Lean burn with negative split timing
#51
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).
![](https://www.rx8club.com/attachments/series-i-engine-tuning-forum-63/171926d1305237724-lean-burn-negative-split-timing-screen-shot-2011-05-12-2.44.45-pm.png)
Kawahara, et al, Proceedings of the Combustion Institute, 2007
#52
Again, this is a tangential issue, but if you use negative split in the main timing tables and don't change the idle tables, it looks like this:
![](https://www.rx8club.com/attachments/series-i-engine-tuning-forum-63/171932d1305240170-lean-burn-negative-split-timing-graph.png)
I thought the idle felt weird that way, but I guess it is subjective.
![](https://www.rx8club.com/attachments/series-i-engine-tuning-forum-63/171932d1305240170-lean-burn-negative-split-timing-graph.png)
I thought the idle felt weird that way, but I guess it is subjective.
#53
![Smilie](https://www.rx8club.com/images/smilies/smile.gif)
#54
Max fuel efficiency being at 16.x makes sense for aviation, when you are talking about what fuel consumption is needed to maintain a given air speed.
Think the problem domain is different for cars with fixed transmission ratios. Operating in lean burn gives another way to vary output so that it is closest to the minimum required to maintain vehicle speed at a fixed engine speed. Anyhow, I know Honda's operate at AFRs in the 20s.
Think the problem domain is different for cars with fixed transmission ratios. Operating in lean burn gives another way to vary output so that it is closest to the minimum required to maintain vehicle speed at a fixed engine speed. Anyhow, I know Honda's operate at AFRs in the 20s.
#55
For anyone wanting to try this, I think the best solution would be to get a wideband controller with programmable outputs, like the Innovate LC-1, and connect one to the ecu in place of the stock one.
However, I've just confirmed that you can run any AFR you choose by eliminating closed loop and de-calibrating the MAF scale changing the VE tables. A precisely jetted carburetor is what it'll be like then.
However, I've just confirmed that you can run any AFR you choose by eliminating closed loop and de-calibrating the MAF scale changing the VE tables. A precisely jetted carburetor is what it'll be like then.
Last edited by oltmann; 05-13-2011 at 09:00 AM. Reason: duy
#56
Registered
iTrader: (1)
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.
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.
#58
For the time I was willing to try it, cars runs nicely at indicated 19:1.
Realized I have nearly no clue how the closed loop works, there are many different schemes it seems, none well-documented that I can find.
May have to dive back into IDA Pro, if I can't suss out the logic empirically.
Realized I have nearly no clue how the closed loop works, there are many different schemes it seems, none well-documented that I can find.
May have to dive back into IDA Pro, if I can't suss out the logic empirically.
#59
Banned
iTrader: (3)
So, go ahead and do that for the rest of your driving and see what you come up with. You may be surprised at how good your consumption rate is at idle.
Otherwise, turn the engine off when you are just sitting there.
#60
Registered
iTrader: (2)
Max fuel efficiency being at 16.x makes sense for aviation, when you are talking about what fuel consumption is needed to maintain a given air speed.
Think the problem domain is different for cars with fixed transmission ratios. Operating in lean burn gives another way to vary output so that it is closest to the minimum required to maintain vehicle speed at a fixed engine speed. Anyhow, I know Honda's operate at AFRs in the 20s.
Think the problem domain is different for cars with fixed transmission ratios. Operating in lean burn gives another way to vary output so that it is closest to the minimum required to maintain vehicle speed at a fixed engine speed. Anyhow, I know Honda's operate at AFRs in the 20s.
A car cruising at a constant 70 mph presents a more difficult problem in that the power has to be adjusted quite frequently. The well-recognized most efficient solution is to freeze the throttle position (~constant power output) and let the speed vary as the road rises and falls. The advantage with cars is that they have much more sophisticated engine management controls than the entirely manual controls on most aircraft.
The latest development in car engine management is the trend to replace the direct link between the throttle pedal and throttle plate, whether mechanical or electronic, with making the throttle pedal a power-command servo. In other words, when the driver pushes the pedal, the request goes to the ECU which then adjusts engine parameters (including possibly, but not necessarily, the throttle plate) to achieve the desired power. The RX8 of course is not this sophisticated, so adjusting afr as a way to vary power output is I suspect not easy to fine tune. As such, what I'm hoping for is a map for daily driving to get best mpg's and a different map (MM) for best power, loading one or the other as needed. The Cobb "Economy Tune" hints at that solution, though opinions vary about its effectiveness.
#61
I don't think there is much need for two maps. The ecu already has so much flexibility. Optimize low-load and closed loop for fuel economy, and high-load open loop for power. On a track you will almost never use the former, and if you drive conservatively, the latter.
Right now, I target 1.05 lambda in closed loop. With the factory wideband, the ecu can maintain this.
Lean-burn works by reducing pumping/throttling losses at very low loads. If you need 20hp to maintain cruising speed, and 5hp is wasted creating vacuum pull air in, overall efficiency increases even though you get less energy per unit of fuel at 20:1 than at a richer mix.
Rotaries have great flexibility in intake design, as good as the RX8s intake is for power, it could probably be better at low loads. I'll just go ahead and trademark "Portronic" right now.
Right now, I target 1.05 lambda in closed loop. With the factory wideband, the ecu can maintain this.
Lean-burn works by reducing pumping/throttling losses at very low loads. If you need 20hp to maintain cruising speed, and 5hp is wasted creating vacuum pull air in, overall efficiency increases even though you get less energy per unit of fuel at 20:1 than at a richer mix.
Rotaries have great flexibility in intake design, as good as the RX8s intake is for power, it could probably be better at low loads. I'll just go ahead and trademark "Portronic" right now.
#62
Banned
iTrader: (3)
Right now, I target 1.05 lambda in closed loop. With the factory wideband, the ecu can maintain this.
Lean-burn works by reducing pumping/throttling losses at very low loads. If you need 20hp to maintain cruising speed, and 5hp is wasted creating vacuum pull air in, overall efficiency increases even though you get less energy per unit of fuel at 20:1 than at a richer mix.
Lean-burn works by reducing pumping/throttling losses at very low loads. If you need 20hp to maintain cruising speed, and 5hp is wasted creating vacuum pull air in, overall efficiency increases even though you get less energy per unit of fuel at 20:1 than at a richer mix.
The best lambda for economy - established through hundreds of hours of testing - is between .92 and .95.
Nominal torque is more important that fuel/time.
The way you establish your "lean-burn" is through timing split.
That said, the heating that happens in the combustion chamber - while helpful for economy and emissions - is dangerous with regards to flame front speed when the charge homogeneity and load suddenly increase (and lambda decreases). When the throttle is suddenly open and everything changes, that heat becomes immediate detonation.
Last edited by MazdaManiac; 05-14-2011 at 06:35 PM.
#63
That is too bad. If negative split is not useful for maintaining lean burn, can you explain how Mazda did it with the lean burn RX4 that they released in 1980, or how they are doing it with their prototype Renesis?
http://www.mechanicalengineeringblog...enesis-engine/
That would be useful.
I have run my motor at an indicated 19:1+. Negative split timing did not change the indicated AFR per se.
http://www.mechanicalengineeringblog...enesis-engine/
That would be useful.
I have run my motor at an indicated 19:1+. Negative split timing did not change the indicated AFR per se.
#67
Also, as an advisory to anyone following along, cruising lean produces more oxides of nitrogen (though rotaries are known for producing lower levels), and the reduced CO levels will reduce the efficacy of the rhodium catalyst.
However, cruising rich will increase CO and HC output, while also greatly heating and potentially causing substrate meltdown. Do not attempt to cruise rich unless you are using a midpipe.
Some people seem to have interpreted this as a fuel economy thread, if you're reading just for that reason I don't want anyone taking any bad advice.
However, cruising rich will increase CO and HC output, while also greatly heating and potentially causing substrate meltdown. Do not attempt to cruise rich unless you are using a midpipe.
Some people seem to have interpreted this as a fuel economy thread, if you're reading just for that reason I don't want anyone taking any bad advice.
#68
Ouch! Actually, they don't have a basement, but I'll be driving to a party in Echo Park tonight. Should be on the freeway for at least 100 miles, I'll post my mileage either way. What are you up to this evening?
![Smilie](https://www.rx8club.com/images/smilies/smile.gif)
#69
Banned
iTrader: (3)
The CAT was expected to operate in an .90 lambda environment for 100k+ miles. It really isn't part of the equation.
The point I was making was that the actual combustion AFR is not the observed AFR from your example because of the effect of the negative split. You don't have enough data to post a useful observation from your example.
I have BTA (Beer Time Attack) with the NASA peeps and some of the RTA competitors tonight.
The point I was making was that the actual combustion AFR is not the observed AFR from your example because of the effect of the negative split. You don't have enough data to post a useful observation from your example.
I have BTA (Beer Time Attack) with the NASA peeps and some of the RTA competitors tonight.
#73
Got 20.6mpg using the lean limit for CL on a 90% hwy trip. Essentially the same as I've gotten on similar trips in the past. I blame E10 and my habit of going 85-90. Used stock timing.
Seeing no gains, I decided to try targetting .95-92 in CL above .50 load. Made me realize that stoich or leaner really sucks with just about any kind of acceleration at all. No results on that yet, but I don't really care. Yes, you did tell me so, MazdaManiac.
I've read that Honda Insights run stoich in CL even at WOT. Sounds punishing. I've realized that stoich is good if you used lots of Aquanet in the 80s and you feel really guilty about what you did to the ozone layer. Otherwise, it sucks.
I still haven't satisfied my intellectual curiosity about lean burn, but I'm not sure I'm willing to blow two bills on a programmable wideband right now. I guess I'll just read this 315 page report on the NASA SCORE project for the time being.
http://protonet.org/doc/NASA_1994029725.pdf
It occurs to me that Mazda might be working on lean burn just for emissions reasons. All emissions drop when you go really lean, add a two-way cat and a NOx trap and you make the EPA happy. Might be a needed change to keep rotaries viable regardless of whether lean burn gives rotaries the same fuel economy gains as piston engines.
Seeing no gains, I decided to try targetting .95-92 in CL above .50 load. Made me realize that stoich or leaner really sucks with just about any kind of acceleration at all. No results on that yet, but I don't really care. Yes, you did tell me so, MazdaManiac.
I've read that Honda Insights run stoich in CL even at WOT. Sounds punishing. I've realized that stoich is good if you used lots of Aquanet in the 80s and you feel really guilty about what you did to the ozone layer. Otherwise, it sucks.
I still haven't satisfied my intellectual curiosity about lean burn, but I'm not sure I'm willing to blow two bills on a programmable wideband right now. I guess I'll just read this 315 page report on the NASA SCORE project for the time being.
http://protonet.org/doc/NASA_1994029725.pdf
It occurs to me that Mazda might be working on lean burn just for emissions reasons. All emissions drop when you go really lean, add a two-way cat and a NOx trap and you make the EPA happy. Might be a needed change to keep rotaries viable regardless of whether lean burn gives rotaries the same fuel economy gains as piston engines.
#75
As has been pointed out, I have no idea about "actual" AFRs. ![Smilie](https://www.rx8club.com/images/smilies/smile.gif)
However, my ecu (2005 federal) limits closed loop to a mid-15 reading from the WB O2 sensor. I can target anything up to that in closed loop targets A, and it works apparently indefinitely, for many hours at least. Targeting higher only works for a split second, although I did once have it work for longer. I believe that happened because it was just above the exit load for closed loop, and just below the next open loop load point. Programmers call this a fencepost error.
I don't mess with the other closed loop table anymore, I don't think they are fuel targets, but rather set feedback gain for the reading from the wbo2.
Or are you comparing readings from the stock wbo2 and an aftermarket one?
![Smilie](https://www.rx8club.com/images/smilies/smile.gif)
However, my ecu (2005 federal) limits closed loop to a mid-15 reading from the WB O2 sensor. I can target anything up to that in closed loop targets A, and it works apparently indefinitely, for many hours at least. Targeting higher only works for a split second, although I did once have it work for longer. I believe that happened because it was just above the exit load for closed loop, and just below the next open loop load point. Programmers call this a fencepost error.
I don't mess with the other closed loop table anymore, I don't think they are fuel targets, but rather set feedback gain for the reading from the wbo2.
Or are you comparing readings from the stock wbo2 and an aftermarket one?