oltmann

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 ******* 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.

oltmann

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.

oltmann

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.

stinksause

iTrader: (5)

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

Brettus

iTrader: (2)

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

oltmann

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.

Brettus

iTrader: (2)

so which table did you tweek to get that ?

oltmann

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 ****. 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.

olddragger

iTrader: (3)

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.

reddozen

iTrader: (1)

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 ****.

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.

HiFlite999

iTrader: (2)

[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. [/quote]

"Nuts" doesn't necessarily mean "wrong"!

olddragger

iTrader: (3)

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

Brettus

iTrader: (2)

which fuel table were you working in ?

Rote8

I had the lean blow once, 15 to 1 under a 190% load at 9250rpm.

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.

Brettus

iTrader: (2)

Oh noes - what was the damage ?

Mazmart

iTrader: (12)

Very little stroke (Eccentricity in this case) equals minimal engine braking. It's totally related to torque production as well.

Paul.

oltmann

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.

olddragger

iTrader: (3)

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

HiFlite999

iTrader: (2)

Torque yes, but engine braking, not so sure. As I understand it, most of the engine braking comes from the resistance presented by the closed throttle plate. It takes work (in the physics sense) to compress the air sans fuel mixture, but most of that work is recovered on the "power" stroke, though intake/exhaust overlap can alter that along with VVT. Diesels are generally long stroke but have very little native engine braking because of their lack of throttle plates.

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. 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.

TeamRX8

iTrader: (25)

It's exactly as Paul stated

PhillipM

Has twice the engine braking compared to the old 2.3 4-pot we used to run...

Brettus

iTrader: (2)

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 .

bse50

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

HiFlite999

iTrader: (2)

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. Rotating mass, whether in flywheel or rotor, can only have a temporary effect (consider using engine braking to control a long mountain descent). Diesels have heavy flywheels and zip for engine braking. There are many references discussing the lack of a throttle plate as the reason. Both piston and rotary gas engines have throttle plates. Anyone have a reference to a paper or something explaining a difference in braking?

The lean-thing is very interesting! I'd love to have even 132 hp for freeway cruising at 30 mpg.

Brettus

iTrader: (2)

I'm struggling to see any difference from my 8 and any 2L 4pot as far as engine braking goes - and it seems like plenty to me . Interestingly a 2L 4 makes about the same torque as well .