Page 131 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Transmissions and driveline CHAPTER 5.1

Engine
torque Maximum torque line

Economyline

Lines of constant specific fuel consumption

Engine speed Lines of constant power

Fig. 5.1-26 A typical petrol engine torque speed characteristic.

are very efficient, it only takes a small additional loss for as sporty cars even in standard form. In this category, the
the effect to become significant. performance margins are smaller and most manufacturers
The other factor is the way that the CVT powertrain is have not controlled the engine to rigidly follow the
controlled and the resulting driver’s feel or perception of economy line and have compromised this to allow a larger
the vehicle behaviour in response to accelerator pedal torque margin for acceleration. The effect of transmission
demands. This characteristic is called driveability, and is efficiency is also more significant in this category with
very much conditioned by the individual driver’s previous a greater parasitic power lost. The result has been that
experience and expectations. People moreused to vehicles most early production CVT vehicles have neither given
with manual transmissions are not as comfortable when better economy nor performance relative to vehicles with
driving a vehicle with any AT, because they feel they are manual transmissions. However, a more valid comparison
less in control. There is also a significant compromise re- is with a conventional ATand here the CVT is the winner
quired between vehicle driveability and the possibility of for both economy and performance. More recently larger
achieving the desired economy line operation. A study of engined vehicles like the Audi A6 have shown that CVT is
the engine map in Fig. 5.1-26 shows that in most regions not just a viable alternative to ATs but can deliver the
there is very little extra torque available between the improvements in both economy and performance even
economy line and the maximum torque curve. When relative to the manual version (Audi’s own figures). What
a driver demands acceleration in this area then the system is more, even road test reports in motor magazines have
has to use this excess torque to first accelerate the engine begun to report the virtues of CVTs as being realized ‘.
before a significantly higher power can be transferred to a transmission that for once actually enhances the car’s
the driving wheels. This gives a delay between a driver performance’ (Autocar, 2000).
pressing the accelerator and the vehicle responding, this is
also accompanied by the sound of the engine speeding up 5.1.5.2 Hydraulic transmissions
but nothing apparently happening. Many drivers find this
disconcerting; particularly those used to manual trans- There are three main categories of transmission mecha-
missions. Road test reports have made comments about nism that can be used to provide the power and speed
‘the rubber band effect’ and ‘. the car’s initial sluggish range for vehicle use: hydraulic systems, variable radius
response .’. pulleys and traction drives. The first of these, described
Most vehicles fitted with CVTs have been with smaller in this section, can be further sub-divided into categories
engine sizes, generally below 1.5 litre, and not those noted of hydrostatic transmissions. Both of these are capable of

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