ACL / Techtrend



2015-06-24 21:14

Manual gearboxes account for about a half of the more than 80 million transmissions manufactured by the global auto industry each year; they are the cheapest and simplest form of transmitting engine power to the road and offer the best mechanical efficiency. So, in today’s acutely CO2-conscious world there would appear to be a bright future for a low-cost technical development that promises to address the already economical manual’s few significant disadvantages. These include poor drivability in the hands of inexperienced drivers, an openness to abuse by aggressive drivers, incompatibility with the very latest CO2-saving developments such as city stop-start and engine shut-off in motion (sailing), and safety priorities such as the decoupling of the driveline to improve the operation of ABS and stability control systems.

The development in question comes under the umbrella description of e-clutch, and several suppliers are working on systems and have pre-production systems under evaluation with carmakers. Common to all the systems is some form of programmed intervention in the clutch operation to allow the clutch to be released safely and comfortably to prevent mechanical damage to the driveline, and to enable stall prevention at launch and a degree of creep in traffic, if required. In some applications, programmed clutch action is superimposed over the driver’s commands only for stopping and starting, while more sophisticated interpretations will automate all clutch operations unless the driver overrides the system.

LuK, a member of the German Schaeffler group, is prominent in the field and has been working with OEMs such as Ford, GM, Suzuki and Volkswagen to develop production applications for e-clutch. At the recent CTI Symposium on Advanced Transmissions in Berlin, Dr. Roland Welter, Schaeffler LuK vice president of Total Clutch Systems, presented a progress report on the group’s work on e-clutch systems. He noted in particular that the new World Light Vehicle Test Procedure (WLTP) soon to be adopted in Europe would highlight one of the principal benefits of e-clutch, that of saving fuel while sailing (the vehicle coasting from speed with the engine switched off). While the current NEDC test cycle has a lot of stop time with the engine potentially switched off, it does not permit coasting. According to figures provided by Getrag-Ford Transmissions GmbH, the WLTP has less stop time but allows up to 18 percent CO2 saving by coasting with the engine either on idle or stopped completely.

With automakers placing a high emphasis on each gram of CO2 emitted, the benefit is clear, says Dr. Welter. Manual transmission e-clutch models show fuel savings of more than 5 percent compared with their conventional equivalents in the WLTP cycle, and e-clutch opens the way for simpler and cheaper mild hybridization using manual transmissions and 12 or 48 volt electrification. Dr. Welter identifies three different levels of e-clutch technology, the simplest of which Schaeffler terms MT Plus. This enables sailing and start-stop functionality, including clutch protection on hill starts; also enabled is micro-slip, especially if the clutch is a wet clutch, to allow lower engine speeds to be used without excess NVH through the driveline.

This solution, says Welter, is aimed at entry-level vehicles in developed markets. The step up to Clutch by Wire (CbW) adds engine stall prevention, ensures clutch pedal load and bite point remain consistent over time, and enables weight saving as driveline components no longer have to be over-dimensioned to withstand the large shock forces of a sudden clutch release at high engine rpm. The most sophisticated application is ECM, or electronic clutch management. This dispenses with the conventional clutch pedal to allow gearshifts to take place without the need to work the clutch. A survey of 5,700 taxi drivers operating in Indian cities revealed clutch operation to be the most-hated aspect of urban driving; shifting gears was not felt to be a problem, but the high cost of a full automatic was prohibitive. For these reasons, said Welter, ECM has potential for markets in the developing world where traffic is dense or highly congested.

Cross-section from Getrag-Ford shows how a wet clutch solution could be integrated into a manual transmission.

The programmed operation of the clutch brings the potential for several other benefits besides those of comfort and direct CO2 saving. Transmission ratios can be set taller for lower engine rpm without the fear of judder or vibration, different clutch engagement profiles such as economy or sport could be selected, and launch-control race starts would become a possibility. In addition, e-clutch could protect against engine or gearbox damage following the selection of the wrong gear, and off-road driving in SUVs could be assisted by active control of the clutch. And the option of automated clutch action could make seven- or even eight-speed manual transmissions more feasible in everyday use.

Getrag-Ford Transmissions is exploring the potential of both dry and wet clutch systems, with FWD manual transmissions director Frank Casimir listing the advantages of the wet clutch type to include lower inertia, lighter weight and more compact packaging, faster shifting and better tolerance of extended slipping for hill holding and creep. But while dry clutch systems could effectively be add-on solutions, a wet clutch arrangement would be more expensive and would require some re-engineering to integrate it into the gearbox.

On the all-important question of cost, said Casimir, the total cost of an e-clutch-equipped manual transmission solution would be well positioned midway between a standard manual and a full automatic. However, neither engineer was prepared to predict when the first e-clutch models would reach the market. “Future developments are focused on making the actuator even more compact and powerful, reducing its costs, and equipping it with an integrated control unit,” said Shaeffler’s Dr. Welter. “Initial design drafts have already been prepared, and preliminary testing has begun. An interchangeable head for alternatively operating hydraulic or mechanical systems is planned. Demands made on mechanical assemblies, such as the spindle and electric subsystems, are equally high and require the in-depth mechatronic know-how that Schaeffler has to offer.”

Hydrostatic clutch actuators draw on experience gained in dual clutch applications.