[Diy_efi] Re: open loop maps MAY cause damage.
phil at injec.com
Mon May 30 13:32:00 CDT 2005
It is important to understand why narrow band closed loop is used.
The answer is only catalytic converters.
There was a time back in the last century when 99% of all EFI
systems were made by Robert Bosch GMBH. Yes I know about the US
systems that came and went in a blink.
The first was D Jetronic, it used sets of breaker points to fire the
injectors was entirely linear electronics and as Bosch had not yet
manufactured the O2 sensor had none fitted.
They were supplied to about 30 different manufacturers world wide
who used them on all their sporty / upmarket models.
Imagine what would have happened when the world found out that
the Queen of England's Rolls Royce had dropped it's lunch because it
was running that dangerous open loop.
Then we discovered that poisoning the planet can ruin your weekend.
So some bright spark came up with an oxidizing and reducing catalyst
fitted after the engine exhaust manifold.
This cannot work without that rich/lean switching that takes place
at a center A/F of 14.7 that meant there was now a need to make
a sensor that detects the crossing point b/w richer and leaner than
The use of O2 for global map correction is usually (and most sensibly)
limited to prevent engine damage caused by a faulty sensor. That is
regardless of the sensors output the correction cannot exceed a few
percent either way, sometimes systems will have a greater capacity to
go richer as the fuel is paid for by the customer and warranties by the
Just because you see BLM tables doesn't mean that the ECU accepts all
values between 0 and FF and calculates the fuel accordingly. There are
hard coded limits on acceptable values of every sensor or input the ECU
How the ECU deals with "wild" values is not just an engineering
issue. When I did some work for one of the Big US OEMs I was shocked
when I was told that my Failure Mode Strategies had to be signed off
by the marketing dept. as if they would know what to do.
Well they do know a lot about customers and warranties and made me
not use the check engine light more times than I care to remember.
Learning about Engine Management by partially reverse engineering
a product that suits only a very small segment of the market can
result in all sorts of funny ideas most of which are harmless. Not all
Inferring that something "works" because some business sells it is
the reason that snake oil is made in the quantities that it is.
cogito ergo zoom,
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