A new procedure for the real-time estimation of the EGR rate and
charge oxygen concentration has been developed, assessed and
applied to a low-compression ratio GMPT-E EURO V diesel engine.
High EGR rates are usually employed in modern diesel engines to
reduce combustion temperatures and NOx emissions,
especially at medium-low load and speed conditions. The EGR rate is
usually calibrated in steady-state conditions, but, under transient
conditions, it can be responsible for misfire occurrence or non
optimal combustion cycles, if not properly controlled. In other
words, combustion instabilities can occur, especially during tip-in
maneuvers, which imply transition from high EGR (low load) to low
EGR (high load) rates. Misfire is determined by a temporary
reduction in the intake charge oxygen concentration during the
closure of the EGR valve.
Therefore, a model-based approach for real-time estimation of
the EGR rate and intake charge oxygen concentration is a powerful
tool that could allow the engine ECU to prevent misfire
occurrence.
In this paper, a semi-empirical correlation has been developed
to estimate the EGR rate under steady-state and transient operating
conditions, on the basis of the measured pressure in the intake
manifold, of the measured pressure and temperature upstream from
the EGR valve, and of the duty cycle signal of the EGR valve. The
intake charge oxygen concentration has been estimated on the basis
of the measurements of the air mass-flow rate and injected fuel
mass.
The proposed technique has been applied to a modern EURO V
diesel engine, in order to analyze two different engine transients:
a severe tip-in maneuver with misfire occurrence and an
acceleration ramp during the ECE cycle. The methodology has proved
to be effective in the real-time monitoring of the EGR rate and
intake charge oxygen concentration and to be simple enough to be
implemented in the engine ECU in order to diagnose misfire
occurrence in advance.