・ The torque combiner which sums the torque and mass polar moment of inertia of two or more female mechanical interfaces to a single equivalent one.
2.2.2 Thermo-fluid group
The proposed modeling method isolates the geometry of the fluid container from the working fluid model. Also the heat transfer model is provided externally using heat transfer elements, enabling great flexibility to the modeling approach and detail required. Another target of the proposed method is to isolate the working fluid properties in a different element, thus enabling the modeling of the process to be carried out independent from the working fluid properties This approach enables to adopt different calculation method for the properties of the working fluid, without other modifications to the mathematical models of the thermofluid elements. The thermofluid elements introduced here are:
・ The open thermodynamic system which models a uniform composition working fluid within a boundary. The fluid exchanges mass1 and heat through the boundary2 while the volume is externally provided from a geometry element3. Typical uses are the modeling of the gas inside a cylinder, or the intake and exhaust manifolds. Note that the properties of the working fluid are provided externally through the gas properties element.
・ The valve which models a flow restriction between two open thermodynamic system elements. Heat transfer is considered before and after the restriction and is implemented using two heat transfer connections. The heat flux is defined externally using heat transfer elements by taking into account the gas temperature and thermal properties.
・ The compressor which models a compressor as a stand alone unit, where the power required to drive the compressor is obtained through a shaft. The compressor takes air from the volume with the lower pressure (denoted as downstream) and delivers it to the volume with the higher pressure (denoted as upstream). The compressor performance is described using a map which provides the mass flow rate through the compressor and the corresponding isentropic efficiency for various pressure ratios and angular velocities. Heat transfer is implemented using one heat transfer male interface, through which the heat flux is defined externally using heat transfer elements, assuming a mean gas temperature and thermal properties.
・ The turbine which models a turbine also as a stand alone unit, where the power developed is obtained through a shaft. The turbine performance is described using two maps. The first map gives the swallowing capacity (mass flow rate) of the turbine while the other gives the efficiency of the turbine. Heat transfer is implemented using one heat transfer male interface, through which heat flux is defined externally, using heat transfer elements.
・ The fluid sink which models a fixed fluid condition where the fluid state is kept constant regardless of the mass flow entering or leaving this element. The gas properties are provided externally using a gas properties element. Typical use of such element is for modeling the atmosphere.
・ The fluid source which models an idealized constant mass flow rate into a thermodynamic volume regardless of the thermodynamic condition (gas pressure, temperature etc) inside this volume.
・ The heat exchanger models a fluid heat exchanger based on the efficiency. Typical use of this element is as an intercooler after a compressor.
・ The combustor which models the combustion process. The combustion process although normally takes place inside the open thermodynamic system here is provided externally through this element. Taking a closer look to the combustion modeling in the emptying-filling method we can see that the combustion process is typically a heat release and a mixture composition change, thus the separation of the models is possible. This separation provides added simulation stability (since no modifications are required to the open thermodynamic system), more than one combustion element can be used (modeling two or more injectors with different injection timing) and different combustion models can easily be implemented and added to the system. Two different combustion models were used. The first models the heat release using the S function (or curve), while the second models the heat release using experimentally obtained data.
・ The gas properties element which provides the properties of the working fluid to other elements. This provides flexibility, by allowing the gas properties calculation method to be independent of the modeling method of the elements.
・ The thermofluid combiner which sums the mass flows of two or more female thermofluid interfaces to a single equivalent one.
2.2.3 Geometry group
As already stated, the geometrical data of the space the working fluid occupies are provided externally to the open thermodynamic element. This provides greater flexibility in the modeling process. The geometry group of elements includes among others elements responsible for transferring the power between the working fluid and the mechanical elements.
The geometry data provided to the open thermodynamic element are the volume the working gas occupies, the volume change rate, a reference linear position and a reference linear velocity. The latter are very useful for providing reference values for the moving parts of the engine which influence gas velocity and thus heat transfer and gas mixing speed. The geometry elements introduced here are:
・ The cylinder geometry element which calculates the volume and volume change rate of the working gas, using the angular position and velocity provided from the female mechanical interface (the volume and volume change rate is equally treated and calculated for the upper and lower volumes). Also the cylinder geometry element, calculates the torque developed (and thus provided to the female mechanical interface) due to gas pressure and angular position.
