Lean burn with negative split timing
05-09-2011, 08:36 AM
#27
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.
05-09-2011, 03:10 PM
#30
Registered
Join Date: Dec 2007
Location: UK
Posts: 863
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
05-09-2011, 11:03 PM
#34
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?
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?
Last edited by stinksause; 05-10-2011 at 12:08 AM.
05-10-2011, 01:32 PM
#35
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.
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.
05-10-2011, 04:49 PM
#37
Registered
Join Date: Dec 2007
Location: UK
Posts: 863
Likes: 0
Received 0 Likes
on
0 Posts
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.
.
.
05-10-2011, 07:15 PM
#39
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.
05-10-2011, 07:56 PM
#40
I do intend to get an EGT monitor, however the best evidence I can find indicates that EGT/CHT peaks at stoich.
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.
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.
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.
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.
05-10-2011, 08:57 PM
#42
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.
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
05-10-2011, 11:08 PM
#43
05-11-2011, 11:33 AM
#46
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).
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).
Last edited by MazdaManiac; 05-11-2011 at 11:57 AM.
05-12-2011, 05:18 AM
#49
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).
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).
05-12-2011, 11:43 AM
#50
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.