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Computational Fluid Dynamics (CFD) modeling was used to investigate the effects of the direct injection of wet ethanol at various injection timings during the intake stroke in a diesel engine with a shallow bowl piston. Thermally Stratified Compression Ignition (TSCI) has been proposed to expand the operating range of Low Temperature Combustion (LTC) by broadening the temperature distribution in the cylinder prior to ignition. TSCI is accomplished by injecting either water or a water-fuel mixture with a high latent heat of vaporization like wet ethanol. This current study focuses on isolating the effects that injecting such a high heat of vaporization mixture during the intake stroke has on the distribution of temperature and equivalence ratio in the cylinder before the onset of combustion. A CONVERGE 3-D CFD model of a single cylinder diesel research engine using Reynolds Averaged Naiver Stokes (RANS) turbulence modeling was developed and validated against experimental data.

The combustion phasing of Homogeneous Charge Compression Ignition combustion is incredibly sensitive to intake temperature. Controlling the intake temperature on a cycle-to-cycle basis is one-way to control combustion phasing, however accomplishing this with an intake air heater/intercooler is unfeasible. One possible way to control the intake temperature is through the direct injection of fuel. The direct injection of fuel during the intake stroke cools the charge via evaporative cooling. Some heat is absorbed from the incoming air, lowering the in-cylinder temperature, while some heat is absorbed from the piston/cylinder walls if the spray reaches the walls. The amount of heat that is absorbed from the air vs. the walls depends on the spray penetration length. The available spray penetration length can be controlled by the injection timing during the intake stroke.

Thermally Stratified Compressions Ignition (TSCI) is a new advanced, low temperature combustion concept that aims to control the thermal stratification in the cylinder in order to control the heat release process in a lean, compression-ignition combustion mode. This work uses “wet ethanol”, a mixture of 80% ethanol and 20% water by mass, to increase thermal stratification beyond what naturally occurs, via evaporative cooling of a split direct injection. TSCI with wet ethanol has previously shown the potential to increase the high-load limit when compared to HCCI. The experiments conducted in this paper aim to fundamentally understand the effect that injection strategy has on the heat release process in TSCI. TSCI employs a split-injection strategy in which an injection during the intake stroke allows the majority of the fuel to premix with the air and an injection during the compression stroke introduces the desired level of thermal stratification to control the heat release rate.

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 (NVO) can be used to facilitate ignition of high octane number fuels and control pressure rise rates by diluting the mixture with hot residual gas and introducing some thermal stratification. Controlling the thermal stratification results in sequential autoignition, reduced heat release rates, and operating range extension. Therefore, fundamental understanding of thermal stratification in HCCI combustion with high levels of internal residuals is necessary, along with the development of appropriate models to simulate thermal stratification and its effects on HCCI combustion. A 3-D Computational Fluid Dynamics (CFD) model of a 2.0 L GM Ecotec engine (LNF type) engine cylinder, modified for HCCI combustion, was developed using CONVERGE CFD.

A CFD investigation has been conducted to study the efficiency and emissions characteristics of Thermally Stratified Compression Ignition (TSCI) combustion with direct water injection. The motivation for using this new low temperature combustion mode is its ability to control the heat release process by introducing a forced and controlled thermal stratification beyond what would occur naturally. In this case, TSCI is enabled using direct water injection. The added degree of control over the combustion process allows for a significantly broader operable load range compared to HCCI. The effects of injection parameters including the pressure, start of injection (SOI) timing, and spray pattern have been shown previously to affect the heat release of TSCI and its induced thermal stratification. In the present work, the efficiency and emissions considerations were investigated in detail, and the effects of injected mass are presented.