The control of the engine air system is an essential part for
meeting the emission levels of current and upcoming legislation. Up
to now different strategies were presented in the literature and
also applied on real systems. Starting from simple
single-input-single-output structures in combination with
feedforward parts leading to advanced multi-input-multi-output
approaches. Nevertheless, independent of the used control approach
for each of them suitable references are necessary. Although it
seems adequate to directly use the emission target quantities in a
closed loop air system control, a fast and accurate measurement is
seldom available. An alternative is to use intermediate quantities
as references, like fresh air mass flow or oxygen concentrations,
which represent the state of the air system. However, for control
purposes each of these quantities has to be determined, i.e.,
measured or calculated. Moreover, it has to be considered that each
sensor has different dynamics and accuracy in dependency of the
given range and furthermore also system dynamics can influence the
sensor readouts.
In this work a method for controlling the exhaust gas
recirculation valve of a diesel engine is proposed, where in
contrary to standard approaches not only one but a combination of
different reference quantities is used, with the aim to maintain
predefined tailpipe nitric oxides emissions. The idea is to use
allocation techniques and to combine different measurements with
respect to their accuracy and dynamic properties, thus ensuring
that in each operating point and time instant the most accurate
quantity with respect to NOx is used. The determination
of the static accuracy of each quantity was carried out by a
NOx sensitivity analysis and the use of Gaussian error
propagation. Considering the dynamic properties, response and rise
times of the different sensors and physical quantities were taken
into account. Finally, the control strategy was implemented and
tested on simulation models and on an EU5 passenger car diesel
engine on an engine testbed, showing satisfactory results.