Table 4-2: Flow Coefficients – Directions
4.3. Mechanical Connection
The Mechanical Connection couples two Mechanical Components (Engine-Turbine, TurboCharger-ElectricalDevice-Mechanical Consumer-Vehicle) either via their Power Exchange (Simplified Model) or their Rotational Speed (Full Model).
While the Simplified Model requires only input for Mechanical Efficiency, the Full Model needs also Input for Gear Ratio ( = SpeedUpward/SpeedDownward; Mechanical Connection Arrow indicates the Direction from Up- to Downward) and Slip (Difference of ideal driven
component speed and actual driven component speed related to the ideal driven component
Figure 4-7: Engagement Time
• Friction Coefficient Ratio between slipping and sticking transmission
• Maximum transferable Torque for sticking transmission. If a constant value is specified it is assumed that this value decreases linear to 0 with increasing Clutch Release Position; a different dependency can be specified by means of a Table.
4.4. Cylinder
The specifications for the cylinders cover the basic dimensions of the cylinder and the cranktrain (bore, stroke, compression ratio, conrod length, piston pin offset, firing order), plus information on the combustion characteristics, heat transfer, scavenging process and the valve/port specifications for the attached pipes. Furthermore, initial conditions for the calculation in the cylinder must be specified.
If a standard cranktrain is used, the piston motion is calculated from the stroke, conrod length and piston pin offset. The direction of positive piston pin offset is defined as the direction of the rotation of the crankshaft at Top Dead Center (TDC).
Figure 4-8: Standard Cranktrain
Considering blow-by from the cylinder, an equivalent effective blow-by gap must be specified as well as the average crankcase pressure. The actual blow-by mass flow is calculated from the conditions in the cylinder and the pressure in the crankcase, and from an effective flow area which is calculated from the circumference of the cylinder and the effective blow-by gap. The blow-by mass flow is lost. No recirculation to the intake may be considered.
Basic dimensions of the cylinder and the cranktrain.
Piston Pin Offset If a standard cranktrain is used, piston motion is calculated from the stroke, the con-rod length, and the piston pin offset. The direction of positive Piston Pin Offset is defined as the direction of the rotation of the crankshaft at TDC.
Effective Blow By Gap / Mean Crankcase Pressure
For the consideration of blow-by from the cylinder, an equivalent Effective Blow-By Gap and Mean Crankcase Pressure should be specified. The actual blow-by mass flow is calculated from the conditions in the cylinder, the pressure in the crankcase and the effective flow area calculated from the circumference of the cylinder and the effective blow-by gap. The blow-by mass flow is lost. No recirculation to the intake may be considered.
User Defined Piston Motion
A user-defined piston motion to be specified which allows the user to simulate an unconventional powertrain. For a user-defined piston motion the relative piston position should be specified over crank angle. The relative piston position is defined as the distance of the piston from the TDC position relative to the full stroke. Zero degree crank angle corresponds to the Firing TDC of the selected cylinder.
The cylinder piston motion may be specified alternatively depending on degrees crank angle. The piston position is expressed relatively with 0 meaning piston in TDC and 1 piston in BDC.
Chamber Attachment
Select if an engine with divided combustion chamber is to be simulated. The pre-chamber data can be specified under the
54Chamber sub-group.
Scavenge Model Three scavenging models are available (Figure 4-9 shows a comparison of the scavenging efficiency curves of the perfect displacement and the perfect mixing models.):
Perfect mixing: The gas flowing into the cylinder is mixed
immediately with the cylinder contents. The gas leaving the cylinder has the same composition as the mixture in the cylinder. The perfect mixing model is the standard scavenging model for the simulation of 4-stroke engines.
Perfect displacement: A pipeline model is used to determine the exhaust gas composition. This means that all residual gases in the cylinder are exhausted first. Only when no more residual gases are left in the cylinder, is fresh charge lost to the exhaust.
User-defined scavenging model: For the simulation of 2-stroke engines, the specification of the scavenging efficiency over scavenge ratio is required to define the quality of the port arrangement with respect to scavenging flow. This data are usually taken from literature or from the results of scavenging tests. The scavenging efficiency is defined as the volume of fresh air in the cylinder related to the total cylinder volume. The scavenge ratio is defined as the total volume of air which entered the cylinder related to the total cylinder volume.
Figure 4-9: Scavenging Models
4.4.2. Initialization
As initialization the cylinder conditions (pressure, temperature and gas composition) at the
Setting of Mass The user specifies mass and pressure in the cylinder at SHP, the corresponding temperature is calculated.
Pressure at SHP Specifies the pressure at SHP.
Air Massflow Specifies the air massflow for the given cylinder.
Fuel Massflow Specifies the fuel massflow for the given cylinder.
Trapping Efficiency Air
Multiplier to tune the actually trapped air mass (i.e. two stroke engines, large valve overlap in four stroke engines,...)
Trapping Efficiency Fuel
Multiplier to tune the actually trapped fuel mass (i.e. two stroke engines, large valve overlap in four stroke engines,...)
Mass Fraction of Residual Gas at SHP
Specifies the mass fraction of EGR at SHP
Setting of Temperature
The user specifies temperature and pressure in the cylinder at SHP, the corresponding mass is calculated.
Pressure at SHP Specifies the pressure at SHP.
Temperature at SHP
Specifies the temperature at SHP.
SHP Gas Composition
Specifies the air composition at SHP.
4.4.3. Combustion Model
For the specification of the combustion characteristics, either a heat release approach, a theoretical combustion cycle, a user-written subroutine or a truly predictive model can be selected from the pull down menu.
For selected Vibe Parameter Fitting option Vibe Parameters for the calculated Rate of Heat Release (based on Combustion Models or Table input) are fitted and available as Transients results.
Thereby the total heat released during the combustion is calculated from the amount of fuel which is burned in the cylinder and the lower heating value of the fuel:
For engines with internal mixture preparation the fuel is injected directly into the cylinder and the fueling is therefore part of the cylinder specification. For convenience, the fueling may be specified as the fuel mass which is injected into the cylinder or as a target A/F ratio, where the actual fueling is calculated every cycle from the mass of air in the cylinder and the specified target air/fuel ratio.
In the case of external mixture preparation, the fuel is fed to the intake system and the total heat supply is calculated from the amount of fuel in the cylinder at intake valve closing. For modeling of gasoline direct injection engines, fuel may be added to the cylinder charge directly. In this case In Cylinder Evaporation (4.4.3.19) must be selected and the normalized rate of evaporation must be specified. The rate of evaporation defines the addition of fuel vapor to the cylinder charge.
The specified curve is normalized, so that the area beneath the curve is equal to one. The actual amount of fuel added is either defined directly or by the target A/F-Ratio.
As for engines with internal mixture preparation, the evaporating fuel mass or the target A/F-ratio can be set by the user. If the target A/F-ratio is selected, the injected fuel mass will be determined as the fuel mass required in addition to the aspirated fuel mass to achieve the desired A/F-ratio. If the A/F-ratio is already lower than the target A/F-ratio, no fuel will be added. The evaporation heat is used to calculate the cooling of the cylinder charge due to the evaporation of the fuel. The following table may be used to determine the evaporation heat of different fuels: