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Homework Help: Otto vs. 4 stroke

  1. Dec 4, 2005 #1
    This is for a 300 level thermodynamics class. I am supposed to write a short paper comparing the 4 stroke engine to the otto cycle.

    I thought these were the same thing though?
    For my paper, i wrote what the 4 stroke are: intake, compression, power, exhaust...and described each one.

    Then i listed the 4 processes of the otto cycle:
    Process 1-2 is the Compression stroke.
    Process 2-3 is the firing of the spark plug.
    Process 3-4 is the Power stroke
    Process 4-1 is the Exhaust stroke and the Intake stroke

    Then I wrote a paragraph explaining why the ideal otto cycle doesn't exist in the real world because of heat transfer and friction.

    Does this sound right or am i missing something big?
    Last edited: Dec 4, 2005
  2. jcsd
  3. Dec 5, 2005 #2


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    Ok, you're half-right.

    Firstly, remember that the Otto cycle is an ideal air standard type cycle, and that applying it to a real cycle is an approximation.

    Secondly, remember that the four-stroke cycle is not the only one used in real life applications. Did you remember about the two-stroke cycle, which can also be modelled by the Otto cycle? And did you remember the Diesel cycle, which can also be a four-stroke?

    You must consider the fact that the discrete processes you summarised aren't quite like that in real life, and because things like ignition and combustion happen over time, and nothing really occurs at constant pressure or volume, that describing the Otto cycle does not in any way actually compare it to an actual 4-stroke cycle.

    The key point you've missed is that Otto/Diesel and Two/Four Stroke are not mutually exclusive terms, and that Otto and Diesel are air standard cycles and that real life isn't like the textbook.
  4. Dec 6, 2005 #3
    The question to be answered in the paper is confusing. I dislike these school type questions. They want you to be aware of some riddle to be answered, but at times the teacher doesn't understand how confusing the actual question statement is. Perhaps you could ask the teacher to rephrase the question without giving you any answer. The question is like asking you to compare a car to a Corvette. It's not a reasonable type question because the Corvette is a car too, and a car can be a Corvette. Do you compare the Corvette to all cars, or some cars, and really what is the question. If the question was what makes the Corvette different than most cars, the question would make more sense.

    Brewnog is right that the 4 cycle and Otto cycle are slightly different in scope. The definition really of an Otto cycle is a spark ignition 4 cycle engine, because there are other 4 cycle engines such as the compression ignition diesel. It appears the Otto cycle can be either throttled, or hit and miss, or direct injection stratified charge.

    In that way you might rephrase the question yourself to mean what makes the Otto cycle different than 4 strokes in general. Or you could talk about what makes the theoretical Otto cycle different than actual Otto engines.
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