A Model for Fuel Optimal Control of a Spark-Ignited Variable Compression Engine
Variable compression engines are a mean to meet the demand on lower
fuel consumptions. 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. A vital question is thus what combination of
compression ratio and ignition angle should be used to achieve maximum
engine efficiency.
Fuel optimal control of a variable compression engine is studied and
it is shown that a crucial component is the model for the engine
torque. A model for the produced work that captures the important
effects of ignition and compression ratio is proposed and
investigated. The main task for the model is to be a mean for
determining the fuel optimal control signals, for each requested
engine torque and speed. The contribution is a model suitable for
finding this optimal combination. This model consists of well-known
components, and the novelty lies in the compilation and validation of
the control oriented efficiency model for a variable compression
engine.
Despite the models simplicity, it describes the indicated work with
good accuracy, and suits its purpose of finding optimal control
signals. The evaluation shows that a fuel optimal controller based on
the proposed model will miss the optimal IMEP with only $1.5\%$, and
that the corresponding loss in engine efficiency is less than 0.5
percentage units.
Ylva Nilsson, Lars Eriksson and Martin Gunnarsson
SAE World Congress 2006,
2006
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