Manic Mechanic

iTrader: (3)

Has anyone run across a custom built Miller Cycle engine or considered/done it yourselves? I'm looking for any experience from race shops or other hot rodders. This is a different approach to valve timing that holds the intake open way past BDC to the point that some of the cylinder charge is actually pushed back out. This has the effect of reducing physical compression but when coupled with a high efficiency supercharger pushing a lot of boost you still get great power due to dynamic compression more than making up for it.

I know Mazda did it well with their Millinia sports sedan getting 100 HP per liter in stock trim but not everybody has their R&D capabilitys. I've done a bunch of searching but found very little. How Stuff Works goes into more detail than anyone else and that's pretty sad since it's very little information. Link: http://auto.howstuffworks.com/frame...rticleZoneID=92 There's a race engine shop in Ontario that admits they use Miller Cycle cams in their supercharged engines (the boat kind at least). But of course they don't just give out thier trade secrets. Have any of you Canadians heard of "Bullet Engines" and know anything about them? Link: http://www.bullet.on.ca/performance.html

I'm thinking of having a custom cam ground for my truck with Miller Cycle specs. This would allow me to run my Whipple with high boost levels and the existing 9.1 to 1 compression ratio and iron heads. I also have a Duramax intercooler that will be bolted on then intergated into the charge air plumbing. I'm not quite sure where it would all end up but based on Miller Cycle theory it might even improve milage under cruise due to the reduction in pumping losses on this mamoth 8100 cc V8. This in addition to the large increase in power eqivilent to a low compression motor running 10-15 lbs. of boost.

Mazda used a 10.1 static compression ratio but that was with aluminum heads. I'm not sure if by running a point under this if I should slightly shorten the intake event to capture more charge or just stick with the standard Miller Cycle intake valve closing range because of the higher combustion chamber temps and more detonation prone engine. And what about fuel octaine rating? This setup should allow for high boost with pump gas on a 9.1/1 ratio motor.

I also wonder if the delayed intake closeing to allow some charge reversion would play havoc with fuel injector timing. I surmise that with the MAF system A/F ratios will be maintained and if any fuel is pushed back out that it will be used on the next intake cycle anyways or by that of other cylinders therefore the PCM should be able to addapt to any of this quickly.

Some Japanese power plant engineers converted their large stationary turbocharged gas engines to Miller Cycle with only a year to do so. The language translation and numerous mathmatical formulas make the site hard for me to understand, but the I believe overall jist of it is that it was relatively easy to do and works well. I think only valve timing and minor tuning were required to increase power and efficiency by a viable amount. Here's the link: http://www.itepsa.com/samples/AFE199907_CH1_MAIN.PDF

I'm wondering what the rest of you forced induction guys might think on this? By the way I've way into the Thunderbird SC scene for many years now so working with forced induction is nothing new to me. This may just be a way to have cake and eat it too when bolting a big blower onto a higher compression engine than usual like most of us dealing with a stock short block.

Thanks,
Vernon

Maggie

Vernon,
The Diesel racers may be a good source on Miller Cycle Event engines. Those guys have done some amazing things with the high compression boosted applications. I know an engineer that's working on a 18:1 compression Miller engine that is boosted by two 3 1/2" turbos that spin a larger 4" turbo for about 300# of boost
Also, you could check out Waukesha Dresser and see if they have anything up on the net. Those guys have done huge amounts of research into the miller cycle event engine.
I have always found the Miller cycle cam interesting, Just haven't had the incentive to address it's virtues... Although; Years ago I did have some limited involvement in a R&D project on multi fuel engines that switched from Otto cycle operation to Miller cycle...but that's been way to long ago for me to offer any insight to your questions.
I do expect that we will be hearing alot more on the topic of Miller Cycle Cams in the future with the increased interest in high boosted applications and the need for cleaner emissions. But then again, the way we think of the internal combustion engine will take on a whole new scientific discipline and the savy builder will be thinking in terms of enthalpic reaction, thermodynamic efficience and Kcals/Kg of fuel...

later,
Maggie

BJM

iTrader: (1)

This new forum is great, where else could such things be debated.

I am clear on the following. The key thing with the Miller cycle is that the expansion stroke is longer than the intake stroke so you can run a very high static compression ratio (really this is the expansion ratio on the combustion stroke). You can run arbitrarily high "compression" by tuning the intake closing so that you keep compression pressure to an octane friendly limit. If you convert your existing gas engine by expanding the gases much further than normal "Otto" cycle, you gain efficiency, not power. Your combustion chamber when compressed is smaller and it has less charge in it so power output falls.

I am not quite clear on the next part - forced induction. To make up for the power loss from having less charge in the cylinder, you supercharge by some means (turbo, supercharger, etc). To do this you need to reduce the "compression" somewhat as you need to avoid detonation. After all is said and done do you wind up with more power than the Otto cycle engine you started with or a more efficient engine?

Manic Mechanic

iTrader: (3)

Hey thanks for the leads Maggie, I'll check it out. Any hard info would be great where ever it comes from. It helps to cut down on the SWAG. I agree that combustion engine science is really becoming common knowledge. The topics discussed on forums like this are things I've spent years explaining to freinds and family so they can understand WTH I'm telling them about when I soup up some of our junk. It's nice to find others who know more than I do and read up.

BJM, the expansion stroke is longer than the COMPRESSION stroke. Since the intake valve is held open for more than 180* the intake stroke will be the longest stroke of all, encompassing the end of the exhaust stroke and the near middle of the compression stroke. Another way to look at it is that the compression stroke doesn't start UNTIL the intake valve closes.

It turns out that compressing charge with the piston produces more heat than compressing it with a effecient supercharger. So use the blower to build compression, not the piston. But since the actual exansion ratio is unchanged you get the benefit of a high compression combustion chamber with a dence "super" charge without the usual heat. It just takes a lot of blower boost to accomplish this since much of the physical cylinder volume that could hold charge is not used. So what is had better be packed tight. This engine won't work without high boost because you give away charge, enough that without boost (instant boost at that) you would have an effective compression ratio of 6-7 to 1. What's the point of that? This design actually raises the compression limit of a boosted engine by not generating as much heat in the charge. Mazda ran 20 PSI with 10:1 on 93 octane, getting 100 HP per liter. So you can actually raise the compression to make up for the lost charge making more power with a more effecient engine.

I know I'm probably not clear, sorry if it takes a while to get what I said.

Thanks for the replies guys, I was starting to wonder.
Vernon

BJM

iTrader: (1)

You are right, I knew what I meant but typed it wrong.

I see the point now. Very interesting. But like you say you need to be able to produce large boost efficiently. If you are using turbo chargers and since you depend on such large boost, you will have to be careful not to wind up with a slug when you are off boost.

Manic Mechanic

iTrader: (3)

From what I understand it doesn't seem like turbocharging would work well for this type of valve timing in an automotive application where RPM must constantly be regained. Like you said off boost it will be a slug, and as such it would take a while for the turbo(s) to spool. Miller Cycle is used with turbos in stationary and industrial engines where the slow initial ramp up is acceptable. For an automotive type engine you pretty much have to have a positive displacement supercharger to have low end torque. It has to be able to quickly provide high boost at a low RPM and be able to effectively produce the high boost across the needed RPM range. A low RPM engine could get by with a big roots by driving it fast to make the high boost needed at the low crank speeds but a roots would run out of thermal effeciecy at higher crank speeds. The Lysom Screw is almost mandetory, with high boost and a good efficiency at high speeds it's about the only supercharger that can keep up at maximum engine speeds. I'm actually not sure how big the Whipple blower I have for my truck is yet. I know it's one of their larger units but I don't know if it's large enough to produce 15-20 PSI on a 496 and still handle 4800 RPM at the crank. Keeping the big engine below it's factory rev limit will help with running the blower at the high overdrive ratio necessary for this to work. Of course a small block with a big twin screw blower could run RPM levels at mechanical limits. This blower on an LS1 could really scream with obscene boost levels. I'll leave that one to someone else.

Vernon

rjw

iTrader: (3)

In researching cam options for SC'd application, it seems that a lot of people are already running cams that MAY be converting their engines to a Miller cycle. For instance , at least one 224/224 cam times the intake valve to be at .006" at around 69 or 70 degrees ABDC. Maybe they aren't aware of the Miller cycle, but they seem to be empolying it to some extent.

Let me quote someone else's work ...

BY closing the intake so late, the heat generated on a 10 to1 static CR engine can be reduced to that of a 7 to static CR engine.

However, the expanion ratio is still that of a 10 to 1 engine. In other words, pretty much the same bang.

The advantage is that you can run lots of boost without the heat created by compression.

The disadvantage is that you need lots of boost down in the lower rpm range, or low end power will suffer.

The twin screw is ideal for this, in that boost happens down low and the smaller volume of air is more dense due to compression made by the blower.

Manic Mechanic

iTrader: (3)

That sure sounds like it. Who's running that? Thanks for the find.

Vernon

rjw

iTrader: (3)

If you're addressing me, then ...

A lot of the vette guys are running 224/224's or 224/230's from vendors like possibly 21 CMC, Cartech and many more.

The reason I say possibly, is beacuse many like to keep there full specs secret.

Personally I spoke to several cam company's and some of the vendors and I have a custom grind coming that has 224 intake duration @.050. It has ZERO overlap which is what many recommend for FI. Intake valve closing @ .006 is almost 70 degrees ABDC.

Does it sound like Miller time ?...

P Mack

iTrader: (6)

This thread reminds me of the static vs dynamic CR thread. People judge whether they'll be able to run pump gas or not based on their dynamic CR. In other words they can get away with running 12:1 static as long as their cam is big enough. I think that may be the reason people who switch to 224/228 sized cams and 11:1 compression end up getting better gas mileage than stock.

Is the supercharger an essential part of the miller cycle? I guess it would help with the low rpm loss of torque with a cam with a lot of overlap. But would it still be called miller cycle with a late intake valve closing and high compression ratio?

I find that hard to believe, do you have anymore details on this? I thought that compressing air with a piston was near isentropic.

rjw

iTrader: (3)

Anyone, please feel free to correct me if you disagree...

The Miller cycle is normally used with a positive displacement supercharger.

Bottom line is heat in the chamber, whether it is created by CR or boost, or both. Just under 1100 degrees (Absolute) seems to be the temperature where meltdown begins... (0F = 460 ABS, 90F = 460 + 90 = 550 ABS)

A blower with an efficiency rating of 70% will add about 14.5 or so degrees (Absolute) temp rise for each pound of boost.

To find the temperature in the chamber created by CR, the formula is

T = CR (to the power of .28 [gas constant]) X 550 (assuming ambient is 90F)

Knowing and/or choosing a combination of static CR, cam and blower @ ?PSI, you can easily detemine if you are creating a recipe for meltdown.

Ambient and intercooler efficiency also come into play of course...

P Mack

iTrader: (6)

If the statement was changed to "It turns out that compressing charge with the piston produces more heat than compressing it with a effecient supercharger and intercooler" then I could see that.

samgm2

I've considered the Miller Cycle for quite some time and have studied it for quite a while.

I have quite a bit of experience with Mazdas, including Miatas, Proteges, and yes, the millenia S. I've torn down the Millenia engine more than once (2.3L supercharged).

I have several of the supercharges from millenias and have quite a bit of experience with miller engine.

I also attended school for too many years and too many years ago(Chemical Engineering) and studing towards Biomollecular Engineering (Ph.D.)

I have come FULL circle from believing that this design has ANY merit.

I currently have an LS6 installed in my Miata (4l60E) transmission. My plan was to convert that miata to Miller Cycle LS1. I have since abandoned this concept altogether having proven to myself - that it is a COMPLETE waste of time.

I now believe that the Miller Cycle is little more than an ENGINEERING MISTAKE. I do not believe that any power is saved by keeping the valves open longer, then having the supercharger force air back in using a high efficiency supercharger save absolutely NO WORK. The BEST you could hope for is 100% adiabatic efficiency of the supercharger and same pistons that would normally compress the charge have to turn the supercharger! So at best, the efficiency is only as good as NOT having the supercharger in there! It goes down hill from there. Think Mazda used the twin screw by accident. It would have been a REAL embarassment if the used an EATON and ended up with 16 MPG!

In the end, you have the same pressure in the chamber and you have to pay the piper somewhere. The law's of thermodynamics, pretty much guarantee this is the case.

If the supercharger was allowed to be switched off most of the time, than a savings (via a mediocre implimentation of the Atkinson Cycle) will result in a minor improvement in efficiency, BUT ONLY WHILE THE SUPERCHARGE IS OFF! In the millenia however, there is almost NO TIME that the supercharge is not in use.

As for the ATKINSON cycle (no supercharger), the SAME EFFECT can be induced FAR MORE SIMPLY by adding an obstuction to the intake and without going to the trouble of making a custom cam. In effect, you create a vacuum in the combustion chamber and for all intents and purposes, you now how less air in the chamber and a longer power stroke in comparison. HO HUM. All that says is that an engine running at lower RPM and putting out less power is more efficient. This is in fact what the throttle plate does.

The proof is in the pudding. There are LOTS of v6 (3.1 to 3.8) liter engines out there that beat the PANTS off the Millenia in terms of both power and Mileage and without the gloriously stupid and complex supercharger implimentation. God help you if you EVER have to change a set of plugs in that pile of garbage called a Millenia. Mazda should be embarassed by that implimentation. There is almost no job on that motor that doesn't require pulling the engine. It is PROOF that so long as no one understands what you are doing and it SOUNDS COOL, people will buy it.

Even after tuning a Millenia to DEATH. The best free way mileage I've seen is only about 22 MPG - if I take it easy!

This is PATHETIC for a car that is supposed to be good for power and efficiency. On the contrary, it confirms what I state about the cycle. That it is an engineering embarrassment. I am happy to be proven wrong and it would be very nice if other engineer jump on board for this discussion.

rjw

iTrader: (3)

While I am only a lowly BSC&E, and have no interest in debating the Millenia, let me ask a question.

In your opinion, what are the similarities and/or differences between a Miller cycle engine and an LSx with PD blower and a cam that holds the intake valve open until around 70 degrees ABDC?

PS. Are you on Miata.net? (I also have a Miata)

samgm2

I'm on Miata.net. I am LS1Bound - almost all of my posts are in the v8 conversion section.

This is one of my Miatas:

http://www.sammichael.net/ftp/painted/jlofront.jpg
http://www.sammichael.net/ftp/painted/jlorear.jpg
http://www.sammichael.net/ftp/painted/jlorear2.jpg

If what you are asking is: Do I think that an engine can be made more efficient by holding the intake open longer? My answer is YES. IF the supercharger is ONLY used under high demand (WOT). In that case, efficiency DOES increase because it is in essense an ATKINSON cycle (albeit not the best implimentation). Once the SC is allowed to apply boost, it is no longer efficient and in fact LESS efficient than simply having an NA engine. Again, the fact that Millenias get such mediocre mileage is in my humble opinion evidence that Mazda didn't have enough R&D in their implementation or even understanding the miller cycle.

Whether or not, the car gets any better mileage will depend on how often the driver forces the SC to boost the engine (when it is less efficient than NA).

I have spent plenty of hours looking at Mazda's godawful supercharger control setup. That supercharger runs about 90% of the time - even at 40% load - accounting for its lousy efficiency. If the supercharger is disabled, the car is gutless and will throw enough codes that it goes into limp home mode and no real testing is possible.

Really, for a gasoline engine, the LS1 is not bad. I wouldn't mind playing with GM's displacement on demand engines (DoD), tho' I doubt that there will be a tremendous improvement in mileage there either. You still have the extra cylinders and you have the friction from all those dead pistons. Seems like a very small step in the right direction, - we really need much more from a major auto manufacturer like G.M.

If you really want to improve efficiency, figure out a way to keep the piston closer to top dead center for longer percentation of the cycle or dump the piston engine altogether. I've been looking at the quasiturbine design and all I can say is WOW! Why didn't I think of that?! I don't believe we will see the quasiturbine in a car in the near future and perhaps its time has already come and gone before it ever goes into production.

As I said before, I'm happy to be shown that I'm wrong, but from where I sit, unless I've missed something, I see no reason that the "miller cycle" is ANY more efficient. In fact, it appears to be less efficient. I don't know of an implimentation of the Miller cycle that actually gets good mileage - but then again, I don't know of a major model besides the Millenia (now gone).

This solution for increasing efficiency comes at a cost of maximum power. The supercharger, if used judiciously, can compensate for this and bring maximum engine potential back to NA level and beyond. So long as its use is the exception, rather than the rule, there would be increase fuel efficiency.

One of the mistakes that Mazda made was in the selection of the engine itself. They elected to use a 2.3 liter engine and then cut its combustion air! It is effectively about a 1.4-1.5 Liter engine in a 3500 pound car! This left them with insufficient power to operate the vehicle under normal circumstances, let alone under heavy load. This mean that they had to engage the supercharger almost all the time just to keep up with traffic.

The LS1 is a different story. It is a large displacement engine and it could easily operate with half of its air/fuel mixture missing. If you are judicious with the SC, you may be able to eat your cake and still have it.

So for 90% of the time you would be operating a version of the Atkinson Cycle and for 10% of the time you would operate a Miller cycle (which is less efficient but if the SC is a very efficient SC) you will not be hurt tremendously and you will be able to recover your full power when you need/want it.

I have been working on an implimentation of the Atkinson Cycle for NA engines that doesn't have a large loss of power and doesn't require a supercharger to bring engine output back to stock NA level. My initial results have shown some promise, but I'm not ready to release it into the public domain yet.

Sam

P Mack

iTrader: (6)

How long does the intake valve have to stay open for it to be considered atkinson cycle instead of otto cycle.

samgm2

I'm not sure that's ever really been defined. You have to keep in mind that the Atkinson cycle refers to a real engine made a long time ago.

There is nothing particularly magical about the Atkinson cycle. The real reason for the atkinson cycle was to avoid a patent violation for the otto cycle.

Its been along time since I looked at the Atkinson engine, but from what I remember, the atkinson cycle differs from the otto cycle simply in that it "allows" (albeit probably unintentionally) the stroke lengths to be different and completes all 4 cycles in 1 revolution of the crank. Through a moderately clever arrangement of the crank and connecting rods two of the cycles have a different stroke length than the other two.

In fact, the intake and exhaust stroke in the atkinson cycle were longer than the compression and power strokes (not what you want for greater efficiency). Again, the purpose had nothing to do with greater efficiency and everything to do with patents.

Today, the Atkinson cycle means a lot of things (mostly, it means a difference in stroke length for different cycles). This can be accomplished in many ways, but basically, anything that allows the power stroke to be "longer" than the intake stroke without additional boost has been called an atkinson cycle by someone. LONGER generally means EFFECTIVELY longer. Even though in general the piston moves exactly the same distance for all the strokes, it is compressing a smaller volume of air.

There are MANY ways of accomplishing this and efficiency gains are small. The piston engine is a REALLY inefficient engine.

I SERIOUSLY doubt that Atkinson ever intended for his engine to be more efficient because it wasn't - and it would have been horribly misbalanced. In his case, the piston really did move a different distance. The crank also serves as the cam worsening the balance problems.

Sam

Maggie

What I find interesting is that there seems to be a trend in this new world of "fuel effecient", "Highbreds" and "super efficient" power plants from Ford, Toyota, etc. being touted as Atkinson Cycle engines. I have even seen multidimensional cam non-throttle valve engines discribed as Atkinson cycle engines!?! Some of these seem to me to be far and away from James Atkinsons design and could be better described as Miller Cycle? It seems that "Atkinson Cycle" has come to represent any anomaly of event in either Miller Cycle or Otto Cycle?????

Maggie