Talk:Miller cycle

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—Apparently, the combustion engine in the Toyota Prius also uses the Miller cycle? Can anybody confirm this and, if so, add it to the page? Lio 07:17, 29 December 2005 (UTC)

It appears to be the Atkinson cycle and was already mentioned on that page Lio 07:22, 29 December 2005 (UTC)

The article said the piston gets the same compression with 70% of the work, but that can't possibly be correct. Work = force * distance, and while the Miller cycle does cut out 30% of the stroke, it cuts out the part when the pressure, and therefore the force, is smallest. The total work saved is probably more like < 5%.

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[edit] Fuel Efficiency

I'm curious about something, and maybe somebody here can answer my questions.

If the Miller Cycle engine forces air/fuel mixture back out of the piston and into the intake manifold, won't that decrease fuel efficiency? Won't the charge return to the intake manifold, mix with the clean air, and when it goes back into the piston, have more fuel injected into it?

This portion of the Miller Cycle, I felt, was not explained very clearly, only that the A/F Mixture was forced back out of the cylinder.

Two possibilities come to mind, but this is just conjecture: The engine could be carbureted, with the carburetor typically coming before the supercharger, or the engine could have direct fuel injection (like a diesel). In either of these cases, you avoid the problem of the air passing the intake port twice.

I have a question about this article as well: The article mentions how the intake valve is left open longer than with the typical Otto cycle, but I know the typical Otto cycle has the intake valve open for a significant portion of the piston's upstroke due to the intertia of the gases and the restriction of the intake port. I was curious how this compared to the Miller cycle valve timing.

[edit] Wrong information!

Hello

I guess someone got this wrong here! The information on this site describes an Atkinson-Cycle Engine. In the Miller-Cycle the intake port is not closed later but earlier! Bernd

No, both cycles leave the intake valve open during part of the compression stroke so that the engine is compressing less against (or none at all?) the pressure of the cylinder walls. That's what increases efficiency. The Automotive Engineering International Online do not distinguish between the two in this page saying "The engine employs the Atkinson/Miller high-expansion principle by means of late intake-valve closing...". There must be some minimal difference between the two, maybe about supercharging vs. normal aspiration. We must ask some engineer in the wiki community. 85.107.29.61 07:37, 25 October 2006 (UTC)

============ Muddy thinking Let's not confuse power(Corvette) with efficiency (Prius)!

As I understand it, for maximum efficiency you want the presure of the *hot* waste gas at the end of the power stroke to be exactly one atmosphere. (if it's more, the overpresure is wasted when the exhaust valve opens) To get there, you can either start with a *cold* fuel/air mixture that is inconveniently less than one atmosphere, or you can start with less than a full cylinder. Leaving the intake valve open and returning a bit of fuel/air to the intake manifold for later use accomplishes the second option nicely.

Supercharging gets more power out of a small engine, but with obvious efficency penalties. (if you boost by 50%, that final 1.5+ atm presure is wasted.)

The way to compensate for the slight loss of power in the miller cycle is by scaling the engine size. (or use variable valve timing to disable the optimization when max power is needed)

Supercharging the miller cycle seems silly to me. Why should one compressor be more efficent that another?

As a side note, this may explain why turbochargers are more efficent that superchargers: Both end with, say, 2 atmospheres of overpresure in the exhaust manifold, but a turbo charger recovers that energy with a turbine and uses it to compress the intake gas. A supercharger needs to rob power from the engine to provide the boost in the intake manifold.


Hello, sorry to say that the this article is so flawed in discribing the Miller Cycle that it can't be fixed with a few corrections. The article needs to be rewritten completely. If one wants to know the real facts, go to "how stuff works". That explaination is 100% correct. james33759 19:26, 17 January 2007 (UTC)

Two things:
  • howstuffworks.com has a whole two paragraphs on the Miller Cycle engine. Those two paragraphs are accurate. However, they are the equivalent of describing a tsunami as a humongous wave: technically correct, but somewhat less than what one would expect from an "encyclopedia".
  • If the description here is wrong, then so is the one on howstuffworks.com. Have you read this in its entirety? What would you say is incorrect (other than "this article is so flawed in discribing the Miller Cycle that it can't be fixed with a few corrections", that is)?

Andy Nguyen 19:49, 19 January 2007 (UTC)

Ok I’ll try and throw in my two cents worth to clear a few things up...

Firstly, the Miller cycle acts on the same principles as the Atkinson cycle, just with the addition of turbo or supercharging.

Both cycles incorporate the design involving a delay in the closing of the inlet valve to increase efficiency. However, the traditional Atkinson cycle engine (which operates all cycles in one crankshaft revolution and has a variable power and compression stroke) compensates for the reduction in efficiency due to reverse flow by an increase in the length of the stroke.

Ralph Miller first used these principles in a supercharged engine, which allowed the inlet valve closing to be delayed and the compressed air to be held in the cylinder by merely the pressure of the induction air – therefore reducing the work required by the piston

As further described in the article, this has the added benefits of allowing a delay in ignition timing, since the onset of pre-ignition is less likely due to the reduced charge temperature.

Nik lawry → 3rd year auto engineering student, Australia

[edit] Potential Inaccuracy

A line in the article states:

"Due to the reduced compression stroke of a Miller cycle engine, a higher overall cylinder pressure (supercharger pressure plus mechanical compression) is possible, and therefore a Miller cycle engine has better efficiency."

As far as I understand, a Miller cycle engine has the same working compression ratio as an Otto engine, it just takes a different path to get there. While a Miller engine may have a geometric compression ratio >10 (like modern Atkinson engines), the effective compression ratio drops due to an open intake valve. LostCause 21:27, 2 November 2007 (UTC)

Well, after reading the Mazda article (which is the best source of information I've found so far), apparently a Miller cycle generally has a compression ratio of <10:1 (Mazda uses ~8:1). The difference is made up by a greater charge density provided by a compressor + intercooler. While that density is a function of the compressor's compression ratio, they are definitely not the same thing. Density is the goal, pressure and temperature are (some of) the tools.
This article doesn't make a very clear distinction that a Miller cycle gains its efficiency through several key, albeit minute, areas. Many of the areas are mentioned, but the article is so piece-mealed together by contributors that only vague descriptions of important aspects are mentioned. The article should probably be broken down into areas suggested by Mazda's article...Let's allow the company who has actually produced viable engines "speak" rather than cobble together an article by self-proclaimed experts. Using the Mazda webpage as a guideline would probably help a lot, imo. LostCause 21:27, 2 November 2007 (UTC)

[edit] Mechanical Advantage

Hey Aqn, Artmario2001. I kind of have to agree that the explanation is incorrect/misleading. Think about it. The limit of the mechanical advantage of the crankshaft, as the cylinder aproaches TDC or BDC is infinity. As the crankshaft moves 90 degrees from there, it actually has the least mechanical advantage. So, basically, that section does nothing to explain why compressing near BDC is inefficient. I'm not even convinced it is any less efficient than the rest of the compression stroke. It is, however, the easiest part of the compression stroke to remove, which is what causes the other efficiency gains to apply. Aij (talk) 19:14, 15 February 2008 (UTC)

I have attempted to rewrite this article. I think it is misleading, and the articles from Toyota and Mazda are likewise. I have not yet determined how to add appropriate references nor am I sure of the process for that (or to get this posted). Overview == The Miller cycle is a version of Atkinson cycle. This type of engine was first used in ships and stationary power-generating plants, but has been adapted by Mazda for their KJ-ZEM V6, Subaru, Toyota, General Motors (called LIVC, 'late intake valve closing' ) and Ford. It is basically a super expansion cycle which reduces the energy lost when the exhaust valve opens by making the expansion (power) stroke longer than the compression stroke. This is done in the Miller cycle by reducing the compression stroke by closing the intake valve late. The original Atkinson engine accomplished the same super-expansion by making the expansion and exhaust strokes both longer than the intake and compression strokes. The original Atkinson cycle engine made the expansion and exhaust strokes mechanically longer than the intake and compression strokes by lowering the lower right hand pivot point in the diagram in the animation on Matt Keveney's Web site. The original Atkinson engine also completed four strokes in one crankshaft revolution. Today’s engines typically use the Miller version of the Atkinson cycle - late intake valve closing. A traditional four-stroke cycle (Otto cycle) engine uses four "strokes", the intake, compression, expansion and exhaust strokes (in two crankshaft revolutions). Much of the internal power loss of an Otto cycle engine is due to the energy lost when the exhaust valve opens and releases the hot, high pressure exhaust gas. About 10% more net cycle work could be produced if the power stroke could continue until the pressure inside the cylinder is almost equal to atmospheric pressure. That is where the Miller/Atkinson cycle comes in. In the Miller cycle, the intake valve is left open longer (through about 20% of the compression stroke) than it would be in an Otto cycle engine. In effect, the compression stroke is shortened. The full compression stroke is two discrete cycles: the initial portion when the intake valve is open and final (effective) portion when the intake valve is closed. This two-stage intake stroke creates the so called "fifth" cycle that the Miller cycle introduces. As the piston initially moves upwards in what is traditionally the compression stroke, the charge is partially expelled back out the still-open intake valve. This loss of charge air results in an effective loss of displacement which requires the engine to be larger for the same power. However, in the Miller/Atkinson cycle, this is partially compensated for by the increased expansion ratio. If converted to a Miller cycle, a 3.3 liter engine might draw only as much air as a 2.7 liter Otto cycle engine (the intake volume is reduced to 2.7 liters by the late closing intake valve) but produce the power of a 3 liter Otto cycle engine due to the fact that the expansion volume is still 3.3 liters. Since the compression ratio is usually maintained in the Miller cycle to maximize efficiency and power, a more useful description of the Miller cycle might be that if converted to a Miller cycle (by increasing its expansion stroke), a 2.7 liter Otto cycle engine would still draw as much air as a 2.7 liter Otto cycle engine (the intake volume remains at 2.7 liters with a longer stroke by the late closing intake valve) but produce the power of a 3 liter Otto cycle engine due to the fact that the expansion volume is now 3.3 liters – but the weight and size would be similar to a 3.3 liter engine. The term “Miller cycle” is sometimes used to imply that turbocharging or supercharging is used to regain the lost power to weight ratio of the Miller cycle. Ralph Miller employed the miller cycle to obtain more power from engines, that were already supercharged, without increasing combustion pressures beyond safe limits. The Mazda KJ Miller Cycle V6 engine uses a supercharger, the Subaru B5-TPH uses a turbo charger while the Miller-cycle engine in the Mazda Demio is naturally aspirated. The advantages of supercharging a Miller/Atkinson cycle engine, in terms of power density; and the disadvantages, in terms of complexity and need for low internal compression ratio (and/or the need for high octane fuel) are much the same as for supercharging the usual Otto cycle. A key consideration of the Miller cycle is the compression ratio. Because the combustor chamber volume is not easily varied, engines with non-variable, late intake valve closing will have a constant compression ratio that can be set to the maximum practical value. For variable or optional Miller cycle engines (where the intake valve delay can be reduced to provide increased power), the compression ratio is set to the maximum value that is practical for when the intake delay is least (for maximum power) and is then unavoidably reduced by the reduced effective compression stroke when the intake valves are closed later. The reduced compression ratio for these engines during Miller cycle operation (late intake valve closing) is akin to the reduced effective compression ratio when the throttle partly closed on IC engines. A Miller cycle engine is well suited to hybrid vehicles where the electric motors can provide short term increased power to compensate for the lower power of the Miller cycle. For hybrid vehicles that have a single cruise horsepower requirement (e.g., the Prius), the fixed miller cycle is attractive. For hybrid vehicles that have a multiple cruise horsepower ranges (Hybrid Tahoe with and without a towing load) the variable delay Miller cycle is attractive. In either case, for acceleration and up short hills, the electric motor supplements. During cruise, deceleration and down hill, the batteries are recharged. ToppaTom (talk) 04:37, 2 April 2008 (UTC)