Author Topic: Atkinson Cycle.  (Read 4612 times)

Jocko

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Atkinson Cycle.
« on: June 12, 2017, 08:38:19 PM »
I have been reading about the Atkinson Cycle, having never come across it before. How does Honda get Atkinson Cycle at lower revs and Otto Cycle at higher revs? I was under the belief that the Atkinson Cycle was a different mechanical configuration. Can anyone direct me to a Link explaining the Honda set up?

culzean

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Re: Atkinson Cycle.
« Reply #1 on: June 12, 2017, 09:14:36 PM »
I think Atkinson is lower compression ratio at lower revs,  it can be done mechanically by altering crank throw or position of crank centreline - Honda do it by altering valve timing to leave inlet valves open past top dead centre to allow some mixture to blow from cylinder back into inlet manifold, this effectively reduces CR as there is simply less mixture to compress.

To get back to otto cycle valve timing goes back to 'normal' as revs rise.
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andruec

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Re: Atkinson Cycle.
« Reply #2 on: June 12, 2017, 09:29:35 PM »
It's not a true Atkinson cycle. The engine just does something 'similar' by adjusting the value timings. There's more info on Wikipedia.

Jocko

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Re: Atkinson Cycle.
« Reply #3 on: June 12, 2017, 10:12:44 PM »
That explains things. I'd read the start of that entry but never got to the relevant part!

mikebore

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Re: Atkinson Cycle.
« Reply #4 on: June 18, 2017, 09:36:13 AM »
Why is the lower compression ratio Atkinson cycle more economical?

This link doesn't seem to quite answer that for me:

http://blog.caranddriver.com/what-is-the-atkinson-combustion-cycle-and-what-are-its-benefits/

Thanks

mikebore

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Re: Atkinson Cycle.
« Reply #5 on: June 18, 2017, 09:46:47 AM »
Why is the lower compression ratio Atkinson cycle more economical?

This link doesn't seem to quite answer that for me:

http://blog.caranddriver.com/what-is-the-atkinson-combustion-cycle-and-what-are-its-benefits/

Thanks

Just found this paragraph from Wikipedia article:

The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio allows more energy to be converted from heat to useful mechanical energy, meaning the engine is more efficient.

This helps. Presumably then in an Otto cycle the pressure in the combustion chamber is greater than atmospheric at the end of the expansion stroke, so some energy has been wasted....although output greater.

andruec

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Re: Atkinson Cycle.
« Reply #6 on: June 18, 2017, 07:19:27 PM »
So presumably with an Otto cycle each stroke ends with a final 'phut' as the pressure inside the cylinder equalises with the atmosphere. That is wasted energy that the Atkinson cycle tries to avoid.

Jocko

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Re: Atkinson Cycle.
« Reply #7 on: June 18, 2017, 07:55:10 PM »
Yes, that's why your car needs a silencer.

culzean

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Re: Atkinson Cycle.
« Reply #8 on: June 19, 2017, 08:31:44 AM »
Yes, that's why your car needs a silencer.

+1  love the reply - logic cannot be faulted, same reason a gun needs a silencer LOL
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Downsizer

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Re: Atkinson Cycle.
« Reply #9 on: June 19, 2017, 09:28:02 AM »
In the Atkinson cycle, the closure of the intake valve is delayed, effectively shortening the compression stroke and reducing the energy needed for compression.  The power stroke is therefore "longer" than the compression stroke, extracting a higher proportion of the combustion energy.

culzean

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Re: Atkinson Cycle.
« Reply #10 on: June 19, 2017, 04:28:08 PM »
In the Atkinson cycle, the closure of the intake valve is delayed, effectively shortening the compression stroke and reducing the energy needed for compression.  The power stroke is therefore "longer" than the compression stroke, extracting a higher proportion of the combustion energy.

Honda also do other clever stuff like opening the throttle butterfly and using valve timing to control intake which reduces 'pumping losses' (where the engine is sucking against a closed throttle at lower revs - this is a waste that happens in petrol but does not happen in a diesel engine).
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

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