Talk:Miller cycle

—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%.

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.

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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.

Wrong Information This article consists of a lot half-truths. The authors should read the original written sources from R. H. Miller and J. Atkinson. The original Miller-Cycle, presented by R. Miller in 1947, is a low temperature supercharging technique (for Diesel engines)which can be achieved with variable valve timing. The aim was to maintain as much as possible the pressures occurring in the conventional charged engine, but to increase the charge due to higher density generated by intense cooling. The other option of this technique is to maintain the charge of the engine and to receive a lower combustion temperature. In the Miller system, charge cooling takes place in the engine itself by expansion of the air due to an early intake valve closing. To realize the same charge, the turbocharger has to compensate the filling losses with higher charge pressures. For this purpose it is essential to use a high efficiency charge air cooler to profit from the temperature advantage at increased boost pressure. With this method, the compression work will be shifted from the piston to the turbocharger while cooling between them will take place. Sources: Miller, R. H.: Supercharging and Internal Cooling Cycle for High Output ASME Transactions, July 1947

James Atkinson US-Patent No.367496, August 2, 1887

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)

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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

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)

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)

The Miller Cycle

Here is the scoop on the miller cycle. A combustion chamber is a fixed size. Look at only intake and combustion strokes. Intake draws in air and fuel. Power expands air and fuel. Both strokes happen in the same size chamer. The important fact to consider is the fixed chamber. Ask yourself this. Is the combustion chamber the ideal size for power (combustion) stroke or does it need to be a little larger to make use of more of the expanding gas. The expanding gas is still expanding and is capable of doing more work before the exhaust valve opens. The miller cycle maintains compression ratio but takes in less air and fuel relative to the intake cycle. The combustion stroke volume is in effect larger relative to the intake stroke volume. More air and fuel is burned (less need for catalytic converter) before it is expelled by the exhause stroke. The miller cycle sacrifices power for efficiency and that is the reason it has been used on stationary engines. The amount of power loss is minimal and the miller cycle is the best option for a series hybrid automobile.

Anoziracs (talk) 17:26, 13 August 2008 (UTC)anoziracs

I agree with Anoziracs, that the central benefit of the Miller and Atkinson cycles are that the expansion ratio is greater compared to the effective compression ratio. The amount of fuel/air compression has to be low enough that it does not cause pre-detonation. The fuel/air mixture is only compressed mechanically, but since its expansion is chemical, there is a far greater amount of energy released and the pressure will go up many times higher than the original compression. So for maximum efficiency the expansion stroke needs to be longer (and larger) in order to utilize the gained pressure all the way to bottom dead center. In fact, when the exhaust valve is opened at BDC, there is still plenty of pressure that forces gas out of the exhaust valve before the exhaust stroke begins the expulsion. There is a limit, however, to how long an expansion stroke ought to be, because toward the end of the combustion stroke, the pressure decreases dramatically, therefore to harvest that late-combustion energy is only worthwhile up to the point where it decreases the average overall power output (or theoretically when the extra power generated is no longer greater than the power needed to move the piston the extra distance during the other three strokes). It is proven that the atkinson and miller cycles are more efficient. However, the Otto cycle creates more torque by using only the most powerful portion of the combustion stroke - the beginning - and quickly moving on to the next cycle.

I think the Toyota rep is being misleading on their blog; downplaying increased gains and up-playing decreased losses. http://blog.toyota.com/2008/09/atkinson-meets.html

The energy spent on compression is a moot point - the friction is small compared to the friction during the expansion stroke (which is now longer in the Prius, really) also small compared to the thermal losses of the engine. The real efficiency benefit is to fully utilize the explosive gases. The Otto version of the Prius engine only compressed 10.5:1 - the 13:1 expansion ratio of the Prius engine must be 23% larger for a good reason. This can explain the dramatically increased efficiency of the Prius engine.

The Atkinson cycle engine in the Toyota Prius develops decent torque at high speed, where it generates plenty of power (torque curve is flat and when speed is graphed linearly, power goes up diagonally in a straight line until max RPM is reached), which is why it needs a CVT to keep it close to its peak power curve for acceleration, and close to its optimal loading when efficiency is needed.

The original author does not cite nearly enough references for the all the points that are presented.

--Wongstein (talk) 02:38, 15 December 2008 (UTC)

It isn't awfully difficult to work out the benefit from an Atkinson cycle. It is perceptible... but not enormous. I strongly recommend a thorough read of Heywood, he discusses 'over expanded' engine cycles, both in terms of specific torque and efficiency. Greglocock (talk) 11:01, 15 December 2008 (UTC)

Unreferenced Information

A traditional Otto cycle engine uses four "strokes", of which two can be considered "high power" – the compression stroke (high power consumption) and power stroke (high power production). Much of the internal power loss of an engine is due to the energy needed to compress the charge during the compression stroke, so systems that reduce this power consumption can lead to greater efficiency.

<scratch>No</scratch> Negligible energy is consumed during the compression stroke save leaking rings or friction losses and other inefficiencies. The energy stored from compression is recovered in the power stroke.<scratch> Simple kinetic to potential back to kinetic, no energy is destroyed etc. Gravity, springs, pressures, etc. all work the same, yes even in an engine.</scratch> Please remove this unreferenced conjecture, especially, "Much of the internal power loss of an engine is due to the energy needed to compress the charge during the compression stroke", it is so silly to talk of the power when the author means energy and even sillier to talk of it as though it vanishes into thin air. —Preceding unsigned comment added by 66.186.238.96 (talk) 06:09, 25 October 2008 (UTC)

No, yet again, you have made the same mistake. If the gas were perfectly insulated from the rest of the engine then it would behave like a spring, as you describe. This is called adiabatic expansion and compression, and is 100% efficient. As soon as heat transfer occurs with the surroundings, ie in the real world, the efficiency drops. This is quite a significant effect on practical engines. Greg Locock (talk) 07:37, 26 October 2008 (UTC)

The heat lost when the working fluid is compressed is so minimal as to be considered negligible when compared to mechanical motion converted to heat i.e. friction in the form of pumping losses, metal to metal contact under pressure, pressure loss via leaking air seals, incomplete heat extraction, fixed valve timing, fixed displacement and other inefficiencies. The heat differential between the compressed gas and the cylinder wall, head and piston are so little compounded by the less than 1/8 of the cycle when the temperature differential exists that the heat transfer is minimal. Unless you have documentation to prove otherwise. I will concede that no loss should have been negligible loss instead.66.186.238.96 (talk)

I don't have Heywood to hand, but am 99% sure he discusses the effect of heat transfer on efficiency. Do you have Heywood? Greg Locock (talk) 23:18, 26 October 2008 (UTC)

Suggestions for an improved article

First, the article is titled Miller cycle yet the article speaks only indirectly about the miller cycle. Instead the article speaks primarily about Miller cycle engines; that is, devices attempting to implement the thermodynamic cycle known as the Miller cycle. This is okay in that the implementation is important but perhaps the article could be split into two sections, one dealing with the cycle itself and one with engines. The second confusing thing is the lack of distinction between power and specific power (or power density) both in terms of volumetric and mass densities. This is especially important when the miller cycle is implemented, as is commonly done, using delayed intake valve closing as this technique reduces effective engine displacement in the same size and weight of engine.

Of course, a clear explanation of the Miller cycle as contrast to the Atkinson cycle is probably another required section in the article.

I don't have the time to rewrite the article right now but hopefully someone can make use of the above ideas. 205.189.151.15 (talk) 00:05, 17 March 2009 (UTC)

simple question...

Given that the supercharger is apparently eating up 25-30% of the total engine power (really?! compressing a small volume of air vs accelerating a ton-plus vehicle and pushing its wall-like frontage through the outdoor air?), just how much "wasted" energy on the compression stroke is being reclaimed with this cycle?! It seems to violate some fairly basic principles as well... you want to squash a certain amount of gas into a certain smaller volume, surely it takes about the same amount of force anyway up? You're just pretty much delaying the inevitable - perhaps less force is needed whilst the valve is open, but more is then needed with the valve shut than otherwise might have been. How much of the improvement is actually simply due to the presence of the supercharger & intercooler in the first place? I've seen small, plain Otto cycle engines enjoy 50% power gains on charging + cooling with only moderate degradation of their economy after all... certainly, using less fuel overall than a 50% higher capacity (e.g. a 6 cylinder vs their 4, 2 litre vs 1.3L) engine would have. 77.102.101.220 (talk) 22:38, 16 June 2011 (UTC)