This study utilized a 4-valve engine under HCCI combustion conditions. Each side of the split intake port was fed independently with different temperatures and reactant compositions. Therefore, two stratification approaches were enabled: thermal stratification and compositional stratification. Argon was used as a diluent to achieve higher temperatures and stratify the in-cylinder temperature indirectly via a stratification of the ratio of specific heats (γ = cp/cv). Tests covered five operating conditions (including two values of A/F and two loads) and four stratification cases (including one homogeneous and three with varied temperature and composition).
Stratifications of the reactants were expected to affect the combustion control and upper load limit through the combustion phasing and duration, respectively. The two approaches to stratification both affect thermal unmixedness. Since argon has a high γ, it reached higher temperatures through the compression stroke [1]. Therefore, the result of the stratified composition cases was an indirect thermal stratification.
Over all test conditions, the direct thermal stratification had no noticeable effect on the combustion duration or phasing within the limits of the experiment (a thermal difference of 45°C in the reactant gas mixtures, measured at the valve stems). However, the argon stratification case produced indirect thermal stratifications of sufficient magnitude to keep a constant combustion phasing with a mass-averaged intake temperature up to 14°C lower than the fully homogeneous case. For a given combustion phasing, it also increased the combustion duration by approximately 2°CA (and thereby decreased the maximum pressure rise rate by 20 – 40%). The combined thermally and compositionally stratified case had the same combustion phasing effect (to a smaller degree), and the same combustion duration effect (to a slightly larger degree).