Wondering how the OEMs make the exhaust pop and crackle under deceleration?
Here’s an in-depth guide on the “pops ‘n’ burbles” for you all, courtesy of me. I spent some time reverse engineering how they worked so I could add them to the tuning software for the company I work for.
First off, there’s two generations of burbles. Earlier calibrations could not target different burbles under different drive modes, instead opting to only have them available in Sport/Track mode. Newer cars extended the burble functionality so that it could be modified to run in any of the many drive modes, with differing targets under the drive modes. The system made its first appearance in the Focus RS software strategies, and has outputs that allow it to control both fuel and spark.
First, how does it work? Well, the system lives in what I’ll refer to as 4 distinct modes:
Unavailable is just that, the system is considered unavailable due to not meeting certain conditions, certain failure modes, safety conditions, etc.
To become “Available”:
– Minimum RPM satisfied
– Minimum APP (accelerator pedal position) satisfied
– Minimum Engine Coolant temperature reached
– Catalyst temperature BELOW a maximum value for safety
Any of these not met instantly makes the system unavailable.
To become “Armed”:
– System must be available
– Exceed a certain RPM (newer calibrations configurable by drive mode)
– Exceed a certain APP (newer calibrations configurable by drive mode)
Once armed, the system needs to be triggered to become “Active”:
– APP Rate must exceed a rate to trigger. I.e. You have to lift the throttle fast enough for it to trigger.
Once active the system does the following:
– Sets the target lambda (air-fuel ratio) to a configurable LEAN amount. Lean combustion is slower than normal combustion.
– Sets the target spark torque ratio to a configurable amount. Spark Torque Ratio is used by the vehicle control to retard spark timing for torque reduction. A value of 0.5 for example means to operate the engine at current conditions such that it is down 50% of possible torque using just spark retard. Spark retard causes combustion to begin LATER. This naturally causes exhaust gas temperature to increase as the combustion process spends less time in the cylinder transferring heat to the chamber, and so carries more of its heat with it. If you retard it sufficiently, combustion will still be occurring when the exhaust valve opens, causing it to continue into the exhaust.
– Initiates a configurable countdown before it will become inactive.
Combing the two (lean + spark retard) means causing combustion that is slow enough that it happens in the exhaust, essentially its causing a “lean misfire” giving you “pops and crackles”. Normally a lean misfire sucks, because you feel the cut in power, but in this case, you’re already in a condition where you’re requesting no real torque production and the engine would normally cut fuel anyways (the ultimate lean misfire if you think about it, what’s leaner than no fuel?).
So, the system is using fuel to achieve the effect (whereas normally in this scenario it’d be in DFSO, deceleration fuel shut-off with no fuel), and a side effect of combustion in the exhaust is the heat of the system is moved into the exhaust (some combustion is happening there, instead of in the cylinder where it can transfer heat to the block). This is typically why misfires of any variety are avoided. They get heat into the exhaust in some way or form. A rich misfire puts hydrocarbons in the catalyst, causing exothermic reactions that heat it up. Lean misfires slow combustion down so that it is still happening in the exhaust. This exhaust gas heat can damage things like exhaust valves, turbocharger turbines, oxygen sensors, but the first thing to die and the whole reason cars even monitor for misfire and prevent it, is the catalytic converter.
This heat from late combustion combined with unburned hydrocarbons and excess oxygen will raise the catalyst temperature fairly quickly. This same effect (retarding spark timing, not necessarily running lean) is used during startup for example to quickly bring the catalyst to operating temperature, however, if left to its own accord without monitoring, it’ll overheat the catalyst and it’ll melt down.
This is why catalyst temperature can make the system unavailable. If the temperature is too high, the system disables to prevent further heating of the catalyst. As well, the margin for catalyst overheat is actually quite a bit lower than enrichment cooldown of the catalyst (1472*F vs 1700*F for true “overheat”). This ensures that the catalyst stays at a healthy operating temperature. Because of this built in safety, the system will never allow the catalyst to get damaged, and the other upstream components (exhaust valves, turbocharger turbines) don’t have exothermic reactions, so will be cooler than the catalyst typically. As well, they usually can handle higher EGTs than the catalyst can, so they’re effectively safe as well. So this system is never operated in a way that could be destructive, at least in factory calibration form.
For an example of everything configurable in the calibration, I attach the below:
There’s a master enable/disable, an ability to enable/disable per drive modes, as well as all the conditions and targets the mode uses.
This should hopefully cover everything you could want to know about “pops and burbles”.
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