Methods for Cylinder Pressure Based Compression Ratio Estimation
Three methods for compression ratio estimation based on cylinder
pressure traces are developed and evaluated for both motored and fired
cycles. Two methods rely upon models of polytropic compression and
expansion for the cylinder pressure. It is shown that they give a good
estimate of the compression ratio, although the estimates are biased.
A method based on a variable projection algorithm with a logarithmic
norm of the cylinder pressure, which uses interpolation of polytropic
models of the expansion and compression asymptotes, is recommended
when computational time is an important issue. For motored cycles it
yields the smallest bias and confidence intervals for these two
methods. For firing cycles a user-specified weighting factor is needed
during the combustion phase, which pays off in a smaller estimation
bias but also a higher variance. The third method includes heat
transfer, crevice effects, and a commonly used heat release model for
firing cycles. This method estimates the compression ratio more
accurately in terms of bias and variance. The method is more
computationally demanding and thus recommended when estimation
accuracy is the most important property. In order to estimate the
compression ratio as accurately as possible, motored cycles with as
high initial pressure as possible should be used.
Marcus Klein and Lars Eriksson
SAE Technical paper series SP-2003,
2006

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