[Diy_efi] Re: Diy_efi Digest, Vol 6, Issue 23

[Rick McLeod]

Isn't it common on the newer catalytic equiped
vehicles to use a pair of O2 sensors to monitor
effectiveness of the cat and feed this info via
computer for alerting the driver via a CEL/MIL/DIC
presentation?

--- Bill Washington <bill.washington at nec.com.au>
wrote:

> Mike,
>     In my experience even when the diesel is pumping
> out lots of black 
> smoke - ie unburned or part burned fuel - there is
> still lots of excess 
> oxygen - the reason for the smoke is usually droplet
> sizes that are too 
> large to burn in the available time - ie inadequate
> atomisation so the 
> oxygen can't get to all the fuel during the power
> stroke - In this 
> scenario, for a mechanical injection diesel, even if
> more fuel is 
> injected than there is oxygen to burn there will
> still be excess oxygen 
> because of the droplet size.
>      The big adavantage with the latest generation
> of common rail 
> diesels is the superior atomisation achieved by
> raising the fuel 
> pressure in the distribution rail to extremely (even
> dangerously) high  levels and changing from
> mechanical injection to 
> electronic injection - the mechanical injection
> pumps had no hope of 
> achieving the injection pressures the common rail
> designs use as normal 
> - the pressure difference from mechanical injection
> to common rail is 
> orders of magnitude.
>     Therefore I believe that a normal O2 sensor
> (narrow or wideband) 
> will not work on a diesel because there is always
> 'excess' oxygen. Note 
> also that normally, diesels have wide open air
> induction - ie no 
> throttle butterfly and engine speed and power are
> controlled solely by 
> the volume of fuel injected, therefore the amount of
> air inducted in a 
> normally aspirated diesel per stroke is essentially
> the same across the 
> entire rev/power range (neglecting restrictions in
> the air filter and 
> intake tract) - including these effects means that
> the volume of 
> inducted air will reduce slightly as the revs
> increase.
>     Adding a turbo changes this, of course
>     That being said, I seem to recall reading
> somewhere about some of 
> the common rail diesels using an oxygen sensor but
> no details of type or 
> purpose though it stands to reason that it would/
> could only serve any 
> purpose in a maximum power condition in a modern
> common rail diesel when 
> it may be possible to inject more very finely
> atomised fuel than there 
> is air to burn it, then an oxy sensor could come
> into play to limit the 
> volume of injected fuel.
> Regards
> Bill
> 
> >----------------------------
> >
> >Message: 7
> >Date: Mon, 22 Aug 2005 22:10:37 +0800
> >From: "Mike" <niche at iinet.net.au>
> >Subject: Re: [Diy_efi] Wide band sensor, F1
> >To: diy_efi at diy-efi.org
> >Message-ID:
> <6.2.1.2.0.20050822220408.02951220 at 203.0.178.192>
> >Content-Type: text/plain; charset="us-ascii"
> >
> >At 03:01 PM 8/22/05, you wrote:
> >  
> >
> >>I would say "no", assuming you mean the usual WBO2
> sensor used in petrol engines.  I have two reasons:
> >>
> >>1.    It's calibrated to provide a response which,
> while almost linear immediately about the
> stoichiometric mixture for petrol (c. 14.7: 1?
> memory fails) its response goes rapidly non-linear
> on either side of that figure.  Diesel engines,
> AFAIK, run in a regime where there is always excess
> air to burn the fuel which is expected to be metered
> by the injection pump.  So the sensor would be stuck
> "on the end stops" of its response curve.
> >>    
> >>
> >
> >Damn good point, I forgot all about this,
> power=fuel, not air flow based.
> >
> >I should have known as the first thing I did in
> Malaysia was rent a diesel
> >with turbo and jammed the wastegate off - tut
> tut...
> >
> >  
> >
> >>2    Even if the injection pump is injecting
> excess fuel which cannot all be burnt, it tends to
> come out the exhaust as fine particles of carbon
> rather than as unburnt fuel, which is what the
> sensor is looking for.  I'm not convinced the sensor
> would notice, because of the different combustion
> process (chemically and physically) going on in a
> diesel.
> >>    
> >>
> >
> >mmmm Surely though if there is excess fuel any O2
> sensor will indicate
> >when (allowing for response time) and how far
> assuming it isnt jammed up
> >against the 'rich' stop so quickly... ?
> >
> >So under those conditions would a wideband show
> lower than  say 11:1
> >and not have its judgement clouded (pun) by soot
> particulates ;) ?
> >
> >ie. Even though its nonlinear wouldnt it be
> mappable, ie Calibrated to give
> >a known output, with an error margin, at extreme
> ends of the scale, its been
> >a long time since I looked at any O2 sensors
> admittedly...
> >
> >I wonder what they use for F1 (or do they) when
> running 4:1 or is it
> >with that exception so they use such very rich
> mixtures just to cool
> >the exhaust valves at selected parts of the race
> when acceleration
> >isnt that important,
> >
> >
> >
> >Regards from
> >
> >
> >Mike
> >Perth, Western Australia
> >VL Commodore Fuse Rail that wont warp or melt !
> >Twin tyres for most sedans, trikes and motorcycle
> sidecars
> >http://niche.iinet.net.au
> >  
> >
> >  
> >
> > _______________________________________________
> Diy_efi mailing list
> Diy_efi at diy-efi.org
> http://lists.diy-efi.org/mailman/listinfo/diy_efi
>