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Old 01-27-2009, 12:19 AM   #21 (permalink)
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NO - a Miller cycle engine involves forced induction. What you just described is Atkinson cycle.

Atkinson engines work by use of an ellipsus, which allows the connecting rod's crank end to slip from a smaller stroke journal to a larger stroke journal.

The idea is that the intake stroke should be smaller, and the power stroke should be longer, which gives the best of a long stroke engine (torque) and a small displacement engine (efficiency).

Adding boost came about when it was determined at an Atkinsonized Otto-cycle engine didn't create good power, and may cause other issues with late closing of the intake valve allowing bleeding back into the intake manifold. Adding boost helps to control the intake bleed off, and adds more low-end torque to the engine.

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Old 01-27-2009, 01:29 PM   #22 (permalink)
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Interesting stuff... so could we also close the intake way early instead of late? This would do away with the blow-back, create a vaccuum in the cylinder for a partial (in) stroke and partial (comp) stroke, and effectively reduce the comp ratio without affecting the other strokes.

??
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Old 01-27-2009, 04:09 PM   #23 (permalink)
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Inducing vacuum in the cylinder before the valve opening event does two things - both of which are undesirable -

1. It makes for more pumping losses (try pulling two cups apart that fit perfectly in each other.)

2. It changes airflow profiles for an otherwise nicely tuned intake system, due to the vacuum caused by the cylinder being part way down with no air movement.. it would cause vacuum pulses in the intake that would ruin both harmonics (check the TB spacer thread) and the overall fluid dynamic profile of the intake itself by increasing the vacuum per intake stroke, in a pulse like fashion.

(#2 is partially speculative, and can't be held as gospel. Please research it before assuming it is correct.)
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Old 01-27-2009, 04:11 PM   #24 (permalink)
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amcpacer: The modern implementation of the Atkinson cycle is an *extended* duration intake. A much later intake valve closing than typical to reduce dynamic compression while achieving benefit of high expansion ratio. Like Christ explained the original utilized "dual cranks" for small to typical compression ratio with much higher expansion ratio. The cam solution achieves the same ends without the mechanical complexity of shifting crankshaft throw lengths.

The Miller cycle used forced induction with the extended IVC timing to achieve complete cylinder fill and cut down on pumping losses but was unable to dramatically increase the mechanical compression ratio as there was no decrease of dynamic compression. Slight improvement in thermal efficiency with displacement-specific power on parity or higher than the Otto cycle (Atkinson cycle is much lower)

MazdaMatt: wouldn't do that if I were you. Creating the vacuum on the downstroke from an incomplete fill would result in much higher pumping losses on the intake side. It's a lot easier to pull mixture into the cylinder and bleed a bit of it off than it is to pull the cylinder down against a pair of closed valves. You'd wind up with MUCH less power.
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Old 01-27-2009, 04:14 PM   #25 (permalink)
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uhm... although my idea struck me as stupid at first for the reasons you explained, the beginning of the comp stroke would be just as "pulled up" as the end of the suck stroke (that sounds dirty)... i would expect that it would be equalized.
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Old 01-27-2009, 04:14 PM   #26 (permalink)
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...in fact, that vacuum may help with atomization

(and by "in fact" i mean "in pure speculation")
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Old 01-27-2009, 08:07 PM   #27 (permalink)
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Lexus says the upcoming 2010 RXh uses an Atkinson cycle V6 engine. Anyone know how that was done..?

I would have thought it would have to be a Miller cycle for a V6....
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Old 01-27-2009, 08:11 PM   #28 (permalink)
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V engines are more likely to utilize the technology correctly than inline engines are. In a V engine, the air that gets pushed out can simply cross over the plenum to the next cylinder in the firing order, whereas in an inline engine, the airflow has to change directions 3 times.

Once in the reversion cycle that puts it out of the cylinder, then sideways in the plenum to the next cylinder's opening (which could be against airflow, going from 1-3 and 2-4, at least in Honda engines), then down the next cylinder's runner along with fresh air.
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Old 01-27-2009, 08:48 PM   #29 (permalink)
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Quote:
Originally Posted by Christ View Post
V engines are more likely to utilize the technology correctly than inline engines are. In a V engine, the air that gets pushed out can simply cross over the plenum to the next cylinder in the firing order, whereas in an inline engine, the airflow has to change directions 3 times.

Once in the reversion cycle that puts it out of the cylinder, then sideways in the plenum to the next cylinder's opening (which could be against airflow, going from 1-3 and 2-4, at least in Honda engines), then down the next cylinder's runner along with fresh air.
Look at it this way...

Using the current implementation of the Atkinson cycle, delayed intake valve closing and 13:1 "native" compression ratio, look at what would happen with just a single cyclinder engine. You would have reverse airflow OUT the intake path/duct for each compression stroke, not exactly viable for a MAF/IAT equipped engine.

It appears to me that you need to go to 4 cylinders, inline or whatever, before you arrive at a solution for the reverse flow problem. With 4 cylinders you have an "opposite" cylinder beginning an intake stroke at the same time you need a place to "put" the charge being "exhausted" due to the delayed intake valve closing of the cylinder just begining a compression stroke.

The only alternative to 4 cylinders that I can see is having a one way "reed" valve to block the reverse flow. Or, of course, the Miller cycle as chosen by Mazda in the Millenia S's V6.
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Old 01-27-2009, 09:03 PM   #30 (permalink)
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I think you're looking a little too far into it...

Regardless of engine size (except single cylinder low speed engines) the manifold is constantly under vacuum. When the piston pushes part of the mixture back into the manifold, it's still under vacuum, it's just under slightly less vacuum.

There is still more pressure on the other side of the throttle plate, which keeps positive airflow in the correct direction (into the cylinder, out the exhaust side.)

Even with a 6 cylinder engine, the Atkinson cycle is still effective, as a result of this fact. The only reason for introducing boost is to keep more air/fuel in the cylinder, even though some of it gets pushed back out still. Since boosting engines increases the VE of the engine by adding more air than it could normally induct at the same RPM under vacuum, it also lessens the power loss associated with not having a full cylinder when not under boost.

In other words, the only reason to add boost is to compensate for the Atkinson effect's power loss.

If you add boost, you're increasing the VE of the engine without sacrificing power. If you use Atkinson and Boost (Miller) then you're still getting the FE increase of the Atkinson cycle, while increasing the VE of the engine (less pumping loss).

It's a best of both worlds scenario.

I don't believe the actual Atkinson engine ever intended for the cam to be designed to allow reversion in the intake tract, as this is counter-intuitive for power to be made. As I noted earlier, the real Atkinson design changed only the crankshaft, by adding an ellipsus that would allow the piston to have two effective stroke lengths. A much shorter stroke would be used for intake and compression, then a longer stroke used for combustion and exhaust.

This would yield a result that had less pumping loss than a stroker, but similar torque, with the same fuel use as a smaller displacement engine. VE would also be increased. kinetic energy created per BTU of fuel also increases (energy cannot be created nor destroyed blablabla) due to the longer stroke creating more leverage on the crank during the combustion event.

The idea of the "Atkinson cam" is bunk, essentially. It may increase FE, but it sacrifices power to do it, which is exactly the opposite of the true Atkinson design.

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