Compression ratio estimation based on cylinder pressure data

Four methods for compression ratio estimation based on cylinder pressure traces are developed and evaluated for both simulated and experimental cycles. The first three methods rely upon a model of polytropic compression for the cylinder pressure. It is shown that they give a good estimate of the compression ratio at low compression ratios, although the estimates are biased. A method based on a variable projection algorithm with a logarithmic norm of the cylinder pressure yields the smallest confidence intervals and shortest computational time for these three methods. This method is recommended when computational time is an important issue. The polytropic pressure model lacks information about heat transfer and therefore the estimation bias increases with the compression ratio. The fourth method includes heat transfer, crevice effects, and a commonly used heat release model for firing cycles. This method is able to estimate the compression ratio more accurately in terms of bias and variance. The method is more computationally demanding and is therefore recommended when estimation accuracy is the most important property.