Tracer-based PLIF temperature diagnostics have been used to
study the distribution and evolution of naturally occurring thermal
stratification (TS) in an HCCI engine under fired and motored
operation. PLIF measurements, performed with two excitation
wavelengths (277, 308 nm) and 3-pentanone as a tracer, allowed
investigation of TS development under relevant fired conditions.
Two-line PLIF measurements of temperature and composition were
first performed to track the mixing of the fresh charge and hot
residuals during intake and early compression strokes. Results
showed that mixing occurs rapidly with no measureable mixture
stratification remaining by early compression (220°CA aTDC),
confirming that the residual mixing is not a leading cause of
thermal stratification for low-residual (4-6%) engines with
conventional valve timing.
In the second part of the study, single-line PLIF measurements
performed later in the compression stroke showed that the
distribution of TS and its development are very similar for both
motored and fired operation. This finding indicates that the
mechanism producing the temperature stratification is the same for
both cases, although some differences in magnitude can occur. A
subsequent parametric study proved that these differences can be
attributed to the impact of both incomplete fuel mixing and
cylinder-wall temperature variation, depending on the type of
engine operation (DI skipfiring or premixed continuous firing
respectively).
In the final part of this study, the simultaneous use of the two
lasers allowed correlation of the high temperature zones existing
before TDC with the early reaction zones after TDC. These image
pairs indicated that the first combustion reactions begin in the
highest temperature regions followed by reactions in progressively
cooler zones, proving that TS is the root cause of sequential
auto-ignition in HCCI-combustion engines. All these measurements
demonstrate the feasibility of quantitative tracer-based PLIF
diagnostics in harsh engine environments and provide useful
information for future HCCI engine development.
