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Lean burn with negative split timing
I don't see this topic discussed much lately. Rotarygod has written quite a bit about it, and Mazdamaniac also mentioned it in the past. For that matter, Mazda did a lean-burn RX4, and they've done a lean-burn/hydrogen prototype Renesis. I wouldn't be surprised if they considered a lean-burn Renesis for the RX8, but decided NOx traps weren't worth fucking with.
Anyhow, I put together a calibration that does it, just made the trailing plugs fire 10-30 deg before the leading in low load areas and set the same areas to target 17.5 afr in the closed loop maps. I haven't tested much, and I'm wondering if it isn't discussed because the concept is recondite, or doesn't work as it ought. |
The idle tables need some correction as well. They're set up to taper from neg split idle timing to positive split main timing, that has to be corrected or amusing things happen when these tables are referenced.
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Well. I guess no one cares, but so far it seems the ecu has a lean limit. Once it is on a cell for a bit it drops the AFR to 15.6 or so. Maybe I can find a workaround, but I give up for now.
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I care! This is super great news ... could you post some of the more informative RG posts? Some honda guys have been seeing amazing results by combining FI and lean burn...
you may find this interesting ... http://www.d-series.org/forums/force...g-64-8mpg.html ... although I doubt we will see any figures like that, I do not think it unrealistic to hope to break 40 mpg on our cars with the proper set-up and not too much weight shedding |
sounds like a cool idea - have seen mine run 17.0 (due to an injector staging issue) in closed loop before and there didn't seem to be any negative impact.
I might finally be able to justify the turbo to my wife if I could get better fuel consumption and say it is because of the turbo HEH |
I'll find RG's posts tomorrow. The basic idea is that the fuel gets concentrated near the trailing tip of the rotor, so even though the overall mixture is too lean, it is rich enough near the trailing plug to ignite. The flame front from the first ignition travels too slowly to reach the area around the leading plug, but it compresses the mix enough so that the rest can ignite when the leading plug fires.
The best part is that, apparently, when you get a little lean, EGTs go up, but when you get really lean, they are lower than stoich. I was able to get the car to cruise at about 17.2:1 for ten minutes or so, but it only worked above 4k in 6th. Even so, the instant fuel monitor on my AP was reading in the 30-35 range. Now I can't get the AFR to stay above 15.6:1, it goes to 17 for a while and then drops down. I've seen devices that will trick the WB02 into reading low. There are some discussions of this on ecomodder.com. Seems like an unfortunate hack, though. Even though I can't seem to get lean burn to work, I'm still running up to -25 deg timing in low load. You might not think it would work at all, but it seems to run really well. I've had to tweak basically every table in the ignition section of AccessTuner Race, and I didn't know what most of them did a few days ago. No, don't have a girlfriend. :) |
so which table did you tweek to get that ?
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I'll post everything when I'm more awake. Changed every cell in trailing map, MM's split calc sheet was helpful. I modelled this after a map I found for an FC RX7.
At one point I reduced advance in the leading table, so the trailing advances wouldn't have to be so crazy, but it made no difference. The RX8 already idles with negative split timing, but all the tables are set up to taper into normal split. I had to adjust all of them, other wise there is an odd hesitation going in and out of idle. Alternatively, you can just use normal timing in the trailing map areas around idle. There is also some interaction between idle and the RPM Delta tables. They control ignition retard for coastdown and throttle tip-in. Right now I have them zeroed out. Guess what? Rotaries don't engine brake for shit. I don't quite understand how that works, but the effect is undeniable. I think that RPM Delta A controls high gears, and B low gears, but I haven't tested much. Pardon me if I have some of this confused. I've never tuned before, and I've done 52 maps in the last two weeks, so I'm getting delirious. |
Dang--dude.
Explain more please--rotary doesnt like engine braking? On track it has been shown with fuel/timing adjustments on decel they can reduce egt's. |
If it helps anything, The bridge port idles at 1500 RPMs with about 20 AFRs, and runs fine too (I want it to idle at 1200 or less). Needs a lot of tuning above 6000 RPMs cause it starts hesitating and running like shit.
On the way to Deals gap, I posted like 270 miles on my tank and was probably at an 1/8 of a tank left when I topped off. Usually my light comes on at 260 miles on the highway. So 10 more miles and 1/8 of a tank left based on the gauge. |
[quote=Charles R. Hill;3968890
I mentioned a similar thought to MM a long time ago but I got the sense he thought I was nuts. :lol2:[/quote] "Nuts" doesn't necessarily mean "wrong"! :rock: |
Ok what is happening during engine braking? My mind can run free with this................I have my own ideas, but if anyone has found some hard evidence???
OD |
Originally Posted by oltmann
(Post 3968646)
I'll post everything when I'm more awake. Changed every cell in trailing map, MM's split calc sheet was helpful. I modelled this after a map I found for an FC RX7.
At one point I reduced advance in the leading table, so the trailing advances wouldn't have to be so crazy, but it made no difference. The RX8 already idles with negative split timing, but all the tables are set up to taper into normal split. I had to adjust all of them, other wise there is an odd hesitation going in and out of idle. Alternatively, you can just use normal timing in the trailing map areas around idle. There is also some interaction between idle and the RPM Delta tables. They control ignition retard for coastdown and throttle tip-in. Right now I have them zeroed out. Guess what? Rotaries don't engine brake for shit. I don't quite understand how that works, but the effect is undeniable. I think that RPM Delta A controls high gears, and B low gears, but I haven't tested much. Pardon me if I have some of this confused. I've never tuned before, and I've done 52 maps in the last two weeks, so I'm getting delirious. which fuel table were you working in ? |
Originally Posted by Charles R. Hill
(Post 3968887)
Look up "Chrysler Lean-Burn".
I was logging to see how far the MAF would go, the MAF had stopped at 372, that was about 6800 rpm... Ran great for a few pulls. Never tried with the low comp rotors. :dunno: |
Originally Posted by Rote8
(Post 3969456)
I had the lean blow once, 15 to 1 under a 190% load at 9250rpm. :banghead:
I was logging to see how far the MAF would go, the MAF had stopped at 372, that was about 6800 rpm... Ran great for a few pulls. Never tried with the low comp rotors. :dunno: Oh noes - what was the damage ? |
Originally Posted by olddragger
(Post 3969253)
Ok what is happening during engine braking? My mind can run free with this................I have my own ideas, but if anyone has found some hard evidence???
OD Paul. |
The neg split timing is definitely responsible for the smooth, lean idle in the RX8 (which people still complain about.) I reversed the timing at idle, it becomes very lopey and unsteady.
Reddozen, I bet your car wouldn't idle at all without it. Just don't set your idle above 1700rpm, that is when the timing starts to go "normal." Also, if you can't get the MAF calibrated enough so you can idle normally due to turbulence, you should try making it richer in the fuel VE table. Bigger values make it richer, so try setting it to 1.4 at the idle areas. I tried to use both that table, and the closed loop table to force a 17.5:1 AFR. Open loop fueling max is 1 lambda. Lean burn isn't a new concept, but piston engines need special modification to make it work. The Civic HX had VTEC-E, which closed one of the intake valves to swirl the lean mix. Supposedly rotaries have the ability inherently, but I'm stymied by the ecu. I've read that Mitsu and Subaru Renesas-based ECUs also have a lean limit. |
Thank you Paul--that is a characteristic of this engine I do NOT want to lose. Very little throttle lift oversteer moments with this car.
OD |
Originally Posted by Mazmart
(Post 3969475)
Very little stroke (Eccentricity in this case) equals minimal engine braking. It's totally related to torque production as well.
Paul. By "feel", I'd guess that RX-8 engine braking is in line with what one would expect from a 1.3L piston 4, but it's not immediately obvious to me why. :scratchhe One reason might be that without combustion pressure, the apex seals do not seal as well as piston rings, causing less power off, throttle closed manifold vacuum to brake against. |
It's exactly as Paul stated
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Originally Posted by HiFlite999
(Post 3969796)
By "feel", I'd guess that RX-8 engine braking is in line with what one would expect from a 1.3L piston 4, but it's not immediately obvious to me why. :scratchhe One reason might be that without combustion pressure, the apex seals do not seal as well as piston rings, causing less power off, throttle closed manifold vacuum to brake against. Has twice the engine braking compared to the old 2.3 4-pot we used to run... |
Set my CL maps to give me about 15.3 and that is working fine - no timing changes . Waiting to see if that gives me any better consumption at cruise .
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We're forgetting a great contributor to engine braking...
Flywheel mass. Mazmart has got it right but production cars often have heavy flywheels to reduce "snappy" engine brake :) |
Originally Posted by bse50
(Post 3970045)
We're forgetting a great contributor to engine braking...
Flywheel mass. Mazmart has got it right but production cars often have heavy flywheels to reduce "snappy" engine brake :) The lean-thing is very interesting! I'd love to have even 132 hp for freeway cruising at 30 mpg. |
Originally Posted by HiFlite999
(Post 3970159)
I've tried to find any semi-scientific reference to less engine braking with the rotary, but have only found vague generalities, not explainations of any consequence. .
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They have already told you why Hiflite, it's because engine braking is torque-related as well. Our e-shaft is not that eccentric after all!
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Originally Posted by bse50
(Post 3970292)
They have already told you why Hiflite, it's because engine braking is torque-related as well. Our e-shaft is not that eccentric after all!
I'm thinking that true engine braking (vs inertial effects encountered when the rpm is changing) is about the volume of air one is trying to pump through a closed throttle plate. Because the rotary is a small displacement "rpm" engine (which implies low peak torque), the holding ability at 3000 rpm down a mountain grade is going to be less than that of a V-6 at 3000 rpm simply because that represents less "attempted" airflow. Shifting to up the rpm in the rotary so that it's trying to pump the same amount of air, and you get the same engine braking effect. Because of the rpm difference required to get the same airflow in a more conventional engine one is fooled into thinking there's a smaller effect. (Port/valve timing effects may also play a role.) Or maybe I'm missing something. :dunno: |
That's right but in the end it's the crankshaft fighting to "win" the vacuum. I suck at physics but it should be a matter of levers.
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I'm intrigued by this, and some other discussions I've seen on the forums regarding efforts Mazda made to improve atomization of fuel. I'd welcome such a lean burn for cruise conditions.
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Throttle plate angle, compression ratio, gear ratios, VE at those rpms and displacement are the main influences for engine braking, along with ignition timing when you get down to the phase in point for idle.
Internal friction is a relatively minor component. Tbh, the rotary has pretty much the same engine braking as any other petrol car with 150-160lbft would display. The reason it doesn't have the same braking as a 3.5 V6 at the same rpms is pretty simple tbh, it's not trying to draw as much vacuum, if you put them both in an rpm range where they make similar power at the same roadspeed, and then lift off, it would be pretty similar. |
how did we get from lean burn to engine braking?
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it's an RX8Club tradition to take a thread on a subject that you know nothing about and hijack it with a subject that you know even less about ...
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Originally Posted by stinksause
(Post 3971860)
how did we get from lean burn to engine braking?
No joke... |
so ... let's get back on topic ... I'll ask a question ... I am not an expert, so I will try my humble attempt, as no one else will....
from the honda thread ... (i know the engines are quite different) it is somewhat clear that the biggest hurdle to jump would be the slower flame front speed that's what robs power, especially if you are in the higher RPM speed. (however, we are lucky since our rotors move SLOWER than the speed of the essentric shaft http://www.youtube.com/watch?v=OuId4nuxXaM ... this helps) But first some videos ... Flame Fronts: http://www.youtube.com/watch?v=E841wOOp0iQ http://www.youtube.com/watch?v=sEf8va1S7Sw for the newbs ... note how the spark fires before TDC Diesel: http://www.youtube.com/watch?v=_IFCubco95w http://www.youtube.com/watch?v=-uxB8aiZyTc <- note the secondary flame fronts So what we DO have working for us are... slower moving combustion chamber w.r.t our output shaft 2 spark plugs. Question: Is it possible to fire one of your spark plugs twice to take advantage of the secondary flame fronts? I vaguely recall reading an article about the use of a 3rd spark plug located below the other 2 to take advantage of the blowback phenomena in PP ported engines ... it fires right around the time of the exhaust port opening (either before or after, I can't recall) either way, I think the key is to add another spark event to make sure every last bit of fuel in the lean mixture gets combusted ... this is the general idea behind the prechamber talk in the honda thread ... although that thread is long ... it is VERY informative ... DISCUSS personally, I think the rotary is better suited for lean burn that other engines ... the honda thread alludes to this http://forums.subdriven.com/showthre...nt-propagation Factory lean burn: http://asia.vtec.net/article/d15b/ (last engine on the page) again ... I am somewhat of a newb too, but I will contribute when no one else can/ is willing What is the limit of our ECU in terms of how lean it can go ... does OL or CL make a difference? |
IME you can go to 15.6:1 (actually 1.06 lambda) only in closed loop. 1 lambda is the max for OL fuel tables, at least in accesstuner race.
Interesting point about diesels and engine braking. The RX8 ecu retards timing to increase engine braking. Most modern ecus, including bangers, do the same thing, but to a lesser degree (get it? Degree?) More like 20° vs 57-59° on the RX8. I have no clue how this works, but having eliminated it I can tell you it works. I don't think diesels use spark retard to enhance engine braking. :) I took a break from playing with timing. Normally, I am an 87 octane guy, but I thought I'd get a tank of 91 to give me some safety margin while I played with timing. Naturally, it was the worst gas I've ever gotten, and I was getting low load detonation constantly, even on the stock timing table. I started to think the whole neg split timing thing was an elaborate prank to make me kill my engine, but half a tank of good 91 and half a gallon of acetone has fixed it now, so hopefully I will have more to report on soon. |
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Originally Posted by oltmann
(Post 3972653)
Interesting point about diesels and engine braking. The RX8 ecu retards timing to increase engine braking. Most modern ecus, including bangers, do the same thing, but to a lesser degree (get it? Degree?) More like 20° vs 57-59° on the RX8. I have no clue how this works, but having eliminated it I can tell you it works.
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Hopefully you get it right because no amount of gas mileage savings will cover an engine replacement. Getting it right means installing EGT monitoring equipment. Without it you are flying completely blind.
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Thanks, you're right, I hadn't noticed that. Looks weird in my log though, 10-5=68.5? I've heard that the Subaru Renesas ECUs only report log data at 4800bps. Mazda seems about the same, makes it hard to learn about transient conditions.
https://www.rx8club.com/attachment.p...6&d=1305072821 |
I do intend to get an EGT monitor, however the best evidence I can find indicates that EGT/CHT peaks at stoich.
http://www.n66ap.alexap.com/Mixture_...s/image002.jpg While emissions of all types of pollutants decline as the mixture gets leaner, catalytic converters simply stop working on oxides of nitrogen. I think this is the main reason cars cruise at stoich, it is as lean as you can go without making smog/acid rain. http://ecomodder.com/forum/member-cf...-emissions.jpg Of course, for that very reason, I am a bit reticent to operate my car this way on a permanent basis. Those with midpipes, however, may see a reduction in overall emissions while operating lean. http://www.g33.co.uk/images/fic-af-emissions.gif |
general reciprocating engine data is irrelevant
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:rolleyes: Wankel's have lower thermal efficiency, not different physics, but I'll humor you.
The Mazda will happily run 200 degrees lean of peak with no worry about burning an exhaust valve. There are none of course! Tracy Crook believes the reason it runs so well when leaned out is because the wankel is naturally stratified charge. What that means is the fuel droplets or fuel vapor is separated from the main air charge and is thrown outwards toward the spark plugs by centrifugal force due to the rotating combustion chamber. The intake charge rotates around the center of the engine just inside the rotor housing. This concentration of rich mixture makes it much easier for the spark plugs to ignite the overall lean mixture. At this very lean setting the EGT has dropped down into the piston engine range and prior to the RX8 engine is only slightly less economical than an air cooled piston engine. Tracy Crook estimates the pounds of fuel burned per hour for each HP generated is 0.47 while an air cooled piston engine is about 0.45. If you run the engine at peak power full rich it will burn 0.65 BSFC just like an air cooled piston aircraft engine under the same conditions. The new Mazda RX8 side exhaust port engine is reported to be much more economical. We don't have the BSFC numbers yet but the RX8 car will yield 27 MPG at 55 MPH, 25 MPG at 70 MPH and 20 MPG at 80 MPH. That is equivalent to 2, 2.8 and 4 gallons an hour in aircraft terms. Cars have more drag than airplanes so it is hard to relate this to aircraft service. In the side exhaust port RX8 engine any residual un-burned fuel is re-circulated to the next combustion cycle. The exhaust port timing is also modified in the RX8 engine to allow a longer expansion cycle recovering more energy from the fuel. The RX8 engine has much lower exhaust temperature confirming the lower fuel consumption. http://rotaryeng.net/Sport-av-art.html |
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Originally Posted by oltmann
(Post 3973187)
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not that it matters but I am a mechanical engineer and have been through all that stuff long ago, but I was referring to the Renesis engine relative to its specific durability requirements
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I'm just starting to muddle my way through this thread, but a couple of quick points:
1) Maximum fuel efficiency does NOT occur at the leanest possible burn. In fact, it occurs at a mix slightly rich of stoich. 2) The ignition idle table is independent of the main ignition table. Nothing particularly weird happens if a different split is set between the two - it just switches. 3) Negative split during cruise will change the effective lambda during combustion, even though the resultant lambda at the tailpipe might remain the same. Remember - you are measuring AFR long after it actually matters with an AFR gauge. 4) Idle lambda has little to nothing to do with overall fuel economy. You are chasing your tail if you use the lambda, ignition or fuel trim data from idle for anything other than drivability adjustment. 5) The Renesis has a ton of "engine braking" capability. Certainly as much as any other passenger car I've driven. I often don't even use my brakes when coming to an intersection until the last 10 feet. Even coming down out of the hills around Banning CA, just lifting the throttle slows the car considerably. 6) "Lean Burn" has some rather serious consequences with regards to latent heat in the combustion chamber that doesn't necessarily show up in the EGT. Latent heat is the top culprit in detonation at load transition in boosted motors (followed closely by leading misfire, which can often be caused by lean-burn as well). |
Originally Posted by TeamRX8
(Post 3972027)
it's an RX8Club tradition to take a thread on a subject that you know nothing about and hijack it with a subject that you know even less about ...
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Originally Posted by zoom44
(Post 3974037)
I think i started that about 8 years ago :)
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Originally Posted by MazdaManiac
(Post 3973640)
I'm just starting to muddle my way through this thread, but a couple of quick points:
1) Maximum fuel efficiency does NOT occur at the leanest possible burn. In fact, it occurs at a mix slightly rich of stoich. 2) The ignition idle table is independent of the main ignition table. Nothing particularly weird happens if a different split is set between the two - it just switches. 3) Negative split during cruise will change the effective lambda during combustion, even though the resultant lambda at the tailpipe might remain the same. Remember - you are measuring AFR long after it actually matters with an AFR gauge. 4) Idle lambda has little to nothing to do with overall fuel economy. You are chasing your tail if you use the lambda, ignition or fuel trim data from idle for anything other than drivability adjustment. 5) The Renesis has a ton of "engine braking" capability. Certainly as much as any other passenger car I've driven. I often don't even use my brakes when coming to an intersection until the last 10 feet. Even coming down out of the hills around Banning CA, just lifting the throttle slows the car considerably. 6) "Lean Burn" has some rather serious consequences with regards to latent heat in the combustion chamber that doesn't necessarily show up in the EGT. Latent heat is the top culprit in detonation at load transition in boosted motors (followed closely by leading misfire, which can often be caused by lean-burn as well). |
How do you know what your fuel economy at idle is? How do you measure that?
Are you just sitting there for hours, waiting for the fuel to deplete and then measuring it against time? Fuel economy is usually referenced against distance. Are you just assuming that, since the idle AFR is rich that the "fuel economy" at idle is suffering? I hate to tell you, but engines have a horrible Ve at idle. Rotary engines have almost NO Ve at idle. No matter how lean you get it, it will still require more energy to idle than any piston motor. |
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