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Atkinson vs. Otto Cycle

Discussion in 'Gen 2 Prius Main Forum' started by Rockville1, Feb 28, 2008.

  1. BAllanJ

    BAllanJ Active Member

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    It's so that you can get more energy out of the fuel that's left in there to combust. In most engines, when the piston reaches the bottom of the power stroke and the exhaust valve opens, the pressure in the cylinder is still way above atmospheric pressure and so there's energy that could be used but is thrown away. The improvement is to make the power stroke longer than the intake stroke so the energy is used by letting the explosion expand further... that's Atkinson. The Prius has a modification that isn't quite that but effectively shortens the intake stroke (sorta) by the valve timing thing. The intake charge that's pushed back into the manifold isn't wasted... in comes back on the next intake.
     
  2. phdchp

    phdchp Junior Member

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    Thanks for the reply. What I understand you to say is that the Prius engine has an intake and power stroke that are of equal length. Some of the fuel that is taken into the cylinder in the intake stroke must be pushed back passed the intake valve to keep the pressure low at the end of the power stroke. It can be visualized that the engine is 30% more efficient than a normal ICE since it is not burning 30% of the fuel taken into the cylinder on any given power stroke. The expelled fuel is burned on the next power stroke. Is this correct?
     
  3. ronhowell

    ronhowell Active Member

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    Well, yes and no. Because the expansion power stroke, as well as the intake stroke are both of the order of 15:1, the cylinder pressure at the end of the Atkinson cycle power stroke is of a similar order to what it would be under an Otto cycle with a 10:1 power stroke. 30% less fuel is being burned during the Atkinson power stroke, and around 80% less power is produced, so the efficiency compared to the Otto cycle is 80%/70%, or some 14% higher, when measured as power produced/fuel consumed (also known as specific power output).

    Also remember that the Prius is essentially always in top gear, since it has no gearbox. This is why the electric traction motor is essential, since it produces it's maximum torque at start, or 0 RPM, to assist the ICE initially when moving the car from rest.
    Both mechanically and electrically it is a brilliant engineering solution to the shortfalls of the basic Otto cycle ICE powered car, which always requires a gearbox for acceptable acceleration times.
     
  4. gattaiolo

    gattaiolo Junior Member

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    Hello everyone!

    I was wondering if the differences between a Prius 3 and a Honda Insight 2 in terms of emission of pollutant gases could be explained by the Atkinson/Otto cycles.

    Here are the emission figures:


    Prius 3
    CO = 0,2580 gr/km (Insight = 0,0500 g/km) --> 5x moins!
    NOx = 0,0060 gr/km (Insight = 0,0070 g/km)
    HC = 0,0580 gr/km (Insight = 0,0160 g/km) --> 3,6x moins!


    Insight 2
    CO = 0,0500 g/km --> 5x less!
    NOx = 0,0070 g/km
    HC = 0,0160 g/km --> 3,6x less!
     
  5. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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  6. guinness_fr

    guinness_fr Junior Member

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    moins just means less in French. :D
     
  7. usbseawolf2000

    usbseawolf2000 HSD PhD

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    I am the poster of the thread Jimbo provided. All my information come from California Air Resource Board (CARB ). It is one of the most stringent standard in the world.

    Can you provide the source of your information? Are you sure you did not have it backward?
     
  8. gattaiolo

    gattaiolo Junior Member

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    Hello!

    Yes, sorry for leaving some French stuff in my message. Actually, I copy/pasted the data from an e-mail exchange I had with the Toyota and Honda corportare PR here in Belgium. Anf I also gathered some data from the official websites.

    And I double checked everything: no, the data is not mixed up.

    It's interesting to see that the data in your graph looks very different... of course, your data is in gr/mi and my data is in gr/km (can someone make the conversion?) ;-)
     
  9. MattFL

    MattFL Member

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    My understanding is, starting at bdc on an otto cycle engine, the first several degrees of crank rotation are very mechanically inefficient, meaning it requires a lot of energy for the first little bit of compression. As the crank rotates further the compression becomes more mechanically efficient. With the prius engine the intake is left open so the first several degrees of rotation past bdc do not compress anything and you only compress air during the more mechanically efficient period. This is where the energy savings comes from. There are a few ways to make up for thw loss of power. The most popular are with an electric motor such as in a hybrid car, or you can add a turbo which will bring the power back without the loss of efficiency. A turbo uses extra energy from the exhaust that is otherwise wasted so the addition of a turbo does not decrease efficiency.
     
  10. donee

    donee New Member

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    Hi MattFL,

    Interesting point.

    Allot of the Prius (Miller Valve Train / Atkinson Cycle) energy efficiency comes from the improved heat cycle. The exhaust gases on a Prius are colder, because the piston has a 13:1 expansion ratio. So the Ratio of Tin (hot compressed gas just after combustion) to Tout (cooler expanded gas just before exhaust) in the heat cycle is greater, and thus the efficiency is higher.

    In an Otto engine the expansion ratio is usually below 10:1, and many times down around 8.5:1 .
     
  11. MattFL

    MattFL Member

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    After some more reading, my understanding as to why the atkinson cycle engine is more efficient may not be correct. I don't remember where I read that, it was quite some time ago and I cannot seem to find the article again... The wiki article is similar to your explanation and to me it makes more sense:

    [ame=http://en.wikipedia.org/wiki/Atkinson_cycle]Atkinson cycle - Wikipedia, the free encyclopedia[/ame]

    The Prius also has an offset crankshaft to reduce drag during the power stroke. Interestingly enough, they use this same idea on some high horsepower superbikes, though I'm sure it has more to do with power output and less to do with economy on those things. ;)
     
  12. teeasal

    teeasal New Member

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    2 scenerios:

    1st, we know the Prius struggles when climbing uphill especially when the battery charge is low. Why can't we temporarily change the valve timings to convert it back to the Otto cycle during these "challenging" times and allow the Prius to be more powerful when needed, but at a cost of temporary lowered fuel efficiency?

    2nd, for all other cars, especially those offers upgrading the ICE to a V6 at a premium, why can't they also offer the V6 running the Atkinson cycle, so that the larger ICE produced the same power output as a inline 4, providing the fuel efficiency gain? Seems like this would be an easy sell because the fuel savings would easily offset the marginally higher cost/price of the V6 ICE. And this can be implemented to ALL cars, not just to hybrids. They can even make V8 running Atkinson cycle to replace a V6.
     
  13. qbee42

    qbee42 My other car is a boat

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    Mostly because Atkinson cycle engines have poor low end torque. With a Prius, you have the electric motor to help at low speeds. With a normal car the performance would be unacceptable.

    Tom
     
  14. fuzzy1

    fuzzy1 Senior Member

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    Would it survive the resulting 13:1 compression ratio? What octane rating is needed for such compression?

    BTW, I don't know that the Prius struggles when climbing uphill. And I've climbed a lot of hills with it.
     
  15. austingreen

    austingreen Senior Member

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    There is the problem of knock because of the increased compression and heat. Mazda seems to have solved some of these problems in its sky engines coming out next year. They have licensed toyota psd technology to use with a sky engine, and toyota has licensed the technology in the mazda sky system. So maybe we will. For fuzzy, the sky needs premium at compression of 14:1, but can run regular at 13:1. I'm sure the prius would need premium without a major engine redesign.

    You need to design the high compression into the 6 first, and other tricks are making the new 6's more efficient. The 6's have weight, size, and friction penalties compared to a 4, but today's 6 cylinder engines can provide plenty of hp and torque. Going up in cylinders also adds cost.
     
  16. teeasal

    teeasal New Member

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    Thanks everybody for your inputs.

    I was thinking the bigger badder 6 on Atkinson might be able to compensate the lower low end torque and act just like a 4. If still not good enough, make it even bigger bore. Howabout adding a small MG like the Honda IMA for lower end assist? (Oh well, this stumbles into the "hybrid" territory!)

    Oh yes, the Prius does struggles climbing long and steep (I mean STEEP) hills. Maybe most cars struggles too except those really powerful bad asses. But the Prius groans so loud you'd have to wear ear-muffs or risk hearing lost.

    If Toyota cross license with Mazda exchanging HSD with SKY that'll be nice. Can the ICE also be configured to be Atkinso/Otto variable hybrid cycle so that the max compression ratio is limited to what 87 octane gas can be run without pinging? (like, 12:1 commpression, 13:1 expansion). It seems very doable since it's just a matter of changing the valve timing.

    As for the added weight, cost of the 6, charge more money. Wouldn't the fuel savings of the Atkinson cycle attract customers in spite of the (moderate, I hope) higher price? The cost difference between a 6 and a 4 is minimal, I think. Car manufacturers had been able to power their econo-big-boxes with v6's like the cheapo Dodge Caravans selling at sub-compact prices.
     
  17. PriusSport

    PriusSport senior member

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    This is probably the source of the gas mileage advantage in the Prius over other hybrids.
    The ICE is more efficient. The Prius is just a brilliant piece of engineering. It is what attracts the car to sophisticated owners.
     
  18. teeasal

    teeasal New Member

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    I think this is even better: make the Prius ICE auto-detect what kind of octane fuel you put in the tank, and adjust the max compression ratio allowed accordingly if, and only if, engine load demands more power like climbing steep hills, or even towing your boat!

    So now if the driver is planning to do any extranuous exercises on the Prius, simply fill in the most premium gas. If you don't, then the Prius acts just like the good old Prius.
     
  19. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    To alter the compression ratio 'on the fly' so that the car could be both an Otto and Atkinson engine as load demands, without needing unreasonable Octane, one might do this:

    Variable Compression Engine
     
  20. teeasal

    teeasal New Member

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    That looks cool but why introduce the extra moving parts thus increasing friction/maintenance? I believe an engine will have higher efficiency if it has fewer moving parts. The Prius' transmission sun gear is a good example.

    Why not simply change the timing of the intake valve, making it variably closing at between lowest dead centre (highest compression) and 30% after compression stroke (lowest compression)?