Allison Transmission, of General Motors Corporation, the leading producer of medium and heavy duty automatic transmissions in the world, is bringing its hybrid technology to Europe. Across the United States and Canada the GM Allison Hybrid System has already proven its performance and reliability in a real working environment in one of the toughest roles - the urban bus. Over 500 GM Allison Hybrid System fitted buses are operating across North America on many challenging duty cycles and now with orders from OEMs in Europe, this advanced technology will soon be on Europe's roads.

Fundamentals of Hybrid Technology
Conventional diesel vehicles are powered by a single source comprising an internal combustion engine which converts fuel, in this case diesel, into usable energy. Hybrid electric vehicles (HEV) have two sources; the addition being an electric motor powered by an energy storage device, maintained at an optimal energy level by the primary source, the engine. This configuration lowers the demand placed on the primary power source, enabling both sources to operate at optimum efficiency. There are many possible configurations for HEVs, but they all contain the same key components; a power unit (normally a diesel engine), an energy storage system (battery, or other energy storage) and electric machines (motor and generator). The HEV architecture uses these components as part of the powertrain to recover braking energy and optimize power load on the combustion engine.

The two basic design configurations are series and parallel, although there are architectural modifications on both series and parallel configurations in order to enhance vehicleefficiency. In the series hybrid configuration the combustion engine is not connected directly to the drive train but instead

drives a generator that feeds the batteryor electric motor. The wheels are then driven solely by the power that the electric motor provides. The series hybrid configuration is basically an electric vehicle that is recharged by a fossil fuel power plant (the engine) and a generator that it carries on board. In a parallel hybrid configuration both the engine and the electric motor are linked to the transmission so that either of them, or both at the same time, may provide the power to turn the wheels. Since the parallel hybrid configuration allows the engine to drive the wheels also through a direct mechanical path, it offers better efficiency than a series hybrid configuration, and a more functional and flexible design. The overall benefits of a parallel hybrid configuration tend to outweigh those of series design in road application and many hybrids, including the patented GM Allison Hybrid System, produced now have a parallel configuration.

GM Allison Hybrid System
The two-mode GM Allison Hybrid System provides technical advantages over other parallel hybrid configurations. The real invention is an Ev DriveTM which is designed to combine (torque blend) electrical machine power with engine power. Ev Drive does not have fixed gear ratios as does a typical automatic transmission. Gear ratios, speed ratios, and torque ratios through the Ev Drive are infinitely and continuously variable. This module forms an Electrical Variable Transmission (EVT). The EVT architecture is based on mechanical power splits (modes) and the electrical power path, which can be used to allow continuous control over engine operating speed. There are 57 different functional conditions which can be incorporated to achieve the best power train efficiency over the wider range of vehicle speeds.