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Abstract |
A specific heat ratio model and compression ratio estimation
Cylinder pressure modeling and heat release analysis are today
important and standard tools for engineers and researchers, when
developing and tuning new engines. An accurate specific heat ratio
model is important for an accurate heat release analysis, since the
specific heat ratio couples the systems energy to other thermodynamic
quantities. The objective of the first part is therefore to investigate models
of the specific heat ratio for the single-zone heat release model, and
find a model accurate enough to introduce a cylinder pressure modeling
error less than or in the order of the cylinder pressure measurement
noise, while keeping the computational complexity at a minimum. As
reference, a specific heat ratio is calculated for burned and unburned
gases, assuming that the unburned mixture is frozen and that the
burned is at chemical equilibrium. Use of the reference model in heat
release analysis is too time consuming and therefore a set of simpler
models, both existing and newly developed, are compared to the
reference model. A two-zone mean temperature model and the Wiebe function are used
to parameterize the mass fraction burned. The mass fraction burned is
used to interpolate the specific heats for the unburned and burned
mixture, and then form the specific heat ratio, which renders a small
enough modeling error in $\gamma$. The impact that this modeling error
has on the cylinder pressure is less than that of the measurement
noise, and fifteen times smaller than the model originally suggested
in Gatowski et.al (1984). The computational time is increased with 40
% compared to the original setting, but reduced by a factor 70
compared to precomputed tables from the full equilibrium program. The
specific heats for the unburned mixture are captured within 0.2 % by
linear functions, and the specific heats for the burned mixture are
captured within 1 % by higher-order polynomials for the major
operating range of a spark ignited (SI) engine. The second part is on compression ratio estimation based on
measured cylinder pressure traces. Four methods for compression ratio
estimation based on both motored and fired cylinder pressure traces
are described and evaluated for simulated and experimental data. The
first three methods rely upon a model of polytropic compression for
the cylinder pressure, and it is shown that they give a good estimate
of the compression ratio for simulated cycles at low compression
ratios, although the estimates are biased. The polytropic model lacks
information about heat transfer and therefore, for high compression
ratios, this model error causes the estimates to become more biased.
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 at both low and
high compression ratios. An investigation of how the methods perform
when subjected to parameter deviations in crank angle phasing,
cylinder pressure bias and heat transfer shows that the third and
fourth method can deal with these parameter deviations.
Marcus Klein
2004
Informationsansvarig: webmaster
Senast uppdaterad: 2021-11-10
