reshape combustion chamber
#26
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The problem with going too high in compression is airflow. Not through the intake or exhaust but rather through the basthtub in the rotor faces. Remember, unlike a piston engine we aren't just compressing air in place. We are also flowing it and all of that flow goes through those bathtubs. If it gets too small then we are getting severe pumping losses that are not offset by any compression gains. Mazda has a paper from long ago that shows the power differences between various compression ratios. Power really doesn't change much between about 9:1 compression and 12:1. It's pretty flat in there from a peak power standpoint. The higher compression does make more low end and makes more part throttle power though. That makes sense as the engine isn't at 100% VE and therefore not filling up the combustion chamber anyways. Above 12:1 power falls off pretty fast.
The reason that we've only seen spark ignited diesels is due completely to this phenomenon. You can't get compression ignition as you can't get the compression ratio high enough without incurring severe pumping losses. Small OS .30 rotary engines use glow plugs but they also stay on full time. The old John Deere diesel engines and even the Rolls Royce attempts had to bring the dynamic (effective) compression ratio up through roots supercharging. Remember dynamic and static compression ratios are not the same thing. They only match when the VE is at 100%.
The best chamber shape would actually be narrower, longer, and deeper.
The reason that we've only seen spark ignited diesels is due completely to this phenomenon. You can't get compression ignition as you can't get the compression ratio high enough without incurring severe pumping losses. Small OS .30 rotary engines use glow plugs but they also stay on full time. The old John Deere diesel engines and even the Rolls Royce attempts had to bring the dynamic (effective) compression ratio up through roots supercharging. Remember dynamic and static compression ratios are not the same thing. They only match when the VE is at 100%.
The best chamber shape would actually be narrower, longer, and deeper.
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i apologize ahead of time, i just want to make sure i'm understanding what you're saying correctly. Are you saying the pumping losses occur because that a smaller percentage of the air will end up inside that combustion chamber because it's smaller?
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yeah, there were some great examples given of some great diesel rotary options that have been done over the years. some of them had similar ideas, but none of them seemed to put it all together the way i want to do it. if i can get it to work the way i want it, it would be a pretty simple engine to run. Getting to that point will be a long road though.
#29
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You'll find that raising compression more and more will help with power lower in the rpm range and at lower volumetric efficiency levels but as loads and rpms rise, it'll hit a point where there is no longer a gain and in fact a loss may occur. According to that old Mazda paper, it's right at 12:1 compression.
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thanks for the explanation of flow, looking at how the rotor is positioned during combustion and where the spaces are, that makes perfect sense, so a change in the shape of the combustion chamber is gonna take some research.
You are talking only gasoline engines though, correct rotarygod?
which defense companies,j9fd3s?
p.s. if anyone has any measurements on there 12a housings, i am just trying to make a few scaled drawings to look at. it's really appreciated if anyone happens to have the info, if not, not a big deal.
You are talking only gasoline engines though, correct rotarygod?
which defense companies,j9fd3s?
p.s. if anyone has any measurements on there 12a housings, i am just trying to make a few scaled drawings to look at. it's really appreciated if anyone happens to have the info, if not, not a big deal.
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anyone have any knowledge on commonrail systems for diesels? i've been checking out that option, but what type of pumps do they use to create pressures that high? the injection pumps before common rails i think would be a lot harder to adapt to what i want. Has anyone seen an electric pump capable of that kind of pressure? i've been lookin', but not a lot of luck yet. lots of crazy diesel setups out there in the world though.
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does anyone have the timing from one rotor face firing to the next firing of the next face of the same rotor? i think they fire every 60 degrees, correct? or is it 120* if it's the following face on the same rotor?
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if anyone is interested in taking a look, i was looking at an injection system like this http://delphi.com/manufacturers/auto...rfs/multecdcr/. i think it could be very easily adapted to what i'm trying to do.
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the "hot spots" and detonation problems really have me leaning towards a big surface area, very shallow combustion chamber. very similar to the stock shape, but possible a bit longer to keep that offset. But yes, much, much shallower. The "swirl" effect commonly a big concern in the diesel community for the fuel/air mixture, i think will be deminished by the injection system being used and the sheer pressures of injection.
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i was looking at some of the demo videos on youtube, just to spark some ideas. i notice that simply because of the nature of the motor, the combustion chamber "tub" on the rotor face does not contain all the compressed air like in a piston engine where you only have the remaining distance left on the top of the cylinder (if any) and then the space in the C.C. does anyone have any idea what compression would look like without combustion chambers? i'm not really too interested in doing anything like this, but something i had thought about.
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same rotor shape, with no dishes, yeah. thanks again to everyone who has contributed thoughts, the more ideas out there, the more options there are to attain perfection.
#41
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The problem would be that you'd have so little clearance between the rotor face and the housing surface that you'd get huge pumping losses as air wouldn't be able to flow through there very easily. Keep in mind that it's not all of the air in the chamber that is affected but it's enough. Having no dishes is the same as having a very high compression ratio.
#42
BoostedHULK, it may be easier to understand the importance of the rotor dish and efficiency if you think about it this way.
The point along the minor axis of the epitrochoid (the narrow spot in the 8 of the rotor housing) splits the combustion chamber in half.
Ignition has to happen with some advance of TDC and so ignition occurs when the the upper trailing portion of the combustion chamber has a larger volume than the leading portion.
This means it is the expanding mass of the air/fuel combustion that must pass through the slot of the rotor from the trailing side of the rotor to the leading side of the rotor to finally push the rotor in the correct direction to generate power.
The more restrictive you make the passage to the leading portion of the combustion chamber the more force you exert on the rotor in the opposite direction of the power stroke.
As mentioned, Mazda has found that with the geometry of their Wankel engine 12:1 is the limit.
The only way to increase the compression ratio effectively is to DECREASE the eccentric shaft offset thus increasing the width of the minor axis of the epitrochoid so there is less total volume in the leading and trailing combustion chamber portions.
The point along the minor axis of the epitrochoid (the narrow spot in the 8 of the rotor housing) splits the combustion chamber in half.
Ignition has to happen with some advance of TDC and so ignition occurs when the the upper trailing portion of the combustion chamber has a larger volume than the leading portion.
This means it is the expanding mass of the air/fuel combustion that must pass through the slot of the rotor from the trailing side of the rotor to the leading side of the rotor to finally push the rotor in the correct direction to generate power.
The more restrictive you make the passage to the leading portion of the combustion chamber the more force you exert on the rotor in the opposite direction of the power stroke.
As mentioned, Mazda has found that with the geometry of their Wankel engine 12:1 is the limit.
The only way to increase the compression ratio effectively is to DECREASE the eccentric shaft offset thus increasing the width of the minor axis of the epitrochoid so there is less total volume in the leading and trailing combustion chamber portions.
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thanks a ton for the GREAT responses, they are all greatly, greatly appreciated. bluetII, thanks for the explanation, you seemed to word it just perfectly for it to click in my brain on how to make it work.
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i got the engine tore down to it's bares. gonna pull the internals next weekend. so, basically what i'm looking at right now, is raising the compression, and i'm gonna do that by building up as much as possible above and below the combustion chamber with a coating. a change in the shape of the bath tub is gonna be necessary. i'm also looking at possibly only using the trailing side for actual fuel injection (combustion/ignition) to get it running. Being more hands on with it now, i'm will to hear any other ideas you guys may have as i run into problems with getting it to run.
#45
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It's a futile effort. You aren't going to get anything out of it. There is a way to get highercompression ratios out of a rotary and have them work but you must change the k value of the engine. This means that the shape of the rotor and housings would be totally different and that is well outside the scope of any hobbiest.
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i think saying any effort is futile is just a crappy point of view. you are right though, about changing the rotor and housing shapes. i am going to try to manipulate the shape with coatings. the engine is a spare that i can try as many designs as possible with. i will see if i can get this thing to run.
#48
It's a futile effort. You aren't going to get anything out of it. There is a way to get highercompression ratios out of a rotary and have them work but you must change the k value of the engine. This means that the shape of the rotor and housings would be totally different and that is well outside the scope of any hobbiest.
#49
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You aren't going to fundamentally change the shape with a coating. All you can do is add or take away material from roughly the same areas. Keep in mind the shape of the rotors was 1mm different in radius between the pre-86 and the 86-later 13B engines and is another 1mm different between the last 13B and the Renesis rotors. This is about as much as you'll be able to change them. Since you want higher compression, you are adding. However you can only add so much and can't get what you want. This is all you can do. What you need to do is to completely change the rotor shape and housing geometry. This can't be done through any amount of modification to any existing parts and trying to do so is quite frankly futile. It's not a crappy point of view. It's reality. You can't do what you want unless you completely redesign the engine from scratch and then build it from the ground up with completely new parts that have absolutely nothing in common with existing ones. In other words, a clean sheet engine. As I said, that's outside of the abilities of the backyard mechanic.
Can you design a rotary that can effectively use a higher compression ratio? Yes you can. It would look very different but would fundamentally still work the same.The "peanut" shape of the housings would be less pronounced and more towards a circle and the rotors would look fatter as a result. However now we run into different problems such as rotor dish size and shape. You have to have it. You can't not have one and still have your mixture concentrated towards the spark plugs. If you redesign the entire shape, then you need to determine the best width. Mazda has only recently discovered that the shape they've used for decades has been less than optimal which should be a revelation because it's only taken them with all of their testing nearly 50 years to discover this!
It's not futile if you have the time, money, and resources to devote towards it. However as difficult as it is, it's pretty much a sure thing that the word "futile" is in fact pretty accurate in this context. Others have already welded up rotors and raised compression. There's nothing new in that. There's also a reason why no one is still doing it. Just something to think about.
Can you design a rotary that can effectively use a higher compression ratio? Yes you can. It would look very different but would fundamentally still work the same.The "peanut" shape of the housings would be less pronounced and more towards a circle and the rotors would look fatter as a result. However now we run into different problems such as rotor dish size and shape. You have to have it. You can't not have one and still have your mixture concentrated towards the spark plugs. If you redesign the entire shape, then you need to determine the best width. Mazda has only recently discovered that the shape they've used for decades has been less than optimal which should be a revelation because it's only taken them with all of their testing nearly 50 years to discover this!
It's not futile if you have the time, money, and resources to devote towards it. However as difficult as it is, it's pretty much a sure thing that the word "futile" is in fact pretty accurate in this context. Others have already welded up rotors and raised compression. There's nothing new in that. There's also a reason why no one is still doing it. Just something to think about.
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great supporting evidence and you're right. i need to to flatten out the peanut shape a bit, but more importantly i need to create an easy "flow" of gases from one half of the combustion chamber to the other. The dish does need to be there, or would be better, for keeping the air/fuel mixture near the spark plug, but i don't plan on using a spark plug, and with the high pressures of the injection system being used, i think that the "bath tub" will be less important to combustion. we'll see how it works out though. You may very well be 100 percent correct, i don't want to argue, just don't like to be told not to try, ya know? worst case scenario is, turns the rotor the wrong direction, and maybe breaks an e-shaft, or throws metal or ceramic pieces out the exhaust. Maybe create the first ever, rotary bomb?