The GM Allison Hybrid System has a two-mode compound split parallel hybrid architecture that incorporates two mechanical ranges, an input-split low range and a compound-split high range. This allows the GM Allison Hybrid System to optimize use of the engine to achieve maximum fuel efficiency across the entire range of operation from start-up to maximum road speed. When the driver of a GM Allison Hybrid System equipped vehicle, accelerates the vehicle from a stop, an electrical motor draws power from the battery to set the vehicle in seamless motion. Then as the vehicle picks up speed the engine adds mechanical power directly to the drive train through the Ev Drive as required. The GM Allison Hybrid System's controllers and battery charging system ensure that the battery modules are maintained at optimum charge.

Increased efficiency
Internal combustion engines are torque deficient at low rpm, for example during vehicle start-up so normally a large engine is required in order to support the start-up function, larger than is needed for cruising speeds. However, electric motor/generators can exhibit maximum torque at stall, providing powerful acceleration during vehicle launch from a stop. The motor/generators and energy storage of the electrical path perfectly complements the engine and planetary gears of the mechanical path, enabling a more versatile operating vehicle.The motor/generators and energy

upon duty cycle. When applicable, smaller engines, that have fewer and smaller combustion chambers and smaller moving components, can provide lower fuel output and therefore reduce unnecessary fuel wastage. Another major factor in hybrid efficiency comes from the system controllers maintaining engine operation within its most efficient torque and temperature ranges.

Further contributing to hybrid efficiency is the regenerative braking system. When the driver brakes to slow the vehicle, the kinetic energy that is normally dissipated in the brakes as heat is transferred back into usable electric energy by the electric motor/generators which, operate as a generator to charge the battery. Allison Transmission has estimated that 40 percent of the energy used to accelerate a bus can come from energy saved during regenerative braking.

GM Allison Hybrid System components

System Controllers
The whole system is controlled by comprehensive energy management software, which acts like the 'brain' of the GM Allison Hybrid System. Two system controllers process information from system components and driver inputs, providing system command and control for precise vehicle propulsion and energy production. They also provide diagnostic and reprogramming functions.