Multi-dimensional Modeling of Combustion in Compression Ignition Engines Operating with Variable Charge Premixing Levels

Premixed combustion modes in compression ignition engines are studied as a promising solution to meet fuel economy and increasingly stringent emissions regulations. Nevertheless, PCCI combustion systems are not yet consolidated enough for practical applications. The high complexity of such combustion systems in terms of both air-fuel charge preparation and combustion process control requires the employment of robust and reliable numerical tools to provide adequate comprehension of the phenomena. Object of this work is the development and validation of suitable models to evaluate the effects of charge premixing levels in Diesel combustion. This activity was performed using the Lib-ICE code, which is a set of applications and libraries for IC engine simulations developed using the OpenFOAM® technology. In particular, a turbulence-chemistry interaction model, based on the simple Eddy Dissipation Approach, was introduced to account for the effects of turbulent mixing on chemical reaction rates. It is a tentative solution to represent the effects of sub-grid mixing on the chemical reaction rates when detailed reaction mechanisms are adopted. Chemical reaction rates were computed by a robust semi-implicit extrapolation method for integrating stiff Ordinary Differential Equations with monitoring of both local and global error to adjust stepsize. To reduce the CPU time when detailed chemistry was used, both the ISAT (in-situ adaptive tabulation) and DAC (dynamic adaptive chemistry) techniques were adopted in combination. Simulations were performed by varying the charge premixing level from the typical diesel combustion mode towards an almost completely premixed/HCCI mode using n-heptane, whose injected mass was split between portinjection and direct-injection. This allowed a detailed investigation of the “mixed injection conditions”, that are typical of dual fuel configurations without employing fuels of different chemical nature, composition and ignition tendency. The choice of using a single fuel was motivated by the need to isolate the effects of different premixing levels and the resulting interaction between the charge and the fuel spray. Measurements for validation were collected by means of specific experiments on a fully instrumented single cylinder research engine, having the injection and the combustion systems architecture typical of the current light duty diesel engine technology. To realize homogeneous air-fuel charge, the intake manifold is modified to provide the desired extent of fuel port-injection.