Atkinson Engine

Discussion in 'Clarity' started by ClarityBill, Sep 3, 2019.

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  1. ClarityBill

    ClarityBill Active Member

    There are a couple items mentioned in this article that I wanted to discuss.

    The article: "Atkinson cycle (late-closing intake valves) that sacrifices some low-rpm torque to gain mid-range efficiency. A two-mode variable intake-valve-timing and -lift system provides extra power for acceleration and maximum efficiency during cruising."

    Has anyone evaluated when the 'two-mode variable intake-valve-timing and -lift system' is putting the ICE in Atkinson cycle? What is the maximum RPM /torque for Atkinson cycle? Minimum RPM?

    The article: "Recycling cooled exhaust gas (EGR) also reduces consumption"

    I thought EGR was for reducing NOx, and actually reduced thermal efficiency (increasing consumption). I thought the Clarity used a NOx catalytic converter for NOx reduction.
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  3. MrFixit

    MrFixit Well-Known Member

    Atkinson Cycle is not an operating mode. The engine always operates "in Atkinson Cycle"... the fundamental difference being that all 4 cycles (intake, compression, power, and exhaust) all take place with a single revolution of the crankshaft. In the conventional ICE engine, it takes two crankshaft revolutions to execute all 4 cycles. The variable intake-valve-timing and lift is just an tweak to tune efficiency further. Conventional ICE engines can incorporate similar tweaks.

    I'm not sure what to make of the claim that EGR somehow "reduces consumption".
  4. KentuckyKen

    KentuckyKen Well-Known Member

    FWIW, here is the most succinct explanation I’ve found of James Atkinson’s 1876 improvement on the Otto cycle courtesy of Car and Driver:

    Like countless other 19th-century inventors, entrepreneurs, and tinkerers, British engineer James Atkinson sought ways to improve on the Otto four-stroke combustion engine, first produced in 1876. The engine he patented in 1882 had variable stroke lengths provided by a multilink connecting rod between the piston and the flywheel. While Atkinson’s engines weren’t successful, his thermodynamic cycle is still in wide use, mainly in gas-electric hybrids. The key advantage is higher efficiency than is achievable in an Otto engine, albeit with some loss of low-speed output. The Atkinson cycle is ideal for hybrids because their electric motor(s) make up for the lost low-speed output.

    The Atkinson cycle delays the intake valve’s closing until the piston has completed 20 to 30 percent of its upward travel on the compression stroke. As a result, some of the fresh charge is driven back into the intake manifold by the rising piston so the cylinder is never completely filled (hence the low-speed power reduction). The payoff comes after ignition when the piston begins descending on the expansion (also called power) stroke. Consistent with Atkinson’s original thinking, the shortened intake stroke combined with a full-length expansion stroke squeezes more work out of every increment of fuel.
    In most engines, the compression ratio is set as high as the engine can stand short of detonation in pursuit of power and efficiency. Compression and expansion ratios are the same in an Otto engine. Atkinson wins on efficiency because its expansion ratio is significantly larger than its compression ratio.

    Correct me if I’m wrong, but don’t all Otto cycle engines take 2 revolutions to complete their cycle (4 stroke) and the Atkinson cycle just adds delayed intake valve opening?
    Last edited: Sep 3, 2019
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  5. MrFixit

    MrFixit Well-Known Member

  6. ClarityBill

    ClarityBill Active Member

    IMHO: The Atkinson animation is not what we have in the Clarity... It is just showing one implementation of the Atkinson cycle.

    The Honda engine is conventional 4-stroke engine with delayed intake valve. The delay on the intake valve has two settings, one is Atkinson, the other is 'not'. (The article described it as: A two-mode variable intake-valve-timing and -lift system)

    On the Clarity, Atkinson cycle is an operating mode.
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  8. craze1cars

    craze1cars Well-Known Member

    Agree that Atkinson animation dates to 1880’s and is largely irrelevant today. That’s indeed how the original worked, but it is not at all how a modern Atkinson cycle works in a Prius or a Clarity. As KentuckyKen speculates, today’s Atkinson’s are conventional 4 stroke engines with 2 revs of crankshaft per cycle, and they simply mimic the Atkinson cycle benefits on the combustion side at certain rpms and loads via modern variable valve timing tech.

    Some more explanation here:
    Last edited: Sep 3, 2019
  9. MrFixit

    MrFixit Well-Known Member

    You could be right. Looking for positive confirmation either way.
    It is referred to as an Atkinson-Cycle engine, but I understand your point that it may only utilize some elements.
  10. craze1cars

    craze1cars Well-Known Member

    This is pretty good animation at showing the differences in Otto and Atkinson in the modern application. Crankshafts are no different...conventional 4 stroke setup just maybe longer strokes are kinda typical of modern Atkinson setups. Back in 1880 the insane crankshaft linkage of the engine was so dang complicated it would never be practical to use, which is why it was never really used, and this is no different today.
  11. MrFixit

    MrFixit Well-Known Member

    Seems like a stretch to label this an Atkinson Cycle engine.
    This seems like just a minor modification to the cam...

    Hardly resembles the real Atkinson, but I guess the 'novelty' (if you call it that), is the ability to alter the valve timing dynamically based on the demand .
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  13. craze1cars

    craze1cars Well-Known Member

    Agree entirely. Variable valve timing is something many modern engines first car with variable valve engine was my 2002 Acura TL.

    Atkinson in today’s terminology is just another take on variable valve timing that maximizes fuel efficiency at all costs, including at the expense of max power output. Nothing more.
    Last edited: Sep 3, 2019
  14. craze1cars

    craze1cars Well-Known Member

  15. MrFixit

    MrFixit Well-Known Member

    Then, back to the question originally posed by Bill,

    The max/min RPM/torque - The valve timing results in a lower compression ratio for a given displacement so you are clearly sacrificing horsepower and torque in the interest of efficiency. Not sure if there are enough specifications to quantify this. What about the question of if / how EGR can "reduce consumption"?
  16. craze1cars

    craze1cars Well-Known Member

    I haven’t the foggiest...
  17. ClarityBill

    ClarityBill Active Member

    I thought I saw a 'switch' in the OBDII data that looks like Clarity changing from Atkinson mode, to power mode. I was wondering if anyone else had seen this, and tracked the operation.

    I am guessing that maintaining Atkinson mode would maximize efficiency. I assume it is not in Atkinson mode when the engine is at high RPM's. The question is, how high can the RPM's go in Atkinson?
  18. ClarityBill

    ClarityBill Active Member

    My understanding of EGR is that it cannot improve thermal efficiency: I think the statement in the article is just wrong. (Hard to believe?)

    NOx is created in high-temperatures: EGR is used to lower the combustion temperature. Reducing the maximum temperature of the combustion reduces NOx generation.

    Higher combustion temperatures give higher thermal efficiency, with the same exhaust temperature. Reducing combustion temperature reduces the 'available' energy, and reduces system thermal efficiency.

    I thought the Clarity ran with the higher combustion temperature (and higher thermal efficiency), then treated the NOx in the exhaust gas.
  19. insightman

    insightman Well-Known Member

    There are 4 bulleted strokes listed at the end of the original animation's description of the normal-crankshafted Prius' Atkinson-cycle engine.
  20. Atkinson

    Atkinson Active Member

    This is an Atkinson cycle engine in that it uses a variable late closing intake valve timing to reduce volume on compression and to effectively reduce compression and by extension, power.
    Why would Honda engineer a feature that reduces power?
    If you wanted to leave the throttle open as far as possible for as long as possible under varying loads while minimizing throttling losses, this is a solution.
    Atkinson cycle attempts to control power with valve timing while leaving the throttle wide open.
    The design includes a very high compression ratio to offset the late closing intake valves and offer some room for modulating compression ratio (and power).
    Modulating power via wasteful throttling is avoided.

    EGR is bad for efficiency because it lowers peak combustion temps where NOx (pollutant for our lungs) is formed.
    Emission control device that we have to live with.
  21. ukemike

    ukemike New Member

    Even the original Atkinson engine with weird multi-link connecting rods is still a 4 stroke engine. The strokes refer to the number of times the piston goes up and down during one complete cycle. The thermodynamic cycle of an engine (Otto, Diesel, Atkinson) really has nothing to do with the crank, it is all about what goes on in the cylinder. The main reason he used the bizarre linkage was to avoid infringing on the Otto cycle patents which were still current at the time. His engine design was not successful because the linkage setup was difficult to balance at higher speeds. Can you imagine the rebuild cost on one of those? 5 bearings in place of 1!
    The Miller cycle is similar to the Atkinson cycle in that it closes the intake valves once the piston is on the way back up. Miller made this modification to engines to allow him to use greater supercharger pressure.

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