The heart of the power train is the internal
combustion engine that provides the power required to
move a vehicle. However, this task is made much more
efficient with the aid of the transmission and the other
drive-line components that make up the power train (fig.
2-1). This chapter covers the basic principles of manual
and automatic transmissions, propeller shaft assemblies,
and final drives.
Power from the engine provides the torque
required for the transmission to overcome inertia.
Inertia is a property of matter by which it remains at
rest or in uniform motion in a straight line unless acted
upon by some external force. In this case, the inertia
of the vehicle at rest is overcome by an external
forcethe engine power in the form of torque. Once
the vehicle is moving, acceleration begins and
increases and very little torque is then required. The
bigger the load on the engine, the bigger and more
efficient the transmission must be. Once a vehicle
gains the desired speed, it moves along with very little
effort until something is encountered, such as a grade in
the road, that increases the resistance to its movement.
Now torque is required again and the operator has to
select a lower gear.
The transmission (fig. 2-2) provides the mechanical
advantage that enables the engine to move the vehicle.
It allows the operator to control the power and speed of
the vehicle and allows disengaging and reversing the
flow of power from the engine to the wheels by means
of a clutch.
The clutch engages and disengages the engine
crankshaft to or from the transmission and the rest of the
power train. Engine power to the load must be applied
slowly to allow a smooth engagement and to lessen
shock on the driving and driven parts. After
engagement, the clutch must transmit the engine power
to the transmission without slipping. Additionally, the
engine must be disconnected from the power train in
order to shift gears.
Figure 2-1.-Typical power train.