Guest Nic-Rebel450CA Posted October 7, 2003 Share Posted October 7, 2003 There seem to be lots of people with turbocharged Zs (gee, I wonder why ) I am just curious if anyone has ever considered trying a miller-cycle setup with a Z. What is a miller-cycle? A miller-cycle engine is the same as a standard four-stroke engine, except...A Miller-cycle engine depends on a supercharger.A Miller-cycle engine leaves the intake valve open during part of the compression stroke, so that the engine is compressing against the pressure of the supercharger rather than the pressure of the cylinder walls. The effect is increased efficiency, at a level of about 15 percent. About the only thing that would be needed for this would be a custom made camshaft, but that's not very expensive considering the horsepower gain. I would imagine that gains over 15% would be likely with higher boost levels. Now, I am not very interested in turbos myself, I just like to see Z's go fast and wondered how many people know about this design. BTW, for those of you who dont know about this, it is already in production in some mazdas, it's not just a theory. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted October 7, 2003 Share Posted October 7, 2003 A Miller-cycle engine leaves the intake valve open during part of the compression stroke' date=' so that the engine is compressing against the pressure of the supercharger rather than the pressure of the cylinder walls. The effect is increased efficiency, at a level of about 15 percent. I would imagine that gains over 15% would be likely with higher boost levels.[/quote'] Where have you seen documentation on this? This engine cycle seems suspect - to me, leaving the intake valve open would just reduce your compression, and reduce quantity of intake charge, as it would just leave the cylinder. Who cares if the manifold pressure is running positive? Cylinder compression is MUCH higher than 15 psi (or whatever) - you would just raise the manifold pressure from the reduced flow. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 7, 2003 Share Posted October 7, 2003 http://abcnews.go.com/sections/tech/Geek/geek000320.html I've always been interested in the Miller cycle engine. I got a ride in a Millenia S when they first came out. Pretty spunky for a big car with a 2.3L engine. It looks like the efficiency comes from not having to compress the air/fuel from the bottom of the compression stroke??? I wonder if a large overlap cam on a high pressure turbo would do basically the same??? Never heard of that being a good combo, but maybe the cam timing needs to be just right... Jon Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 7, 2003 Share Posted October 7, 2003 Just realized that the overlap thing won't work. You apparently want the intake open late, but not the exhaust. Jon Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 7, 2003 Share Posted October 7, 2003 Hijacking the topic for a second: Stirling cycle engines are just plain cool, but wouldn't be likely in a car due to power/weight/size. One of the coolest things about the stirling cycle is that if you power the fan on the engine with hand crank, or electric motor, you have a refridgerator. I wonder if old Robert Stirling firgured out that part in the 1800's when he invented it. There is no reason why a miller cycle engine couldn't be turbocharged that I see... it is a neat idea. Hmmmmmm. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 7, 2003 Share Posted October 7, 2003 One reason I just thought of - LAG! Jon Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 7, 2003 Share Posted October 7, 2003 I was thinking about that too Jon, but it could still work. Granted at low boost pressures and RPM, you'd lose efficiency, and since that is probably where most of the driving takes place, it might not be feasible for a daily driver. However, a car that see's highrpm duty and is rarely out of boost SHOULD be able to benefit from this increased efficiency. The difficult part as I see it is to tune the cam profile. Even just going into the mechanism portion of the crank/piston and cam model would be useful, but I would think you'd want to delve much deeper than that, and consider the fluid dynamics of the whole equation...... Hard to do without some very expensive software to say the least. (GT-Power would do it if you had the time/money) And the problem with just testing, is there is no easy way to change cam profiles on the fly. You'd need a few custom ground cams IMHO, and have to swap them and compare back-to-back on a dyno to really know what's working and what isn't.... And even then I'd probably suggest starting with a DOHC engine so you could adjust intake cam timing and get a bit more data that way. OK, so it isn't a project for the light hearted. Looking at some big money and time to find any real results I'd expect! Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted October 7, 2003 Share Posted October 7, 2003 Cylinder compression is MUCH higher than 15 psi (or whatever) ... Not for the first part of the stroke, at the first part of the stroke the mixture isnt compressed at all. It also isnt nearly the volume of the incoming airflow. I wonder if a large overlap cam on a high pressure turbo would do basically the same??? Overlap doesnt get more of a charge into the cylinder because of incoming air pressure, overlap works because the exhaust actually sucks the intake charge into the cylinder. However, overlap would have no bearing on the miller-cycle as the part of the miller-cycle that is different is the end of the intake cycle whereas the overlap would take place at the beginning. There is no reason why a miller cycle engine couldn't be turbocharged that I see It would, in this verbage supercharger and turbo are synonymous. A turbo is just an exhaust-driven supercharger. The difficult part as I see it is to tune the cam profile. This could be mathematically calculated, all you would have to do is calculate the point at which your cylinder pressure is greater than the intake flow. I would imagine this would be a factor of the pressure and volume versus the incoming air pressure and the volume of the incoming flow in CFM. Would that be pounds per cubic inch per minute? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 7, 2003 Share Posted October 7, 2003 Doesn't the Mazda engine run around 25psi boost? That's why I think a turbo wouldn't work well. You need some rpms to get that kind of boost from a turbo. A drag only car might be able to use a turbo, but probably not a street car. I'm sure if someone wanted to try it you could get the cam profile from a Millenia and make a pretty good guess on the cam. I like Drax's idea of a DOHC motor though, because you could tweak the cam timing easier. Jon Quote Link to comment Share on other sites More sharing options...
Guest BigE Posted October 7, 2003 Share Posted October 7, 2003 From my textbook, Engineering Fundamentals of the Internal Combustion Engine, for a class I took in I.C.E.'s: For either variation of the (Miller) cycle to work efficiently, it is extremely important to be able to close the intake valve at the precise correct moment in the cycle. However, the point where the intake valve must close changes as the engine speed and/or load is changed. This control was not possible until variable valve timing was perfected and introduced. Automobiles wih Miller cycle engines were first marketed in the latter half of the 1990s. A typical value of the compression ratio is about 8:1, with an expansion ratio of about 10:1. The first production automobile engines operating on Miller cycles used both early intake valve closing methods and late intake valve closing methods. Several types of variable valve timing systems have been tried and are being developed. At present, none of these offer full flexibility, and major improvements are still needed. end The book also says that due to the valve closing either after BDC or before BDC, a supercharger is used to counteract the effects of low output per displacement. Due to the complexity involved (VVT), I would think a Miller engine would be counterproductive for an individual to attempt. The book (title above, ISBN 0-13-570854-0) is written by Willard W. Pulkrabek. I highly recommend it if you're interested in the thermo, fluid dynamics and heat transfer involved in internal combustion engines. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 7, 2003 Share Posted October 7, 2003 Well that puts a damper on things, doesn't it. Jon Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted October 8, 2003 Share Posted October 8, 2003 I would think that the referrence quoted is referring to the ability to get maximum performance. It's kind of like how the demand for overlap increases throughout the RPM range. I dont see any reason why you wouldnt be able to have at least a small amount of miller-cycle. Certainly you wouldnt be able to take full advantage as there is no VVT, but there should be a notable performance gain, even if it is not maximum capability. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted October 8, 2003 Share Posted October 8, 2003 My impression was this "keep the valve open longer" trick occurred much later following the intake stroke, where you would lose the inertia from the intake, and actually have flow back out the valve. Correct me if I'm wrong, but that doesn't seem to be flow efficient - and for maximum power out of an engine, you simply try to put in the most air (and fuel). I'm looking at my turbo cam spec card, and the intake valve actually closes 52* ABDC (at 0.050" camlift). And I know that flows up high - it loves to spin 6000. If it made more power by delaying the valve closure even further, wouldn't they have done it, instead of stopping at 52*?? Quote Link to comment Share on other sites More sharing options...
Dan Juday Posted March 26, 2005 Share Posted March 26, 2005 "I'll have a Miller, please" I dug this out of the archives in case somebody might be interrested. I bought my daughter a car last week with a blown engine. The plan is she rebuilds it (with my supervision) and gets an education and her first car all in the same deal. Local shop had a nice looking Mazda Millenia buried in the repo lot. '95, leather, sunroof, all the options. $400, SOLD. Get it home and find out it has one of these cool Miller-cycle mills in it. 2.3L V6, four cams, and wierd supercharger. It started right up! But it blows blue clouds of Valvoline. We'll be pulling the motor out next week. If anyone is interrested we'll give updates with pics. Quote Link to comment Share on other sites More sharing options...
Moridin Posted March 26, 2005 Share Posted March 26, 2005 I'd like to see that that. Be very interesting indeed Quote Link to comment Share on other sites More sharing options...
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