Definition of Hybrid System
A hybrid system is a vehicle powertrain that relies on two distinct power sources to propel the vehicle. These power sources are typically a gasoline-powered internal combustion engine combined with an electric motor. The control system selects the best combination of these power sources depending on driving conditions. Hybrid vehicles use energy far more efficiently than conventional gasoline-powered systems. Additional hybrid system components include a special hybrid transaxle, an inverter assembly, a hybrid battery and electronic control components.
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System Operation
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Hybrid systems combine the best operating characteristics of internal combustion engines with those of electric motors. This reduces emissions and dramatically increases fuel efficiency.
When starting and at low speeds, battery power drives the vehicle (unless battery power is low). The engine typically starts when speed increases above 15 to 20 miles per hour.
Under normal driving conditions, energy from the engine is divided between driving the wheels and charging the battery. The system controls the energy distribution to maximize efficiency.
During acceleration, engine power to the wheels is supplemented by battery power, providing the extra necessary power.
During deceleration and braking, the wheels drive the generator to recover energy otherwise lost to heat in the brakes. The recovered energy is stored in the battery, supplementing the power generated by the engine.
Engine
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The internal combustion engine in a hybrid system is smaller and more efficient than those on conventional vehicles, but the basic operation is the same. The air intake, fuel delivery, ignition, cooling and emissions systems include the typical standard components, but are more efficient due to the smaller engine size and lower resource requirements.
One difference in many hybrid systems is adjustable valve timing. Conventional timing is enhanced by holding open the intake valve well into the compression stroke, forcing cylinder volume back into the intake manifold. This allows the hybrid system to intelligently control timing to maximize fuel efficiency (and reduce emissions), while still producing maximum power when required.
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Transaxle
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A hybrid transaxle assembly includes the electric motor/generator that drives the vehicle, a second motor/generator for charging the battery, a planetary gear set to split power and provide variable gear ratios, a reduction unit and a differential.
The two generators are highly-efficient alternating current synchronous generators and electric motors. The generator that works with the engine to drive the wheels has strong torque characteristics for dynamic performance, and also recovers and regenerates the braking energy. The other generator both regulates the energy for battery charging, and serves as the engine starter.
Inverter Assembly
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The inverter changes high-voltage direct current from the battery into alternating current for the transaxle. The assembly typically includes a boost converter. The inverter assembly also includes a cooling system shared with the vehicle's two electric motor/generators.
Battery
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Hybrid systems require a high-voltage battery, typically a nickel-metal hydride (NiMH) unit that provides both high power density and longevity. The system controls charging and discharging rates to maintain a constant state of battery charge. Hybrid battery power cables to the electric motor/generators must be high-voltage, high-amperage cables with special shielding to reduce electromagnetic interference.
Engine Control Components
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Hybrid systems require all of the engine control components found in any other modern automobile, plus extra sensors and modules for optimizing the hybrid operation. The On-Board Diagnostic (OBD) system includes several hybrid-specific diagnostic trouble codes, in addition to the typical OBD-II codes.
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References
- "Electric and Hybrid Cars"; Curtis D. Anderson and Judy Anderson; 2010
- "Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory, and Design"; Mehrdad Ehsani, Yimin Gao, and Ali Emadi; 2009
- "How Do Hybrid Cars Work?"; Richard Hantula; 2009
- "Hybrid Vehicles"; James L. Benson; 2008
Resources
- Photo Credit Detailansicht Hybrid Motor image by 3rdTwin from Fotolia.com
