6 MTZ 12|2006 Volume 67

Authors:Lucas Flückiger, Stephan Tafel and Peter Spring

Hochaufladung mit Druckwellenlader für Ottomotoren

You will find the figures mentioned in this article in the German issue of MTZ 12|2006 beginning on page 946.

Pressure-Wave SuperchargedSpark-Ignition Engines

The modern generation of pressure-wave supercharger(PWS) called Hyprex offers a new approach for super-charging gasoline engines. This type of PWS has the po-tential to replace complex and costly two-stage super-charging systems. Thereby, the base engine may be keptsimple and small, with minimized on-costs. This articledocuments the results of the collaboration between Swiss-auto Wenko AG, the Measurement and Control Laboratoryof ETH Zürich, and Robert Bosch GmbH.

1 Introduction

Due to the increasing uncertainty in thecrude oil market and the rise in the gaso-line prices fuel consumption becomes animportant aspect of the purchasing deci-sion for the customer. At the same time,customers do not accept compromises indriveability, dynamics, or comfort. Thecurrently favoured diesel technology isseen with a critical view since the increas-ing complexity in both exhaust gas af-tertreatment and injection systems willcause further cost increase. These repre-sent major challenges for the car manufac-turers/OEMs, since in the high-volume seg-ment of the market, the substantial add-oncosts for the required technologies cannotfully be passed to the end user.

Due to the aforementioned reasonsidentified above, gasoline engines havebeen regaining ground once lost to dieselengines. The technologies for achievingthe lower consumption targets for gasolineengines are well known and documented[1-4]. Measures to reduce fuel consumptionon spark ignited (SI) engines have the em-phasis on reducing the pumping losses –the main efficiency drawback compared tocompression ignition engines. Hereby theso-called “downsizing” with its benefit inpart load operation in higher mean effec-tive pressure ranges with better efficiencyis one of the most attractive means. There-by the base engine can be kept relativelysimple with an aftertreatment system withwell proven technology. The possible re-duction in swept volume (downsizing fac-

DEVELOPMENTSupercharging

7MTZ 12|2006 Volume 67

tor), which indicates the relative reduc-tion, is primarily limited by the torquecompensation potential of the supercharg-ing device. The sufficient drive-awaytorque, transient torque build-up and max-imum torque and power out put are in par-ticular limiting. These performanceboundaries are hard to overcome with con-ventional, single stage charging systems.Combinations of such supercharging tech-nologies can improve the situation [5, 6],but quickly result in an unacceptable cost-to-benefit equation.

The PWS offers essential advantagesand can solve the trade-off problem. Witha single supercharging stage, the down-sized engine can simultaneously achievehigh specific torque and power outputwhile maintaining the SI – engine per-formance characteristics. Since PWS donot have a surge limit and exceed the en-gine relevant pressure ratios, they becomean attractive solution for the conflict of ob-jectives. The Hyprex PWS is a derivative ofthe known Comprex technology and al-lows the matching of the PWS process tothe temperature and mass flow spread ofthe SI-engine. Due to its simple layout, costeffective solutions for engines of less than1 l displacement can be realized.

The article documents recent resultsand demonstrates the potential of the PWSHyprex technology.

2 Downsizing and Supercharging

Downsizing, i.e. the reduction of the en-gine’s displaced volume, is one of the mostcost effective solutions for the fuel con-sumption reduction of gasoline engines[7]. The associated torque and power reduc-tion is compensated by supercharging.Thereby, the achievable boosting pressureand therefore the torque just above idlespeed are limited by mechanical bound-aries or system complexity of the chargingdevice. Therefore the charging device be-comes the main focus when looking atsuch engine systems. Currently availablecharging technologies cannot fulfil theperformance goals as individual systemsdue to unattractive cost-benefit trade off.[8, 9]. Combinations of gasoline direct-in-jection and variable valve-timing with tur-bocharging create synergy effects with apositive outlook in market [10].

More advanced supercharging systemssuch as two-stage turbocharging and elec-trically supported charging systems im-prove the torque dynamics, which aremainly limited by the charging device it-self. Further, high boosting pressure and

two fluids of different pressures arebrought into direct contact, pressureequalization is faster than mixing.

In simple terms, the pressure-wave pro-cess of the PWS can be divided into two pri-mary elements: the high-pressure, and thelow-pressure processes, Figure 2. In the for-mer, the exhaust gas enthalpy available inchannel 3 is used for supercharging thefresh air flowing into channel 2. Leading aportion of the exhaust gases through thegas pocket valve on the one hand reducesthe exhaust gas pressure in channel 3 andtherefore lowers the compression of thefresh air in channel 2. On the other handthe exhaust gas enthalpy flowing throughthe gas pocket valve increases the pressurein the cell just before the low-pressure sec-tion starts, which improves scavenging.The desired function of the low-pressuresection is to scavenge the cell towardschannel 4 and to fill it up again with freshair from channel 1. Under normal operat-ing conditions, channel 4 is not only filledwith exhaust gases, but also with fresh air,which has been compressed in the high-pressure section and did not flow intochannel 2. The part of fresh air in channel4 is called scavenging air. It can be reducedby lowering the pressure in channel 1. Theless scavenging gasses present, the higherthe temperature in channel 4. This behav-iour may be used to control the oxidationconditions in the catalytic converter,which is located downstream of the PWSunit.

Through continued improvements ofthe PWS, the known problems are elimi-nated to a large extent and both, theachievable pressure ratio and efficienciesare increased. Table 1 shows the main mod-ifications relative to the Comprex andtheir influences.

5 Results

This article focuses on the potential of thesupercharging device. A discussion of thetypical engine maps is therefore skippedsince these maps largely depend on the in-trinsic performance of a given internalcombustion engine.

The system under consideration con-sists of a 1.0 l, 4-cylinder 4-stroke spark-ig-nition engine and a HX95 PWS. The engineis manufactured by VW Brasil and is usedin the VW Gol where it is originallyequipped with a Garrett GT-14 turbocharg-er. In this layout the engine is rated at apeak torque of 158 Nm at relatively low2000 rpm. The peak power of 77 kW at5500 rpm is however state-of-the-art.

good transient response can be achieved atlow engine speed. Aside from the costs,these systems suffer from high complexityand other drawbacks. In Figure 1, qualita-tive steady-state, achievable full load pres-sure ratios for different single-stage super-charging devices at 1500 rpm and 2000rpm are plotted over engine displacement,in typical layouts.

The combination of a mechanicalcharger (MC) and a turbocharger (TC)arranges two devices that suffer from poorefficiency at low mass flows. Electricallyassisted turbochargers (EATC) consumevery significant amount of power for a sub-stantial improvement in the dynamic per-formance and are based on a single-stagecompressor with its well known draw-backs. A similar situation exists with two-stage concepts with an electrically assist-ed, second compressor. The PWS repre-sents an appealing solution to this trade-off problem.

3 The Pressure-Wave Supercharger

Initial experiments of the wave rotor appli-cation as a topping unit for locomotive gasturbines were made by Brown Boveri Com-pany (BBC) in the 1940s [11]. Under the su-pervision of Kantrowitz and Berchtold, sev-eral units were successfully manufacturedand tested on vehicle diesel engines from1947 to 1955 by the I-T-E Circuit BreakerCo. in the USA. In the 1970s, a vast varietyof prototypes were built and tested by BBCon truck engines, on passenger car dieselengines and on tractors. In 1987, the PWSactivity was sold to Mazda. That companythen produced 150,000 diesel passengercars equipped with PWSs. In 1998, therights to the PWS were sold to SwissautoWenko AG. Later known as Swissauto Engi-neering SA (SESA), this is the only compa-ny known to the authors that is currentlyproducing modern versions of PWSs. Thenew generation of wave rotors known asHyprex PWS is designed for small gasolineengine applications. The cover pictureshows an example of a PWS for gasoline en-gines rated from 1.0 to 1.2 litres displace-ment and peak power of 100 kW.

4 Supercharging with PWS

In pressure-wave machines such as thePWS, pressure-wave exchanger or wave-ro-tor energy is transferred between twogaseous fluid streams by short-time directcontact of the fluids in narrow flow chan-nels, the so-called cells. Pressure-wave ma-chines make use of the physical fact that if

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8 MTZ 12|2006 Volume 67

For better comparison, only the ancil-lary devices of the engine have been modi-fied. Table 2 summarizes the technical dataof the engine system.

The PWS’s main advantages over theturbocharger are the non-existence of asurge limit, the short response time to a

driver’s demand and higher pressure ra-tios over a very wide mass flow range. Inaddition, overall system complexity is re-duced since PWS neither require connec-tions to the cooling water nor to the oilsupply of the engine. The exhaust gas af-tertreatment concept that goes with the

Hyprex boosting system contains a three-way catalytic converter in front of the PWSfor fast light off and an oxidation catalyticconverter after the charger. In conjunctionwith the scavenging air in the exhaust gas,conversion rates of 0.99 are obtained forhydrocarbon emissions up to full load andrated speed.

Some of the advantages can be visual-ized in the compressor map. In Figure 3 thefull-load line of the turbocharged 1-literengine (red) shows the typical trend in theGT-14 compressor map. The maintaineddistance to the surge line is inevitable inorder to ensure safe operations of the TCduring transients. The nominal power po-tential is limited by the choke line. Thelines of constant engine speed indicate thedifferent trends in pressure difference overthe engine. When choosing a compressor,the trade-off between avoiding any surge,ensuring a minimal distance to the chokeline and operating the compressor at max-imum efficiencies needs to be considered.

In comparison, the full load line of thePWS (blue) is plotted with the correspon-ding compression efficiencies. It becomesclear, that the PWS offers benefits on lowengine speeds (PWS line is partially out-side the surge line of the TC map, boostpressure limiting is the available exhaustgas enthalpy) as well as at rated speed(PWS line outside the choke line).

Therefore the PWS as single-stage boost-ing device realizes both, high specifictorque and high power output simultane-ously. The excellent compression efficien-cies (0.66 to ∼1) of the PWS is rooted in thecooling effect of the rotor by the scaveng-ing air.

As expected, the engine results are ex-traordinary. As shown in Figure 4 the 1.0-liter engine reaches 2350mbar (absolute)boost pressure at 1500 rpm which is trans-formed to a break mean effective pressure(BMEP) of 26 bar or 200 Nm engine torque,respectively. The peak power of 100 kW isproduced at 5000 rpm. Compared to theturbocharged engine, these are gains of 14bar BMEP at 1500 rpm and 7 bar at 5000rpm. Hence, the PWS engine produces 200Nm and 100 kW per litre engine displace-ment with a single stage charging device.

The favourable pressure difference overthe engine allows very rich air-to-fuel ra-tios to be avoided, which reduces fuel con-sumption.

The dynamic behaviour is shown in Fig-ure 5 via a load step at 1500 rpm. 1 secondafter the immediate opening of the throt-tle body, the Hyprex engine achieves 1.5times the boosting pressure of the turbo

Typ I-4, SI (VW Golf)

Displaced volume dm3 0.998

Bore/stroke mm 67.1/70.6

Cam timing (In-/Outlet) °KW 83*/110/113

Valve diameter (In-/Outlet) mm 25.5/20.0

Valve lift (In-/Outlet) mm 8.4/7.6

Compression ratio – 9.2

Close coupled, three way catalyst mm x mm/CPSI/ 103 x 125/400

Catalyst downstream of Hyprex mm x mm/CPSI/μmm 110 x 110/300/50

* Early position

PWS type HX95

Number of cycles per revolution – 2

Drive – 12-V E-Motor

Maximum speed 1/min 18,000

Number of cells per cycle – 54

Rotor length mm 93

Rotor diameter (effective) mm 95

Load control GPV

Cold start/EGR casing offset

Table 2: Technical data of the PWS and engine

Comprex Hyprex Advantages

Rotor drive Fixed ratio from Variable speed, driven Optimal rotor speed forcrank shaft by electric motor all operating conditions

Rotor noise Reduced number Increased number of Reduced noise emissionsof cells, symmet- cells, asymmetrically from the PWSrically arranged arranged

Boost pressure Waste gate Gas pocket valve Boost pressure controlcontrol -> Exhaust gas is -> Exhaust gas is does not damage

passed by the passed by the scavenging processcharging device charging process only Higher boost pressure at

low engine speedsImproved compression efficiencies

Casing offset Fixed Variable, 0 to 20° Faster boost pressurebuild-up in warm-up.

Reduced EGR

Exhaust gas Not used Closed coupled three- High conversion rates up toaftertreatment way catalytic converter full load (hydrocarbons >0.9)

Optional oxidation Fast light offcatalytic converter downstream of PWS

Table 1: From Comprex to Hyprex: improvements at the PWS

9MTZ 12|2006 Volume 67

charged engine. Thereby it needs to be con-sidered that for better comparison onlythe periphery of the engine was changed,namely the exhaust piping with the charg-ing device. No measures were taken on theengine side.

6 Transient Phenomena and Model-Based Control

Exhaust gas recirculation (EGR) is a prob-lem occurring typically when PWSs areused as boosting devices for internal com-bustion engines Figure 6. During hard ac-celeration, critical situations arise whenev-er large quantities of exhaust gas are recir-culated over the charger from the exhaustmanifold to the intake manifolds of the en-gine. Such recirculations cause the enginetorque to drop sharply and thus severelyaffect the driveability of the vehicle.

The analysis of steady-state measure-ment data leads to the useful relation be-tween EGR rate, i.e. the exhaust gas massflow compared to the mass through Chan-nel 2, and the so-called scavenging rate,which is an indicator for the amount offresh air leaving through the exhaustChannel 4 [12-14]. High scavenging airmass flow not only reduces the exhaustgas temperature in Channel 4, but also in-dicates a low EGR rate. Alternatively, insuf-ficient scavenging causes the exhaust gasremaining in the cell to be wound up andreleased into the compressed air of Chan-nel 2. Figure 7 shows such a winding-upprocess based on the finite-difference mod-el developed at the Measurement and Con-trol Laboratory (IMRT) at the ETH Zürich[13]. An EGR control system was then de-signed and verified based on the fact thatthe EGR rate is linked to the scavengingrate, which allows the scavenging rate x_scto be used as the control variable. A lowEGR rate can therefore be guaranteed bykeeping the scavenging rate above a cer-tain level [14].

Modern PWS devices such as theHyprex provide the means to arbitrarilyset the offset angle between air and gascasing and the rotor is driven independ-ently of the engine speed by an electricmotor. Both these features allow the pres-sure-wave process to be tuned to match thechanging thermodynamic boundary con-ditions. The optimal choice of these con-trol actions is not so insignificant that apurely experimental approach is possible.The model-based open-loop control systempresented in [15] is able to predict the pres-sure wave dynamics and it defines optimalreference values for the two actuators.

7 Summary

In the near future, charging will become amajor focus also on SI engines. With thePWS, the downsizing potential for small SIengines can be assets in a fresh view. Thesimplicity of the hyprex charging deviceand its potential to simultaneously achievehigh torque and power densities make thePWS an attractive technical solution. In ad-dition, the combustion engine can be fur-ther reduced in displacement and the fuelconsumption lowered. Hereby, cost bene-fits relative to “multiple” stage chargingsystems can be expected. Under considera-tion of these boundary conditions, drive-trains can be evaluated anew, which willlead to reduced system costs.

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Berlin/Heidelberg: Springer, 2005[2] Bandel, W.; Fraidl, G.K.; Friedl, H.; Kapus, P.E.: Mehrw-

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[3] Flückiger, L.; Bohac, S.; Cowland, C.; Nehmer, D.:Development of a Hydraulic Valve Actuation EnginePart II: Impact on MPFI Engines. 2002 Global Power-tain Congress Ann Arbor, MI

[4] Guzzella, L.; Wenger, U.; Martin, R.: IC-EngineDownsizng and Pressure Wave Supercharging forFuel Economy. SAE 2000-01-1019

[5] Hagelstein, D.; Theobold, J.; Michels, K.; Pott, E.:Vergleich verschiedener Aufladeverfahren für direk-teinspritzende Ottomotoren. 10. AufladetechnischeKonferenz Dresden 2005

[6] Krebs, R.; Szengel, R.; Middendorf; H.; Fleiss, M.;Laumann, A.: Neuer Ottomotor mit Direktein-spritzung und Doppelaufladung von Volkswagen. In: MTZ 66 (2005), Nr. 11, S. 978-999

[7] Fraidl, G.; Kapus, P.; Piock, W.; Denger, D.; GasolineEngine Concepts Related to Specific Vehicle Class-es. ImechE Conference „21st Century EmissionsTechnology“ 2000, London

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[9] Müller, M.; Sumser, S.; Fledersbacher, P.; Rössler, K.;Hertweck, G.; Fieweger, K.; Bauer, H.J.: Ist quasi-drehzahlstationäre Abgasturboaufladung für Pkw-Ottomotoren möglich? 10. Aufladetechnische Kon-ferenz Dresden 2005

[10]Geiger, J.; Habermann, K.; Lang, O.; Vogt, B.;Wittwer, M.: Aufladung und Direkteinspritzung. In: MTZ 65 (2004), Nr. 12, S. 970-977

[11]Real, R.: The 3000 kW Gas Turbine Locomotive Unit.In: Brown Boveri Review 33 (1947), Nr. 10, S. 270-271

[12]Amstutz, A.:. Geregelte Abgasrückführung zurSenkung der Stickoxid- und Partikelemissionen beimDieselmotor mit Comprex-Aufladung. Zürich, ETH,Dissertation No. 9421, 1991

[13]Spring, P.: Modeling and Control of Pressure-WaveSupercharged Engines Systems. Zürich, ETH, Disser-tation No. 16490, 2006

[14]Spring, P.; Onder, C.; Guzzella, L.; submitted for pub-lication. EGR Control of Pressure-Wave Super-charged IC Engines. IFAC Journal of Control Engi-neering Practice

[15]Spring, P.; Onder, C.; Guzzella, L.; submitted for pub-lication. Fuel-optimized Control of a Pressure-WaveSupercharger: A Model-based Feedforward Ap-proach. Special Issue on Automotive Controls by theIEEE Transactions on Control Systems Technology