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Archive for December, 2009

After much heated debate over the choice of powertrain, the hybrid design was finally chosen. Initially, it was thought that it was best to use a series design considering the specialties of our group members. With five electrical engineers to three mechanical engineers it made sense to make a design based on … electricity. While the parallel design is a mostly mechanically based design, it makes the most sense to us considering we are competing in a race.

The parallel design is most efficient at high speeds. It also allows us the most instantaneous power as there is a electric motor and ICE coupled mechanically. This is going to be a problem for our MEs because in addition to suspension, braking, and chassis now they have to deal with the powertrain, but the EEs are just going to have to step up and work in areas outside their experience.

Below is a diagram of the power flow in our parallel hybrid design contained within our chassis.

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Hybrid Powertrains

While petroleum fueled transportation has been a great solution for a long time, citing petroleum shortages, supply instability, high prices, and pollution, hybrid powertrain designs are becoming much more popular. Each type of design incorporates regenerative braking. This is done by using electric motors to absorb a vehicles motion to generate electricity. This electricity can later be used to propel the vehicle. However, there are several further variation between hybrid powertrains that each have their advantages and disadvantages.

The three general types of hybrid designs are

  • Series
  • Parallel
  • Series-Parallel
  • SERIES HYBRIDS

    1996 – 1999 GM EV1,  Diesel-Electric Trains

    A series hybrid drivetrain is a drivetrain in which two power sources feed a single powerplant (electric motor) that propels the vehicle. Most commonly, a combustion engine will drive a generator which will charge a battery. The battery then drives the electric motor. This setup mechanically decouples the engine from the wheels. The motor can be used as motive power source or as a generator while braking.

    Advantages of this design include that the engine can be run at any point in its speed-torque range. This allows it to run at its most efficient point at all times. In addition, there is much less complexity in the drive train as it is completely controlled by electric motors. There is no need for a drive shaft or differentials as each wheel could be potentially driven by its own motor.

    Disadvantages of this design include inefficiencies introduced by the conversion of energy from chemical (petroleum fuel) to electric (battery) to mechanical (wheels). Also, the generator attached to the ICE adds additional weight and cost. A larger electric motor must also be chosen because the mechanical energy required to move the vehicle depends solely on electric power.

    PARALLEL HYBRIDS

    Honda Hybrids

    Parallel hybrids couple mechanical and electrical sources mechanically. This can be done many ways. Many automakers choose to couple the power sources through a differential. Thus the torque or speed from each system can be added.

    Advantages of this system include weight savings that are offered by the elimination of the generator. Also, a much simpler power converter can be used as there are not as many paths for electric power to flow. At high constant speeds this design acts similarly to a series platform because the engine can run near its top efficiency. A smaller battery can be used

    Disadvantages include that in stop-and-go driving the parallel system becomes much less efficient because the engine will not be able to run at its top efficiency. This type of design also requires a much more robust engine that includes a more robust transmission.

    SERIES-PARALLEL HYBRID

    Toyota Prius, Ford Escape

    A series-parallel hybrid combines the efficiencies and complexities of both types of systems. The ICE and electric motor are combines both mechanically and electrically.

    Advantages of this system are that it can act like both a series or parallel system depending on the driving conditions. At high speeds the system will function in parallel, routing power from the engine directly to the wheels, where the engine can run near its highest efficiency. In slow or stop-and-go conditions the system will run in series, routing power from the engine to the generator to the motors, allowing it to run at its highest efficiency. Therefore, this is the most fuel efficient design.

    Disadvantages of this system include a higher complexity and cost. An generator is required in addition to a mechanical coupling system such as a differential.

    Thanks to M. Ehsani and Yimin Gao and their text “Hybrid Trivetrains”

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