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  1. #1

    Hybrid Gasoline Engine - Atkinson Cycle

    In an article on the Prius in the local newspaper, the writer referred to the Prius as having an (Inventor's Name - which I cannot recall) - cycle type gasoline engine coupled to an electric motor.

    The article seemed to suggest that there is something unique or different about the gasoline engine in the Prius by referring to it as an (Inventor's name) cycle type of gasoline engine. Automotive writers being notoriously uninformed and ill-equipped to write about their subject - I wonder if there truly is anything unique about the engine used in the Prius, or was it just another gas-bag writer trying to impress us with his knowledge.

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  3. #2

    Prius Gasoline Engine


    While I do believe there are a lot of writers that don't go the extra mile and give details on most subjects, the jist of what the reporter was saying is probably true--although I haven't read the article.

    The Prius, and I believe the Escape, both use an Atkinson cycle engine. Most gasoline engines use the Otto cycle. The Atkinson cycle, I believe, is an Otto cycle engine that has been modified to allow the intake and exhaust valve timing to be variable. This allows the engine to operate at a maximum efficiency at variable power outputs. The typical Otto cycle engine has peak power and peak efficiency at different locations in its operating envelope. I.e., you'll get a lot of power out of the engine at some rpm and at some fuel input (gas pedal location) but it won't be operating at maximum efficiency.

    The Atkinson engine tries to achieve maximum efficiency at whatever power output the engine is operating. I.e., you can have maximum efficiency whether you need 20 hp to travel on level ground or 70 hp to accelerate while going up a hill.

    I hope this helps, and I hope others can correct me if I am wrong.

  4. #3

    Hybrid Gasoline Engine - Atkinson Cycle

    Typically the Atkinson cycle has a lower "compression" ratio than its "expansion" ratio. This allows more of the energy from the fuel combustion to be captured. Practically this is achieved by leaving the intake valve open during part of the compression stroke, pushing a portion of the intake charge back into the manifold to be used by the next cylinder.

    A by-product of this is that Atkinson cycle engines actually run best on lower-octane fuels. The slower flame propegation of high-octane fuel is detrimental to such an engine, and Prius owners who have made the mistake of running premium fuel in their cars have been rewarded with misfires, check engine and hybrid system warning indicators.

  5. #4

    Hybrid Gasoline Engine - Atkinson Cycle


    You might be interested in visiting this web site http://www,creedproject.org/stream_v7n2.pdf page 10and observe that whike every thing you say is true testing a prius on E85 actually incresed power by 20%.

    Every thing in life involves trade offs

  6. #5

    Hybrid Gasoline Engine - Atkinson Cycle

    Remind me not to lend those guys _my_ car.

    You can do lots of things that will increase power, but that doesn't mean it is a good idea! Toyota does not recommend E85. There are cars that can run on E85 (a lot of Ford engines labeled compatible, for example) but the Prius is not one of them...

  7. #6

    Hybrid Gasoline Engine - Atkinson Cycle

    As stated by Steve its the larger expansion ratio of this simulated Atkinson Cycle that achieves the improved efficiency of the gas engine. Combined with lighter components that are used in the engine IMO. I suspect that the 13.5:1 c.r. is actual piston stroke versus TDC volume and wonder what the effective compression ratio is. Ordinary engines loose heat energy down the exhaust pipe, this Toyota engine gathers some of this with its larger expansion ratio and converts it into kinetic energy.
    The trade off is reduced power vis-a-vis the engine capacity. However this does not matter with the electric motor to supply acceleration torque.
    Although the engine cycle is called an Atkinson Cycle, it is not an Atkinson engine IMO. The Akinson engine per se is more defined by it's awkward linkage which proved to be impractical but did give actual variable suction and compression strokes. The Toyota engine arrives at the cycle through the back door so to speak by pushing some of the charge back into the inlet manifold as previously stated.

  8. #7

    Hybrid Gasoline Engine - Atkinson Cycle

    Further to previous. If there was only some way to make a simple reliable compact mechanism that would connect a piston to a crank and provide a longer expansion stroke than suction stroke, then we would have perhaps a significant design breakthrough. The Atkinson cycle would really come to the front. One hesitates to say that it is impossible but the fact is that the Atkinson gas engine developed by Messrs. Manlove and Alliott around 1906 to 1910 is the only instance that I can find. It's funny the way that things turn around over time. The Atkinson engine was originally developed, I believe, as a way of overcoming an Otto cycle engine patent.
    Can anyone add more to the subject?

  9. #8

    Hybrid Gasoline Engine - Atkinson Cycle

    All cars will run better on E-85. Ethanol is a great oxygenate, and will increase output and power. On the flip side, is that it will lower mileage since it is "less explosive"

  10. #9

    Hybrid Gasoline Engine - Atkinson Cycle

    Pictures always help - and Matt Keveney, someone with whom I am not at all familiar, has given us that gift. For easy comparison of the Otto 4-cycle vis-a-vis the Atkinson 4-cycle - as well as a wonderful look at all the other engines Matt has drawn, see his Animated Engines web collection at http://www.keveney.com/Engines.html and enjoy.

  11. #10

    Hybrid Gasoline Engine - Atkinson Cycle

    It's my belief that the Prius engine would be more accuratley called a Miller Cycle engine. That is a engine that has a shorter compression stroke than a power stroke. This is most often accomplished by holding the intake valve open in the early part of the compression stroke. I understand an Atkinson Cyle engine to be one that accomplishes intake, compression, power and exhaust in a single turn of the crankshaft.
    Mazda which I believe has built engines which shorten the compresssion stroke has referred to
    them as Miller Cycle engines.
    I would like to hear comment on this from someone more knowledgable than I.

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