Next generation of Gasoline turbo technologies

Dipl.- Ing. K-H. Bauer; C. Balis, M.S.E. G. Donkin, Donkin, C.Eng; P. Davies, 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 limits of conventional turbocharger design when it comes to providing more low speed torque, transient response and partial load efficiency. The increased drive towards higher BMEP at very low engine speeds forces turbocharger engineers to rethink modern boosting layouts. Honeywell Turbo Technologies has taken a fresh look at the design of the gasoline turbocharger 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 inertias has provided significant improvements in engine transient t orque 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-

33. Internationales Wiener Motorensymposium Motorensymposium 2012

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.

33. Internationales Wiener Motorensymposium 2012

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 standard 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 engines 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 transient torque performance. Any enhancements in boosting systems that improve the engines‟ 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 aerodynamic 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 centrifugal 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 without 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 delivers 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 approximated 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

E q u a t i o n ( ii ) – A c c e l e r a t i o n P o w e r

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 b) Down-sizing c) Combined case

14% from 1750 to 1500rpm 11% from 1,8 to 1,6 Litre

F i g u r e 1 : - D o w n - s i zi n g t a r g e t s

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

Baseline a b c

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]

1.8 1.8 1.6 1.6

1750 1500 1750 1500

16.8 19.5 18.8 22.0

240 280 240 280

2.70 2.23 2.32 2.00

2.13 1.86 1.91 1.71

42 75 59 95

168 188 162 185

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.

E q u a t i o n ( i ii ) - T u r b o K i n e t i c E n e r g y

and expanding the power term to P accel

P turb P comp

E q u a t i o n ( iv ) - A c c e l er a t i o n t i m e

P brg gives

E q u a t i o n ( v ) – A c c e l er a t i o n t i m e ( ex p a n d e d )

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 unusual. 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.

F i g u r e 2 :- P r e s s u r e , M a s s f l o w & U / C o v s . C r a n k s h a f t r o t a t i o n s

3.2

Turbocharger Problem Statement

To conclude, in order to enable downsizing and down-speeding a new turbocharger design 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 ce ntrifugal 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 nonzero inlet angles for the blade. They are also intrinsically low in inertia (Fig 3b).


Axial

Radial Radial Axial

Fig. 3a :- Turb ine efficienc y vs. U/Co

Compressor side

Fig. 3b :- Inertia vs. Turb ine Flow

TM

DualBoost

Turbine side

S t an d a r d R o t o r F i g u r e 4 : - O u t l i n e o f S t an d a r d a n d D u a l B o o s t T M R o t a ti n g G r o u p

The DualBoost™ team at HTT has exploited all these phenomena and its‟ new axial turbine 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 balances the aero-dynamic thrust load in the machine; to give a quasi „zero‟ axial load concept in steady-state and thirdly it has lower inertia again than an equivalent flowing, larger


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 Peak Power

280 Nm (22 Bar BMEP) 1500-4500 RPM 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 devices. 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. Load step from 1500rpm

1,6L I4 Gasoline

DualBoostTM 600ms

450ms

Standard Turbo

F i g u r e 5 a : - S t ea d y - S ta t e P e r f o r m a n c e

Figure 5b:- Transient Torqu e

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 5090% 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.


1,6L I4 Gasoline

Standard Turbo

1000ms

450ms

DualBoostTM

Figure 6:- Sum mary 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 vehicle simulation to assess the potential impact of the DualBoost TM superior performance on fuel economy. The baseline Powertrain had a Final Drive Ratio (FDR) 4,067. It was calculated that lengthening 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 vehicle. 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.


1,6L I4 Gasoline Simulation

+1,8%

+2,7%

+2,5%

+2,6%

F i g u r e 7 : - F u e l ec o n o m y w i t h s h o r t e r FD R

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 DualBoost TM 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 acceler ation that shows the significant advantage brought by the DualBoost™ around 1500ms after the kick-down at t = 2 seconds.


2,0L I4 Gasoline

~ +400rpm

~ +5kph

~ +1m/s²

Figu re 8:- 0-60kph , Wide Open Thro ttle 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 transient 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. 2,0L I4 Gasoline

~ +130Nm

~ +200kg/h

DualBoost Acceleration 150k rpm/s

Competitor

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


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 engine. 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. 2,0L I4 Gasoline

~ +95Nm

th

Figu re 10:- Tip-in, 1500 rpm , 4 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 manual transmissions require sufficient immediately available low speed torque for the takeoff event. Insufficient engine torque requires increased slip speeds, which lead to overheated launch clutches. The tip-in behavior at 1200 rpm engine speed is a good measure of the launch performance 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 6 th 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.


2,0L I4 Gasoline

~ +95Nm

th

Figu re 11:- Tip-in, 1200 rpm , 6 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 rotational acceleration of the turbocharger rotational group. As already discussed in the description of the DualBoost™ concept, it is the combination of the excellent bearing efficiency, 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, Honeywell 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 reduction that are not achievable with conventional turbochargers with compromising driveability. 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.


8.

References / Literatur

[1]

J. Lotterman, N. Schorn, D. Jeckel, F. Brinkmann and K.-H. Bauer: New Turbocharger 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 Turbochargers, 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).


Next generation of Gasoline turbo technologies

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