boosting the starting torque of downsized si engines gt ...guc2002-fev.ppt boosting the starting...

GT-Suite Users Conferences 2002

Rohs/Habermann/Lang/Rauscher/Schernus 1

Boosting the Starting Torque of Downsized SI EnginesGT-Suite User‘s Conference 2002

Hans RohsInst. For Combustion Engines (VKA)RWTH Aachen

Knut Habermann, Oliver Lang, Martin Rauscher, Christof SchernusFEV Motorentechnik GmbH

Acknowledgement:Some of the presented results are taken from a research program funded by the European Community (GRD1-1999-10272)

Good morning Ladies and Gentleman.

This presentation shows results of a study about boosting the starting torque of downsized SI engines.

First, I want to acknowledge the contribution of my fellow co-authors Knut Habermann, Oliver Lang, Martin Rauscher and Christof Schernus and please note some of the presented results are taken from a research program funded by the European Community.

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tBoosting the Starting Torque of Downsized SI EnginesContents

Turbocharging Challenges of SI Engines

Additional Boosting Device: e-booster

Additional Boosting Device: Roots blower

System Comparison

Summary

Now, let me then outline the structure of this presentation.

As an introduction I’d like to give a brief overview about the challenges regarding the turbocharging of SI engines especially under downsizing circumstances.

Then I’m going to show you how these challenges can be met with the implementation of an additional boosting device, either an e-booster or a Roots blower.

For both of these I will discuss steady state and transient system performance and a comparison of the two systems will be presented.

Finally I will summarize the results.

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tBoosting the Starting Torque of Downsized SI Engines

rpm

BM

EP

rpm

BM

EP

TC

NA

6 8 10 126

8

10

12

14

16

18

20

nat. aspirated SI engines

Turbos

BM

EP

@ 2

000

rpm

(ba

r)

BMEP @ 1000 rpm (bar)

Starting Torque Characteristics of TC and NA Engines

This plot shows the starting torque characteristics of turbocharged and naturally aspirated SI engines.

To illustrate how to read this plot two sample full load curves are added. The

main plot contains the steady-state full load BMEP values for 1000 rpm 8

and 2000 rpm.88

Therefore the further to the upper left an engine is located in this plot the steeper the descend of BMEP towards lower engine speeds is.

Compared with naturally aspirated engines the turbocharged engines generally have a lower relative BMEP at 1000 rpm.

Now, if the turbocharging is applied as part of a downsizing concept this results in less then desired torque at very low engine speeds even if torque and power at higher engine speeds match or even surpass those of a non downsized naturally aspirated engine.

And this gets even worse if you look at the transient behaviour of such an engine concept because of the turbo lag.

The goal of this study is to show how both, the steady-state and the transient performance of a downsized SI engine, can be significantly improved with the implementation of an additional boosting device.

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tBoosting the Starting Torque of Downsized SI EnginesSimulation model

Intercooler

Intake manifold

Exhaust manifold

Wastegate

TC

Exhaust system

��

�

�

��

��

��

BMEP at full load

BM

EP

(ba

r)

Engine Speed (rpm)

This GT-Power model of an 1.8 litre 4 cylinder turbocharged SI engine isused as the base model for these investigations.

The highlighted components are8 the turbocharger, 8 the intercooler, the

8 intake and exhaust 8 manifolds, 8 the wastegate and 8 the exhaust system.

8 The calculated full load BMEP curve of this engine illustrates the lack of

starting torque. While at slightly above 2000 rpm 8 the engine has a BMEP

of 20 bar this drops to approximately 8 11 bar at 1000 rpm.

To evaluate the transient behaviour of this engine we have performed load step simulations at multiple engine speeds.

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0

4

8

12

16

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.4

0.8

1.2

1.6

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.8

1.0

1.2

1.4

1.6

transient BMEP steady state BMEP

(bar

)

TC shaft speed

1000

rpm

boost pressure

(bar

)

Time after load step (s) Time after load step (s)

TC compressor

PR

Turbo Engine Load Step Response Example

Load Step BMEP=2 bar to Full Load @ 1750 rpm = const.

For instance, this slide shows the load step response at 1750 rpm from 2 bar BMEP to full load.

The transient model is set up so that the simulation converges at a base operating point of 1750 rpm and 2 bar BMEP. Then at the start of the load step the throttle is opened and the wastegate is closed (both within 0.2 seconds) and the engine speed is kept constant.

This slide shows the engine BMEP in the upper left plot 8 , the boost

pressure in the lower left plot8 , the Turbocharger shaft speed in the upper

right plot8 and the Compressor pressure ratio in the lower right plot8(all plotted versus the time after the load step).

With the opening of the throttle the boost pressure instantly increases up to the ambient pressure like a naturally aspirated engine would behave, but from that time on the pressure increase continues much slower.The turbocharger takes time to get up to speed from the initial value of below 50000 rpm up to the steady state full load value of approximately 100000 rpm.

Therefore even 3.5 seconds after the load step the engine hast not reached its steady state BMEP value.

To generate a map of the transient capability of the engine we have selected

discrete time values 8 after the load step and marked the respective reached operating points in the full load BMEP map.

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1000 2000 3000 4000 5000 60000

2

4

6

8

10

12

14

16

18

20

22

Turbocharged SI Engine steady state full load

BMEP response after 0.0 s (BMEP=2 bar) 0.25 s 0.5 s 1.0 s 2.0 s 3.0 s

BM

EP

(ba

r)

Engine Speed (rpm)

Steady State Performance and Load Step Response

Lo

ad S

tep

@ 1

750

rpm

0.00 s

0.25 s0.50 s

1.00 s

2.00 s3.00 s

By repeating the load step calculation at different engine speeds, we gain a map of both, steady state performance and load step response.

If you have a look at the dark blue line for instance you can read the maximum BMEP 1 second after a load step from 2 bar BMEP which issignificantly lower then the full load value.

To use this engine in a downsizing concept the low end torque and the transient performance would both need a significant boost.

To achieve this, several solutions come to mind. As the low performance is mainly a result of insufficient air supply a modification of the boosting system is necessary.

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tBoosting the Starting Torque of Downsized SI EnginesRequired Boost Pressure in Compressor Map

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.201.0

1.5

2.0

2.5

3.0

desired boost pr.

steady state full load

Surge L

imit

180000 rpm

140000

100000

60000

40 %

60 %

50 %

η = 70 %

75 %

corr. flow rate (m3/s)

Com

pres

sor

PR

If we have a look at the compressor map with the steady state full load curve

and 8 the desired boost pressure, we can see that an electrically assisted turbocharger won’t be capable of increasing the steady state BMEP at low engine speeds because the operating points of the base engine are already very close to the surge limit.

Thus the application of an additional boosting device is more promising.

As I said in my introduction we have investigated the use of an additional e-booster as well as an additional roots blower.

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tBoosting the Starting Torque of Downsized SI EnginesImplementation of e-booster in GT-Power model

Bypass

Turbocharger

E-Booster

Electrical load

This picture shows the implementation of the e-booster in the GT-Power

model. The turbocharger 8 remains the same as in the base model.

The e-booster 8 is applied upstream of the turbocharger and consists of the compressor, a free shaft and a torque object.

The respective electrical load for 1.0 kW effective compressor power is

subtracted from the engine cranktrain 8 for steady state operation.

A bypass 8 is provided for operating points where the e-booster is idling, switched off or causes a pressure loss (This can occur in transient operation).

For the load step simulations the following assumptions have been made:

At 2 bar BMEP the bypass is fully opened and the e-booster is idling. This is realised in the GT-Power model by running the e-booster at as little speed as possible in the provided compressor map.

At the start of the load step a constant power of 1.0 kW is provided instantly while the closing of bypass and wastegate as well as the opening of the throttle take 0.2 seconds.

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Additional e-booster: Full load and load step response

1000 2000 3000 4000 5000 60000

2

4

6

8

10

12

14

16

18

20

22

Turbo + e-booster steady state full load


BM

EP

(ba

r)

Engine Speed (rpm)

This figure shows the response map of the turbo engine with the e-booster compared to the results of the engine equipped only with the turbocharger,

the latter ones being dimmed in the graph. 8

This shows a significant increase in steady state and transient performance.

8 The area between the black and the grey line marks the gain in steady state full load BMEP. At 1000 rpm the system with e-booster has more then 15 bar BMEP which is an increase of approximately 4 bar. The maximum BMEP of 20 bar is reached at 2000 rpm versus 2250 rpm without the e-booster.

8 A comparison of the transient results shows that, while for the first quarter of a second there are no benefits detectable, half a second after the load step the system with e-booster begins to pull ahead.

8 One second after the load step the system with e-booster already has a BMEP of 11 bar at 1000 rpm which equals the maximum steady state value for the turbocharged base engine at this speed.

8 Another second later the system with e-booster has 2 to 4 bar more BMEP for the whole engine speed range below 2000 rpm.

8 And this stays that way further on, illustrated here, three seconds after the load step.

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tBoosting the Starting Torque of Downsized SI EnginesE-booster steady state @ engine speeds 1000-2000 rpm

0.00 0.01 0.02 0.03 0.04 0.05 0.061.0

1.2

1.4

1.6

engine speed

surg

e

100000 1/min

120000 1/min

70 %

60 %

compressor wheel diameter: 37 mm

PR

corr. flow rate (m³/s)

0.00 0.01 0.02 0.03 0.04 0.05 0.061.0

1.2

1.4

1.6

60 %

70 %100000 1/min

surg

e

120000 1/min

compressor wheel diameter: 43 mm

corr. flow rate (m³/s)

Comparison of Compressor Wheel Diameters 37 and 43

Our simulations have shown that choosing the right compressor size for the e-booster is critical.

The plot shows an efficiency map for for a compressor with a wheel diameter of 37 mm and the operating points for steady state full load from

1000 rpm (8 the upper left point) to 2000 rpm (8 the lower right point).

This configuration results in compressor speeds up to 120000 rpm which is above the current technical limit of 100000 rpm maximum for electric motors used in e-boosters.

With an increase of the chosen wheel diameter to 43 mm 8 the required speeds drop to more favourable values but the operating point at 1000 rpm

8 engine speed is getting close to the surge line.

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tBoosting the Starting Torque of Downsized SI EnginesRequired Kin. Energy for Transient E-booster Operation

0 200 400 600 800 1000 1200 1400

1.00

1.05

1.10

1.152.0 s

1.0 s

0.25

s

0.5 s

8000

0 1/

min

6000

0 1/

min

d = 37 mm d = 43 mm

Com

pres

sor

PR

Energy (J)

Load Step BMEP=2 bar to Full Load @ 1750 rpm = const.

Choosing a wrong compressor size can also have negative impacts on the transient behaviour.

This plot shows the required kinetic energy for the e-booster during a load step from 2 bar BMEP to full load at 1750 rpm.

Marked are the time since the load step with the green lines and compressor speeds with the blue lines.

The black line represents a compressor with a wheel diameter of 37 mm and the dashed red line with 43 mm.

8 Both are operated with a starting speed of 40000 rpm which equals a kinetic energy of about 200 J. The slight differences in the kinetic energy are due to the different moments of inertia.

8 The bigger Compressor reaches a pressure ratio of 1.05 after an increase

in the kinetic energy of approximately 200 J. 8 By contrast the smaller compressor requires more then double that amount to reach the same pressure ratio due to the much higher speed needed.

On top of that the pressure ratio of the smaller compressor even drops to 8values below 1.0 for a significant amount of time because the compressor speed is not high enough for the required air mass flow.

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Breathing characteristics similar to engine:

high boost @ low revs

Simple: purely speed controlled

Booster engaged only at low revs:

½No delay at start

½No turbo lag

½Boost pressure gradient only depends on throttle

½Small power consumption

Small blower operating speed range ⇒ high average efficiency

Large gear ratio ⇒

compact booster

1000 1500 2000 2500 30000

4000

8000

12000 Gear Ratio Engine/Booster = 1:4

Disenga-gement

Engagement

boos

ter

spee

d (r

pm

)

engine speed (rpm)Long full load

operation

Additional Roots blower

Secondary airinjection capabilityLow backpressure

sensitivity

Let me list some of the properties of this system.

The Roots blower, which has the benefit of8 similar breathing characteristics to the engine, is connected to the belt drive with a clutch

which can be engaged and disengaged automatically at selected speeds. 8

That way a high gear ratio can be chosen to support the turbocharger at low engine speeds and the roots blower can be disengaged at higher speeds where

the turbocharger is in full effect.8 This results in no delay at the start of the blower and thus no turbo lag. The boost pressure gradient only depends on the throttle. The low speeds mean low power consumption.

8 Additionally the roots blower has secondary air injection capability and low bachpressure sensitivity.

8 As the blower is operated only in a small speed range it can be optimized for a high average efficiency.

8 As the required power is provided by the engine itself there is no problem with long full load operation.

8 The large gear ratio results in a compact booster.



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tBoosting the Starting Torque of Downsized SI EnginesImplementation of roots blower in GT-Power model

Bypass

Turbocharger

Roots blowerMechanical connection

The implementation of the roots blower into the GT-Power model is handled the same way as with the e-booster.

The Roots blower 8 is applied upstream of the turbocharger 8 and a

bypass 8 is provided.

The compressor object is connected to the engine crankcase with a gear connection and an additional torque object is implemented to simulate the mechanical friction of the Roots blower.

Again for 2 bar BMEP the bypass is fully opened and the Roots blower is disengaged. In the GT-Power model the gear ratio of the gear connection is set as low as possible with the available compressor map data. The Torque object to simulate the friction is disconnected entirely.

Again the switching of throttle, bypass and wastegate takes 0.2 seconds and in the same amount of time the clutch is engaged, meaning that the gear ratio for both the Roots blower and the torque object are set to the chosen ratio of 4.2 : 1.

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1000 2000 3000 4000 5000 60000

2

4

6

8

10

12

14

16

18

20

22

Turbo + Roots steady state full load


BM

EP

(ba

r)

engine speed (rpm)

Additional Roots blower: Full load & load step response

This plot shows the simulation results for the system with roots blower. 8

An overlay of the results of the base engine8 reveals a similar performance increase as with the e-booster.

8 The steady state BMEP at 1000 rpm is increased to 15 bar and the maximum BMEP of 20 bar is reached at 2000 rpm.

We can see, that in transient operation it is desirable to keep the Roots blower engaged for speeds up to 3000 rpm. The load step simulation results are even more impressive then with the e-booster.

8 After half a second the system with the Roots blower has gained up to 3 bar BMEP versus the base engine..

8 Another half a second later it has already surpassed the steady state full load curve of the base engine for all speeds below 2000 rpm.

8 Two seconds after the load step the BMEP is approaching its steady state full load values for engine speeds above 1500 rpm.

Now, what is the cause for this impressive behaviour?



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0.2 s0 s

1.4 s

0.4 s

0.2 s

speed gain TC

pressur e increase r ootsblower

clutch engagement

max transient PR

Steady state and transient operation in Roots map

0.00 0.01 0.02 0.03 0.04 0.05

1.0

1.1

1.2

1.3

1.4

2000

1750

1250 engine speed (rpm)

steady state full load,

1500

1000 effective p

ower rootsblow

er (kW)

speed rootsblower (rpm)

55 %50 %40 %

8000

6000

4000

2000

2.0

1.5

1.0

PR

volumetric flow rate (m3/s)

Lets have a look at the compressor map of the Roots blower.

The black squares connected with the black line are the operating points at steady state full load. The red lines mark the effective power of the Roots blower which is about 1.0 kW at 1000 and 2000 rpm Engine speed and up to 1.3 kW in-between. Now, what happens during a load step, for example at 1000 rpm engine speed?

8Within the first 0.2 seconds the clutch is engaged and the Roots blower is accelerated to the respective speed of 4200 rpm with the gear ratio of 4.2. So far the pressure ratio stays at 1.0 as the bypass is just closing.

8 Now, with the throttle fully opened and wastegate and bypass closed, the Roots blower quickly builds up pressure. 1.4 seconds after the load step the

pressure ratio is above the steady state value at this speed.8 Further on as the turbocharger gets up to speed it slowly drops back to the steady state value.

8 If we repeat this procedure for multiple engine speeds and connect the

operating points with maximum pressure ratio 8 we can see that the transient operating range of the Roots blower significantly surpasses its steady state operating range.

Especially at 2000 rpm engine speed the maximum effective power of the Roots blower is more then double the steady state value.

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tBoosting the Starting Torque of Downsized SI EnginesSteady state performance w/o and w/ additional booster

��

�

��

��

��

Turbo only Turbo + e-booster Turbo + Roots

BM

EP

(ba

r)

engine speed (rpm)

A comparison of the steady state full load curves shows that at 1000 and 2000 rpm engine speed both systems deliver about the same BMEP.But in-between the Roots blower benefits from his bigger power supply. As we learned on the previous slide the Roots blower consumes up to 1.3 kW in these operating points while the e-booster has to stay at 1.0 kW maximum.

But both system significantly increase the performance of the engine. 8

For the transient comparison the load step from 2 bar BMEP to full load at 1750 rpm is chosen.

The steady state full load BMEP at 1750 rpm of all three systems given in

this plot … 8



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0

4

8

12

16

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.4

0.8

1.2

1.6

2.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.8

1.0

1.2

1.4

1.6

transient BMEP steady state BMEP

(bar

)

TC shaft speed e-booster speed

1000

rpm

Turbo Turbo + Roots Turbo + E-Booster

boost pressure

(bar

)

Time after load step (s) Time after load step (s)

TC compressor Roots e-booster

PR

Comparison of Load Steps @ 1750 rpm

… show up in this plot as dashed lines in the upper left diagram.

For both systems it is obvious, that the additional boosters have two major

benefits for the transient behaviour of the system. 8 They directly increase

the boost pressure after the closing of the throttle 8 and they increase the air massflow and the available exhaust energy resulting in a faster acceleration of the turbocharger during the load step.

However, while the full load values for the two systems with additional boosting devices are not to far apart at 1750 rpm, the transient performance shows another picture.

8 Half a second after the load step the system with the Roots blower has approximately 1.5 bar more BMEP then the system with the e-booster.

8 Another half a second later the lead increases to 2.5 bar.

The cause for this is visible in the lower right plot. 8 As the roots blower is forced to its full load speed immediately with the engagement of the clutch its pressure ratio rises above its steady state value, while the e-booster at the same time is still accelerating and only slowly gets up to its steady state pressure ratio.

This results in a significantly higher boost pressure with the Roots blower.

Three seconds after the load step the system with the e-booster catches up to the system with the roots blower.

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tBoosting the Starting Torque of Downsized SI EnginesSummary

½ Supplementary booster supports TC compressor and enhances

the available exhaust energy

⇒ faster TC acceleration

½ E-booster developed to good maturity, easy integration but

demanding to power supply and battery

½ Additional mechanical booster provides high boost pressure at

low engine speeds (transient & steady state). High gear ratios

enable compact boosters to be integrated into belt drive.

½ Generally, additional costs for the additional booster have to be

small to achieve good acceptance in the market

Let me briefly summarize the results of our study.

.

.

.

.

Thank your very much for your attention, I’m looking forward to your

questions. 8

boosting the starting torque of downsized si engines gt ...guc2002-fev.ppt boosting the starting...

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