|A variable ratio pulley allows improvements in cost and functionality. Click to enlarge.|
UK-based engineering consultancy Integral Powertrain has developed a new generation of belt-integrated starter generator (B-ISG systems) for stop-start applications using a new multi-mode, variable ratio drive pulley. The company anticipates that the total CO2 reduction achieved by the system will be greater than that possible with current B-ISG systems.
The core of the new system is the Variable Ratio Pulley, which packages a two-ratio epicyclic gearbox into the belt drive pulley hub. To start the engine, the high ratio (3.1:1 in the development system) is engaged to substantially reduce the torque requirement, allowing a medium-sized diesel engine to be cranked without the need for a high belt tension and the associated friction and wear.
Existing B-ISG systems provide refined stop-start operation, but they have significant disadvantages. This is mainly due to the requirement to generate very high torque and transmit it through the accessory belt, which impacts cost, packaging, weight and efficiency. A further issue is that engine-off comfort is compromised unless an expensive electrical air conditioning pump is fitted.—Integral Powertrain technical director Luke Barker
Tests have shown that engine start with the Integral system occurs in less than 0.3 seconds. When the engine is running, the ancillary drive ratio returns smoothly to 1:1 or can be maintained at the high ratio during low engine speed operation if accessory loading is high. The ability to maintain an efficient drive speed enables lighter, more compact ancillaries resulting in further efficiency improvements.
The Variable Ratio Crank Pulley also provides cost savings for vehicles that have an electric drive capability, Integral says. When the engine is off, the freewheel facility allows the ancillary drive belt to be powered by the motor, allowing belt-driven ancillaries such as hydraulic power steering, air conditioning and coolant pumps to continue to function in the normal way. This eliminates the need to specify electrically-driven ancillaries and provides the additional comfort and safety of continuous operation of these systems during engine-off operation.
Traditional systems also need a substantial electrical machine and inverter, with a battery or ultracapacitor system to provide a high current for fast starting in cold conditions.
The new B-ISG system allows the use of permanent magnet machines where previously the combination of high stall torque and high maximum speed has made this type of machine impractical for stop-start, Barker said. Integral Powertrain is developing a new high-efficiency permanent magnet motor specifically for this application.
With a diameter of 150mm and integrated low-cost, water cooled power electronics, the new machine can directly replace a standard alternator. As well as starting, the machine (rated at 5-10kW) will provide efficient regenerative braking and torque enhancement at a price that will make it affordable in volume sectors.