Piston-Liner Crevice Geometry Effect on HCCI Combustion by Multi-Zone Analysis

Abstract

A multi-zone model has been developed that accurately predicts HCCI combustion and emissions. The multizone methodology is based on the observation that turbulence does not play a direct role on HCCI combustion. Instead, chemical kinetics dominates the process, with hotter zones reacting first, and then colder zones reacting in rapid succession. Here, the multi-zone model has been applied to analyze the effect of piston crevice geometry on HCCI combustion and emissions. Three different pistons of varying crevice size were analyzed. Crevice sizes were 0.26, 1.3 and 2.1 mm, while a constant compression ratio was maintained (17:1). The results show that the multi-zone model can predict pressure traces and heat release rates with good accuracy. Combustion efficiency is also predicted with good accuracy for all cases, with a maximum difference of 5% between experimental and numerical results. Carbon monoxide emissions are underpredicted, but the results are better than those obtained in previous publications. The improvement is attributed to the use of a 40-zone model, while previous publications used a 10zone model. Hydrocarbon emissions are well predicted. For cylinders with wide crevices (1.3 and 2.1 mm), HC emissions do not decrease monotonically as the relative air/fuel ratio (λ) increases. Instead, maximum HC emissions are obtained for an intermediate value of λ. The model predicts this relative air/fuel ratio for maximum HC emissions with very good accuracy. The results show that the multi-zone model can successfully predict the effect of crevice geometry on HCCI combustion, and therefore it has applicability to the design of HCCI engines with optimum characteristics for high efficiency, low emissions and low peak cylinder pressure.

Spatial Analysis of Emissions Sources for HCCI Combustion at Low Loads Using a Multi-Zone Model

“…The model predicts with good accuracy pressure traces and heat release rates for the 12 cases being analyzed. Agreement is similar to the results obtained in previous publications [8,12]. Other combustion parameters such as burn duration and combustion efficiency are also well predicted.…”

“…The mass distribution for f=0.26 in Figure 10 shows results that correspond closely with those previously studied for typical HCCI operating conditions [8,12]. Under these relatively high load conditions, burnt-gas temperatures are sufficiently high throughout the core gases for reactions to go to completion, forming CO 2 .…”

“…Christensen et al [7] varied the geometry of the piston top-land crevice and concluded that it was the source of most of the HC emissions for their operating conditions (premixed HCCI using isooctane at equivalence ratios of f = 0.22 and higher). Similar results were found by Aceves et al [8], using a multi-zone modeling approach to analyze the experimental results of Christensen et al [7]. The modeling results in Aceves et al [8] also indicated that a substantial portion of the CO was due to the crevices and boundary layers at these conditions.…”

“…Similar results were found by Aceves et al [8], using a multi-zone modeling approach to analyze the experimental results of Christensen et al [7]. The modeling results in Aceves et al [8] also indicated that a substantial portion of the CO was due to the crevices and boundary layers at these conditions. For lower load operation, however, CO emissions can rise dramatically, reaching levels equivalent to 60% of the fuel carbon at fueling rates typical of idle in diesel engines (f = 0.1 to 0.12) [4,5,9,10].…”

Fast Prediction of HCCI Combustion with an Artificial Neural Network Linked to a Fluid Mechanics Code

“…Unburned fuel and air are compressed and heated by the expansion of hot reaction products. Compression heating from the burned gases typically generates temperatures hot enough to combust the fuel (except for fuel in the crevices and boundary layers [7,8]), and is the main mechanism for spreading combustion in HCCI engines. Flame propagation (diffusion of energy and radicals from the burned zone to the unburned zone [9]) plays a secondary role in spreading combustion.…”