Page 114 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 5 .1 Transmissions and driveline

It should be noted that in the lower gears these graphs Emissions and fuel consumption can be heavily influ-
can indicate that very steep hills can be climbed. In enced by the driver’s gear selection.
practice, it may not be possible to actually start from Clutch operation and changing gears can be tiring, es-
rest on anything like these gradients because of the pecially when in heavy traffic.
capacity of the clutch and the difficulty of achieving Not suitable for all drivers; controls on larger vehicles
just the correct engine conditions. On two-wheel-drive can be heavy and most require some dexterity during
vehicles, the available grip from the tyres can also be operation.
a limiting factor. Even on the 4 4 example we are
considering, in practice, the low ratio in the transfer
gearbox would be required at gradients much steeper 5.1.3.1 The front-wheel-drive passenger
than, say 1 in 3. car gearbox

Fig. 5.1-6 is a cross section of a ‘typical’ front-wheel-
5.1.3 The manual gearbox drive transmission. The features it contains are typical of
those found in many, if not all, gearboxes. The essential
Most people who drive will be able to describe some elements are the three shafts that take the drive from the
aspects of one of these. As the name suggests, the driver engine (on the centre line of the crankshaft) to the
has to change between one gear ratio and another, as the output of the gearbox. From here, the driveshafts con-
vehicle requires, when using this type of gearbox. The nect the gearbox to the (driven) wheel hubs. As men-
different gears have different ratios that allow different tioned in Section 5.1.2.1, this configuration of gearbox
relative speeds between the engine and road wheels. can also be used in rear-wheel-drive, ‘mid-engined’
There are several distinct types of these transmissions; vehicles, the installations being very similar. Two of the
including ‘transverse’ or ‘transaxle’ front-wheel-drive obvious features of this gearbox are the integral differ-
gearboxes and ‘inline’ gearboxes used in rear and four- ential and the final drive gear pair. The function of the
wheel-drive vehicles. Four-wheel-drive vehicles will have differential is described in Section 5.1.6. Because of the
an additional transmission unit on the rear of the gearbox final drive, the overall ratio of the front-wheel-drive
to enable the drive of both front and rear axles. gearbox has typical reduction ratios of around 12:1 in 1st
Uses gear and around 3:1 in top gear. In comparison, the
Inline gearboxes are used in a wide range of vehicles from typical rear-wheel-drive gearbox will have a reduction of
small passenger cars up to large trucks, while the vast 3 or 4:1 in 1st and an overdrive ratio of around 0.8 in top
majority of transverse gearboxes is used in passenger cars gear. Fig. 5.1-7 is a schematic of the front-wheel-drive
and small vans. It should be noted that manual gearboxes gearbox to show the drive path in the gearbox.
are nowhere near as common in the US and Japanese Drive passes from the engine, via the clutch to the
passenger car markets as they are in Europe. This is input shaft. The various gear pairs then transmit torque to
particularly the case with small to medium cars. In the the intermediate shaft. The final drive pinion is on or part
past, the majority of larger passenger and commercial of this shaft and in turn drives the final drive wheel. The
vehicles in Europe used manual gearboxes of one type or final drive wheel is part of the differential assembly. Each
another. This particular area of the market is changing of the driveshafts is connected by a spline to the side gears
and becoming dominated by ATs. in the differential. This allows the two driveshafts (and
hence the wheels) to rotate at differing speeds to each
Advantages
other although the average speed will always be the same
Usually have high mechanical efficiency. as the final drive wheel. It should be noted that the three
Arguably the most fuel-efficient type of transmis- shafts rarely lie in a single plane, and, viewed from the end
sion, although this depends on the driver selecting the shafts, would lie in a ‘V’ shape. In this way, the centre
the most appropriate gear. distance and relative position between the input and
Relatively cheap to produce – possibly only half of output shafts can be designed to suit the installation.
the equivalent automatic. Drive is only engaged between the input and in-
Light weight – typically 50–70% of the equivalent au- termediate shafts using one set of gears at any one time.
tomatic weight. For example, when 1st gear is engaged, only that par-
Smaller and hence usually easier to package in the ticular gear pair carries the drive between the two shafts.
vehicle. This is achieved using the synchromesh assemblies,
which can be seen in the section illustration above. In this
Disadvantages
example, the synchromesh assemblies are on the in-
Some driver skill required – ask anyone who only termediate shaft, although they can be positioned on the
drives autos! input shaft.

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