“Review of thermal management of catalytic converters”

[Ascari32]

"Review of thermal management of catalytic converters to decrease engine
emissions during cold start and warm up."



I found this document on the Web and it throws more light on the choice of valve timing chosen for the 3.2 JTS.

The static timing of the 3.2 JTS is 11.5deg. atdc inlet opens, 9deg. atdc exhaust closes, giving a NVO of 2.5deg.

Late opening of the inlet results in a over - rich mixture, if the ECU has not adjusted the AFR based on the MAF output signal.

Early opening of the exhaust valve results in gases at a higher temperature as the pressure has not fallen due to the piston travelling further down the bore.

These two conditions, an over - rich mixture and its early release into the man - cats, results in the catalytic converters reaching "Light - off" much more quickly.

This has a great deal of bearing on the sound level of the exhaust system on startup and in particular when manifold cats are no longer utilised.

These angles are maintained until the engine has gone through a warm - up cycle, which I presume is temperature controlled.

Which goes some way to explain why Dynamometer plots of the AFR indicate lean burn at low revs, yet the exhaust tail pipes are heavily sooted.

Additionally, it is difficult to maintain light - off when slowing from high speed to a crawl on the M25, or for that matter when on the north circular. So the valve timing rotates back towards their static timing values.

Reading this article indicates, there are two factors in maintaining light - off; a rich mixture burning at the exhaust port/Cat input, and a chemical reaction within the cat which reduces unburnt fuel whilst generating heat.

This is all well and good with standard exhaust systems, but what of those that have been extensively modified?

Well, experiments have shown, that the quantity of fuel used to maintain Light - off can be reduced to a level where the exhaust noise; at start - up, is much more civilised. This can be done by reducing the MAF output voltage at idle. But it is limited to a value, beyond which the system fails - EML's, Hill Start, VDC, ESR and Fuel gauge failure.

However, an appropriate value can improve sound levels and produce a more accurate AFR at higher revs, ie - more power. But the system is extremely sensitive and the value programmed into the ECU makes it difficult to select the right value quickly as the ECU is continually trying to counter this and restore the operating conditions to the Status Quo.

The time taken for the cats to reach light - off can be of the order of 90Sec., and I believe I read in the handbook for the 3.2JTS that there is no need to allow the engine to come up to temperature  - simply drive off, whilst not being over zealous with acceleration. This, as indicated in the article, and from other research is because the chemical reaction within the light - off process, produces heavy deposits/an excess of particulates - hence the sooty deposits on tail pipes.

The question remains however: to what degree can a heavily modified exhaust system be "Silenced"? It appears, sound levels and sooty deposits can be reduced, a) by trimming the MAF output signal, and b) by reducing the temperature at which the ECU deems the engine/cats are sufficiently heated to allow the valve timing to rotate to they "normal operating angles"

But, where is the device/sensor which governs this???

By implying, one should "Drive - off immediately" suggests the intention is to get away from these angles which on start - up, produce excess particulates, yet it should be possible to limit the time the valve timing creates the conditions for their production, when idling/driving in slow moving traffic. Whilst trying to maintain light - off, not only is excessive pollutants produced, but poor economy is also a consequence of this philosophy.

I fail to see the difference between being invited to "drive off immediately" and limiting the time the engine produces environmentally toxic emissions, during idle or low - Rev driving. They are one and the same surely.? It is acknowledged the bulk of harmful pollutants are generated at tick - over, low revs. If temperature control is what dictates this, then surely by changing the temperature, they can be reduced.

Again I ask, does anyone know where the temperature sensor is?