When to go into Closed-Loop?

[Evert Rosseel]

> >   How do the "big boys" know when to go into closed loop?
> 
> There is no rule set in stone for determining when to go to closed loop. 
>  Most do it based on engine run time and coolant temp. at start-up (the 
> warmer the engine at start-up, the less they wait).  However, this is 
> not necessarily the best strategy (but its sufficient for their 
> emissions controls).  Another strategy I have seen counts number of 
> engine revolutions instead of time, so if you start the engine and 
> operate it at high RPM, the system would go to closed loop faster than 
> if you were just idling.  Yet others use calculated average mass air 
> flow, and I think that's probably the most accurate method.  Remeber 
> that going to closed loop does not necessarily mean better emissions if 
> the catalyst is not up to temperature.
> 
> Also, almost all the systems I have seen have a simple diagnostic check 
> that if the sensor voltage is too low or too high, they ignore the 
> reading and stay in open loop.
> 
> Best Regards, Mazda
> 

Basically you need 3 conditions to be able to go to closed loop :

1 The engine should be warmed up somewhat, so that no enrichment is 
necessary to get a combustible mixture. 
This depends on the coolant temperature (easily measured) and also 
(to much less extent) on the air temperature.

2 The catalyst should be warmed up sufficiently, otherwise no 
benefits are obtained by going to closed loop.
As far as I know, no catalysts with temperature sensors built in exist, 
although this seems the most logical solution.
In the most modern systems with both a lambda sensor before and after 
the catalyst (for on board diagnostics OBD, in which case the correct 
functioning of the catalyst during use must be monitored (legal 
requirement)), it should be possible to measure when the catalyst lights 
off, but I have not seen any implementations of this yet.

3 The lambda sensor should be warmed up sufficiently. This can be 
(and often is) solved by using electrically heated sensors.

Especially 1 and 2  depend strongly on the ambient temperature. In 
some cases engine warm up is limiting, in other cases the catalyst 
light off. This also depends on the engine (distance between engine 
and catalyst, special measures taken like retarded ignition to get 
higher exhaust gas temperatures etc.).

Although in practice good results can be achieved, this is not always 
the case. Since emission limits are only legally enforced measured 
for one test condition, even the simplest of systems is good 
enough. (This is changing however : "cold" tests will become 
necessary in the near future).
For driveability, closed loop operation is not essential. In fact, 
running slightly rich helps to get good response. Therefore, without 
test equipment and tests with different "ambient" temperatures, it is 
not possible to know how good the products from different manufacturers are.

Evert
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*    Dr. ir. Evert Rosseel                                             *
*    Laboratory for Machines                                   *
*    Department of Mechanical and Thermal Engineering          *
*    University Gent                                           *
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