Page 118 - Automotive Engineering

Transmissions and driveline CHAPTER 5.1

driveline may not be the whole figure. The main gearbox, Packaging restrictions in the vehicle and on the
however, invariably sees the whole of the engine torque; engine ancillaries, if the casing has to be altered.
so, the number of drive wheels can be ignored when Availability – as discussed above.
considering this. The performance of a vehicle is very rarely simply
This torque value can then be calculated back up the a matter of top speed and acceleration!
driveline, taking account of the transmission ratio(s) and Selection of the lowest ratio – 1st gear
efficiency to give the torque required at the engine (at This governs the starting performance of the vehicle
the clutch or end of the crank). By considering the rolling and will depend on:
radius of the road wheel and the ratios in the trans-
mission, the engine speed can also be calculated. Gradient of hill required to be climbed – worst case.
There are a variety of conditions at which the vehicle Gross (fully laden) weight of the vehicle.
has to operate that determine the gear ratios to be Weight of any trailer required.
chosen. These are likely to be modified by certain prac- Characteristics of the engine at low engine revs – i.e.
tical considerations within the transmission, but for this minimum engine speed for effective air inlet ‘boost’
purpose, we can consider the initial requirements in on pressure-charged engines.
order to determine the ratio set for the transmission. Selection of top gear ratio – typically 5th in passenger cars
Engine characteristics.
5.1.2.4 Changing ratios – matching of the Economy requirements at cruise.
transmission to the vehicle In-gear performance – is the driver expected to
change gear on overtaking?
It is important to appreciate that the choice of gear ratios in Top speed to be achieved in top or next gear (usually
a transmission is often dictated, in practice, by what is 4th) – is top gear an ‘overdrive’?
available or what is already in production. This situation
occurs because of the large expense involved in engineering The intermediate gears are usually spaced to provide an
new gearsets, and installing or modifying the manufactur- even, comfortable spread between these extremes. In
ing plant to make the new parts. There are some cases that theory, the ratios are often chosen to give constant
do necessitate a change, however. These may include speed or varying speed increments, between the gears
a change in the engine, for example, from petrol to diesel, (Fig. 5.1-3). By using constant speed increments, the
or a significant change to the weight of the vehicle in which engine would reduce by a consistent speed change each
the gearbox is to be installed. Obviously, the finances time the driver changed up. For example if a driver
available within the vehicle manufacturer and the volumes changed up while accelerating every time they reached,
involved will have a very large influence in this decision. say 3000 rev/min, the engine speed would be the same
Wherechanges can beaccommodated,theymaybelimited after each gear shift. With variable speed increments,
to one or two gear ratios, leaving the intermediate ratios as this would not be the case, usually meaning that the
is, hence not necessarily optimized. Finally, before looking change in engine speed with each gearshift would get
at how the ‘ideal’ ratio may be chosen, the other limitation progressively smaller as higher gears were engaged. The
on ratio choice is the gear design itself. An example is a first following figures illustrate this. The ‘upshift’ points
gear pair where there could be a limit on how small the are shown as constant for illustration, although this is
drive pinion might be in order to withstand the shock obviously not necessarily so in practice.
loading which can occur in the gearbox. The particular vehicle requirements or limitations of
There are a number of decisions that need to be made the transmission selected can modify this spacing, for
when deciding what gear ratios should be fitted in a par- example, due to:
ticular transmission unit. A similar process has to be done Complexity requirements – existing ratio sets
for manual transmissions, automatics and CVTs. There is may limit choice on new vehicles, especially for
more flexibility in an automatic or a CVT because of the lower-volume vehicles.
effect of the torque converter and/or the shift map. These, In gear acceleration requirements – provision of par-
inadditiontothegearratios, influencethe effective, overall ticular characteristics at certain vehicle speeds, for
ratio at any point in the operating regime. The factors, example, achievement of 0–60 mph/100 kph with-
which have to be taken into account, are: out too many gear changes.
The performance requirements of the vehicle. Casing limitations on gear sizes.
The weight, rolling resistance and other parameters Emission and fuel economy requirements, i.e. engine
of the vehicle. conditions during the legislated drive cycle.
The restrictions that exist on the design of the Refinement issues at particular engine or driveline
transmission. speeds.

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