Page 115 - Automotive Engineering

CH AP TER 5 .1 Transmissions and driveline

Continuously variable transmission (CVT) – CVTs 5.1.2.1 The layout of the vehicle
are able to vary the ratio between input and output in
a stepless manner rather than having a number of discrete The position of the powertrain components within
ratios. the vehicle has implications both for the engineering of
Infinitely variable transmission (IVT) – Essentially the vehicle and the driveline components including the
a CVT which has the additional ability to operate with transmission itself. Effects include:
zero output speed, hence negating the need for a separate the space available for the powertrain and how it is
starting device. packaged within the vehicle including the location
This chapter is going to look at the transmission
systems used in cars. The rest of the driveline will not be of ancillary components;
considered in any detail; so there will be no detail on such the weight distribution, since the powertrain compo-
things as axles or 4 4 transfer gearboxes. nents are relatively heavy;
the structure to support the powertrain and react
against the driving torques;
5.1.2 What the vehicle requires vehicle handling and ride both from weight distribu-
from the transmission tion and the location of the driven wheel set;
safety structure and passenger protection.
According to some engine colleagues, the transmission is a The choice of vehicle layout is determined principally by
large, expensive bracket to stop the engine from dragging thetargetmarketsectorandbrandimagethatthevehicleis
on the road. However, we will, hopefully, demonstrate required to project. Possible alternatives include saloons,
that transmissions are much more interesting than the ranging from large luxury saloons to micro or town cars,
other, less significant, part of the power train! sports coupe ´s or convertibles, estate cars or off-highway
Essentially, the transmission or driveline takes the vehicles. In many cases, the same vehicle platform will be
power from the engine to the wheels and, in doing so, used for several of these variants. The vehicle layout must
actually makes the vehicle usable. The functions that also be sufficiently flexible to accommodate different
enable this include: engine and transmission options that are offered with
Allowing the vehicle to start from rest, with the many vehicles.
engine running continuously. The main vehicle configurations in use are shown in
Leting the vehicle stop by disconnecting the drive Fig. 5.1-1. The most widely used currently is the ‘stan-
when appropriate. dard’ front-wheel-drive layout shown in Fig. 5.1-1(a). This
Enabling the vehicle to start at varied rates, under has an engine mounted transversely to the vehicle axis
a controlled manner. with the transmission also transverse and in line with
Varying the speed ratio between the engine and wheels. the engine. The differential can be incorporated into
the transmission casing. Another possibility is shown in
Allowing this ratio to change when required.
Fig. 5.1-1(b) with a longitudinal engine transmission as-
Transmitting the drive torque to the required wheels.
sembly, again including a differential and the drive being
The transmission needs to perform all of the above taken to the front wheels. This configuration is used for
functions and others in a refined manner. The structural larger front-wheel-drive vehicles where the size (i.e.
aspects of the transmission, predominantly the casing, length) of the engine gearbox assembly makes installation
often contribute significantly to the structure of the across the vehicle impossible. It also allows front-, rear-
powertrain and the vehicle as a whole. This is important and four-wheel drive vehicles to be developed easily from
when it comes to engineering for the lowest noise and the same vehicle platform as the engine installation and
vibration. The stiffness of the powertrain assembly itself front structure of the vehicle can remain the same in each.
is important in determining the magnitude and the The main alternative, however, is the classic front-engine
frequency of the vibrations at the source (the engine). rear-wheel-drive layout as in Fig. 5.1-1(c). The engine and
This stiffness (and indeed the strength) can also be transmission are still in line but mounted longitudinally
important to the integrity of the vehicle in a crash. with a connecting shaft to a separate rear mounted final
Particularly with front-wheel-drive vehicles, the way in drive and differential that are a part of the rear axle. A
which the body collapses on impact has to be engineered common variant amongst two-seater sports vehicles is
very carefully, and the presence of a large rigid lump shown in Fig. 5.1-1(d) with the engine and transmission
such as the powertrain has a critical influence on the transversely mounted to the rear of the vehicle and driving
way this occurs. The size, shape and orientation of the the rear wheels. If the engine is in front of the rear axle
unit also affect the intrusion into the passenger space then this is usually referred to as a mid-engine layout.
after an impact. The final example shown in Fig. 5.1-1(e) is a four-wheel

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