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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.Eng Honeywell 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 higher BMEP 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 turbine stages. It has been able to increase overall turbo efficiencies, especially at low speeds and 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 never been achieved with conventional gasoline waste gate turbochargers. The concept demon-


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

Kurzbeschreibung: Die in den letzten Jahren erzielten Fortschritte beim Downsizing von Benzinmotoren haben die Grenzen des konventionellen Turbolader-Designs aufgezeigt, wenn es darauf ankommt, mehr Drehmoment bei niedrigen Geschwindigkeiten sowie ein effizientes Einschwing- und Teillastverhalten zu realisieren. Der zunehmende Trend zu einem höheren BMEP bei sehr niedrigen Motorgeschwindigkeiten zwingt die Entwickler von Turboladern, 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 der Turbinenphasen neu definiert. Das Unternehmen konnte die gesamte Turboeffizienz verbessern, insbesondere bei niedrigen Geschwindigkeiten und in transienten Zuständen. In Kombination mit der wesentlich verringerten mechanischen Materialträgheit hat dies zu maßgeblichen Verbesserungen der transienten Drehmomentreaktion des Motors geführt. Diese Präsentation weist ein Maß an Motor- und Fahrzeugleistung auf, das mit herkömmlichen Wastegate-Benzinturboladern niemals erreicht werden hätte können. Das Konzept demonstriert Durchbrüche im Bereich der transienten Motorleistung ohne den Einsatz exotischer Materialien, wie etwa Titanaluminiden, oder die zusätzliche Komplexität von Turbinen variabler Geometrie.


1. Introduction The main reason to boost any engine is to increase its‟ specific torque and power density to drive downsizing and down-speeding, which in turn lead to better fuel economy whilst maintaining the vehicles dynamic performance. Turbocharging has long been the stan-dard technology used to boost diesel engines in passenger vehicles, On-Highway trucks and Off-Highway machines. The majority of gasoline engines however are still naturally aspirated 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 for gasoline for several reasons. Higher exhaust gas temperatures in gasoline engines are of course a factor, cost is another but the main reason is that the air mass flow varies much more than in a gasoline engine than in a diesel. A ratio of 80:1 from idle to rated power for a 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 values this 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 or down-speeding. This in turn can realize the further reductions in fuel consumption and CO2 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 has equivalent overall efficiencies to its conventional competitors but boasts higher turbine efficiencies 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 of variable 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 the same or better full-load steady-state torque and BSFC. This paper presents both the concept and the major effects before going on to present engine and vehicle results that substantiate these claims.


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 to achieve 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 if they are to perform in the same way.

Equation (i) - Vehicle Kinetic Energy Equation (ii) – Acceleration Power

This simple concept allows us to calculate the target Power, BMEP and Time to Torque curves for a typical downsizing and down-speeding problem statement. The baseline used is 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 1500rpm b) Down-sizing 11% from 1,8 to 1,6 Litre c) Combined case

Figure 1 :- Down-sizing targets

The numerical results are shown in Table 1 below. The combined case produces targets of 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.


Engine

Size Speed BMEP Torque

Time to

Torque

Time to

Torque

50-90%

Boosted

Torque

Slope

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 75 188

b 1.6 1750 18.8 240 2.32 1.91 59 162

c 1.6 1500 22.0 280 2.00 1.71 95 185

Table 1 :- Down-sizing 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 the turbocharger rotor. Thus the equations become.

Equation (iii) - Turbo Kinetic Energy Equation (iv) - Acceleration time

and expanding the power term to brgcompturbaccel PPPP gives

Equation (v) – Acceleration time (expanded)

3.1. Turbine Efficiency

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


The bulk of the energy in the exhaust is in the high pressure portion of each pulse as seen in 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 extract energy and accelerate quickly. Improving the turbine efficiency at low U/Co conditions would clearly benefit both the transient and steady-state performance of the turbocharger and engine.

Figure 2 :- Pressure, Mass flow & U/Co vs. Crankshaft rotations

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 a given 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 cen-trifugal compressor paired with a radial turbine. Axial turbines have the advantage over radials 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).


Fig. 3a :- Turbine efficiency vs. U/Co Fig. 3b :- Inertia vs. Turbine Flow

Compressor side DualBoostTM

Turbine side

Standard Rotor

Figure 4 :- Outline of Standard and DualBoostTM Rotating Group

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 an equivalent flowing radial turbine.

Pairing it with a double-sided parallel flow compressor serves multiple purposes. Firstly, it accelerates the turbine further up the U/Co curve as its rotational speed is higher for a given 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

Radial

Axial


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

5. Engine Test Results A DualBoost™ turbocharger has been tested against a conventional radial device. The testing 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 a 2:1 expansion ratio. Fig. 5a shows that both were capable of achieving the target full-load steady-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™ rises much more steeply than for the standard turbocharger. 180Nm was reached 450ms earlier and 270Nm was attained more than 600ms before the baseline.

Figure 5a:- Steady-State Performance Figure 5b:- Transient Torque

Combining the load step results from different engine speeds the overall improvement that the 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 engine speeds. This is due to the ever reducing amount of turbine exhaust energy available to accelerate the rotor group and the increasing significance that reduced inertia has at these operating points.

450ms

600ms

Load step from 1500rpm 1,6L I4 Gasoline

Standard Turbo

DualBoostTM


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 to go 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 performance on 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 the DualBoostTM 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 the range of 1,8 – 2,7% across these cycles. The more dynamic cycles like FTP75 and US06 naturally show the largest improvements.

450ms

1000ms

DualBoostTM

Standard Turbo

1,6L I4 Gasoline


Figure 7:- Fuel economy with shorter 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 concept further. Standard back to back tests were made to evaluate the vehicles performance and drivability. 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 0-60 kph in 1st gear. The first thing to note is that the acceleration took approximately 3 seconds. Both the engine and vehicle speed curves show improvement but it‟s the vehicle accelera-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


Figure 8:- 0-60kph, Wide Open Throttle in 1st Gear

Figure 9 takes a more detailed look at the same maneuver. The classic jump in „naturally aspirated‟ 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 and boosted torque, after approximately 1000ms. The immaturity of the calibration is clear to see as the turbo speed drops in the later part of the acceleration, before accelerating again towards the end, showing that the results from Figure 8 are probably understated.

Figure 9:- 0-60kph, Wide Open Throttle in 1st Gear

~ +1m/s²

~ +400rpm

~ +5kph

~ +130Nm ~ +200kg/h DualBoost

TM

Acceleration

150k rpm/s

Competitor

2,0L I4 Gasoline

2,0L I4 Gasoline


6.2 Vehicle Drivability Standard test procedures have been developed by the car industry over many years to describe the transient behavior of an engine. The metric for the gasoline engine is typically the 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 notable how 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.

Figure 10:- Tip-in, 1500 rpm, 4th Gear

There is a definite limit to the downsizing of a gasoline engine, which is determined by the capability 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 losses in 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 and faster than the production turbocharger. At this lower speed and higher gear there is still some delay but after 1500ms the developed torque is 95Nm higher than the competitor.

~ +95Nm

2,0L I4 Gasoline


Figure 11:- Tip-in, 1200 rpm, 6th Gear

The key technical enabler for the rapid increase in engine torque is the faster rise of the boost 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 entire rotor 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 competitor

provide 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 and down-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 compressor and turbine and is also engaged in qualifying the concept for series production.

~ +95Nm

2,0L I4 Gasoline


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 stationären Motorprüfstand 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 (John Wiley & Sons, New York, 1st edition).

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