Charging Elements

4.14.1. Turbocharger

Two types of turbocharger models are available: Simplified Model and Full Model.

4.14.1.1. Simplified Model

This model is only suitable for steady state simulations. BOOST considers the mean compressor and turbine efficiencies over the cycle in order to calculate the turbocharger energy balance. The advantage of this model is that it only requires limited data to describe the turbocharger performance characteristics. Furthermore, this model provides three modes for the turbocharger simulation:

Boost pressure calculation

The boost pressure is calculated from the specified turbine size and turbocharger efficiency.

Turbine layout calculation

The required turbine size is calculated from the target pressure ratio across the compressor and the turbocharger efficiency by adopting the turbine size multiplier.

Waste-gate calculation

The waste-gate mass flow is calculated from the target pressure ratio across the compressor, the turbocharger efficiency and the specified turbine size. If the target pressure ratio cannot be achieved even with the waste-gate closed, the boost pressure which can be achieved will be calculated from the specified turbine size.

Input data and calculation result relative to the turbocharger mode are shown in the following table:

Boost Pressure Turbine Layout Waste Gate Turbine size

multiplier

input result input

Compressor pressure ratio

result input input

Turbine to total mass flow rate

input input result

• Discharge Coefficient

• Corrected Mass Flow

• Corrected Volume Flow

I case of selected Discharge Coefficient Type the effective flow area of the turbine is calculated from the equivalent discharge coefficient (constant or function of turbine expansion ratio) and a turbine reference area The Turbine Reference Area can either be directly input or is calculated from the cross-section of the pipe representing the turbine outlet and the Pipe Area Scaling Factor.

I case of selected Corrected Mass Flow or Corrected Volume Flow Type the input Reference Conditions is required. If no expansion ratio dependency for Corr.

Mass/Volume Flow is selected the constant input value is assumed to be at the Reference Pressure Ratio.

The conversion of the swallowing capacity taken from the turbine map at a certain pressure ratio to an effective flow area is done with Equation 4.14.1:

1

A

eff effective flow area

po To

m^• swallowing capacity

R

gas constant

ψ

pressure function

The pressure function

ψ

is evaluated at the pressure ratio at which the effective flow area is to be determined. Typical values for the gas constant R and the ratio of specific heats of combustion gases are 287 J/kgK and 1.36 respectively. When evaluating the pressure function

ψ

it must be observed whether the pressure ratio is supercritical. In this case,

ψ

max must be used instead of

ψ

.

To determine the swallowing capacity from an effective turbine flow area obtained by a turbine layout calculation, Equation 4.14.1 must be solved for ^po

To m

•

.

The Turbine Size Multiplier is applied to determine the final swallowing capacity.

The Turbine Efficiency can either be specified directly (constant or as a function of the turbine expansion ratio) or by means of the Turbocharger Overall Efficiency which is the product of compressor efficiency, turbine efficiency and mechanical efficiency. The turbine efficiency can be taken either from a full turbine operating map (if available), or from any equivalent information provided by the turbocharger supplier

For twin entry turbines and multiple entry turbines, the reduction of the turbine efficiency due to the unequal flow distribution at unequal pressure ratios across the flows is taken into account by a reduction of the turbine efficiency. Figure 4-59 shows the factor by which the turbine efficiency is multiplied depending on the pressure ratio between the flows.

Figure 4-59: Deterioration Factor of a Twin Entry- or Multiple Entry Turbine

) ^Note:

The turbine efficiency output in the global results or in the transients is the mass flow weighted average of the calculated efficiency over one cycle.

For the BOOST Pressure Calculation the Pressure Ratio at the compressor only represents an initial value for the start of the calculation. Similarly, the Turbine Size Multiplier only represents an initial value for the Turbine Layout Calculation and the Turbine to Total Massflow an initial value for the Waste Gate Calculation.

In the case of a twin entry turbine or a multiple entry turbine, an inlet flow coefficient must be specified in order to describe the interference between the attached pipes. The inlet interference flow coefficient is related to the cross-section of the pipe representing the turbine inlet. For radial type turbines, an inlet interference flow coefficient of 0.2 is recommended and for axial type turbines a value of 0.05 is recommended.

The attachment type of each pipe (compressor inlet/outlet, turbine inlet/outlet) is known from the sketch of the model and can be checked in the Pipe Attachments sub-group. If it needs to be changed, reattach the pipes to the correct side of the turbocharger.

4.14.1.2. Full Model

Mechanical Efficiency

The ratio between the torque available at the compressor end of the driveshaft related to the driving torque of the turbine. It may be defined as a function of the turbocharger wheel speed by means of

a Table .

Moment of Inertia The MOI related to the compressor drive shaft may be specified in different units using the pull down menu.

Initial Speed As an option the setting of the initial rotor speed is available. If not selected the intersection of initial compressor pressure ratio with the surge line is used for initialization.

This model requires the input of the entire compressor and the entire turbine map.

4.14.1.2.1. Compressor

For the specification of the compressor map points with the following data has to be input by the User:

Corrected compressor speed

Corrected mass flow or volume flow

Pressure ratio across the compressor (total to total) Isentropic compressor efficiency (total to total)

Corrected Speed The compressor speed related either to the square root of the inlet temperature (if No Reference is set) or related to the square root of the ratio of the reference temperature to the inlet temperature (if Reference is set). In any of the two cases the suitable unit has to be set with the pull down menu.

Corrected Mass Flow

The mass flow through the compressor times the square root of the entry temperature divided by the inlet pressure. If a reference condition is specified it is the ratio between the entry conditions and the reference conditions to which the mass flow is related.

Corrected Volume Flow

Similar to mass flow, the volume flow at the entry divided by the square root of the entry temperature.

Massflow Scaling Factor

The x-axis of the compressor map may be scaled.

Efficiency Offset The compressor efficiencies specified may be modified additively.

Reference Conditions

In any case the pressure and temperature have to be defined together with the units by the User.

In document BOOST UsersGuide (Page 171-175)