Thermally stratified compression ignition is a new advanced, low-temperature combustion mode that aims to control the heat release process in a lean, premixed, compression ignition combustion mode by controlling the level of thermal stratification in the cylinder. Specifically, this work uses a mixture of 80% ethanol and 20% water by mass, referred to as “wet ethanol” herein, to increase thermal stratification via evaporative cooling of areas targeted by an injection event during the compression stroke. The experiments conducted aim to both fundamentally understand the effect that a late cycle injection of wet ethanol has on the heat release process, and to use that effect to explore the high-load limit of thermally stratified compression ignition with wet ethanol. At an equivalence ratio of 0.5, injecting just 8% of the fuel during the compression stroke was shown to reduce the peak heat release rate by a factor of 2, subsequently avoiding excessive pressure rise rates. Under pure homogeneous charge compression ignition using wet ethanol as the fuel, the load range was found to be 2.5–3.9 bar gross indicated mean effective pressure. Using a split injection of wet ethanol, the high-load limit was extended to 7.0 bar gross indicated mean effective pressure under naturally aspirated conditions. Finally, intake boost was used to achieve high-load operation with low NOx (oxides of nitrogen (NO or NO2)) emissions and was shown to further increase the high-load limit to 7.6 bar gross indicated mean effective pressure at an intake pressure of 1.43 bar. These results show the ability of a split injection of wet ethanol to successfully control the heat release process and expand the operable load range in low-temperature combustion.

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The operating range of Homogeneous Charge Compression Ignition (HCCI) engines is limited to low and medium loads by high heat release rates. Negative valve overlap can be used to control ignition timing and heat release by diluting the mixture with residual gas and introducing thermal stratification. Cyclic variability in HCCI engines with NVO can result in reduced efficiency, unstable operation, and excessive pressure rise rates. Contrary to spark-ignition engines, where the sources of cyclic variability are well understood, there is a lack of understanding of the effects of turbulence on cyclic variability in HCCI engines and the dependence of cyclic variability on thermal stratification. A three-dimensional computational fluid dynamics (CFD) model of a 2.0L GM Ecotec engine cylinder, modified for HCCI combustion, was developed using Converge. Large Eddy Simulations (LES) were combined with detailed chemical kinetics for simulating the combustion process. Twenty consecutive cycles were simulated and the results were compared with individual cycle data of 300 consecutive experimental cycles. A verification approach based on the LES quality index indicated that this modeling framework can resolve more than 80% of the kinetic energy of the working fluid in the combustion chamber at the pre-ignition region. Lower cyclic variability was predicted by the LES model compared to the experiments. This difference is attributed to the resolution of the sub-grid velocity field, time averaging of the intake manifold pressure boundary conditions, and different variability in the equivalence ratio compared to the experimental data. Combustion phasing of each cycle was found to depend primarily on the bulk cylinder temperature, which agrees with established findings in the literature. Large cyclic variability of turbulent mixing and spatial distribution of temperature was predicted. However, both of these parameters were found to have a small effect on the cyclic variability of combustion phasing.

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High heat release rates limit the operating range of homogeneous charge compression–ignition engines to low and medium loads. Thermal stratification has been shown to stagger autoignition, lower heat release rates, and extend the operating range of homogeneous charge compression–ignition engines. However, the dependence of naturally occurring thermal stratification on the engine size, speed, and internal residual dilution is not fully understood. A three-dimensional computational fluid dynamics model with large eddy simulations and detailed chemical kinetics was developed using CONVERGE. This model was used to simulate two different engines: (1) a light-duty 2.0 GM Ecotec Engine modified for homogeneous charge compression–ignition combustion in one of the cylinders and (2) a medium-duty Cummins B-series engine modified for homogeneous charge compression–ignition combustion in one of the cylinders. For the light-duty engine, five consecutive modeled cycles were compared with experimental data from 300 consecutive cycles using residual gas dilution at 2000 r/min. For the medium-duty engine, five consecutive modeled cycles were compared with experimental data from 100 consecutive cycles using air dilution with intake heating at 1200 r/min. In the light-duty engine, it was found that incomplete mixing between fresh charge and residual gas increased thermal stratification early in the compression stroke for residual dilution compared to air dilution. Residual stratification at the onset of ignition was small and not directly coupled with thermal stratification. Heat losses to the walls were the dominant source of thermal stratification at the onset of ignition. The reduced oxygen concentration due to residual dilution, increased the temperature requirement for autoignition, which increased heat transfer losses and increased the thermal stratification around top dead center. The thermal stratification before ignition reduced when the engine speed increased because of the lower heat transfer losses. The light-duty engine was found to have larger portion of the fuel energy lost to heat transfer than the medium-duty engine, which resulted in larger thermal stratification before ignition.