Modelling for Fuel Optimal Control of a Variable Compression Engine
Variable compression engines are a mean to meet the demand on lower fuel
consumption. A high compression ratio results in high engine efficiency, but also
increases the knock tendency. On conventional engines with fixed compression
ratio, knock is avoided by retarding the ignition angle. The variable compression
engine offers an extra dimension in knock control, since both ignition angle
and compression ratio can be adjusted. The central question is thus for what
combination of compression ratio and ignition angle the maximum efficiency is
achieved, considering the set of compression ratios and ignition angles that give
a sufficiently low knock intensity.
Four knock detection methods are proposed, compared and evaluated with
respect to robustness for noise and choices of parameter values. Three of the
knock detectors are categorised as on-line, and are designed for giving feedback
about knock occurrence to the engine control unit. The methods can determine
both whether or not knock is present and the crank angle at knock onset.
A study of the relationship between knock oscillation properties and knockonset
is performed. It is concluded that the logarithm of the normalised knock
energy depends almost linearly on the rate of knock occurrence.A new formulation
of multi-zone engine models is presented. The formulation makes it easy to increase
or decrease the number of zones during the simulation. One of many possible
applications is the investigation of engine efficiency.
An analysis of experimental data shows how the engine efficiency changes
with compression ratio and ignition angle. An engine torque model is developed
and validated, from which the optimal choice of compression ratio and ignition
angle can be calculated with high accuracy.
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Last updated: 2020-10-13