Abstract
Keywords: Powershift transmission, customer requirements, development tools, fuel consumption, product features
Apart from the internal combustion engine and the axle system, the driveline of a road vehicle is primarily defined by the transmission. In today´s city buses powershift automatic transmissions are predominantly used, especially due to their high level of comfort and easy operation. New challenges for transmission systems of today are originated by the improvements of the combustion engines, especially their increasing torque and coolant temperatures, which are driven by emission regulations. On the other hand, life cycle cost for the vehicle systems are getting more and more important and lead to technological challenges, e.g. higher reliability and better fuel consumption.
In conjunction with overall requirements like optimum cost, comfort etc., this leads to a very complex spectrum of requirements. These demands are partly different for vehicle manufacturers and end users like line operators, respectively. As a consequence, we started the development of a new powershift transmission, which was aligned with long-term targets, strongly oriented on customer´s demands, e.g. more comprehensive vehicle applications, reduced life cycle costs, and reduced application efforts required. After a general analysis with the QFD methodology (quality function deployment), the transmission type, a powershift transmission with torque converter, planetary gear set, and primary retarder was found to be the adequate solution to the multiple and diverging requirements. We developed a transmission system which offers specific and new solutions in all of its design and manufacturing details.
Latest development tools and methods, which are briefly described, were used to define the concept. The result is a modular powershift transmission with a 6-speed planetary gear set; it features a torque converter with lock-up clutch and optional torsional vibration damper. The new powerful cooling system is equipped with two heat exchangers, providing the unit with a high level of reliability and improved braking torque for heavier buses as well as higher average speeds, long-term performance, and comfort for the primary city bus retarder. The modern ECU is directly mounted to the transmission which generated a minimum number of electrical interfaces. As a conclusion, the advantages of the new powershift transmissions are highlighted because they represent a higher level of performance for today´s and future powertrains, as well as improved life cycle costs and excellent comfort; moreover, they excel thanks to additional safety characteristics and the respective environmental protection contributions. To emphasize this, we named it EcoLife, which stands for both, higher economical and ecological benefits as well as improved lifetime.