Luxury goods
Daimler’s latest seven-speed automatic transmission has made the four-cylinder S-Class a reality
- Published in Features.
Box of tricks: This automatic cuts noise, vibration and carbon emissions
Hybridisation can realise large efficiency gains but it’s expensive and usually brings an increase in weight and packaging requirements. So in parallel engineers are finding smaller but less costly improvements from optimising systems such as the base transmission.
This is Daimler’s approach. Its seven-speed planetary automatic, known internally as the W7B700, is being replaced by an updated version – the W7C700. It’s still a seven-speed unit but reduced friction and drag losses, a new torque converter and lower viscosity oil contribute to fuel consumption reductions of 7%.
“Our overriding goal is CO2 reductions in the fleet,” said Gerhard Henning, Daimler’s director of transmissions, drivetrain and hybrids. “With a six-cylinder diesel in the S-Class we’ve saved 17g/km with conventional measures that can be easily implemented without making major changes. You don’t have to come up with entirely new technologies – you can optimise the details.”
The new torque converter module is probably the most significant – of the 7% fuel consumption reduction, 3% comes from here. The friction discs in the lock-up clutch are more widely spaced, reducing drag torque. And contact pressure when closed has been cut by 15%, reducing the oil pump’s energy consumption.
But the really clever engineering is in the vibration damper. Its torsional angle has been increased from the existing version’s 17° to 42°, which improves its decoupling performance; but the most significant change, Henning said, is the adoption of a pendulum damper, developed with Tier One supplier LuK.
“Our objective was simple – we wanted to achieve zero slip,” said Henning. “We were the drivers, but LuK contributed its ideas. The pendulum damper is the key element in the drivetrain.”
Ordinary spring-mass oscillators respond only to specific excitation frequencies; a pendulum’s natural frequency depends on its length and on gravitational acceleration so it will change in accordance with engine speed, enabling the main drivetrain excitation frequencies to be damped.
The result is that around 90% of vibrations are decoupled. This means that torque converter slip is no longer necessary to deliver the required levels of NVH. This improves driveline efficiency and allows lock-up at much lower engine speeds without any booming or vibration being felt or heard in the cabin. This means that engines can be downspeeded even further.
The extra driveline refinement has also enabled Daimler to offer a four-cylinder diesel engine in the S-Class luxury sedan for the first time. The 2,143cc unit develops 150kW/500Nm and will take the S250 CDI to 240km/h but CO2 emissions on the NEDC test cycle are 149g/km – the same as a base model Volkswagen Golf.
