Very many thanks for posting the link Iain. I have little to no experience of working with modern engines or knowledge of their control stratedgy.
The text in the link is even more verbose than I, and seems not to be aimed at those seeking a basic practical understanding, though perhaps more suited to those who may be seeking to create an answer via a formulaic process.... which is outside my comfort zone... (-:
I picked up from the text that primarily modern exhaust systems have greater restrictions to exhaust gas flow built into them in order to comply with EU vehicle construction and use regulations. I guess the statement that was an eye opener for me was that in some circumstances during the exhaust cycle a cylinder may not fully evacuate all the exhaust gasses created at the end of the ignition/power stroke, due to those exhaust system restrictions...!
The mention of turbo`s loosing efficiency due to those same restrictions seems entirely logical given the speed of the turbo is determined by the rate of the flow of the exhaust gasses driving it`s vanes, and logically directly affecting the boost available.
I note that a turbo diesel is mentioned and wonder if the author has ever been near a BIG diesel engine running out of control when it`s turbo oil seal fails leaking the engines lubricating oil into the intake which is then burned as fuel thus it`s fuelling is beyond control.... not unusual for engines to self destruct once they have burned the majority of their lubricant..Ouch..! In that situation formula may be less handy than a pair of overalls jammed into the intake to choke it to death before it starts throwing bits of rods etc about the place..(-:
Sorry I digressed..
I can understand the logic of the situation where.... if the restriction in the exhaust is considerably increased perhaps by a cat breaking up and partially blocking the passage of gasses trying to exit the system, and that the number of cylinders feeding into that cat will be increasing the "back-pressure" as the engine continues to run...?
Using nothing other than basic mechanical logic.... IF that pressure could ever be higher at any time during the exhaust cycle of any one cylinder feeding into that exhaust system, say at the end of that cylinders exhaust stroke, where it`s exhaust valve would be opened to some extent, in that event I guess there could be bits of cat circulating in the disturbed gas flow within the exhaust system, and that exhaust system gas being at a higher pressure than within the cylinder with the not yet closed exhaust valve, then it seems possible if not probable that some of the bits of cat will end up being forced into that cylinder..????
As the over-run is mentioned as playing a part, I guess the volume of air in the cylinder will have been reduced due to the air intake (throttle plate) being closed, creating a vacuum in the inlet manifold the fuel being shut off also will likely reduce the volume of air in the cylinder waiting to be expelled during the exhaust stroke, perhaps providing more risk of the pressure in the exhaust system exceeding that produced in the cylinder near the end of it`s exhaust stroke...???
Of course my logic could be entirely flawed, and I am more than happy to be corrected...
Just thinking in type..
Thanks again Iain. and MisterCorn.. :thumb: