Overrun lean out when return to idle

[Sprocket]

Having problems with leaning out on prolonged overrun when returning to idle.

The engine is supercharged with the injectors pre supercharger. This is not an unusual condition on this setup using carbs or a previous 'other' ECU. We suspect that the draw of the supercharger on the manifold is so great, on a closed throttle, the fuel film is being stripped from the excessivly large surface area of the charger, manifold and plenum during prolonged overrun (off scale lean). The VE map is very good under normal driving conditions in all areas where this lean off occurs during overrun. Returning to idle gently there is no lean off. Prolonged overrun down to idle see's a huge time delay in Lambda returning to good values (many seconds, where normally very good!)

We are running Alpha-N/ MAP compensation because of the nature of this setup with the throttle and injectors infront of the supercharger, and in general works a treat.

We did have a thought about using the RPM/MAP correction table and setting richer values in the overrun region where it leans out, but unsure if this would cure the problem. ultimately we ran out of time on the dyno. For another time perhaps.

As soon as I can get a log together, I'll get one up, but for now, is there anything we can look into to perhaps start improving this problem? we do have an idle air stepper valve on the manifold, but due to the way this has been fitted, has no real useable effect. We are looking into resolving this soon with a different valve, but not sure whether it would have any bearing this.

Any help welcome.

PS the engine is a 1969 Mini Cooper 998 with a supercharger conversion. 12PSi boost and water/ methanol injection. We made 96hp at the wheels. Not bad for a 1 litre

PS. Firmware 1.2.31

[gunni]

The dyno really isn´t the place for drivability tuning in my opinion. That is best left to logs and road/track mapping

In your case I would inspect the overrun rpm points and potentially richen up the lowest load and rpms just before it hits idle to re-create the film again.

[Sprocket]

I already understand your point about the accelleration and deceleration on the dyno, and we will have next Saturday to play about with it on the road. While we had the dyno for the day, by the end before pulling the car off the dyno we played about with the AE to get it somewhere near without causing too much of a drivability issue. We upset some of the neighbours at home driving up and down, in and around LOL.

The overrun lean off is different, it doesnt matter whether under load or without load, by the time revs return to idle, it goes massivly lean for a good few seconds and then eventually recovers if it hasnt already stalled.

We'll have a look at the lower end, but looking at the logs when lifting off throttle from higher rpms, it goes off scale rich and then straight to off scale lean. We might have to use the four gas analyser just to see if it is infact lean or rich, but it doesnt pop or bang and the overrun fuel cut is disabled at this time.

[gunni]

You can tell if its lean or rich based on pulsewidth, do you have overrun fuel cut ?

[VEMS]

I suspect this is caused by the injectors being so far up the inlet track (even before the supercharger), after overrun ends it will take a long time (certainly near idle range where airspeed is low) for the fuel to reach the engine (when resumed), the fuel film being robbed from the entire track before valve (after injector) does not help either. My advise, either disable overrun or use it only in a specific way higher rpm than idle range e.g idling around 1000 rpm, resume fuel around 2000-2500.

Best regards, Dave
 

[Sprocket]

You can tell if its lean or rich based on pulsewidth, do you have overrun fuel cut ?

Fuel cut has been disabled

I suspect this is caused by the injectors being so far up the inlet track (even before the supercharger), after overrun ends it will take a long time (certainly near idle range where airspeed is low) for the fuel to reach the engine (when resumed), the fuel film being robbed from the entire track before valve (after injector) does not help either. My advise, either disable overrun or use it only in a specific way higher rpm than idle range e.g idling around 1000 rpm, resume fuel around 2000-2500.

Best regards, Dave

I totally agree. however overrun fuel cut is already dissabled. What we need is deceleration enrichment?.............. but how to achieve this?........ hence the idea of using the RPM/ MAP MAP correction table on the overrun area since TPS reaches ~0% , yet MAP dives very low in comparison to normal idle. MAP compensation only acts above 100KPa



Shared log uploaded http://vems.hu/vemstune/sharingcenter/reports.php?cmd=view&key=1741X1

[Sprocket]

[S6onVEMS]

Fuel cut has been disabled

What we need is deceleration enrichment?.............. but how to achieve this?........ hence the idea of using the RPM/ MAP MAP correction table on the overrun area since TPS reaches ~0% , yet MAP dives very low in comparison to normal idle. MAP compensation only acts above 100KPa

So how about tuning the bottom row of the VE table properly to hit the lambda target and then setting the bottom row lambda to 0.98 or something like that? You will need to be certain you have a bottom row that meets your lowest vacuum kPA and will suffer some in efficiency, but I think this would meet your goal. (of course I haven't looked at your log so if you are running Alpha-N this won't be a possibility.)

[gunni]

You can tell if its lean or rich based on pulsewidth, do you have overrun fuel cut ?

Fuel cut has been disabled

I suspect this is caused by the injectors being so far up the inlet track (even before the supercharger), after overrun ends it will take a long time (certainly near idle range where airspeed is low) for the fuel to reach the engine (when resumed), the fuel film being robbed from the entire track before valve (after injector) does not help either. My advise, either disable overrun or use it only in a specific way higher rpm than idle range e.g idling around 1000 rpm, resume fuel around 2000-2500.

Best regards, Dave

I totally agree. however overrun fuel cut is already dissabled. What we need is deceleration enrichment?.............. but how to achieve this?........ hence the idea of using the RPM/ MAP MAP correction table on the overrun area since TPS reaches ~0% , yet MAP dives very low in comparison to normal idle. MAP compensation only acts above 100KPa



Shared log uploaded http://vems.hu/vemstune/sharingcenter/reports.php?cmd=view&key=1741X1

I haven´t looked at the log but here is a trick I do.

drive to as fast as legally possible, put it into a gear that will reach pretty close to rev-limit and let of the throttle, monitor the lambda and tune the appropriate cells accordingly, let the car come as close to idle as possible before pressing the clutch or pressing the clutch back in. This will give excellent indication of where your fuel numbers need to be on overrun to maintain lambda (along with fuel film) , results will vary a little based on what gear you do this in and if you let go in neutral, but all close enough and should help from stalling.

[Sprocket]

OK so I have spent a very long time looking at the mass of data we gathered from both the dyno and the road.

The big problem here is that the area we are looking at where it stalls runs into the idle region, which we know is good. There is a lean spot on the map (I think this is from the dyno when we were running overruns through those cells on the rollers), that we will richen up, but after the amount of data I have looked at, I am more and more convinced it is down to the MAP diving down to 30kpa when throttle closed, where it would normally be 75kpa at or around idle (remember we are running Alpha N). We tried Speed density to begin with, but the nature of the setup it actually runs better with Alpha N/ MAP correction. It is funny, as I had almost the exact same issue with the TBi manifolds for the Mini, but that was way back when and Megatune! and yes, it was down to the bottom end of tune in the fuel map.

We'll richen up the lean spot on the map first, but the next step I think is to add fuel using the RPM/MAP MAP correction table in the overrun region. Not sure how this will work, but it won't hurt to try.

[Sprocket]

No idea what to do to fix this. We are now convinced it is a physical symptom of the engine set up, which we know is less than ideal. maybe if we spent several days on it we would come to some comprimise, but at the moment it is what it is.

[jrbe]

Depending on how uneven your injectors are at very light pulsewidths (you might be surprised how much) you may be aiming at too lean of a lambda target. You may have to richen it up more than expected to fire the unevenly lean cylinders.

Adding fuel during decel can end up loading up the engine to the point that it stalls no matter how well tuned the idle cells are.

You could try to open up the spark gap a bit and add a bit/bunch of ignition timing in higher decel areas of the map which can help.

[Sprocket]

Im not too clear on what you are saying And im not too sure if you fully understand how this is packaged.

This is an Austin 'A' series engine with 2 'siamesed' inlet ports. Port injection on one of these is a whole other lengthy subject. Getting round this most go the route of 'wet manifold' or 'throttle body' injection with the injectors some distance from the valves.

In our case wwe have placed a supercharger infront of the throttle and pllaced two injectors between the throttle and supercharger which mashes the air and fuel before it reaches the engine. That with a fairly wild cam for the engine size has made for some symptomatic issues some i have discussed here.

I do feel that we can tune round it, but it needs time spent on it. The owner is looking at other improvements over winter, and i have discussed with him the choice of cam, since there is a poor engine vacuum at idle meaning we had to use Alpha-N in the lower map region changing to speed density in the high map (works really well).

The thing is, the owner tells me that it has always had this cutting out issue even with a host of different carbs, and last 2 years, fuel injection. A host of head gasket failures over the last two years and the owner was about to set fire to the car in frustration.

Things we have done this year..... replaced the heavily skimmmed and modified small valve head with a lghtly modified un skimmed head, ripped out A anothers hardware, installed Vems, installed a better IAC stepper valve, added a supercharger recirculation solenoid (only small), moved the injectors much closer to the supercharger, installed makeshift water/meth injection, and installed the IAT sensor in the plenum after the supercharger. These changes seen a gain in Hp of 10%, more important, tbe car idled realy well, better than it ever did, dives much much better, and actually survived the years events including 1 trackday which has always previously seen  the head gasket fail. The owner is very happy with the results so far as he described how he now has to brake so much more before a particular corner because he is going so much faster than previous years.

This really really is just a breif explanation of whats gone on before, and the only niggle we have now is this overrun stall. We are looking at changing the cam for something a little milder in the hope we can change to full speed density at the cost of a few spare Hp.

Roll on 2016 season 

To add, the target lambda never gets leaner than 0.96, this car being the age it is only requires a visual emmissions test and it idles so much better there tban it does at lambda 1

[jrbe]

I got that the injectors are quite a way from the valves with lots of stuff in between.

What i was getting at is pulsing a group of 4 of the "same" injectors at 1.25ms you can see a 20%+ difference in injector flow.  If you're tuning for stoich you can have the leaner injectors of the group misfiring those cylinders.  If you aim rich it can work around this.  It was just something i've seen previously that sort of matches the issues you mentioned (not knowing about the Siamesed ports.)
 I didnt realize it was only 2 injectors and didnt realize it is a siamesed port engine.

I cant imagine how you could tune that engine well everywhere except maybe injector per cylinder running really well tuned sequential injection.  Still would be hard to get around uneven flow and weird reversion problems from the Siamese stuff.

They were just some ideas for you not realizing some of the special parts of the engine.