Making Sense of the Competing Hybrid Systems
As the number of hybrid models increase, and all the manufacturers get into the game, it seems that the amount of information around how hybrid systems work, and the differences between them becomes increasingly impenetrable. Toyota, Ford, and General Motors each produce their own unique hybrid systems, and, of course, each of them tout their systems as offering the greatest fuel economy increase, but what really are the differences between these systems? How can a fleet manager who is considering hybrid vehicles make the best-informed decision about which hybrids are right for their fleet?
Without getting into all the details of vehicle mechanics, there are some key differences between the Toyota Hybrid System (THS-2), Ford Hybrid System (FHS), and the GM Two-Mode Hybrid (AHS-2).
THS-2: In reverse, and at low speeds, the engine is off, and power is supplied by one of the electric motors, while the other regenerates the battery. A single set of gears transfer power from the motors and engine to the wheels, and the electric motor is the main power source; the engine increases output to supplement acceleration and assist with higher cruising speeds, while attempting to minimize its output. Both electric motors feed power through the single set of gears.
FHS: The Ford system augments the THS architecture by adding a set of output gears to the system, increasing torque and adding the ability to power heavier vehicles and increase the towing capacity. Before the introduction of the hybrid Escape in 2004, which had a 1,500 lb towing capacity, hybrid vehicle had virtually no towing capabilities. One electric motor feeds power thru the first set of gears with the engine while the second electric motor supplies power thru the output gears with power from the first set of gears.
AHS-2: By adding clutches to the hybrid system, the Two-Mode platform adds another layer onto the hybrid propulsion system. The two electric motors can now operate in one of two modes: one motor feeding power into each of the sets of gears, or with one electric motor feeding power into both sets of gears and the second motor sending power thru the second set of gears.
So, what does all this mean? Each of these systems are designed to minimize the power output of the engine to increase fuel economy, but take different approaches to meet performance requirements. The THS-2 system uses electric motors to supplement a small engine’s power output to manage acceleration and overcome increased loads while constantly using a second motor as a generator. By redirecting some of the power output of the second motor to a set of output gears, the FHS increases torque and towing capacity by sacrificing some of the capacity for the second motor to act as a generator. The AHS-2, or Two-Mode, system opens up the output gears to both motors providing additional torque, boosting output, but diverting generating capacity to higher speeds. With heavier vehicles and loads, and increased towing requirements, more power must be directed to the output gears to increase the systems capacity. By examining the requirements of your fleet and comparing those requirements to these systems fleet mangers should be able to align their business needs with the optimal hybrid system for their application.
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