In this study, experimental and simulation investigations on the
roles of charge density (ρtdc),
temperature (Τtdc) at the top dead
center and oxygen concentration (φO2) on the combustion paths,
emissions and thermal efficiency of a high load operation diesel
engine were conducted. Experimental engine was a modified
single-cylinder engine equipped with variable mechanisms of boost,
exhaust gas recirculation (EGR) and intake valve closing timing
(IVCT) to regulate the Ptdc, φO2
and Τtdc. Simulations of engine
combustion processes were performed with an ECFM-3Z combustion
model.
The results revealed that higher
Ptdc, leading to lower overall
fuel/oxygen equivalence ratio (Φm), enhanced the rate of
mixing and chemical reaction and benefited improvement of the
thermal efficiency. It was found that increasing charge density
played two opposite roles in NOx formation: one was
inhibiting combustion temperature rise due to increased total heat
capacity of the charge and another was increasing the air
entrainment rate resulting mainly in raising mixture temperature.
The role of reduced φO₂ by using EGR was essentially to decrease
the chemical reactivity of the fuel/gas mixture and to retard the
phase of heat release rather than to increase heat capacity and to
lower mixing rate. A phenomenon of formation and maintenance of a
large amount of incomplete combustion products, i.e., CO, was found
in the high load operation engine during the combustion process and
was named as "cold storage of carbon-monoxide," which
retarded the heat release phase and decreased the burning gas
temperature, which led to decreased NOx emissions. It
was also found that the engine exhaust soot correlated with the
amount of the "cold storage of CO."