Air intake - any advantage changing to an air pod

[bonno]

Warsch I will send you a PM on several contacts.

[johnl]

I said:
"I also find the engine may well perform better (on partial throttle, so feel 'livelier') when it's still cold, which is I think because it's running in open loop mode, rather than closed loop. This means the ECU isn't yet trying to maximise fuel efficiency, and open loop maps tend to be richer than closed loop, and richer - within reason - tends to be better for power."

And, it now occurs to me, open loop ECU mapping for cold start air / fuel ratio will be quite rich because this causes a high exhaust gas temperature and consequently promotes a rapid heating of the catalytic converters (i.e. not just a 'safe' default AFR, but something more deliberate for a specific reason).

Is this the single instance of an emmissions control measure improving engine performance... Maybe, though ignition timing is not unlikley to simultaneously be considerably retarded for the same reason (high exhaust temperature until the cats 'light up'), so perhaps a gain on the roundabout and a loss on the swing?

Regards,
John.

[Citroënbender]

I thought rich mixtures burned at a lower temp than lean mixtures. Seriously, have I got this wrong for the last 25 years?

[johnl]

I don't think you're wrong, but I don't think it's entirely as simple as that.

My understanding from this and that I've read and heard, FWIW, right or wrong:

A lean mixture burns more completely and hotly inside the cylinder because lean combustion is rapid and all available fuel gets burnt in cylinder. The lean burn exhaust gas may still be very hot as it exits the cylinder (maybe hot enough to burn valves), but the temperature quickly decreases as it passes into the exhaust system. This is because all the fuel was consumed and it is no longer burning in the pipe.

A rich mixture burns cooler in the cylinder, but more fuel is left uncombusted. Since rich burn combustion tends to be slower than adiabatic or lean burn, remnant unburned fuel and oxygen can keep burning as it passes into the exhaust pipe. This cooler / slower rich burn creates lower in cylinder temperature, but higher (than for lean burn) temperatures further along in the pipe, and so exhaust gasses are hotter when they reach the catalytic convertor, which heats it up somewhat more quickly. This is why excessive fuelling can burn / melt cat cores (especially if there were also to be some misfiring, or a pre cat exhaust leak, which means a lot of oxygen as well as fuel in the exhaust stream).

I have heard that in order to promote a rapid cat warm up, some engines (Mazda I think, rings a bell) deliberately misfire some of the cylinder 'firing' strokes on cold start so that more unburnt fuel and oxygen is introduced into the exhaust. This is not so much because of the extra unburnt fuel from the 'misfire', but it introduces the oxygen needed to more fully burn any fuel that is in the pipe. Something similar can be done by artificially introducing air into the exhaust with a pump, and running rich at start up.

At any rate, I have noticed significantly better partial throttle performance (power, throttle response, general 'eagerness') when the engine is stone cold than exists after the engine has warmed up somewhat (well, I never floored it with a cold engine...). I only suspect this is due to the AFR being richer, for whatever reason. It may be that it is only richer to enhance ignition at lower charge temperature, or it may be that it is also richer to enhance cat warm up. I don't really know, just guessing based on snippets I at least think I know.

Speculating; if an engine were runing significantly lean, then at high load the exhaust headers might glow red hot (due to the hot in cylinder burn) for the first few inches of their length only because the heat quickly goes out of the exhaust gas though the pipe wall. On the other hand, if an engine were running significantly rich, then at high load the exhaust might glow red for a substantially greater part of its' length, because the fuel is still burning in the pipe and heat is being generated actually inside it, so the pipe gets hotter instead of cooler? Maybe.

Regards,
John.

[bazzbazz]

Trivia Time -

Without realising it John has just explained how a DPF Regeneration works. During the regeneration extra fuel is injected into the system, the unburnt fuel continues to burn through the exhaust system, including the DPF, and raises the exhaust system temperature to the point where it burns out all the deposits in the DPF, and it very often will glow red hot !

It is also the reason that when doing a Forced Regeneration one must make sure the vehicle is on a concrete/bitumen surface and that there are no flammable items near the vehicle. So unless you want to do some planned back-burning one should not attempt this on the front lawn !