
Innovate Motorsports Application Note 19:
New Fuel Control Strategies Enabled by Innovate’s “Direct Digital” Wideband
Controls
Patrick Thompson - Innovate Motorsports
To view the
original white paper in PDF format, click here
Oxygen sensors are
critical components in every internal combustion vehicle on the road.
Innovate’s unique approach (aka “Direct Digital”) can be used to control
oxygen sensors in a manner that differs greatly from traditional, mostly
analog, technologies.
This, in turn,
enables new strategies in fuel controls. While the initial commercial
acceptance of the Innovate technology has been in the performance
aftermarket, the biggest gains will be realized in the OEM market, where
factory-original ECUs can be designed and programmed to utilize Direct
Digital wideband technology.
Background
Current Innovate products use the same wideband zirconiumdioxide oxygen
sensors as current production vehicles, but the control methodology is
completely different. US Patent #6,978,655, titled “System, Apparatus, and
Method for Measuring an Oxygen Concentration of a Gas,” fully details the
inventions summarized below.
The Innovate measurement principle does not use the regular PID
(proportional-integral-derivative) feedback mechanism to control the
wideband sensor. Instead, the pump current is positive until the reference (Nernst)
cell shows < Lambda 1 in the measurement chamber of the wideband. Then the
polarity of the pump current is reversed until the reference cell shows >
Lambda 1. This is done with a small hysteresis. This way the measurement gas
in the measurement chamber oscillates at about 100-500 Hz around
stoichiometric. The oscillation frequency depends on the constant (but
changing polarity) pump current, hysteresis, the sensor itself, and Lambda.
The frequency has a max at Lambda 1. This is basically a 2-point regulator,
or in digital electronic terms, the operating principle of a delta-sigma
analog to digital converter, except that here the analog value measured is
directly the exhaust gas.
The duty cycle PWM of that oscillation is calculated with (t1 - t2) / (t2 +
t2), therefore has a range of +/- 1.0. t1 is the duration of positive
polarity of pump current, t2 the duration during negative current polarity.
Both measured with 16 bit accuracy. With PWMair (duty cycle in air) the O2
flow rate of the pump cell can be directly calculated with PWM / PWMair and
therefore Lambda can be calculated from that. Because the sensor is only
used with constant and relative high Ip, but with changing polarity, PWM is
completely linear with O2 flow, and independent of the Lambda/Ip curve of a
particular sensor after normalizing to PWMair. Because of the oscillation,
there is no equilibrium state in the cell which would slow down the
diffusion. Also because of the oscillation, there is no electrostatic charge
buildup on the measurement cell that causes drifts during operation at
Lambda < 1.0. The Lambda/Ip curve of a wideband sensor has a singularity at
Lambda 1.0. This causes instabilities in the normal PID feedback mechanism.
The Innovate method does not show those instabilities. A conventional PID
feedback loop needs to be tuned to the speed response of the controlled
system. The best one can do is to achieve critical damping, otherwise it
would lead to wild oscillations and over swings. The Innovate principle
approach basically makes specific use of those by running the feedback loop
deep into those normally undesired oscillations.
In a conventional PID feedback wideband controller the temperature is
regulated via the AC impedance of the reference cell (usually measured with
a small AC current of 1-4 kHz). This measurement frequency has to be
filtered back out of the reference cell signal with low pass filters and
that cause further latency in the
measurement loop.
The Innovate system uses the AC impedance of the pump cell, which shows the
same temperature behavior as the reference cell. With the Innovate
measurement principle already essentially impresses an AC current on the
pump cell, it’s possible to measure its temperature for temperature
regulation without any further hardware costs. A further slowdown of the
conventional PID measurement loop of a wideband comes because many wideband
controllers drive the pump cell with a low impedance voltage source (op-amp
output) directly. The pump current is then measured with a measurement
resistor. But the pump cell in a wideband acts also like a Nernst cell that
produces a counter EMF that’s dependent on the Lambda value in the
measurement chamber. This counter EMF is essentially shorted by the low
impedance source and then causes a reduction of the pump current during
Lambda transitions, which is in turn compensated by the changing error value
in the reference cell. This means, in a regular PID implementation of the
measurement loop of a conventional wideband controller, many (often
hundreds) loop passes are made through the delay between pump cell and
reference cell. With the Innovate “2-point regulator” implementation only
two passes are needed for a complete Lambda measurement as Lambda can be
calculated after every oscillation period. This is the major reason the
Innovate measurement principle is so fast.
Response Time
The
fast response time of Direct Digital technology enables new fuel control
strategies. The biggest impact is in two areas:
1) Rapid load-transition Periods. Due to the slow response time of PID
systems, even the most sophisticated modern vehicles must go “open-loop” for
up to 500ms second after major throttle transitions (for example,
“Acceleration Enrichment” period). Such open-loop operation often results in
excessive fuel injection (and therefore reduced MPG, and increased
emissions) during transitions. In most regular driving, transitions are
quite frequent. It is estimated that eliminating open-loop operation
acceleration enrichment improves average fuel economy by as much as 7%, and
reduces
emissions by the same amount.
2) Injector balancing. OEMs allow up to 5% cylinder-to-cylinder variation in
new fuel injectors. This is less than ideal to begin with, as some cylinders
will be lean (producing excessive NOx), and some will be rich (producing
excessive CO). The real problem however, is that this condition deteriorates
over time (injector clogging, etc.). Cylinder-to-cylinder variation climbs
to 10% or more, stressing the catalytic converter, damaging engines, and
producing excess emissions. Direct Digital technology is fast enough to
enable a single sensor to detect individual-cylinder lambda as the “slugs”
of exhaust pass the sensor. This allows the ECU to vary each injector’s duty
cycle, and precisely manage each cylinder.
The benefits of full-time closed-loop operation are clear- essentially you
have a self-tuning, adaptive engine that can handle major variations in fuel
composition, barometric pressure, and component aging. However, until Direct
Digital, many of the gains from closed-loop operation were unattainable.
Full-time closed loop functioning does not preclude the ECU from building
and maintaining a safe “fallback” fuel map. If an oxygen sensor fails during
normal operation, the ECU will then be merely as smart as the best current
ECU.
Conclusion
Designers of new fuel-control systems should fully explore the benefits of
implementing full-time, closedloop injection strategies based on Direct
Digital Wideband systems.
To view the
original white paper in PDF format, click here
Thank you for shopping with us,
Tuner Tools
management

