistance curves to determine steady-state operating conditions.

Total resistance force for a given vehicle mass, tire pressure, ground traction condition, a
gradient, and vehicle speed can be expressed as,

F resist = F aero (V vehicle ) + F gradient + F roll (3.259)
The total resistance force is determined by the three components as shown in Figure 3.21.
For different gradients and traction conditions, we would have different curves (i.e. A, B,
C) as shown in Figure 3.22. As a vehicle changes gradient and ground conditions while it
is traveling, the total resistance to it changes between these curves. If the vehicle is desired
to maintain a certain speed despite these variations, the output torque (or equivalent force
delivered to the tires) must be large enough to overcome the total resistance.
Therefore, as the operating conditions change, the gear ratio must be changed in order
to meet the demand. The gear ratio decision can be made either by the driver or by a control
computer. The main principle is to overlay the torque–speed curves for different gear ratios
over the actual resistance load curves. Let us assume that the vehicle is operating in gear 4 at
full throttle along the load curve A, at steady-state speed corresponding to about 1500 rpm
at the engine output shaft (point X), where the torque output from engine–transmission is
equal to the load. Consider that the road gradient changes and that now the load curve is the
curve B. At that speed, the load is larger than the torque capacity of the engine–transmission
at the current gear. If no gear change is made, the vehicle speed (and engine speed) will
decrease until the load curve and torque curve intersects at a point (point Y). At that point
engine speed is reduced to about 1250 rpm. If the gradient further increases to load curve C,
the engine–transmission cannot maintain the vehicle speed at the current gear ratio, because
the maximum output torque capacity at the current gear (gear 4) is always smaller than the
load. If the gear is not reduced, the vehicle stops and the engine stalls. The only way to
keep the vehicle moving is to reduce the gear ratio to gear 3 (or a lower gear) so that the
torque output capacity of the engine can meet the load, hence the engine speed stabilizes
at the steady-state speed of about 950 rpm (point Z).

3.8.2 Automotive Transmission: Manual Shift Type
A typical automotive manual transmission has five gear ratios. Figure 3.23 shows a view
of a manual transmission concept. The input shaft is the crank shaft from the engine. The