Effects of Injection Timing on the Conditions at Top Dead Center for Direct Injected HCCI 2003-01-3219
A numerical approach has been applied to investigate direct injected HCCI combustion in terms of estimating the conditions at Top Dead Center (TDC) for three different injection timings during a calculated compression stroke in the commercial CFD software FIRE.
The paper first describes reproduction of the initial spray, which is introduced at different angles during the calculated compression stroke. These spray shapes have been validated by comparison to experimental investigations in a pressurized constant volume vessel with densities in accordance with the cylinder conditions. The sprays origins from a colliding flow nozzle, which is of interest in direct injected HCCI because of increased spray dispersion and turbulence.
In the calculated compression stroke two fuels were compared: n-dodecane and n-heptane. The injected amount of fuel, 117 mg, results in a generally lean distribution at TDC. For early injection the spray is distributed more homogenously with values close to the overall equivalence ratio, ϕgl=0.44, which is attractive for direct injected HCCI. When the fuel is injected later approaching TDC, a distribution with both lean and relatively rich fractions, ϕ>0.5, appears. These rich fractions are most likely to produce NOx.