No doubt about it: The modern internal combustion engine has a lot of demands on it, not the least of which are low emissions,
good fuel economy and useable power. And it all boils down to getting the right air/fuel mixture packed into the combustion
chamber.
The on-board monitor responsible for keeping an eye on all this is the fuel system monitor, and it does its job continuously,
watching for excessive corrections to the fuel quantity delivered by the Engine Control Module (ECM) and setting Diagnostic
Trouble Codes (DTCs) when things get out of hand. Diagnosing fuel trim related faults using fuel trim Parameter Identifications
(PIDs) can help us in diagnosing other driveability issues.
What is Fuel Trim?
The inside of a lambda sensor is a microcosm of technology at work.
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Every tech learns early on about proper air/fuel ratio, that magic number where the engine works at its best. This is the
ratio of air mass to fuel mass, and for a gasoline engine, that magic number is 14.7:1, or 14.7 grams of air for every gram
of fuel. This "ideal" is called the stoichiometric ratio, and it's different for different fuels. The stoichiometric ratio
for 10 percent ethanol fuel, for example, is 13.85:1. Stoichiometric defines the ratio that provides just the right amount
of fuel to utilize all of the oxygen drawn into the combustion chamber.
When engineers are programming the ECM for a given engine, they are able to model the quantity of air flowing into that engine
at various engine speeds (rpm) and throttle openings (load). These theoretical air charges are programmed into a map with
corresponding fuel injection pulse widths to form an rpm/load look-up table that the ECM will use as a base to determine how
much fuel to supply. To make sure the fuel charge is correct, the ECM relies on a feedback measurement of oxygen in the exhaust
stream. If needed, it will then "'trim" the pulse width to maintain the correct value for lambda (the term used to define
air/fuel ratio relative to stoichiometry).
A healthy lambda sensor is key in maintaining overall peak performance.
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A stoichiometric air/fuel ratio would result in a lambda of 1.0. A lambda less than 1.0 indicates too much fuel was supplied,
and the mixture is rich. A lambda greater than 1.0 indicates the opposite: Too little fuel was supplied, and the mixture is
lean. A gasoline engine produces its best power when lambda is 0.85-0.95 (slightly rich), but its best fuel economy at 1.10-1.20
(slightly lean).
That seems like a fairly wide range, doesn't it? We still have to consider emissions, however, and to meet current standards,
a three-way catalytic converter has to receive exhaust gases from a much narrower lambda range: 1.000 +/- 0.005! Lean of this
range, oxides of nitrogen (NOx) emissions begin to increase. A mixture that's too rich can lead to increased hydrocarbon (HC)
and carbon monoxide (CO) emissions, as well as potential catalyst damage.
On Board Diagnostic 2 (OBD2) regulations require that the ECM be capable of monitoring the systems that could cause variances
in vehicle emissions, and report unacceptable variances to the driver by turning on the Malfunction Indicator Lamp (MIL).
In addition to all of this, the designers have to take into account vehicle aging and its effect on lambda, building in a
means to adjust the base fuel injector pulse width to compensate.
This adjustment is the definition of fuel trim. The ECM adjusts, or "trims" the injector pulse width to slightly increase
or decrease the amount of fuel that would otherwise be injected based solely on the amount programmed.
