Next generation of Gasoline turbo technologies

8/20/2019 Next generation of Gasoline turbo technologies

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33. Internationales Wiener Motorensymposium 2012

Dipl.- Ing. K-H. Bauer; C. Balis, M.S.E.G. Donkin, C.Eng; P. Davies, C.EngHoneywell Transportation Systems

The Next Generation of Gasoline Turbo Technology

Die nächste Generation der Benzin-Turbotechnologie

Abstract:

The progress in downsizing of gasoline engines in recent years has demonstrated the lim-its of conventional turbocharger design when it comes to providing more low speed tor-

que, transient response and partial load efficiency. The increased drive towards higherBMEP at very low engine speeds forces turbocharger engineers to rethink modern boost-ing layouts.

Honeywell Turbo Technologies has taken a fresh look at the design of the gasoline turbo-charger and has redefined the aerodynamic layout of both the compressor and the turbinestages. It has been able to increase overall turbo efficiencies, especially at low speedsand in transient conditions and this combined with substantially reduced mechanical iner-tias has provided significant improvements in engine transient torque response.

This presentation demonstrates a level of engine and vehicle performance that have neverbeen achieved with conventional gasoline waste gate turbochargers. The concept demon-


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strates breakthroughs in transient engine performance without the use of exotic materialssuch as Titanium Aluminide or the additional complexity of variable geometry turbines.

Kurzbeschreibung:

Die in den letzten Jahren erzielten Fortschritte beim Downsizing von Benzinmotorenhaben die Grenzen des konventionellen Turbolader-Designs aufgezeigt, wenn es daraufankommt, mehr Drehmoment bei niedrigen Geschwindigkeiten sowie ein effizientesEinschwing- und Teillastverhalten zu realisieren. Der zunehmende Trend zu einemhöheren BMEP bei sehr niedrigen Motorgeschwindigkeiten zwingt die Entwickler vonTurboladern, moderne Turbolader-Layouts zu überdenken.

Honeywell Turbo Technologies hat dem Design des Benzinturboladers ein neues Aussehen verpasst und das Aerodynamik-Layout sowohl des Verdichters als auch derTurbinenphasen neu definiert. Das Unternehmen konnte die gesamte Turboeffizienzverbessern, insbesondere bei niedrigen Geschwindigkeiten und in transienten Zuständen.In Kombination mit der wesentlich verringerten mechanischen Materialträgheit hat dies zumaßgeblichen Verbesserungen der transienten Drehmomentreaktion des Motors geführt.

Diese Präsentation weist ein Maß an Motor- und Fahrzeugleistung auf, das mitherkömmlichen Wastegate-Benzinturboladern niemals erreicht werden hätte können. DasKonzept demonstriert Durchbrüche im Bereich der transienten Motorleistung ohne denEinsatz exotischer Materialien, wie etwa Titanaluminiden, oder die zusätzliche Komplexitätvon Turbinen variabler Geometrie.


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1. Introduction

The main reason to boost any engine is to increase its‟ specific torque and power densityto drive downsizing and down-speeding, which in turn lead to better fuel economy whilstmaintaining the vehicles dynamic performance. Turbocharging has long been the stan-

dard technology used to boost diesel engines in passenger vehicles, On-Highway trucksand Off-Highway machines. The majority of gasoline engines however are still naturallyaspirated today, though the market penetration for boosted engines is growing rapidly.

The last 15 years have seen a strong move towards variable turbine geometry for diesel.However, fixed geometry waste gate controlled turbines have remained the standard forgasoline for several reasons. Higher exhaust gas temperatures in gasoline engines are ofcourse a factor, cost is another but the main reason is that the air mass flow varies muchmore than in a gasoline engine than in a diesel. A ratio of 80:1 from idle to rated power fora gasoline engine compares to just 6:1 in a passenger car diesel.

One of the primary challenges to further downsizing and down-speeding of gasoline en-gines is the necessity to preserve the vehicles‟ dynamic performance. The driver valuesthis as “fun to drive” and it must be maintained. At the engine level this translates to tran-sient torque performance. Any enhancements in boosting systems that improve the en-gines‟ transient torque response can be used to increase the levels of downsizing ordown-speeding. This in turn can realize the further reductions in fuel consumption andCO2 necessary to meet consumer and regulatory demands.

With this in mind, Honeywell Turbo Technologies (HTT) has developed a new aerodynam-ic concept called DualBoost™, that promises to make a step change in the industry. It

represents a paradigm shift from the classic aerodynamic solution of a single sided centri-fugal compressor and a radial inflow turbine that the industry has used for 35 years.

It uses a double-sided compressor wheel in combination with an axial turbine. It hasequivalent overall efficiencies to its conventional competitors but boasts higher turbineefficiencies under low speed unsteady conditions and up to 50% less rotating inertia with-out the use of exotic materials such as Titanium Aluminide or the additional complexity ofvariable geometry turbines. This means it still reaches regular steady-state targets but de-livers exceptional transient performance improving “time to torque” by 25-35% for thesame or better full-load steady-state torque and BSFC.

This paper presents both the concept and the major effects before going on to presentengine and vehicle results that substantiate these claims.


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2. Power Train Needs

In the ideal case the work done to accelerate a vehicle from state 1 to state 2 can be ap-proximated to the change in its kinetic energy. Also, the work done by the engine toachieve this can be considered to be the area under the Power vs. Time curve. For two

vehicles with different engines but identical performance, the work done must be equal ifthey are to perform in the same way.

Equat ion (i) - Vehicle Kinet ic Energy Equat ion (ii ) – Accelerat ion Power

This simple concept allows us to calculate the target Power, BMEP and Time to Torquecurves for a typical downsizing and down-speeding problem statement. The baseline usedis a modern 1.8L GDI gasoline engine with VVT developing 240 Nm (~17 Bar BMEP) @1750 RPM.

Figure 1 highlights the results for 3 cases that were studied.

a) Down-speeding 14% from 1750 to 1500rpmb) Down-sizing 11% from 1,8 to 1,6 Litrec) Combined case

Figure 1 :- Down -sizing targets

The numerical results are shown in Table 1 below. The combined case produces targetsof 30% increase in BMEP and 26% reduction in Time to Torque and a doubling of the

„boost slope‟ which offers turbo machine designers a challenging problem statement.


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Engine

SizeSpeed BMEP Torque

Time to

Torque

Time toTorque50-90%

BoostedTorqueSlope

Torque

@ 1s

[L] [RPM] [Bar] [Nm] [s] [s] [Nm/s] [Nm] Baseline 1.8 1750 16.8 240 2.70 2.13 42 168

a 1.8 1500 19.5 280 2.23 1.86 75 188b 1.6 1750 18.8 240 2.32 1.91 59 162c 1.6 1500 22.0 280 2.00 1.71 95 185

Table 1 :- Down-siz ing targets

3. Turbocharger Targets

A similar kinetic analysis can be applied to a turbocharger by replacing the mass-velocity(mv²) term for the vehicle with a polar moment of inertia-rotational speed (Iω²) term for theturbocharger rotor. Thus the equations become.

Equat ion (i ii ) - Turbo Kinet ic Energy Equat ion (iv) - Acc elerat ion t ime

and expanding the power term to brg compturbaccel P P P P gives

Equat ion (v) – Acc elerat ion t ime (expand ed)

3.1. Turbine Efficiency

Turbine efficiency is a function of Blade Speed Ratio (U/Co), where U is the turbochargerspeed and Co is the speed of the inlet gas. It has been degraded over the years becauseof the need for increasing compressor diameters as specific engine power increases aswell as the use of downsized „low inertia‟ turbines. This issue is exacerbated in a moderngasoline engine by operating the turbine in a highly pulsating flow environment.


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The bulk of the energy in the exhaust is in the high pressure portion of each pulse as seenin figure 3. A U/Co ratio of 0,2 on the arrival of a pulse at the start of a transient is not un-usual. Turbine efficiency at such conditions is normally poor making it difficult to extractenergy and accelerate quickly. Improving the turbine efficiency at low U/Co conditionswould clearly benefit both the transient and steady-state performance of the turbocharger

and engine.

Figure 2 :- Pressure, Mass f low & U/Co vs. Crankshaft rotat ions

3.2 Turbocharger Problem Statement

To conclude, in order to enable downsizing and down-speeding a new turbocharger de-sign is required that minimizes inertia, optimizes turbine efficiency at low U/Co and for agiven engine operating point, runs the turbocharger faster (higher U thus higher U/Co).

4. The DualBoost™ Concept

HTT went “back to basics” and questioned the traditional aerodynamic concept of a ce n-trifugal compressor paired with a radial turbine. Axial turbines have the advantage overradials of having better turbine efficiency at lower U/Co values (Fig 3a), especially when

the designer takes advantage of their intrinsically lower mechanical stresses to utilize non-zero inlet angles for the blade. They are also intrinsically low in inertia (Fig 3b).


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Fig. 3a :- Turb ine eff icienc y vs. U/Co Fig. 3b :- Inert ia vs. Turb ine Flow

Compressor s ide DualBoost TM

Turb ine s ide

Standard Ro tor

Figure 4 :- Out l ine of Standard an d DualBoo stTM Rotating Grou p

The DualBoost™ team at HTT has exploited all these phenomena and its‟ new axial tur-bine has better turbine efficiency at low U/Co and up to 50% less rotating inertia than anequivalent flowing radial turbine.

Pairing it with a double-sided parallel flow compressor serves multiple purposes. Firstly, itaccelerates the turbine further up the U/Co curve as its rotational speed is higher for agiven engine operation point than that of a conventional single wheel. Secondly it bal-

ances the aero-dynamic thrust load in the machine; to give a quasi „zero‟ axial load con-cept in steady-state and thirdly it has lower inertia again than an equivalent flowing, larger

Axial Radial

Radial

Axial


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diameter, conventional compressor. The result can be seen from the outline of the rotorgroups in Figure 4. The DualBoost™ while longer is clearly the „low inertia‟ concept andachieves this without using any exotic materials.

5. Engine Test Results

A DualBoost™ turbocharger has been tested against a conventional radial device. Thetesting took place on a Ford 1.6L I4 Gasoline GDI (λ=1) with Dual VVT.

Rated Torque 280 Nm (22 Bar BMEP) 1500-4500 RPM

Peak Power 132 kW @ 4750-5500 RPM

5.1 Steady-state & Transient Load Steps

Both turbochargers were sized and matched to have the same corrected mass flows at a2:1 expansion ratio. Fig. 5a shows that both were capable of achieving the target full-loadsteady-state torque and power target. The full data showed that they had similar Engine ΔP and BSFC as well. Fig. 5b however, shows the real difference between the two de-vices. In a load step from 1500rpm, the transient torque curve for the DualBoost™ risesmuch more steeply than for the standard turbocharger. 180Nm was reached 450ms earlierand 270Nm was attained more than 600ms before the baseline.

Figure 5a:- Steady-State Performance Figure 5b:- Transient Torqu e

Combining the load step results from different engine speeds the overall improvement thatthe DualBoostTM delivers can be summarized in Figure 6, in the form of „Time from 50 -90% Torque‟. The effect of the new architecture widens dramatically at lower enginespeeds. This is due to the ever reducing amount of turbine exhaust energy available toaccelerate the rotor group and the increasing significance that reduced inertia has at theseoperating points.

450ms

600ms

Load step from 1500rpm1,6L I4 Gasoline

Standard Turbo

DualBoostTM


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Figure 6:- Summary of Transient Performance

5.2 Fuel Economy simulation

At this stage of the project the engine calibration has not yet been optimized sufficiently togo into formal vehicle testing. Honeywell has however had the opportunity to use full ve-hicle simulation to assess the potential impact of the DualBoostTM superior performanceon fuel economy.

The baseline Powertrain had a Final Drive Ratio (FDR) 4,067. It was calculated that leng-thening the FDR to 3,8:1 would be sufficient to neutralize the transient advantage of theDualBoostTM but still respect the launch performance and gradeability of the baseline ve-

hicle.

Four principle cycles, NEDC, FTP75, US06 and Highway cruise at 70mph were studied.The results, in Figure 7, show that Fuel Economy can be expected to increase in therange of 1,8 – 2,7% across these cycles. The more dynamic cycles like FTP75 and US06naturally show the largest improvements.

450ms

1000ms

DualBoostTM

Standard Turbo

1,6L I4 Gasoline


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Figure 7:- Fuel econ omy w ith sho rter FDR

6. Vehicle Test Results

A production vehicle equipped with a 2.0 l 155 kW gasoline engine and a competitor‟s production turbocharger was chosen to study the advantages of the DualBoostTM conceptfurther. Standard back to back tests were made to evaluate the vehicles performance anddrivability. It should be noted that no change to the production calibration was made and

therefore the DualBoostTM performance shown here is not yet considered to be optimized.

6.1. Vehicle Performance

Figure 8 shows a direct comparison for a wide-open throttle (WOT) acceleration from 0-60kph in 1st gear. The first thing to note is that the acceleration took approximately 3seconds.

Both the engine and vehicle speed curves show improvement but it‟s the vehicle acceler a-tion that shows the significant advantage brought by the DualBoost™ around 1500ms af-ter the kick-down at t = 2 seconds.

+1,8%

+2,7%

+2,5%

+2,6%

1,6L I4 Gasoline Simulation


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Figu re 8:- 0-60kph , Wide Open Thro tt le in 1st Gear

Figure 9 takes a more detailed look at the same maneuver. The classic jump in „naturallyaspirated‟ engine torque is clearly visible immediately just after kick-down for both cases.The DualBoostTM turbochargers‟ acceleration starts immediately because of superior tran-sient efficiency and low inertia. Its acceleration rate is evident to see, approximately 2x

faster than the benchmark competitor unit. This in turn is matched by rises in airflow andboosted torque, after approximately 1000ms. The immaturity of the calibration is clear tosee as the turbo speed drops in the later part of the acceleration, before acceleratingagain towards the end, showing that the results from Figure 8 are probably understated.

Figur e 9:- 0-60kph , Wide Open Thro tt le in 1st Gear

~ +1m/s²

~ +400rpm

~ +5kph

~ +130Nm ~ +200kg/hDualBoost

Acceleration

150k rpm/s

Competitor

2,0L I4 Gasoline

2,0L I4 Gasoline


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6.2 Vehicle Drivability

Standard test procedures have been developed by the car industry over many years todescribe the transient behavior of an engine. The metric for the gasoline engine is typicallythe response to a sudden throttle opening from equal and low constant speed and torque.

Figure 10 demonstrates the engines torque response to a WOT step from 1500 rpm en-gine. The 2x faster response of the DualBoost™ is again obvious to see. It is also notablehow smooth and harmonious the rise in engine torque is compared to the production unit. A delta of around 95Nm of torque was measured after just 1000ms.

Figu re 10:- Tip-in, 1500 rpm , 4 th Gear

There is a definite limit to the downsizing of a gasoline engine, which is determined by thecapability of the engine and available transmission to launch the vehicle. Specifically ma-nual transmissions require sufficient immediately available low speed torque for the ta-keoff event. Insufficient engine torque requires increased slip speeds, which lead to over-heated launch clutches.

The tip-in behavior at 1200 rpm engine speed is a good measure of the launch perfor-

mance of an engine. The faster the boost pressure is available the lower the heat lossesin the launch clutch.

Figure 11 demonstrates the DualBoost™ performance with the vehicle in 6th gear under

these launch conditions. The turbocharger speed again rises more spontaneously andfaster than the production turbocharger. At this lower speed and higher gear there is stillsome delay but after 1500ms the developed torque is 95Nm higher than the competitor.

~ +95Nm

2,0L I4 Gasoline


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Figu re 11:- Tip-in, 1200 rpm , 6 th

Gear

The key technical enabler for the rapid increase in engine torque is the faster rise of theboost pressure in the inlet manifold. This pressure rise is a direct result of the fast rota-tional acceleration of the turbocharger rotational group. As already discussed in the de-scription of the DualBoost™ concept, it is the combination of the excellent bearing effi-ciency, the increased aerodynamic efficiency at low U/C0 and the low inertia of the entirerotor that enable this extraordinary transient performance.

7. Summary and Outlook

By re-examining the fundamental aerodynamic design of a gasoline turbocharger, Honey-well has been able to demonstrate a new turbocharger concept that :-

has equivalent steady-state and fuel economy to a conventional turbo.

has superior low speed transient efficiencies

has 50% less inertia compared to a conventional turbocharger

uses only conventional materials and simple fixed geometry.

As a result of this it can :-

accelerate 2 times faster than its benchmark competitorprovide more than 25% reduction in „time to torque‟ at low engine speeds

deliver more than 20% more torque after the first second of a high gear transient.

Thus the concept is believed to be a key enabler for gasoline engine down-sizing anddown-speeding which in turn will deliver improvements in fuel consumption and CO2 re-duction that are not achievable with conventional turbochargers with compromising drive-ability.

HTT is continuing to improve and mature the aerodynamic designs of both the compressorand turbine and is also engaged in qualifying the concept for series production.

~ +95Nm

2,0L I4 Gasoline


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8. References / Literatur

[1] J. Lotterman, N. Schorn, D. Jeckel, F. Brinkmann and K.-H. Bauer: New Turbo-charger Concept for Boosted Gasoline Engines, 16th Supercharging Conference,Dresden, 2011.

[2] Sonner, M., Wurms, R., Heiduk, T., Eiser, A. : Unterschiedliche

Bewertung von zukünftigen Auflandekonzepten am stationärenMotorprüfstand und im Fahrzeug. 15. Auflandetechnische Konferenz,Dresden, 2010

[3] Kapp, D., 2009, Powertrain Strategies for the 21st Century, “Focus on the Future” Automotive Research Conference, Univ. of Michigan

[4] Grebe, U., Könegstein, A., Wu, K-J., Larsson, P-I., 2008, Differentiated Analysis of

Downsizing Concepts (MTZ 062008, vol 69).

[5] Baines, N., 2002, Radial and Mixed Flow Turbine Options for High Boost Turbo-chargers, 7th International Conference on Turbochargers and Turbocharging.

[6] Hagelstein, D., Theobald, J., Michels, K., Pott, E., Vergleich verschiedener Aufladeverfahren für direkteinspritzende Ottomotoren.

[7] Balje, O.E., 1981, Turbomachinery: A guide to Design, Selection and Theory (JohnWiley & Sons, New York, 1st edition).

Next generation of Gasoline turbo technologies

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