TECHNICAL NOTES ON THE EEC-IV MCU - Auto diagnostics

Technical Notes on The EEC-IV MCU
SENSORS
Compiled by Tom Cloud
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EGO
Eectch98-Part3.fm
Can the HEGO respond quickly enough to monitor each cylinder during engine operation?
Cylinder-by-cylinder HEGO function can sometimes be seen during testing on
engine dynamometers, but only at certain points. HEGO response time is 300 to 1500
milliseconds for a delta Lambda step of 0.1, either rich-to-lean, or lean-to-rich.
Driven into rich or lean saturation, the sensor can require several seconds to
recover. Assuming best case 300 milliseconds response time, in a V8 firing 2 cylinders
per crankshaft revolution with stereo EGO’s, the highest RPM one could hope to
discern individual exhaust pulses would be about 300. Getting this information from
the HEGO into EEC is also made difficult by the low pass filtering on the HEGO
inputs, which are typically -3dB at 1 to 20 Hz, application dependent. This is to
filter out HEGO buzz, which is the rapid output oscillation that HEGO’s produce when
at stoichiometry.
Another problem that cylinder-by-cylinder mixture correction would create is with
closed loop control bandwidth: An engine is basically an air pump with a finite
response time to external stimulus, such as moving the throttle plate. Let’s say
it’s about 1 second. Basic control systems theory says that if you put said air pump
into a closed loop control system, the control system bandwidth must be slower than
the air pump response time, else the system will hunt, or be unstable.
With regards to lean operation with an O2 sensor, the O2 sensor’s output is not particularly
useful far from stoich. The time-honored way of biasing the mixture is to
vary the switch voltage threshold (in software), and to low-pass filter the O2 sensor
signal (again in software) and try to keep its average to a calculated value.
This can vary the mixture sufficiently to optimize the operation of a catalyst. A
normal (heated or unheated) O2 sensor cannot tell you if the mixture is rich enough
for best power or not. The O2 sensor just goes to RICH, and you can’t tell whether
you are at 9:1 or 14:1 from the sensor though in the 14 to 15:1 region you may get
some indication. Typically, the system extrapolates from the highest load at closed
loop fuel trim value and applies this same trim value when power enrichment is on.
This simple method works because injectors tend to be linear. If, at some operating
point, it takes an 8 ms ’valid’ injection time (take the offset of the injector
away) for a 16:1 mixture, then a 16 ms injection time would, ideally, give you an
8:1 mixture.
The L03 1989 Caprice (GM) uses a strategy called lean on cruise. This alternates
five minute periods of operation at about 16.5:1 with one minute intervals of stoich
operation. This allows the ECM to run lean most of the time, but correct for
changes in conditions. Interestingly enough, the O2 voltage does not ’spike’ much
when going from open to closed loop: the engine’s operating conditions don’t change
that much.
EGR
The EGR reduces pumping loss by reducing manifold vacuum. It does, however, lower
NOx by reducing the concentration of O2 in the cylinder. In essence, it lowers the
oxygen content of the air by some 10 to 15%. This added ’inert’ gas absorbs the
energy from burning the fuel and oxygen, releasing it on the expansion stroke. It
EEC-IV Technical Notes: Sensors 46 last edited: 9/29/98