Effects of Mixture Stratification on Ignition and Combustion in a GCAI Engine

Fuel consumption and NOx emissions of gasoline engines at part load can be significantly reduced by Controlled Auto-Ignition combustion concepts. However, the range of Gasoline Controlled Auto-Ignition (GCAI) operation is still limited by lacking combustion stability at low load and by high pressure-rise rates toward higher loads. Previous investigations indicate that the auto-ignition process is particularly determined by the thermodynamic state of the charge and by stratification effects of residual gas, temperature, and air-fuel ratio. However, little experimental data exist on the direct influence of mixture stratification on local ignition and heat-release rate (HRR) in direct-injection (DI) GCAI engines, because it is challenging to measure all the relevant charge and combustion parameters quasi-simultaneously with sufficient spatial/temporal resolution and precision. In the present article, a newly designed laser-diagnostic approach is therefore presented, in which one-dimensional Spontaneous Raman Scattering (SRS) is combined with high-speed chemiluminescence imaging. SRS yields the composition of the charge along a line in terms of air-fuel ratio, exhaust-gas content, and temperature. Thereby, the mixture formation process is analyzed. Additionally, high-speed chemiluminescence imaging is conducted in the same experiment, in order to characterize the ignition and combustion process with relatively high temporal and spatial resolution. In particular, the influence of the three charge parameters, which are measured shortly before combustion, on the ignition process is investigated by exploiting the combined results of both optical techniques. The results show that first auto-ignition in each cycle is basically determined by the gas-phase temperature, which depends on both the residual-gas distribution and local vaporization cooling. Thus, large-scale thermal stratification of the charge leads to a staged combustion process, which progresses from high-temperature to low-temperature regions. However, the progress of combustion and the resulting maximum heat-release rate also depend on the small-scale inhomogeneity in the mixture, which can be controlled by the fuel-injection timing. Furthermore, it is demonstrated that the stability of combustion phasing is dependent on the cyclic variability in the gas-exchange process.