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Thread: What is hybrid vehicles real fuel economy

  1. #1 What is hybrid vehicles real fuel economy 
    Forum Professor
    Join Date
    Apr 2009
    I`ve seen quite different info on fuel economy of HEVs.
    For example this site Fuel Economy: Where the Energy Goes
    states that loses for braking are 4-5% and for idling just 3% in
    combined city/highway cycle.
    If those loses are such negligible why to bother with hybrids?
    Does anyone has real HEV and could tell here how much economy
    in reality does it give?

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  3. #2  
    Forum Ph.D.
    Join Date
    Jun 2011
    Behind the enlightening rod.
    Energy recovered from braking can be used to help accelerate, so smaller chemical fuel engine operating in optimized range is another savings. Poor acceleration, though, there will be no exciting hybrid drag races for quite some time. Small loss there.

    The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides
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  4. #3  
    M is offline
    Forum Junior
    Join Date
    Sep 2006
    Braking is a smaller part of it, but 5% seems understated. Most losses are inherent to the cycle (the Otto cycle for gas engines only gives you about 30% efficiency under ideal conditions), but I would expect the losses due to braking to be more significant than stated in the article. Think about it this way. You accelerate the car to a certain speed (incurring a lot of loss), you cruise (incurring some more loss), and then you decelerate to zero. The deceleration destroys the lot of perfectly good kinetic energy to zero without any benefit. That's a loss of 100% of kinetic energy. My gut feeling says that should count more than 5% of total loss, unless the kinetic energy of your car accounts for only 5% of the consumed energy (a scary thought I can't rule out). Braking is not the main selling point of hybrids, though. In fact, most so-called hybrids don't even have a regenerative braking system (real hybrids, like the Prius, do). The Otto engine is just embarassingly inefficient. The smaller you keep the contribution of the Otto engine, the better, but no one wants to drive a car with less than 100 HP any more. The electric motor allows you to use a smaller Otto engine and boost it when absolutely needed. Of course, all power still comes from combustion, but the hybrid approach allows for a smarter management of that power (and therefore let's you get away with having a smaller combustion engine).

    For fuel efficiency statistics of various car models, check out this website:
    Fuelly | Share and Compare Your MPG
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  5. #4  
    Comet Dust Collector Moderator
    Join Date
    Mar 2011
    New Jersey, USA
    Actual overall Prius results are 48-50 mpg.

    Energy is not only recovered during braking, but when coasting to a stop. (In fact, the actual energy used by the brake pads is wasted as heat). When I drive Ann's car, I make it a game to see how little braking I can actually get away with by timing lights and traffic to maximum energy recovery benefit. On a good day, I can average 52 mpg.
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