rotarygod

iTrader: (1)

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.

boostedHULK

iTrader: (1)

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?

j9fd3s

iTrader: (3)

no, there are a couple of defense oriented companies that have diesel rotaries now

rotarygod

iTrader: (1)

Nope. When the rotor moves to it's smallest point in rotation, all of the air in the chamber has to move from one side of the rotor through the rotor dish to the other. Don't confuse this with apex seal sides. Air moves within the same chamber due to rotor movement and housing shape. It even momentarily flows through the dish backwards (from bottom to top)! The fact that it is getting compressed more isn't an issue as so many high compression diesel engines can attest to. The issue is with the actual movement of air through the rotor dish. Since we have compression and flow internally, we have 2 things to worry about.

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.

boostedHULK

iTrader: (1)

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.

boostedHULK

iTrader: (1)

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.

rotarygod

iTrader: (1)

The common rail pumps are belt driven 3 cylinder plunger style (much like a piston) pumps.

boostedHULK

iTrader: (1)

thanks, do they have a specific name? i know that older diesels used "lift pumps".

boostedHULK

iTrader: (1)

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?

boostedHULK

iTrader: (1)

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.

boostedHULK

iTrader: (1)

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.

rotarygod

iTrader: (1)

We don't have a swirl effect in our engines and no amount of intake swirl remains by the time the rotor face gets over to the plug side of the engine. A better shape would be narrower, longer, and deeper.

boostedHULK

iTrader: (1)

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.

rotarygod

iTrader: (1)

When you say "without combustion chambers", what exactly do you mean? Are you wondering what combustion would look like with a different rotor shape but no dishes in the rotor face?

boostedHULK

iTrader: (1)

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.

rotarygod

iTrader: (1)

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.

BLUE TII

iTrader: (9)

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.

boostedHULK

iTrader: (1)

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.

boostedHULK

iTrader: (1)

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.

rotarygod

iTrader: (1)

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.

boostedHULK

iTrader: (1)

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.

PvillKnight7

iTrader: (16)

What type of 'coating' are you using?

fidelity101

iTrader: (11)

yeah try and outsmart the many engineers about their "peanut" shaped housing for an improved design because I'm sure they would love to know too lol.

rotarygod

iTrader: (1)

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.

boostedHULK

iTrader: (1)

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?