2-stage turbo match

8/2/2019 2-Stage Turbo Match

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Ricardo Josef Boek Research Center

2-Stage Turbocharger Matching for a Light Duty Diesel Engine

The main aim of this work is to develop and describe detailed and easily applicable method for two

stage turbocharger matching. It means finding an optimal twin of turbochargers low and high

pressure ones. This work describes techniques that can be used for compromise solution following

from minimum fuel consumption and maximum transient response together with satisfying of all

given constraints such as maximum turbine and compressor temperatures, turbochargers speed,

maximum combustion pressures and smoke limit. Moreover the engine has to reach also the target

power curve.

Two acceptable configurations are described and compared in the next part of this paper. Slightly

different turbocharger control strategies are used to obtain the same power curve. The first

configuration is equipped by inter-stage cooler and only one actuator controls the engine. The second

configuration without inter-stage cooler has to be controlled by two controllers that enable complete

high pressure turbocharger (HPT) by-passing at highest engine speed.

Engine

The engine is internally called JRC engine with swept volume of 2 dm3. This currently developing

engine has an unusual engine block. The block structure is constructed from cross-beams and is called

skeletalfor its shape. This construction is used in order to provide weight saving. The plastic cover is

planned to keep oil filling inside. This project is a part of cooperation between Ricardo Inc. and Josef

Bozek Research Centre. The target specific power of 77.5 kW/L moves the currents engines

performance limits any further. The 2-stage turbocharging is necessary to provide required boosting

pressure. The Figure 1 shows the described skeletal block.

Boost Control Elements

Three main control elements were chosen for boosting system governing see Figure 2.

Hight pressure turbine by-pass (HPT by-pass)

Low pressure turbine waste-gate (LPT waste-gate)

Hight pressure compressor by-pass (HPC by-pass)

HPT by-pass is in principle the most important actuator. Ideally it could control engine alone without

any other control elements help. It decreases the boosting pressure within high pressure stage and so

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Figure 1: Cylinder block for highly turbocharged diesel engine JRC (Josef

Bozek Research Engine).


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controls whole system. LPT waste-gate is the second in importance sequence. It decreases the LP

turbocharger speed and avoids its over-speeding. HPC by-pass enables complete by-passing of HP

stage together with HPT by-pass. In other words said it is able to switch off the hight pressure stage.

This usually happens at high engine speed when the HPC is too small for extensive mass flow.

Thus the engine controlling management can be assembled from combination of only one, two or

three actuators.

Turbocharger Matching Introduction

Compressors and turbines are defined as maps within simulation 1-D/0-D codes like Wave or GT-

Power. A compressor / turbine change demand means a changing of particular map. The constructing

of each map consists of filling of column speed mass flow pressure ratio efficiency matrix and

its interpolation respectively extrapolation. Different compressor or turbine means different map. It is

clear that the manual optimisation (testing of available compressor and turbine maps) is time

consuming even for the single stage turbocharging. On the other hand it is commonly used. One

turbocharger matching has two optimised variables (degrees of freedom) compressor and turbine

map. The number of all variants for manual match could be still acceptable even it is time consuming.

There exists an approach that can fasten the turbocharger match. It uses mass flow multipliers for

scaling of TC maps. The multiplier scales directly the mass flow column of the map matrix and a

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Figure 2: 2-Stage turbocharger engine scheme of diesel engine with three highlighted mainboost control elements.


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fictive compressor / turbine is created then. Its parameters like efficiency, pressure ratio, speed, surge

line, etc. do not correspond to the reality, but when the multiplier is chosen reasonably (usually not

too high or low) this approach can be used with good results. Resulting mass flow multiplier helps to

find the closest TC map from production series. The neighbouring maps should be subsequentlychecked whether the chosen TC is really the optimum choice. The two multipliers map dependences

could be easily created - for example x-axis ~ compressor multiplier and y-axis ~ turbine multiplier

for chosen engine speeds. Desired parameters and constraints could be plotted within these maps and

their intersections show the acceptable operating area for compressor and turbine size.

2-Stage TC matching

When described approach is applied to a 2-stage TC system it gives 4 degrees of freedom (optimised

variables): two compressor multipliers and two turbine multipliers. It is not easy to find and optimum

within 4 dimensional space with respecting of all constraints (TC max. speed, turbines and

compressors temperatures, Pmax, etc.). One of possible approach of doing this is described below in 3steps (approximations).

Step 1: Finding of a working TC configuration

The first step of a 2-stage turbocharger system building is a finding of a relatively working twin TC

(non-scaled) configuration. The point is that the constraints should not be overstepped significantly

even some of them will be probably slightly out of range. Ricardo database twin turbo 1.9dm 3 engine

turbochargers were used for the first approximation in this case. The assumption is that this couple

turbochargers should work with developing 2dm3 engine.

Step 2: A rough TC matchingThe second step deals with a rough TC matching. The number of 4 degrees of freedom to optimise is

too high that is why the following simplification is chosen. The number of optimising parameters is

decreased to number 2. These chosen variables are: mass flow multiplier of the high TC stage (the

same multiplier used for HPC and HPT) and mass flow multiplier of the LP stage (the same number

applied to both LPC and LPT). The maps of selected parameters are cross-plotted for these two

optimised variables (x-axis HP TC mass flow multiplier and y-axis LP TC one). The Figure 3 shows

maps for different engine speeds.

The LP, HP = 1 present non-scaled couple of turbochargers belonging to 1.9dm3 engine. The coloured

contours separate the areas where the required BMEP (Table 1) is matched. Compressor stalling

operation cases shown in Figure 3presents another constraint which has to be take into account. The

stalling magnitude has to be 0 (no stalling) ideally at all engine operations.

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Table 1: Target parameters for 2-stage TC engine. BMEP - Brake mean effective pressure, AFR - Air to fuelratio defines the smoke limit.


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Air to fuel limit values that presents the smoke limit shown in Table 1 are chosen according

experience with using common rail injection system. A further decreasing of these numbers would

lead to the extensive smoke production.

The overall summary of varied parameters, constant and optimised variables in the 2 nd step are

collectively shown in Table 2.

The control is primary undertaken by HPT by-pass that keeps (tries to keep) the full load BMEP

according Table 1. LPT waste-gate avoids LP stage over-speeding if this menaces. The AFR is held

constant according Table 1. Thus the target BMEP and stalling magnitudes are optimised parameters.

Note that the magnitude of compressor stalling is defined as a fraction of the cycle for which

compressor was operating beyond the stall limit. Stalling magnitude of 0 means no stalling and thevalue of 1 full stalling. The aim is to have compressors without any stalling.

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Figure 3: TC mass flow scaling maps for a rough 2-stage TC matching. X-axis: HP (High pressureturbocharger mass flow multiplier), Y-axis: LP (Low pressure mass flow multiplier).

Table 2: Summary of varied, constant and optimised parameters for the second - therough TC match.


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Figure 4 shows an intersection across engine speeds where the target BMEP was reached for two

tested configurations - with and without inter-stage cooler. The variant without inter-stage cooler

predicts HPC temperatures above limits at both inlet (~200C@3500min-1) and outlet

(~300C@4500min-1) compressor side. The common compressor limit is around 180C at its outlet.

This configuration is not further optimised for this purpose and a possible feasibility of its usage will

be discussed in the next chapter.

Generally the compressors stalling reduction can be achieved by compressor size reduction it means

using of a smaller mass flow scaling factor. Turbine size remains unchanged then.

The particular compressors and turbines maps were chosen from Garrett database according to

optimised mass flow multipliers - Figure 4.

Step 3: Fine TC matching

The TC maps were chosen mainly on the BMEP basis in the previous step. The fully functional

system satisfies all main requirements and constraints defined this way. However the fuel

consumption or preliminary transient response was not taken into account. Moreover the using of twoidentical multipliers would not be able to find the best solution of TC maps. Hence fine and detailed

TC match is performed now, when the system is expected near its optimum. The 4 parameters

optimisation is selected. It means a finding of 2 compressor multipliers and 2 turbine ones. The range

for these multipliers stays between 0.8 and 1.2.

WaveDoE tool was used for an optimum mass flow multipliers finding. WaveDoE is an SPM

(Stochastic process model) tool that creates substitutive model of given system from in our case

computed working points defined in DoE matrix. The filling of DoE matrix is done by number of

Wave runs in our case 200. The varied parameters for DoE matrix are LPC, LPT, HPC and HPT

mass flow multipliers see Figure 5. The variables in DoE matrix are used as an input for SPM model

of chosen parameters creating fuel consumption and BMEP. The graphical output of SPM system is

then used for the optimum multipliers finding.

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Figure 4: Maps showing the area where the required BMEP is reached for configuration with and without inter-stage cooler.
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The SPM model was created for the 2 key engine speeds (2000 and 4000 min -1) to save Wave

computational time. It means 2x200 Wave run points.

Resulting SPM models for both engine speeds with optimised points are shown in Figure 6. Black

points correspond to turbocharger maps chosen in step 2 it means all four mass flow multipliers

equals number 1. The arrows show more suitable (better) operating point that is logically different for

both engine speeds. From Figure 6 follows that the rough turbocharger match was quite sensitive. The

maps for lower 2000 min-1 engine speed recommends smaller turbochargers and at 4000 min -1bigger

ones that confirms at about compromise setting. The final resulting compromise mass flow multipliers

are shown in Table 3. From this table follows that only LPC and HPT are changed. The mass flow

multipliers presented there were used for finding of an appropriate LPC and HPT maps from Garrettdatabase.

Wave model is controlled by two PD (proportional-derivative) controllers where the first one sets therequired BMEP using HPT by-pass actuator and the other one watches about LP TC speed (prevents

over-speeding) using of LPT waste-gate actuator.

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Figure 5: Design of experiment for 4 variables (LPC, LPT, HPC and HPTmass flow multipliers). The blue points present the 200 Wave runs for SPMmodel creating.

Table 3: Optimal mass flow multipliers for 2-stage TCsystem. Chosen from SPM model.


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Figure 6: SPM model of BSFC at 2000 and 4000min-1. The arrows show an optimum setting forparticular engine speed.


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Control Variants

The previous chapter refused the usage of the 2-stage boosting system without inter-stage cooler

because of too high temperatures at compressor. There exists a possibility of control strategy

adjustment to reduce this temperature. It has to be tested by simulation run, because it is not

guaranteed that it will really work. The aim of it is to fully by-pass the HP turbocharger at high

engine speeds and so decrease HP temperature. This will work only if the LP turbocharger is alone

able to provide enough boosting pressure to meet the given performance targets. For that reason a

HPC by-pass has to be set into the boosting engine hardware, of course it is places between HPC and

LPC. The simulation runs show that this variant really should work. However a higher temperature of

190C is indicated at HP compressor but this should be enabled in certain circumstances, because the

common compressor temperature maximum stays around 180C.

Table 4 describes the two available variants (configurations) of 2-stage system controlling with main

control elements description. The 1st configuration is the one used for TC matching and it is equipped

by only 1 boost pressure controller the HPT by-pass and it has inter-stage cooler. The 2nd

configuration has 2 boost pressure control elements - HPT and HPC by-passes and it has not any

inter-stage cooler. The LPT waste-gate remains closed in both variants and could be taken off since

the LP TC maximum is not reached.

The schemes of both variants are shown in Figure 7. The actuators are highlighted by green

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Table 4: Chosen control variants of 2-stage turbochargerboosting system.

Figure 7: The schemes of chosen control variants for 2-stage turbocharger boosting system of a diesel engine.


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background colour. The simulation results (the runs of selected output parameters and actuator

values) are presented in Figure 8. They simply explain the main differences between both

configurations. The blue line demonstrates the 1st variant with one HPT by-pass actuator + inter-stage

cooler and the red line illustrates the complete by-passing of HP turbocharged of the 2nd variant athigh engine speeds (more than 3000min-1) by two actuators HPC and HPT by-pass. Presented lines

come up to full load where the turbocharger match was performed as well.

A fuel saving of 2nd configuration is visible at highest engine speeds. Other parameter runs are not

shown here for space saving. However it should be claimed that all engine parameters for these

presented variants stay within all watched constraints except already mentioned slight higher

compressor temperature in the 2nd configuration.

The compressor maps for both configurations are shown in Figure 9 and Figure 10. The operating

points of HPC and LPC with the total pressure ratios are shown in there, as well.

The second variant HP turbocharger speed is quite low (at about 20 000min-1) at high engine speeds. It

would be better (from transient response point of view) to keep a high HP TC speed at all engine

speeds. These low TC speed would cause low transient response, especially at full load, when engine

speed decreases down to 3000 min-1. There will be probably a torque drop down caused by HP

turbocharger speed increase up to high values. This situation could be easily evaluated as HP turbo

lag. The HP TC speed could be raised by HPT by-pass closing (HPC by-pass stays opened no need

of additional boosting pressure). But when this is performed, the HPC temperature increases even itgives almost no compression. So in fact the HP TC has to be by-passed as much as possible to push

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Figure 8: Full load comparison of the two selected control variants for 2-stage turbocharged engine.


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down the compressor temperature.

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Figure 9: 1st configuration: One HPT by-pass actuator + inter-stage cooler.

Figure 10: 2nd configuration: HPT and HPC by-pass, no inter-stage cooler.


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From Figure 8 and above comments follows that each configuration has its advantages and

disadvantages. The first variant will be definitely more expensive which is caused by the inter-stage

cooler. But just one control element (HPT by-pass) makes the setting of ECU (electronic control unit)

quite easy. However a theoretical trouble with transient responses could be indicated here as well.The setting of inter-stage cooler and the ducts connecting it to the both compressor means quite big

cumulative volume that will probably cause a significant transportation lag.

The second configuration without inter-stage cooler will be cheaper and without transportation lag.

But already mentioned HP turbocharger lag at full load when decreasing engine speed would impact

the transient response significantly. Moreover the usage of two actuators is more ECU demanding

(more difficult to control and set up the engine management). Higher compressor temperature could

be probably accepted as was explained before.

The summary comparison with smiles is shown in Table 5. The question mark warns of the fact that

this should be further investigated either by simulation or measurements.

Conclusions

This work presents an easy applicable procedure which could be easily used for 2-stage turbocharger

matching. From the content follows that there exist more working configurations which give

reasonable results. Two of them were picked up and compared together. LPT waste-gate was not used

in any of those presented variants.

Acknowledgement

This work has been supported by the Josef Boek Research Center, No. LN00B073. This help is

gratefully appreciated.

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Table 5: Pro-and-cons comparison for the two described 2-stageboosting variants.

2-stage turbo match

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