Atkinson vs. Otto Cycle

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

  1. Rockville1

    Rockville1 Silver Pine Mica

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    I'm posting this in the Main Forum (vs. Technical) as I thought this would be of general interest to Prius Owners. I was also surprised to find so few threads about the Atkinson cycle in the Prius Chat Forum (most are 2 or more years old now).

    For those who don't know, the Prius uses an Atkinson Cycle combustion engine, combined with the electric motor, to provide power. The Atkinson cycle is much different from the typical Otto Cycle that is found in most internal combustion engine (ICE) cars today. The advantage of the Atkinson cycle is that it is much more efficient than the Otto cycle. The disadvangtage is that it has less power (especially when moving the car from a dead stop). However, combined with the hybrid system, the electric motors provide the necessary power to get the car moving before the ICE is needed.

    If you want to learn more about the Otto vs. the Atkinson cycles, read the following two articles at Wikipedia:

    Four-stroke engine - Wikipedia, the free encyclopedia

    Atkinson cycle - Wikipedia, the free encyclopedia
     
  2. ksstathead

    ksstathead Active Member

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    See also...

    nice graphical treatment of the subject (& other engine types) at

    Keveney.com
     
  3. Rockville1

    Rockville1 Silver Pine Mica

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    Cool site Stathead! Thanks.

    Current Cars that use the Atkinson Cycle:

    Toyota Prius hybrid electric (front-wheel drive) with a (purely geometric) compression ratio of 13.0:1
    Ford Escape/Mercury Mariner/Mazda Tribute hybrid electric (front- and four-wheel drive) with a compression ratio of 12.4:1
    Toyota Camry Hybrid hybrid electric (front-wheel drive) with a compression ratio of 12.5:1
    Chevrolet Tahoe Hybrid hybrid electric (four-wheel drive) with a compression ratio of 10.8:1

    Notice that the Lexus hybrids do not use the Atkinson Cycle engines as Lexus has opted to use the hybrid system to boost performance (when combined with an Otto Cycle engine) rather than maximize efficiency (although their hybrid vehicles are still more efficient than the non-hybrid counterparts, e.g., Lexus RX400h vs RX350).
     
  4. David Beale

    David Beale Senior Member

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    I do have to disagree with the statement that the Atkinson cycle is "much different" from the Otto cycle. It's only slightly different. All you do is construct the engine with a long stroke. To avoid the high compression this would normally give you (and which is undesirable if you want to burn low octane rated fuel), you hold the intake valves open during the start of the compression stroke.

    The result is a long power stroke, allowing more energy to be extracted from the expanding gases, without the high compression complications seen in high power engines and diesels. I'm told efficiency gains are an extra 5-10%, which is quite a bit when you are starting from about 25-30%. The downside is a loss of about 30% of the power output.

    BTW, this was done many years ago (probably unintentionally) in the old GM 6 cylinder engine, though most of the low compression was gained there with an inefficient intake path and largish combustion chamber (and therefore high pollution generation).
     
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  5. Rockville1

    Rockville1 Silver Pine Mica

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    Yes, my mistake for using the phrasing "much different." I guess I was just trying to point out that due to the rather ingenious design (IMO) of the Atkinson engine, there are a number of differences that result in a more efficient, albeit less powerful engine. There are several notable differences that orignally allowed Mr. Atkinson (in 1882) to get around the patents on Mr. Otto's engine, inlcuding the longer power stroke vs. intake and ehaust strokes and the fact that all four strokes are completed with one cycle of the crankshaft.

    What's most interesting is that this efficient design was never able to succeed commercially due to the low power output until electric motors were able to merge successfully with the internal combustion engine.
     
  6. qbee42

    qbee42 My other car is a boat

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    The differences are (or were) originally greater. A true Atkinson cycle engine uses an entirely different crank system than an Otto cycle engine. It is more complicated but allows for a longer power stroke verses the compression stroke, even without playing with the valve timing. This was originally done to avoid infringing on Otto's patent, but the side effect was the ability to have a slightly more efficient cycle. In more recent times, someone realized they could get the same cycle effect by holding open a valve and pushing part of the charge back out of the cylinder, which was a lot easier to do than the complex gearing on a true Atkinson cycle engine. If you add a super-charger you get a Miller cycle engine.

    Tom
     
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  7. bredekamp

    bredekamp Member

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    Please explain how an engine can be more efficient and less powerful at the same time. Does it have something to do with the amount of twisting force (torque) the engine can provide?
     
  8. kohnen

    kohnen Grumpy, Cranky Senior Member

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    Easy.

    Efficiency = work out / fuel consumed

    Power = work out / time

    So, if you had an engine that puts out only 1/2 the power, but uses 1/3 the fuel, then the engine would be MORE efficient but LESS powerful.
     
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  9. Rockville1

    Rockville1 Silver Pine Mica

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    In very basic terms (because I'm not an engineer!), the Atkinson design reults in increased fuel efficiency but at a loss of low-speed torque and high-end power. Without the electric assist of the hybrid, an Atkinson engine just doesn't produce sufficient low-speed torque to get the car moving fast enough. Traditional ICE engines do but they are very inefficient (energy wise) at low torque. Once the vehicle is up to speed it takes very little power to keep the car moving. Thus traditional ICE or Otto cycle engines are efficient at highway speeds (where low power is needed) but terribly inefficient at low speeds (high power).

    See link below for more technical description:

    Atkinson [email protected]
     
  10. hill

    hill High Fiber Member

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    It's a thread revisited. About 1, or 2 years ago the topic was gone over ... in fact even more recently. Got a little controversial, as I remember, in at lease one of the dozen or so threads.
     
  11. Flying White Dutchman

    Flying White Dutchman Senior Member

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    nice topic!

    good info
     
  12. bredekamp

    bredekamp Member

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    OK, thanks.

     
  13. David Beale

    David Beale Senior Member

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    You had to ask how it could be more efficient. ;)

    If you are, say, racing (even from stoplight to stoplight as most people do), you want lots of power. So you tune the Otto cycle for max. HP.

    In order to achieve this you end up running high compression (for technical reasons ;) ) and this results in increased temps in the combustion chamber. So you must deal with this before it heats a piston enough to weaken it so the pressure will punch a hole through it. Yup, it happens. More often than many racers would admit (interviewer-"Whats the reason you're out of the race?" driver- "I burned a piston"). How do you cool a piston (and valves, which I didn't even mention)? Well, injecting a little more fuel than you actually need is one way that is usually used. Result, lower efficiency.


    Another thing that results from the higher compression is higher friction between the piston and cylinder walls. Power loss and lower efficiency.

    Engine power output (HP) is speced in torque per unit time per distance. Increase torque per second and you increase HP. So run the engine at higher RPM (adjusting the internals as required to live). More power strokes per second results in higher HP. You then end up dumping exhaust that is still burning, wasting yet more energy. You have to because a long stroke would self destruct at high RPM.

    I could go on, but the result is what they are trying to achieve, that is, more HP. The side "benefit" is lower fuel economy, but you have to "pay to play" (a common phrase in the racing and car nut community - for obvious reasons). So they are happy with the result. We would not be.

    There are many trade offs in engineering. Most are not fun.
     
  14. Qlara

    Qlara New Member

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    What you mention is called "Fuel Enrichment".

    A conventional engine mixes air and fuel in the proportion that will burn up all the fuel using all the oxygen in the air. This is called the stoichiometric mixture and the ratio of air to fuel is approximately 14.7 to 1.

    Modern cars maintain this correct mixture using a Mass Airflow Sensor in the intake manifold and residual oxygen sensors in the exhaust. If the air to fuel ratio increases, so that an excess of air enters the engine, the mixture is said to be "Lean". Unburned oxygen in the exhaust does no harm itself, but a lean mixture tends to burn HOT and can damage an engine not designed for it. The high temperature can also cause nitrogen to combine with unburned oxygen and produce oxides of nitrogen that contribute to pollution.

    Conventional engines are biased towards a Rich mixture (ratio less than 14.7 to 1) when power demand is high. This "Fuel Enrichment" makes sure every scrap of air drawn into the engine is used up to get the maximum possible torque. The unburned fuel can be oxidized, up to a point, by the catalytic converter, but its energy is wasted resulting in lower efficiency.​
     
  15. qbee42

    qbee42 My other car is a boat

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    This is on the opposite end of the spectrum from diesel engines, which always pull in the same amount of air for every cycle regardless of the amount of fuel. They have no throttle. To get more power, more fuel is injected during the power stroke.

    Tom
     
  16. rposton

    rposton Member

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    A 1500cc Atkinson also uses less air than a 1500cc Otto Cycle engine. Less air in, less air out.
     
  17. bac

    bac Active Member

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  18. FL_Prius_Driver

    FL_Prius_Driver Senior Member

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  19. CalvinL

    CalvinL 2013 Prius Persona Black Cherry Pearl

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    Ah this is what I was looking for - a hybrid using the rotary/wankel concept and the atkinson fuel cycle. Ahhh thermodynamics - can't get past those pesky laws.
     
  20. Wa1hog

    Wa1hog Old Blind Hippie

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    Does the Atkinson have the same style starter as a Otto? Does it require the same amps from the battery? If so then what battery starts the ICE in the Prius. Seems that it is an instant start?
     
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