You will find the figures mentioned in this article in the German issue of MTZ 06I2007 beginning on page 480.

Miller- und Atkinsonzyklus

am aufgeladenen Dieselmotor

Miller- and Atkinson-Cycle on a

Turbocharged Diesel Engine

Authors:Eberhard Schutting, Andreas Neureiter, Christian Fuchs, Thorolf Schatzberger, Manfred Klell, Helmut Eichlseder and Thomas Kammerdiener

Using a variable valve train on a diesel engine offers a number of alternative gas-exchange strategies. One of these is the shift of intake valve closure, on which this paper will concentrate. It is better known as the Miller-Cycle when shifted to early, or Atkinson-Cycle when shifted to late closure. Both processes were investigated at Institute for Internal Combustion Engines and Thermody-namic at TU Graz within the project “KNET-VKM der Zukunft”. This Austrian project is state-aided within the program Industrielle Netzwerke und Kompetenz-zentren.

1 Introduction

The shift of intake valve closure (IVC) is an established technique for spark ignition combustion systems [1, 2]. However, the present paper discusses the application to a turbocharged passenger-car diesel engine. Thereby the targets and effects partly differ from the SI-application.

The Miller-Cycle and the Atkinson-Cycle are basically two independent gas exchange strategies, but they are closely related. Both aim to decrease the process temperature by means of a lowered effective compression ratio. This can be equated with lower NOx emissions in general.

The first two chapters shall outline the idealised principles of the two strategies. Effects of gas dynamics and wall heat flow are neglected [3].

By closing the intake valve in the bot-tom dead centre the maximum charge mass will be obtained, and the compres-sion will take place with the maximum compression ratio. Shifting the intake valve closure, the inlet mass and the effective compression ratio will decrease. Thereby it

makes no difference whether it is shifted to early or late. The determining factor is the cylinder volume at IVC, see pressure\vol-ume-diagram, Figure 1. In the Atkinson case a part of the charge is expelled back to the intake port (2-3), in the Miller case an ex-pansion and compression occurs (1-2-3). At point 3 the charge condition in the cylin-der is the same for both cases.

A decrease of charge mass is generally not acceptable in diesel application. Hence the decreased effective swept volume has to be compensated by a higher charging pres-sure, Figure 2. The increase of charging pres-sure corresponds thereby to the decrease of cylinder compression. Thus the application of Miller or Atkinson can be seen as an out-sourcing of compression from the cylinder to the external charger.

2 Thermodynamics

The lower temperature due to the de-creased compression ratio would be wast-ed by increasing the charging pressure, since the compression in the external

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charger increases the temperature as well. This is all the more the case, as a charger always has a lower isentropic efficiency than a cylinder. Due to that Miller and At-kinson cycle can only bring a decrease of temperature, when a charge air cooler is applied. Hence one can characterise Miller and Atkinson cycle as follows: The com-pression is split between a charger and the cylinder whereby an intermediate cooling becomes possible.

The changes in state of a conventional cycle and a Miller/Atkinson cycle (M/A) are plotted in a Temperature/Entropy-diagram in Figure 3 (expansion and compression of Miller cycle are neglected). Starting from ambient pressure p0 the air is compressed to p1 for a conventional cycle and to the higher pressure p1‘ for M/A. Thereby a non ideal compressor is assumed. The cooling in the charge air cooler bases upon the same efficiency, but due to the higher tem-perature difference the decrease is higher in the M/A case. The in-cylinder compres-sion raises the pressure to p3, or p3‘ respec-tively. The postulation of constant mass implies the same specific volume v3. The advantage for M/A in temperature at the end of compression is evident.

As a matter of fact, the achievable tem-perature advantage is dependent on com-pressor efficiency and charge air cooler ef-ficiency. An advantage does only occur, if the cooler is able to outweigh the increased compressor outlet temperature. If this is the case, then the potential increases with the efficiencies. In Figure 4 one can see the maximum potential of temperature de-crease versus shift of intake valve closure. The compressor efficiency (left) and the charge air cooler efficiency (right) are plot-ted as parameter.

The capability of decreasing tempera-ture is not only dependent on the path of fresh air, but also on the exhaust gas recir-culation (EGR). As discussed above, the in-crease in charge temperature reduces the temperature decreasing effect of the lower compression ratio. When admixing recir-culated exhaust gas to the compressed fresh air, the intake temperature is no long-er dependent on charging temperature on-ly, but also on exhaust temperature. There-by it happens that the increase of the re-sulting mixing temperature is lower than the increase of pure charging air, Figure 5 (left). Thus the potential of M/A is increased when applying EGR, because the disadvan-tageous charge temperature increase is smaller. In Figure 5 (right) one can see the theoretical potential versus shift of IVC with EGR rate as parameter.

3 Gas Exchange

If effects of gas dynamics and wall heat flow are neglected, then the Miller- and the Atkinson-cycle are identical in their results and impacts. Of course this is not the case in a real engine, various effects bring about a difference between these two strategies.

Pulsations in the intake manifold and effects of inertia definitely have a strong influence. Inertia effects usually bring an advantage for the Atkinson cycle, whereas pulsations can affect both strategies in a beneficial or non beneficial way, depend-ing on operating point and intake valve closing time.

Another difference arises from the finite closing velocity of the intake valve. Due to significantly increasing flow resistance for small valve lifts, the mass flow passing the valve drops already before the nominal closing time – the actual impact on the valve seat. This means that in case of early IVC the expansion starts even earlier, while in case of late IVC the compression starts not that late. Hence Miller is intensified, whereas Atkinson is weakened.

Considering wall heat flow effects one can find a systematic disadvantage for the Atkinson cycle. Atkinson expels heated gas to the intake port, where it is cumulated. Thus the intake temperature of the follow-ing cycle is increased reducing the temper-ature decrease. Figure 6 shows higher com-pression temperature of Atkinson for dif-ferent charging pressures. For large shifts of IVC the hot gas is even re-expelled to the intake manifold.

4 Overall Behaviour

As discussed in chapter 2 one can see the application of Miller or Atkinson Cycle to a turbocharged diesel engine as a kind of splitting the compression between cylin-der and charger. The potential is therefore significantly dependent on the potential of the turbocharger. The IVC can be shift-ed as long as an increase of charging pres-sure is possible. Thereby the limitation can come from the compressor (surge lim-it, rotational speed) or from the turbine (VNT position). An evaluation of the poten-tial of M/A application can only be made by a consideration of the entire air man-agement system.

The presented evaluation of an appli-cation to a passenger car engine has been made by means of a one dimensional gas flow simulation (BOOST/AVL). There the entire air management system is mo-

delled in form of zero and one dimension-al elements.

The considered engine is a 5-cylinder passenger car engine. This engine is equipped with a two stage serial-sequential turbocharging system. As discussed above, the potential of M/A is mainly dependent on the charging pressure potential of the charging unit. A two stage system provides a much higher pressure than a single stage system. Hence it is a very advantageous pre-condition for M/A application.

The results of a simulation of intake valve shift are plotted in Figure 7, exempla-rily for a load point with 2000rpm and 12 bar IMEP. Starting from operating pa-rameters that correspond to typical serial application, namely EGR=15%, charging pressure = 800 mbarrel and air excess ratio of 1.5, the IVC was shifted to early and late. To maintain a constant charge condition, that is to say same Rate of EGR and same air excess ratio, the VNT position and EGR valve were readjusted. The results are plot-ted versus intake valve closing. 540deg cor-responds to BDC and therefore to conven-tional valve timing with maximum air de-livery ratio (does only apply for idealised trapezoid valve lift curve). All earlier clos-ing times correspond to Miller cycles, all later ones to Atkinson cycles.

A shifted IVC results in a decreased volu-metric efficiency. To maintain a constant charge mass the charging pressure was in-creased by closing the VNT. This results in an up to 15K increased charging tempera-ture. In spite of this disadvantage the ad-vantage in compression temperature can reach up to 38K.

Different from conventional valve strat-egy, the increase of charging pressure does not at first effect the indicated specific fuel consumption (ISFC). This is caused by the almost unchanging exhaust temperature due to the constant mass flow. It is not un-til the turbine efficiency drops, that a no-ticeable increase in ISFC can be found. Of course this behaviour is dependent on the engine operating point, but it was noticed in all investigated load points. Furthermore an advantageous shift of turbocharger op-erating points could often be found, at least for little IVC shifts.

However, the main target of Miller and Atkinson cycle application is not the de-crease of compression temperature, but the decrease of engine out NOx emissions. The mean temperature in the combustion chamber is indeed an important factor for NOx formation. Its decrease was therefore regarded as significant result of the simula-tion (at least for constant charge mass and

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Diesel Engines

composition), but there are other influenc-ing parameters like O2-concentration, flame temperature, mixture formation et cetera. An absolutely reliable conclusion about NOx formation is not possible by means of simulation only, hence additional test bed measurements were conducted.

5 Measurement

The engine used was a single cylinder re-search engine with passenger car-size dis-placement [4]. Besides other features this engine is equipped with a fully variable valve train and a fully variable charge con-ditioning system.

The measurements were based upon the previous conducted simulations. The set-tings for boost pressure, exhaust back pres-sure and particularly intake air tempera-ture were taken directly from the simula-tions. Thus it was ensured, that all bound-ary conditions are corresponded to real engine operation. The transferability of these boundary conditions could be veri-fied by comparison of air excess ratio and specific fuel consumption.

The measurement results for the engine load point described in chapter 4 are shown in Figure 8. They are plotted versus a simula-tion-related IVC, due to the difference be-tween theoretical and real valve lift curve.

Most interesting measurement result is the NOx emissions. Thereby a noticeable re-duction has been found. Starting from a NOx level of 3.4g/kWh a minimum emis-sion of 2.4g/kWh could be reached. 2.5 g/kWh are possible with almost neutral ISFC, what means a reduction of 25 %. The Miller cycle could achieve the higher reductions. Smoke and carbon monoxides remain nearly constant, hydrocarbons dropped sig-nificantly.

6 Simulation of In Cylinder Flow

The incylinder flow has a major impact in the functionality of a modern diesel com-bustion system. An influence of Miller- and Atkinson cycle to the incylinder flow has to be taken into account.

To obtain information about these influ-ences, the flow conditions of the intake stroke were investigated by means of three dimensional computational fluid dynamic calculation. These calculations were based upon the above mentioned measurements on the research engine, from which the boundary conditions were taken. The calcu-lation model covers the entire combustion

chamber, the intake and exhaust ports, in-cluding the position of port indication.

In Figure 9 the results of the CFD-Simula-tion are shown, each as side section and for the ignition dead centre. One can see the distribution of turbulent kinetic ener-gy and amount of local velocity. The Miller-Cycle leads to a significant drop of both parameters, whereas the Atkinson cycle results in an almost identical condition, compared to conventional cycle. This dif-ference may be, amongst others, an ex-planation for the different behaviour of emission formation, monitored during the measurements.

7 Comparison of Miller with EGR

The objective of Miller and Atkinson cycle is the decrease of engine out NOx emis-sions. Hence they have to compete against the most commonly used NOx-reducing technique, the external exhaust gas recir-culation.

Comparing these methods one has to consider this comparison carefully, in or-der not to penalize the one or the other. Two aspects are determining: On the one hand M/A and EGR are no isolated meth-ods, but can be combined. On the other hand both are limited in their capabilities mainly by the charging system. Thus the same turbocharger characteristics have to be applied to both, i.e. same VNT position in this case. From these circumstances a procedure for engine simulation was de-rived as follows: Starting from a load point with conventional valve timing and with-out exhaust gas recirculation the EGR valve was steadily opened, while VNT was held constant. This causes an increasing EGR rate and a decreasing boost pressure. In two of the arising load points the IVC was shifted. Thereby VNT was held constant as well, but EGR valve was adjusted to main-tain the initial EGR rate. The results of the simulation served, as before, as boundary conditions for measurements on the re-search engine. Concluding from the previ-ous investigations the IVC was shifted to early only, due to the higher capabilities of the Miller cycle.

In Figure 10 one can see the effects of this variation on gas exchange and emissions. In the left-hand diagram the decrease of boost pressure with increasing EGR rate is clearly to see (1-4). In the load points with 4 % and 11 % EGR an IVC shift was applied (2-2‘ and 3-3‘). This brings about a further decrease of boost pressure, EGR was held constant.

In the right hand diagram the measured CO and particle emissions and the indicat-ed specific fuel consumption are plotted, each versus measured NOx emissions. The Emissions behave in a normal way for the EGR-variation, they increase with decreas-ing NOx. The fuel consumption remains more or less constant. The comparison to the Miller cycle shows, that these correla-tions are less advantageous. CO as well as particle emissions are higher for same NOx. At least the specific fuel consumption re-mained unchanged.

8 Conclusion

The shift of the intake valve closure, ap-plied to a turbocharged diesel engine, is an appropriate technique for NOx reduction, thus without disadvantages in particles, HC, CO and fuel consumption. The poten-tial is thereby almost exclusively limited by the capabilities of the charging system.

However, compared to external exhaust gas recirculation, which is the most com-monly used NOx reducing technique, no advantages could be found, but some disad-vantages. A supplementation or substitu-tion of exhaust gas recirculation is not rea-sonable. The application of Miller- or Atkin-son-cycle to a diesel engine does not make sense at the moment, if an EGR system is applied.

References[1] Herdin, G.; Gruber, F., Klassner, J.; Matsumura, S.;

Kudo, S.; Ippommatsu, M.: Miller Cycle-Efficiency

Potentials for Gas Engines. Cimac Paper 197, Kyoto,

2004

[2] http://www.toyota.co.jp/en/tech/environment/ths2/

engine

[3] Neureiter, A.: Untersuchungen von frühem und spätem

Einlassschluss zur Spitzentemperaturabsenkung bei

aufgeladenen Dieselmotoren. TU Graz, Diplomarbeit,

2006

[4] Kammerdiener, T.; Fürhapter, A.; Unger, E.; Ducellari,

R.: Ein einfaches variables Ventiltriebssystem zur Re-

alisierung alternativer Verbrennungssysteme bei Otto

und Diesel. Haus der Technik in Essen, 14.03.2004

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